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Novel poly(ethyleneglycol)-assisted synthesis of praseodymium oxide nanostructures via a facile precipitation route
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CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Novel poly(ethyleneglycol)-assisted synthesis of praseodymium oxide nanostructures via a facile precipitation route Sahar Zinatloo-Ajabshira, Masoud Salavati-Niasaria,b,n a Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran
b
Received 6 August 2014; received in revised form 20 August 2014; accepted 24 August 2014
Abstract The effect of the polymeric capping agent on the morphology and size of praseodymium oxide precipitated from praseodymium nitrate in the presence of ethylenediamine (en) as precipitator was investigated. En was used as precipitator to fabricate praseodymium oxide nanostructures for the first time. The products have been successfully synthesized in the presence of poly ethylene glycol (PEG) as capping agent via an improved precipitation route. The effect of amount of en on the morphology and size of praseodymium oxide was investigated. It was found that morphology and size of the products could be greatly affected by this parameter. The as-produced nanostructures were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet–visible (UV–vis) spectroscopy, photoluminescence (PL) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, and energy dispersive X-ray microanalysis (EDX). To investigate the catalytic properties of praseodymium oxide nanostructures, the photocatalytic degradation of 2-naphthol was performed. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: C. Optical properties; Ceramic; Praseodymium oxide; Nanostructures; Catalytic properties
1. Introduction In the past decade, much attention has been focused on the preparation and characterization of nanomaterials owing to their interesting properties and potential applications [1–4]. One of these nanometer-scale materials is praseodymium oxide. It was found that praseodymium oxide is an important rare earth metal oxide because of its specific optical and electrical properties [5,6] as well as potential applications, including ceramic pigments, catalysts, promoters and stabilizers in combustion catalysts, oxygen-storage components, and materials with higher electrical conductivity [7–11]. Praseodymium oxide forms a homologous series with a number of stoichiometrically defined oxides: PrnO2n 2, with n=4, 7, 9, n Corresponding author at: Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran. Tel./fax: þ 98 31 55912383. E-mail address: [email protected] (M. Salavati-Niasari).
10, 11, 12,…. Among these oxides, Pr6O11 is the most stable form at ambient temperature and pressure [12]. Up to now, very limited number of methods have been employed to synthesize praseodymium oxide nanostructures, such as, thermal decomposition [13], molten salt [14], hydrothermal [15], electrochemical [16], precipitation [12], and electrospinning [17]. The development of a simple effective route for synthesizing Pr6O11 nanostructures is of great importance to the potential investigations of its physical and chemical properties. It is good to know that properties of nanomaterials depend on their particle size and shape [18–20]. Therefore, exploring suitable methods to synthesize praseodymium oxide nanostructures and controlling their particle morphology and size is important. Herein, Pr6O11 nanostructures are synthesized via a simple precipitation route in the presence of PEG and ethylenediamine (en). The precipitation method is an appropriate synthesis process for preparation of many inorganic nanomaterials. This method is facile, convenient and cost effective synthetic
http://dx.doi.org/10.1016/j.ceramint.2014.08.105 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: S. Zinatloo-Ajabshir, M. Salavati-Niasari, Novel poly(ethyleneglycol)-assisted synthesis of praseodymium oxide nanostructures via a facile precipitation route, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.08.105
S. Zinatloo-Ajabshir, M. Salavati-Niasari / Ceramics International ] (]]]]) ]]]–]]]
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procedure and provides an effective way to the fabrication of uniform nanostructures. To our knowledge, it is the first time that en is utilized as precipitator for the synthesis of praseodymium oxide nanostructures and the effect of its concentration on the morphology and size of praseodymium oxide via a facile precipitation method in the presence of PEG is investigated. 2. Materials and methods 2.1. Materials and characterization In this investigation, praseodymium nitrate (Pr(NO3)3 6H2O) (PN) as praseodymium precursor, ethylenediamine (en), liquor ammonia solution containing 25% ammonia, poly ethylene glycol 600 (PEG 600) were purchased from Merck Company and were used without further purification. Fourier transform infrared spectra were recorded using KBr pellets on an FT-IR spectrometer (Magna-IR, 550 Nicolet) in the 400– 4000 cm 1 range. Powder X-ray diffraction (XRD) patterns were collected with a Philips diffractometer using X'PertPro and the monochromatized Cu Kα radiation (l=1.54 Å). Microscopic morphology of products was visualized by a Hitachi s4160 Japan scanning electron microscope (SEM). The energy dispersive spectrometry (EDS) analysis was studied by XL30, Philips microscope. Transmission electron microscope (TEM) images were obtained on a JEM-2100 with an accelerating voltage of 60–200 kV equipped with a high resolution CCD Camera. Room temperature photoluminescence (PL) was studied on a Perkin Elmer (LS 55) fluorescence spectrophotometer. The electronic spectra of the samples were taken on a Scinco UV–vis scanning spectrometer (Model S-4100). 2.2. Synthesis of praseodymium oxide nanostructures Praseodymium oxide nanostructures were prepared by simple precipitation method. In a typical procedure, 1 mmol of en was dissolved in 20 ml of distilled water and then was drop-wise added to 20 ml solution containing 1 mmol of PN
and 3 mmol of PEG under magnetic stirring for 10 min. The green gel-like product was filtered and washed with distilled water for three times. The final product was dried at 60 1C and calcined at 500 1C for 3 h (sample 1). Schematic diagram of formation of Pr6O11 nanostructures is depicted in Scheme 1. The experiment was carried out by using 2, 3, 4, and 5 mmol of en at the same conditions. To investigate the effect of en, a blank test was performed by NH4OH instead of en. In the blank test, 8 mmol of ammonia dissolved in 20 ml distilled water and was added into a solution including 1 mmol of PN and 3 mmol of PEG dissolved in 50 ml of distilled water. To study the effect of PEG, one experiment was carried out without PEG (blank test 1). The as-synthesized products were characterized by FT-IR, SEM, TEM, XRD, EDS, PL and UV– vis techniques. 2.3. Photocatalytic measurements The photocatalytic activity of praseodymium oxide nanostructures obtained from sample no. 4 was tested by using 2naphthol solution. The degradation reaction was performed in a quartz photocatalytic reactor. The photocatalytic degradation was carried out with 0.0012 g of 2-naphthol solution containing 0.05 g of Pr6O11. This mixture was aerated for 30 min to reach adsorption equilibrium. Then, the mixture was placed inside the photoreactor in which the vessel was 40 cm away from the UV source of 400 W mercury lamps. The quartz vessel and light source were placed inside a black box equipped with a fan to prevent UV leakage. The experiment was performed at room temperature. Aliquots of the mixture were taken at periodic intervals during the irradiation, and after centrifugation they were analyzed with the UV–vis spectrometer. The 2-naphthol degradation percentage was calculated by Eq. (1) as follows: D:P: ðt Þ ¼
A0 At 100 A0
ð1Þ
where A0 and At are the absorbance value of solution at 0 and t min, respectively.
Scheme 1. Schematic diagram of the synthesis of praseodymium oxide nanostructures. Please cite this article as: S. Zinatloo-Ajabshir, M. Salavati-Niasari, Novel poly(ethyleneglycol)-assisted synthesis of praseodymium oxide nanostructures via a facile precipitation route, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.08.105
S. Zinatloo-Ajabshir, M. Salavati-Niasari / Ceramics International ] (]]]]) ]]]–]]]
3. Results and discussion The crystal structure, average crystallite diameter and composition of the as-prepared products were determined by powder X-ray diffraction method. XRD patterns of the sample no. 4 after washing steps, and after calcination are shown in Fig. 1a and b, respectively. Most of the diffraction peaks of the product in Fig. 1a can be indexed to hexagonal Pr(OH)3 (space group P63/m, JCPDS card 83-2304). Besides, five diffraction peaks in the XRD pattern of the sample no. 4 after washing steps can be ascribed to orthorhombic Pr(CO3)(OH)1.68 (JCPDS 45-0217). It seems that the praseodymium hydroxide, formed at the precipitation process, has high affinity to the atmospheric CO2, thus transforming to the hydroxy-carbonate phase during washing steps [12]. All the diffraction peaks in Fig. 1b can be readily indexed to pure cubic Pr6O11 with Fm3m space group (JCPDS 42-1121). No impurities are detected from this pattern. Using XRD data (Fig. 1b), the average crystallite size (DC) of the as-prepared Pr6O11, sample no. 4, was calculated to be 12 nm using the Scherrer equation [21]: DC ¼
kλ β cos θ
3
where β is the breadth of the observed diffraction line at its half intensity maximum, K is the so-called shape factor, which usually takes a value of about 0.9, and λ is the wavelength of X-ray source used in XRD. Fig. 2 shows FT-IR spectra of sample no. 4 after washing steps and after calcination in the range 400–4000 cm 1. The (C–N) stretching vibration band at 1344 cm 1 and (C–H) bending vibration band 1383 cm 1 indicates the presence of en (Fig. 2a). Besides the bands located at 665, 1055, and 1504 cm–1 can be related to carbonate group [12]. They completely disappear after calcination at 500 1C. The absorption band centered at 3416 cm 1 and a weak peak at 1629 cm 1 are attributable to the v(OH) stretching and bending vibrations, respectively, which indicates the presence of physisorbed water molecules linked to praseodymium oxide sample [22]. The characteristic band for Pr–O vibration is at 428 cm 1 [23] (Fig. 2b).
ð2Þ
Fig. 2. FT-IR spectra of praseodymium oxide synthesized by (1:4) molar ratio of PN to en in the presence of PEG (sample no. 4) after washing (a) and after calcinacion (b).
Fig. 1. XRD patterns of praseodymium oxide prepared by (1:4) molar ratio of PN to en in the presence of PEG (sample no. 4) after washing (a) and after calcinacion (b).
Fig. 3. EDS pattern of Pr6O11 prepared by (1:4) molar ratio of PN to en in the presence of PEG (sample no. 4).
Please cite this article as: S. Zinatloo-Ajabshir, M. Salavati-Niasari, Novel poly(ethyleneglycol)-assisted synthesis of praseodymium oxide nanostructures via a facile precipitation route, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.08.105
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S. Zinatloo-Ajabshir, M. Salavati-Niasari / Ceramics International ] (]]]]) ]]]–]]]
To further confirm the chemical composition of the asprepared praseodymium oxide, EDS spectrum was recorded for sample no. 4. In the EDS spectrum of sample (Fig. 3), Pr and O elements are detected. In order to investigate the detailed morphology and particle size of the as-synthesized products, TEM images of the Pr6O11 obtained from the sample no. 4 (PN:en¼ 1:4) were taken, and shown in Fig. 4a and b. The TEM images indicate that quasispherical nanoparticles are sintered together. Moreover, the images show that praseodymium oxide nanostructures consist of nanoparticles with diameter from 30 to 70 nm which is agreeable to determined particle size by SEM study. The optical properties of the Pr6O11 nanostructures were investigated by PL and UV–vis spectra to further assess their quality. The PL spectrum of the sample no. 4 is shown in Fig. 5a. The excitation wavelength was 350 nm. An emission peak at around 405 nm can be observed in the PL spectrum. This emission peak is due to charge transition from the 4f band to the valence band of Pr6O11 nanostructures, which is similar to the literatures [24]. The UV–vis absorption spectrum of sample no. 4 is illustrated in Fig. 5b. In the UV–vis absorption spectrum, the absorption band was observed at 239 nm. Using the absorption data the band gap was estimated by Tauc's relationship [25].
The energy gap (Eg) of the praseodymium oxide nanostructures has been determined by extrapolating the linear portion of the plot of (αhν)2 against hν to the energy axis (inset in Fig. 5b). The Eg value of the Pr6O11 is calculated to be 4.1 eV. The obtained data are in good agreement with previous report [24] (Table 1). In this work, the photocatalytic activity of praseodymium oxide nanostructures (sample no. 4) was studied by photooxidation of 2-naphthol under UV light irradiation. The obtained result is shown in Fig. 6. According to photocatalytic calculations by Eq. (1), the 2-naphthol degradation was about 100% after 12 min irradiation of UV light. This result suggests as-prepared Pr6O11 nanostructures as an interesting candidate for photocatalytic applications under UV light. To investigate the effect of en concentration on the morphology of the products in the presence of PEG, SEM images of prepared praseodymium oxide micro/nanostructures with PN:en molar ratio of 1:1, 1:2, 1:3, 1:4 and 1:5 are taken and illustrated in Figs. 7 and 8, respectively. As mentioned in experimental section, in this study, en was used as both precipitator and co-capping agent. The en can act as both precipitator for Pr(NO3)3 to Pr(OH)3 and co-capping agent for the Pr(OH)3 nanoparticles by hindering the aggregation of nanoparticles. It seems that en with high steric hindrance in the
Fig. 4. TEM images of praseodymium oxide synthesized by (1:4) molar ratio of PN to en in the presence of PEG( sample no. 4).
Fig. 5. PL spectrum (a) and UV–vis absorbance spectrum (b) of the praseodymium oxide prepared by (1:4) molar ratio of PN to en in the presence of PEG (sample no. 4) (inset: the curve of (αhν)2 against hν).
Please cite this article as: S. Zinatloo-Ajabshir, M. Salavati-Niasari, Novel poly(ethyleneglycol)-assisted synthesis of praseodymium oxide nanostructures via a facile precipitation route, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.08.105
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Table 1 Preparation conditions of praseodymium oxide micro/nanostructures. Sample no.
Precipitator
Molar ratio (Pr(NO3)3 6H2O:precipitator:PEG)
1
en
1:1:3
2
en
1:2:3
SEM
Fig. 6. Photocatalytic 2-naphthol degradation of praseodymium oxide nanostructures prepared by (1:4) molar ratio of PN to en in the presence of PEG (sample no. 4) under UV light. 3
en
1:3:3
4
en
1:4:3
5
en
1:5:3
6a
en
1:4
7b
NH4OH
1:8:3
a
Blank test, in the absence of PEG. Blank test 1, in the absence of en.
b
presence of PEG causes nucleation rather than the particle growth (Scheme 1). In our experiment, different molar ratios of PN to en were employed to study en concentration effect on the morphology of samples in the presence of PEG. When 1:1 M ratio of PN to en was used, the coalesced particles/micro structures of praseodymium oxide were formed (Fig. 7a and b). By increasing the molar ratio from 1:2 to 1:5, sponge-like Pr6O11 nanostructures were obtained (Figs. 7c–f and 8a–d). It seems that when en concentration increases, the chance of collision between Pr(OH)3 nanoparticles decreases because of
steric hindrance effect of en. Among these used molar ratios of PN to en, homogeneous sponge-like nanostructures with small nanocrystallite size can be formed by using (1:4) molar ratio of PN to en (sample no. 4). These nanostructures composed of very uniform sphere-like Pr6O11 nanoparticles with small grain size. Although with less and more molar ratios of PN to en ZN (1:2, 1:3 and 1:5), sponge-like nanostructures can be obtained, these samples were not homogenous (composed of less uniform spherical nanoparticles) and were aggregated in some places (Figs. 7c–f and 8c and d). So the molar ratio of PN to en in the presence of PEG plays an important role to control particle size and morphology of praseodymium oxide nanostructures. To study the effect of the PEG on the morphology of the Pr6O11 nanostructures, sample no. 6 was prepared as blank sample without PEG. SEM image of sample no. 6 is shown in Fig. 9a and b. It is noteworthy that less uniform sponge-like nanostructures with large size were formed. PEG 600 limits the size of the Pr(OH)3 nanoparticles and protects them from further aggregation. It acts as a capping agent and plays an important role in the formation of uniform sponge-like praseodymium oxide nanostructures with small nanoparticle size (Fig. 8a and b). When PEG is utilized, polymeric molecules are adsorbed preferentially on the nuclei surface to inhibit aggregation by steric hindrance mechanism [26]. Therefore, an advantage of using PEG is that it produces uniform sponge-like Pr6O11 nanostructures with small nanoparticle size. To investigate the effect of the en on the morphology of the praseodymium oxide nanostructures, sample no. 7 was synthesized as blank test 1 using NH4OH in the presence of PEG. SEM image of sample no. 7 is shown in Fig. 10a and b. It can be seen that in the absence of en, less homogeneous spongelike nanostructures with large size were obtained. As already mentioned, when en used as precipitator (Fig. 8a and b), the chance of collision between Pr(OH)3 nanoparticles decreased because of steric hindrance effect of en, and therefore the size of nanocrystallites decreased. These results indicate that en with high steric hindrance is a suitable co-capping agent to produce homogeneous sponge-like Pr6O11 nanostructures.
Please cite this article as: S. Zinatloo-Ajabshir, M. Salavati-Niasari, Novel poly(ethyleneglycol)-assisted synthesis of praseodymium oxide nanostructures via a facile precipitation route, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.08.105
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S. Zinatloo-Ajabshir, M. Salavati-Niasari / Ceramics International ] (]]]]) ]]]–]]]
Fig. 7. SEM images of praseodymium oxide prepared using (a and b) 1:1 ( sample no. 1), (c and d) 1:2 (sample no. 2), and (e and f) 1:3 (sample no. 3) molar ratio of PN to en in the presence of PEG.
Please cite this article as: S. Zinatloo-Ajabshir, M. Salavati-Niasari, Novel poly(ethyleneglycol)-assisted synthesis of praseodymium oxide nanostructures via a facile precipitation route, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.08.105
S. Zinatloo-Ajabshir, M. Salavati-Niasari / Ceramics International ] (]]]]) ]]]–]]]
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Fig. 8. SEM images of praseodymium oxide synthesized by (a and b) 1:4 (sample no. 4), and (c and d) 1:5 (sample no. 5) molar ratio of PN to en in the presence of PEG.
The probable formation mechanism of praseodymium oxide micro/nanostructures using en in the presence of PEG in aqueous solution can be summarized as follows:
H2 NCH2 CH2 NH2 þ 2H2 O-H3 N þ CH2 CH2 N þ H3 þ 2OH PrðNO3 Þ3 þ 3OH -PrðOHÞ3 þ 3NO3 Δ
PrðOHÞ3 ⟹Pr6 O11 :
4. Conclusions In summary, homogeneous sponge-like Pr6O11 nanostructures have been successfully synthesized via a simple precipitation route in the presence of PEG and ethylenediamine (en). Using of en both as a precipitator and co-capping agent in the presence of PEG is the novelty of this work. When asprepared praseodymium oxide nanostructures were used as photocatalyst, the 2-naphthol degradation percentage was about 100 after 12 min irradiation of UV light. This result suggests as-obtained nanostructures as an interesting candidate
Please cite this article as: S. Zinatloo-Ajabshir, M. Salavati-Niasari, Novel poly(ethyleneglycol)-assisted synthesis of praseodymium oxide nanostructures via a facile precipitation route, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.08.105
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Fig. 9. SEM images of praseodymium oxide prepared in the absence of PEG (sample no. 6).
Fig. 10. SEM images of praseodymium oxide prepared in the absence of en (sample no. 7).
for photocatalytic applications under UV light. FT-IR, EDS, and XRD analyses proved high purity of the as-synthesized Pr6O11. The optical properties of as-synthesized products were also investigated. Acknowledgments Authors are grateful to the council of Dr. Masood Hamadanian from the Mechanical Chemistry Department of University of Kashan for supporting this work by Grant no. (159271/168).
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Please cite this article as: S. Zinatloo-Ajabshir, M. Salavati-Niasari, Novel poly(ethyleneglycol)-assisted synthesis of praseodymium oxide nanostructures via a facile precipitation route, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.08.105
CERAMICS INTERNATIONAL
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Novel poly(ethyleneglycol)-assisted synthesis of praseodymium oxide nanostructures via a facile precipitation route Sahar Zinatloo-Ajabshira, Masoud Salavati-Niasaria,b,n a Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran
b
Received 6 August 2014; received in revised form 20 August 2014; accepted 24 August 2014
Abstract The effect of the polymeric capping agent on the morphology and size of praseodymium oxide precipitated from praseodymium nitrate in the presence of ethylenediamine (en) as precipitator was investigated. En was used as precipitator to fabricate praseodymium oxide nanostructures for the first time. The products have been successfully synthesized in the presence of poly ethylene glycol (PEG) as capping agent via an improved precipitation route. The effect of amount of en on the morphology and size of praseodymium oxide was investigated. It was found that morphology and size of the products could be greatly affected by this parameter. The as-produced nanostructures were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet–visible (UV–vis) spectroscopy, photoluminescence (PL) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, and energy dispersive X-ray microanalysis (EDX). To investigate the catalytic properties of praseodymium oxide nanostructures, the photocatalytic degradation of 2-naphthol was performed. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: C. Optical properties; Ceramic; Praseodymium oxide; Nanostructures; Catalytic properties
1. Introduction In the past decade, much attention has been focused on the preparation and characterization of nanomaterials owing to their interesting properties and potential applications [1–4]. One of these nanometer-scale materials is praseodymium oxide. It was found that praseodymium oxide is an important rare earth metal oxide because of its specific optical and electrical properties [5,6] as well as potential applications, including ceramic pigments, catalysts, promoters and stabilizers in combustion catalysts, oxygen-storage components, and materials with higher electrical conductivity [7–11]. Praseodymium oxide forms a homologous series with a number of stoichiometrically defined oxides: PrnO2n 2, with n=4, 7, 9, n Corresponding author at: Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran. Tel./fax: þ 98 31 55912383. E-mail address: [email protected] (M. Salavati-Niasari).
10, 11, 12,…. Among these oxides, Pr6O11 is the most stable form at ambient temperature and pressure [12]. Up to now, very limited number of methods have been employed to synthesize praseodymium oxide nanostructures, such as, thermal decomposition [13], molten salt [14], hydrothermal [15], electrochemical [16], precipitation [12], and electrospinning [17]. The development of a simple effective route for synthesizing Pr6O11 nanostructures is of great importance to the potential investigations of its physical and chemical properties. It is good to know that properties of nanomaterials depend on their particle size and shape [18–20]. Therefore, exploring suitable methods to synthesize praseodymium oxide nanostructures and controlling their particle morphology and size is important. Herein, Pr6O11 nanostructures are synthesized via a simple precipitation route in the presence of PEG and ethylenediamine (en). The precipitation method is an appropriate synthesis process for preparation of many inorganic nanomaterials. This method is facile, convenient and cost effective synthetic
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Please cite this article as: S. Zinatloo-Ajabshir, M. Salavati-Niasari, Novel poly(ethyleneglycol)-assisted synthesis of praseodymium oxide nanostructures via a facile precipitation route, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.08.105
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procedure and provides an effective way to the fabrication of uniform nanostructures. To our knowledge, it is the first time that en is utilized as precipitator for the synthesis of praseodymium oxide nanostructures and the effect of its concentration on the morphology and size of praseodymium oxide via a facile precipitation method in the presence of PEG is investigated. 2. Materials and methods 2.1. Materials and characterization In this investigation, praseodymium nitrate (Pr(NO3)3 6H2O) (PN) as praseodymium precursor, ethylenediamine (en), liquor ammonia solution containing 25% ammonia, poly ethylene glycol 600 (PEG 600) were purchased from Merck Company and were used without further purification. Fourier transform infrared spectra were recorded using KBr pellets on an FT-IR spectrometer (Magna-IR, 550 Nicolet) in the 400– 4000 cm 1 range. Powder X-ray diffraction (XRD) patterns were collected with a Philips diffractometer using X'PertPro and the monochromatized Cu Kα radiation (l=1.54 Å). Microscopic morphology of products was visualized by a Hitachi s4160 Japan scanning electron microscope (SEM). The energy dispersive spectrometry (EDS) analysis was studied by XL30, Philips microscope. Transmission electron microscope (TEM) images were obtained on a JEM-2100 with an accelerating voltage of 60–200 kV equipped with a high resolution CCD Camera. Room temperature photoluminescence (PL) was studied on a Perkin Elmer (LS 55) fluorescence spectrophotometer. The electronic spectra of the samples were taken on a Scinco UV–vis scanning spectrometer (Model S-4100). 2.2. Synthesis of praseodymium oxide nanostructures Praseodymium oxide nanostructures were prepared by simple precipitation method. In a typical procedure, 1 mmol of en was dissolved in 20 ml of distilled water and then was drop-wise added to 20 ml solution containing 1 mmol of PN
and 3 mmol of PEG under magnetic stirring for 10 min. The green gel-like product was filtered and washed with distilled water for three times. The final product was dried at 60 1C and calcined at 500 1C for 3 h (sample 1). Schematic diagram of formation of Pr6O11 nanostructures is depicted in Scheme 1. The experiment was carried out by using 2, 3, 4, and 5 mmol of en at the same conditions. To investigate the effect of en, a blank test was performed by NH4OH instead of en. In the blank test, 8 mmol of ammonia dissolved in 20 ml distilled water and was added into a solution including 1 mmol of PN and 3 mmol of PEG dissolved in 50 ml of distilled water. To study the effect of PEG, one experiment was carried out without PEG (blank test 1). The as-synthesized products were characterized by FT-IR, SEM, TEM, XRD, EDS, PL and UV– vis techniques. 2.3. Photocatalytic measurements The photocatalytic activity of praseodymium oxide nanostructures obtained from sample no. 4 was tested by using 2naphthol solution. The degradation reaction was performed in a quartz photocatalytic reactor. The photocatalytic degradation was carried out with 0.0012 g of 2-naphthol solution containing 0.05 g of Pr6O11. This mixture was aerated for 30 min to reach adsorption equilibrium. Then, the mixture was placed inside the photoreactor in which the vessel was 40 cm away from the UV source of 400 W mercury lamps. The quartz vessel and light source were placed inside a black box equipped with a fan to prevent UV leakage. The experiment was performed at room temperature. Aliquots of the mixture were taken at periodic intervals during the irradiation, and after centrifugation they were analyzed with the UV–vis spectrometer. The 2-naphthol degradation percentage was calculated by Eq. (1) as follows: D:P: ðt Þ ¼
A0 At 100 A0
ð1Þ
where A0 and At are the absorbance value of solution at 0 and t min, respectively.
Scheme 1. Schematic diagram of the synthesis of praseodymium oxide nanostructures. Please cite this article as: S. Zinatloo-Ajabshir, M. Salavati-Niasari, Novel poly(ethyleneglycol)-assisted synthesis of praseodymium oxide nanostructures via a facile precipitation route, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.08.105
S. Zinatloo-Ajabshir, M. Salavati-Niasari / Ceramics International ] (]]]]) ]]]–]]]
3. Results and discussion The crystal structure, average crystallite diameter and composition of the as-prepared products were determined by powder X-ray diffraction method. XRD patterns of the sample no. 4 after washing steps, and after calcination are shown in Fig. 1a and b, respectively. Most of the diffraction peaks of the product in Fig. 1a can be indexed to hexagonal Pr(OH)3 (space group P63/m, JCPDS card 83-2304). Besides, five diffraction peaks in the XRD pattern of the sample no. 4 after washing steps can be ascribed to orthorhombic Pr(CO3)(OH)1.68 (JCPDS 45-0217). It seems that the praseodymium hydroxide, formed at the precipitation process, has high affinity to the atmospheric CO2, thus transforming to the hydroxy-carbonate phase during washing steps [12]. All the diffraction peaks in Fig. 1b can be readily indexed to pure cubic Pr6O11 with Fm3m space group (JCPDS 42-1121). No impurities are detected from this pattern. Using XRD data (Fig. 1b), the average crystallite size (DC) of the as-prepared Pr6O11, sample no. 4, was calculated to be 12 nm using the Scherrer equation [21]: DC ¼
kλ β cos θ
3
where β is the breadth of the observed diffraction line at its half intensity maximum, K is the so-called shape factor, which usually takes a value of about 0.9, and λ is the wavelength of X-ray source used in XRD. Fig. 2 shows FT-IR spectra of sample no. 4 after washing steps and after calcination in the range 400–4000 cm 1. The (C–N) stretching vibration band at 1344 cm 1 and (C–H) bending vibration band 1383 cm 1 indicates the presence of en (Fig. 2a). Besides the bands located at 665, 1055, and 1504 cm–1 can be related to carbonate group [12]. They completely disappear after calcination at 500 1C. The absorption band centered at 3416 cm 1 and a weak peak at 1629 cm 1 are attributable to the v(OH) stretching and bending vibrations, respectively, which indicates the presence of physisorbed water molecules linked to praseodymium oxide sample [22]. The characteristic band for Pr–O vibration is at 428 cm 1 [23] (Fig. 2b).
ð2Þ
Fig. 2. FT-IR spectra of praseodymium oxide synthesized by (1:4) molar ratio of PN to en in the presence of PEG (sample no. 4) after washing (a) and after calcinacion (b).
Fig. 1. XRD patterns of praseodymium oxide prepared by (1:4) molar ratio of PN to en in the presence of PEG (sample no. 4) after washing (a) and after calcinacion (b).
Fig. 3. EDS pattern of Pr6O11 prepared by (1:4) molar ratio of PN to en in the presence of PEG (sample no. 4).
Please cite this article as: S. Zinatloo-Ajabshir, M. Salavati-Niasari, Novel poly(ethyleneglycol)-assisted synthesis of praseodymium oxide nanostructures via a facile precipitation route, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.08.105
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To further confirm the chemical composition of the asprepared praseodymium oxide, EDS spectrum was recorded for sample no. 4. In the EDS spectrum of sample (Fig. 3), Pr and O elements are detected. In order to investigate the detailed morphology and particle size of the as-synthesized products, TEM images of the Pr6O11 obtained from the sample no. 4 (PN:en¼ 1:4) were taken, and shown in Fig. 4a and b. The TEM images indicate that quasispherical nanoparticles are sintered together. Moreover, the images show that praseodymium oxide nanostructures consist of nanoparticles with diameter from 30 to 70 nm which is agreeable to determined particle size by SEM study. The optical properties of the Pr6O11 nanostructures were investigated by PL and UV–vis spectra to further assess their quality. The PL spectrum of the sample no. 4 is shown in Fig. 5a. The excitation wavelength was 350 nm. An emission peak at around 405 nm can be observed in the PL spectrum. This emission peak is due to charge transition from the 4f band to the valence band of Pr6O11 nanostructures, which is similar to the literatures [24]. The UV–vis absorption spectrum of sample no. 4 is illustrated in Fig. 5b. In the UV–vis absorption spectrum, the absorption band was observed at 239 nm. Using the absorption data the band gap was estimated by Tauc's relationship [25].
The energy gap (Eg) of the praseodymium oxide nanostructures has been determined by extrapolating the linear portion of the plot of (αhν)2 against hν to the energy axis (inset in Fig. 5b). The Eg value of the Pr6O11 is calculated to be 4.1 eV. The obtained data are in good agreement with previous report [24] (Table 1). In this work, the photocatalytic activity of praseodymium oxide nanostructures (sample no. 4) was studied by photooxidation of 2-naphthol under UV light irradiation. The obtained result is shown in Fig. 6. According to photocatalytic calculations by Eq. (1), the 2-naphthol degradation was about 100% after 12 min irradiation of UV light. This result suggests as-prepared Pr6O11 nanostructures as an interesting candidate for photocatalytic applications under UV light. To investigate the effect of en concentration on the morphology of the products in the presence of PEG, SEM images of prepared praseodymium oxide micro/nanostructures with PN:en molar ratio of 1:1, 1:2, 1:3, 1:4 and 1:5 are taken and illustrated in Figs. 7 and 8, respectively. As mentioned in experimental section, in this study, en was used as both precipitator and co-capping agent. The en can act as both precipitator for Pr(NO3)3 to Pr(OH)3 and co-capping agent for the Pr(OH)3 nanoparticles by hindering the aggregation of nanoparticles. It seems that en with high steric hindrance in the
Fig. 4. TEM images of praseodymium oxide synthesized by (1:4) molar ratio of PN to en in the presence of PEG( sample no. 4).
Fig. 5. PL spectrum (a) and UV–vis absorbance spectrum (b) of the praseodymium oxide prepared by (1:4) molar ratio of PN to en in the presence of PEG (sample no. 4) (inset: the curve of (αhν)2 against hν).
Please cite this article as: S. Zinatloo-Ajabshir, M. Salavati-Niasari, Novel poly(ethyleneglycol)-assisted synthesis of praseodymium oxide nanostructures via a facile precipitation route, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.08.105
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Table 1 Preparation conditions of praseodymium oxide micro/nanostructures. Sample no.
Precipitator
Molar ratio (Pr(NO3)3 6H2O:precipitator:PEG)
1
en
1:1:3
2
en
1:2:3
SEM
Fig. 6. Photocatalytic 2-naphthol degradation of praseodymium oxide nanostructures prepared by (1:4) molar ratio of PN to en in the presence of PEG (sample no. 4) under UV light. 3
en
1:3:3
4
en
1:4:3
5
en
1:5:3
6a
en
1:4
7b
NH4OH
1:8:3
a
Blank test, in the absence of PEG. Blank test 1, in the absence of en.
b
presence of PEG causes nucleation rather than the particle growth (Scheme 1). In our experiment, different molar ratios of PN to en were employed to study en concentration effect on the morphology of samples in the presence of PEG. When 1:1 M ratio of PN to en was used, the coalesced particles/micro structures of praseodymium oxide were formed (Fig. 7a and b). By increasing the molar ratio from 1:2 to 1:5, sponge-like Pr6O11 nanostructures were obtained (Figs. 7c–f and 8a–d). It seems that when en concentration increases, the chance of collision between Pr(OH)3 nanoparticles decreases because of
steric hindrance effect of en. Among these used molar ratios of PN to en, homogeneous sponge-like nanostructures with small nanocrystallite size can be formed by using (1:4) molar ratio of PN to en (sample no. 4). These nanostructures composed of very uniform sphere-like Pr6O11 nanoparticles with small grain size. Although with less and more molar ratios of PN to en ZN (1:2, 1:3 and 1:5), sponge-like nanostructures can be obtained, these samples were not homogenous (composed of less uniform spherical nanoparticles) and were aggregated in some places (Figs. 7c–f and 8c and d). So the molar ratio of PN to en in the presence of PEG plays an important role to control particle size and morphology of praseodymium oxide nanostructures. To study the effect of the PEG on the morphology of the Pr6O11 nanostructures, sample no. 6 was prepared as blank sample without PEG. SEM image of sample no. 6 is shown in Fig. 9a and b. It is noteworthy that less uniform sponge-like nanostructures with large size were formed. PEG 600 limits the size of the Pr(OH)3 nanoparticles and protects them from further aggregation. It acts as a capping agent and plays an important role in the formation of uniform sponge-like praseodymium oxide nanostructures with small nanoparticle size (Fig. 8a and b). When PEG is utilized, polymeric molecules are adsorbed preferentially on the nuclei surface to inhibit aggregation by steric hindrance mechanism [26]. Therefore, an advantage of using PEG is that it produces uniform sponge-like Pr6O11 nanostructures with small nanoparticle size. To investigate the effect of the en on the morphology of the praseodymium oxide nanostructures, sample no. 7 was synthesized as blank test 1 using NH4OH in the presence of PEG. SEM image of sample no. 7 is shown in Fig. 10a and b. It can be seen that in the absence of en, less homogeneous spongelike nanostructures with large size were obtained. As already mentioned, when en used as precipitator (Fig. 8a and b), the chance of collision between Pr(OH)3 nanoparticles decreased because of steric hindrance effect of en, and therefore the size of nanocrystallites decreased. These results indicate that en with high steric hindrance is a suitable co-capping agent to produce homogeneous sponge-like Pr6O11 nanostructures.
Please cite this article as: S. Zinatloo-Ajabshir, M. Salavati-Niasari, Novel poly(ethyleneglycol)-assisted synthesis of praseodymium oxide nanostructures via a facile precipitation route, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.08.105
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S. Zinatloo-Ajabshir, M. Salavati-Niasari / Ceramics International ] (]]]]) ]]]–]]]
Fig. 7. SEM images of praseodymium oxide prepared using (a and b) 1:1 ( sample no. 1), (c and d) 1:2 (sample no. 2), and (e and f) 1:3 (sample no. 3) molar ratio of PN to en in the presence of PEG.
Please cite this article as: S. Zinatloo-Ajabshir, M. Salavati-Niasari, Novel poly(ethyleneglycol)-assisted synthesis of praseodymium oxide nanostructures via a facile precipitation route, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.08.105
S. Zinatloo-Ajabshir, M. Salavati-Niasari / Ceramics International ] (]]]]) ]]]–]]]
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Fig. 8. SEM images of praseodymium oxide synthesized by (a and b) 1:4 (sample no. 4), and (c and d) 1:5 (sample no. 5) molar ratio of PN to en in the presence of PEG.
The probable formation mechanism of praseodymium oxide micro/nanostructures using en in the presence of PEG in aqueous solution can be summarized as follows:
H2 NCH2 CH2 NH2 þ 2H2 O-H3 N þ CH2 CH2 N þ H3 þ 2OH PrðNO3 Þ3 þ 3OH -PrðOHÞ3 þ 3NO3 Δ
PrðOHÞ3 ⟹Pr6 O11 :
4. Conclusions In summary, homogeneous sponge-like Pr6O11 nanostructures have been successfully synthesized via a simple precipitation route in the presence of PEG and ethylenediamine (en). Using of en both as a precipitator and co-capping agent in the presence of PEG is the novelty of this work. When asprepared praseodymium oxide nanostructures were used as photocatalyst, the 2-naphthol degradation percentage was about 100 after 12 min irradiation of UV light. This result suggests as-obtained nanostructures as an interesting candidate
Please cite this article as: S. Zinatloo-Ajabshir, M. Salavati-Niasari, Novel poly(ethyleneglycol)-assisted synthesis of praseodymium oxide nanostructures via a facile precipitation route, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.08.105
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Fig. 9. SEM images of praseodymium oxide prepared in the absence of PEG (sample no. 6).
Fig. 10. SEM images of praseodymium oxide prepared in the absence of en (sample no. 7).
for photocatalytic applications under UV light. FT-IR, EDS, and XRD analyses proved high purity of the as-synthesized Pr6O11. The optical properties of as-synthesized products were also investigated. Acknowledgments Authors are grateful to the council of Dr. Masood Hamadanian from the Mechanical Chemistry Department of University of Kashan for supporting this work by Grant no. (159271/168).
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Please cite this article as: S. Zinatloo-Ajabshir, M. Salavati-Niasari, Novel poly(ethyleneglycol)-assisted synthesis of praseodymium oxide nanostructures via a facile precipitation route, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.08.105