Preparation and Characterization of PbS Nanoparticles ... - Springer Link

J Clust Sci (2014) 25:937–947 DOI 10.1007/s10876-013-0674-0 ORIGINAL PAPER

Preparation and Characterization of PbS Nanoparticles via Cyclic Microwave Radiation Using Precursor of Lead (II) Oxalate Masoud Salavati-Niasari • Azam Sobhani Sanaz khoshrooz • Noshin Mirzanasiri

•

Received: 22 October 2013 / Published online: 4 December 2013 Ó Springer Science+Business Media New York 2013

Abstract PbS nanoparticles have been successfully synthesized through a simple, rapid microwave route using lead (II) oxalate as precursor, [Pb(O4C2)]. In this study, four different sulfur sources were applied that including sodium thiosulfate (Na2S2O3), thioacetamide (NH2CSCH3), thioglycolic acid (HSCH2CO2H), and Lcysteine (HO2CCH(NH2)CH2SH). The effects of sulfur sources on morphology and size of products in two different solvents, ethylene glycol (EG) and poly EG (PEG 400), were investigated. The products were analyzed by X-ray diffraction analysis, scanning electron microscopy, Fourier transform infrared spectroscopy and X-ray energy dispersive spectroscopy. Photoluminescence was used to study the optical properties of PbS nanoparticles. Keywords synthesis

Nanoparticles PbS Microwave Lead (II) oxalate Chemical

M. Salavati-Niasari (&) S. khoshrooz Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, P. O. Box 87317–51167, Kashan, Islamic Republic of Iran e-mail: [email protected] M. Salavati-Niasari Institute of Nano Science and Nano Technology, University of Kashan, P. O. Box 87317–51167, Kashan, Islamic Republic of Iran A. Sobhani Department of Chemistry, Kosar University of Bojnord, Bojnord, Islamic Republic of Iran N. Mirzanasiri School of Chemistry, College of Science, University of Tehran, Tehran, Islamic Republic of Iran

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Introduction Inorganic semiconductors are easily accessible, in the last decade a great number of them have been manufactured as nanoparticles. The nanoparticles of inorganic semiconductors are well suited for the development of novel opto-electronic devices, due to their flexibility and simple process ability combined with their optical properties [1, 2]. PbS is one of luminescent materials with a cubic rock salt structure which have novel semiconducting and optical properties. PbS in its bulk form has a near-infrared direct narrow band gap of 0.41 eV at room temperature and its band gap can be widened to the visible region of about 2 eV by forming nanoclusters [3, 4]. Its large exciton Bohr radius (18 nm) [5, 6] and the effects of strong quantum confinement can be achieved even for relatively large structures [7]. Different morphologies of PbS have different properties. They are nanocrystals [8], nanorods [9], nanotubes, nanocubes [10], starshapes [11], dendrites [12], and flower-like crystals [13]. A wide range of techniques has been developed to synthesize PbS with different morphologies. These developments include hydrothermal and solvothermal routes [14], electroless chemical deposition, microwave assisted synthesis and sonochemistry method [15]. Among different ways for synthesis of nanosemiconductors, microwave-assisted synthesis has attracted considerable attention for its rapid volumetric heating, short reaction time, low energy consumption and unusual products as compared with conventional methods [16–18]. When microwave radiation is applied to chemicals, at least one of the components is capable of coupling with the radiation. It is able to promote the reaction rate, and consumes shorter reaction time, comparing to a conventional method. Temperature and concentration gradient are able to be solved by the vibration of microwave radiation. Due to a large amount of microwave energy focused onto solutions, the vibrating electric field applies a force on the charged particles to vibrate accordingly. The radiation can play the role in promoting the reaction kinetic with high efficiency. Recently, metal coordination complexes or organometallic compounds as precursors for the preparation of inorganic nanomaterials have been widely studied as they represent an important interface between synthetic chemistry and materials science [19–23]. Complexation is a popular method of modifying ligands in the original precursor solution. Using a complex as a precursor, we obtain products with the smaller size compared to metal salts, it is because of the chelate effect in the complex [24]. Considering the reaction of the chelating agent, we suggest that oxalate anions coordinated to the Pb2? ions in Pb(NO3)2 inhibit the agglomeration of metal cations and decrease the particle size of the PbS nanoparticles. Therefore, we chose [Pb(O4C2)] as a lead precursor and report a facile method for preparation of PbS nanoparticles via cyclic microwave at short time.

Experimental All the chemicals used in our experiments were of analytical grade, were purchased from Merck and were used as received without further purification. The XRD

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pattern of products was recorded by a Rigaku D-max C III XRD using Ni-filtered Cu Ka radiation. SEM images were obtained on Philips XL-30ESEM equipped with an energy dispersive X-ray spectroscopy. Thermogravimetric-differential thermal analysis were carried out using a thermal gravimetric analysis instrument (Shimadzu TGA-50H) with a flow rate of 20.0 mlmin-1 and a heating rate of 10 °C min-1 in the inert atmosphere. FT-IR spectra were recorded with Shimadzu Varian 4300 spectrophotometer in KBr pellets. EDS analysis with 20 kV accelerated voltage was done. Room temperature PL was studied on a Perkin Elmer (LS 55) fluorescence spectrophotometer. [Pb(O4C2)] was synthesized according to this procedure: lead (II) nitrate, Pb(NO3)2, (2 mmol) was dissolved in 10 ml of distilled water to form a homogeneous solution. A stoichiometric amount of potassium oxalate, K2C2O4H2O, was dissolved in an equal volume of distilled water and added drop-wise to the above solution. After stirring for about 30 min, a white precipitate was isolated. The precipitate was centrifuged, washed out with alcohol and distilled water several times to remove potassium nitrate as a by-product, then dried under vacuum at 50 °C for 5 h. The asprepared [Pb(O4C2)] was characterized by XRD, TGA and FT-IR techniques. Pb(NO3 Þ2 ðaqÞ þ K2 C2 O4 H2 O ðaqÞ ! ½PbðO4 C2 ÞðsÞ þ 2KNO3 ðaq)

ð1Þ

PbS nanoparticles were prepared from 1:1 molar ratio of [Pb(O4C2)] to sodium thiosulfate in EG using microwave irradiation for 10 cycles, with a cyclic duration of 100 s, the radiation was on for 30 s every 70 s interval. At the conclusion of the process, the system naturally cooled down to room temperature. The products were centrifuged, washed out with alcohol and distilled water for three times and dried under vacuum at 50 °C for 5 h. The synthetic pathway is shown in Scheme 1. The purity, morphology, particle size and optical property of the as-prepared products were characterized by XRD, SEM, FT-IR, EDS and PL techniques.

Scheme 1 Shematic diagram illustrating the formation of PbS in different conditions

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Results and Discussion Figure 1 shows the XRD patterns of the [Pb(O4C2)] precursor and PbS nanoparticles (sample no. 1). The XRD pattern in Fig. 1a can be indexed to the anorthic cell ˚ , b = 6.978 A ˚, reported for [Pb(O4C2)] (JCPDS No. 14-0803, a = 6.091 A ˚ c = 50557 A). No significant impurity such as KNO3 was observed, which indicates that pure [Pb(O4C2)] was obtained. All of the reflections of the XRD pattern in Fig. 1b can be indexed to the cubic lattice of PbS (JCPDS No. 77-0244). The intense and sharp diffraction peaks in Fig. 1b suggest that the obtained product is crystallized well. The average crystallite size, d, of the PbS nanoparticles can be calculated by using Debye–Scherrer formula (Eq. 2). The d parameter is the mean size diameter, k is the wavelength of the X-ray radiation, k is a parameter that takes the value 0.899 in cubic systems, b is the width at mid-height of the peak, and h is the Bragg’s angle. The value of d computed with this equation is 36.3 nm from the major diffraction peak of corresponding (200). d ¼

kk b cos h

ð2Þ

Figure 2 shows the TGA curve of [Pb(O4C2)] from ambient temperature to 600 °C. There is only one weight loss step in the temperature range 300–400 °C. The weight loss is about 28.2 % and is due to the evolution of CO and CO2

Fig. 1 XRD patterns of a [Pb(O4C2)] precursor and b PbS nanoparticles

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molecules during the thermal decomposition of [Pb(O4C2)] to lead oxide. The reaction equation of the decomposition of [Pb(O4C2)] is shown in Eq. (3). ½PbðO4 C2 Þ

300400 C

!

PbO þ CO2 þ CO

ð3Þ

The morphology and the size of PbS samples under different conditions were examined by SEM. In this paper, the effect of solvents and sulfur sources on the morphology and the particle size of PbS has been investigated. The results have been listed in Table 1. The SEM image in Fig. 3a shows that the morphology of sample no. 1 is granular. It can be clearly seen that a great deal of PbS spherical nanoparticles with grain diameter about 50 nm exists in nearly even distribution. The results obtained using SEM are in accordance with those provided by XRD. When PEG400 is used instead of EG under the same conditions, the obtained product (sample no. 2) is cube shaped PbS crystals absorbed on the polymer surface. These cubes, as shown in Fig. 3b, have uneven diameters, ranging from 70 to about 200 nm. This shows that solvent plays an important role on the morphology and the particles size of PbS obtained through a microwave route. The morphology of samples no. 3 and no. 4 are shown by SEM images in Fig. 4. It is clear that aggregated nanostructures form, therefore thioacetamide is not a suitable sulfur source for synthesis of PbS nanocrystals with appropriate morphology. These observations indicating that thioacetamide under the microwave radiation releases the sulfur ions faster than sodium thiosulfate and therefore the rate of formation and growth of PbS will correspondingly increase. Figure 5 shows SEM images of PbS obtained from L-cysteine as sulfur source in two different solvents. It is seen that samples no. 5 (Fig. 5a) and no. 6 (Fig. 5b) consist agglomerated nanoparticles with spherical and cubic shapes, respectively. These nanoparticles are not separated well, therefore when sodium thiosulfate is substituted with L-cysteine and other reaction parameters are fixed, crystal morphology of samples is unchanged, but their size and agglomeration are increased.

Fig. 2 TGA curve of the [Pb(O4C2)] precursor

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Table 1 Different conditions for synthesis of PbS samples Sample no.

[Pb(O4C2)] (mol)

Sulfur source (mol)

Time (min)

Power (W)

Solvent

Morphology and size

1

0.01

0.01 (sodium thiosulfate)

5

600

EG

Spherical nanoparticles *50 nm

2

0.01

0.01 (sodium thiosulfate)

5

600

PEG400 ? H2 O (3:1)

Cubes, 70–200 nm

3

0.01

0.01 (thioacetamide)

5

600

EG

Aggregated nanostructures

4

0.01

0.01 (thioacetamide)

5

600

PEG400 ? H2 O (3:1)

Aggregated nanostructures

5

0.01

0.01 (L-cysteine)

5

600

EG

Spherical nanoparticles [100 nm

6

0.01

0.01 (L-cysteine)

5

600

PEG400 ? H2 O (3:1)

Cubes, 50–210 nm

7

0.01

0.01 (thioglycolic acid)

5

600

EG

Microstructures

8

0.01

0.01 (thioglycolic acid)

5

600

PEG400 ? H2 O (3:1)

Cubes *200 nm

When sodium thiosulfate is substituted with thioglycolic acid, in EG, and other reaction parameters remain unchanged, the products are not nanoscale. This may be due to the rapid release of sulfur ions in the reaction. The high-magnification SEM image of sample no. 7 in Fig. 6b, clearly reveals that the surfaces of these samples are perfectly smooth. When PEG400 is used instead of EG, the obtained particles are cubes absorbed on the surface of polymer and their average diameter is about 200 nm. It can be concluded that PEG400 is a suitable solvent, in comparison to the EG, in this condition. Considering different conditions employed to prepare PbS samples, it is obvious that using sodium thiosulfate as sulfur source in EG solvent is preferred for PbS nanoparticles synthesis. FT-IR spectra of K2C2O4H2O, [Pb(O4C2)] and PbS nanoparticles are shown in Fig. 7. The board peaks at ca. 3370–3460 cm-1 in three spectra are representative to the stretching vibrations (mO–H) of water molecules [25]. This observation provides a evidence for the presence of a small amount of water absorption on the surface of [Pb(O4C2)] after handling. The bands around 1,600 cm-1 in K2C2O4H2O (Fig. 7a) are assigned to asymmetric vibrations of the carbonyl group. Upon complex formation, the 1,589 cm-1 (mCO) band shifts to lower region. It is the strongest evidence for complex formation by lead ion through oxygen atom of the ligand. Two bands at 1,371 and 1,287 cm-1, are attributed to ms (C–O) and d(OC=O) modes in the complex. Two distinct peaks at 776 and 507 cm-1 are attributed to the out-ofplane bending mode of water and O–C–O in-plane bending mode of oxalate, respectively [26], which indicate the formation of [Pb(O4C2)]. In the IR spectrum of

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Fig. 3 SEM images of PbS obtained from sodium thiosulfate as sulfur source in: a EG (sample no. 1) and b PEG 400 (sample no. 2)

Fig. 4 SEM images of PbS obtained from thioacetamide as sulfur source in: a EG (sample no. 3) and b PEG 400 (sample no. 4)

Fig. 5 SEM images of PbS obtained from L-cysteine as sulfur source in: a EG (sample no. 5) and b PEG 400 (sample no. 6)

[Pb(O4C2)], no peak appears at ca. 1,400 cm-1, which is a typical absorbance of nitrate anions, indicating the lack of nitrate anions in the [Pb(O4C2)] [27]. The FTIR spectrum of PbS nanoparticles is shown in Fig. 7c. The bands at ca.

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Fig. 6 SEM images of PbS obtained from thioglycolic acid as sulfur source in: a, b EG (sample no. 7) and c PEG 400 (sample no. 8)

Fig. 7 FT-IR spectra of a K2C2O4H2O, b [Pb(O4C2)], c PbS nanoparticles before washing with NaOH, d PbS nanoparticles after washing with NaOH

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Fig. 8 The EDS pattern of PbS nanoparticles

Fig. 9 PL spectrum of PbS nanoparticles

500–1,700 cm-1 in this figure show that oxalate absorbed on the surface of nanoparticles. In order to remove the absorbed [Pb(O4C2)] on the particles, we washed sample with 0.01 N aqueous NaOH solution. Figure 7d shows FT-IR spectrum of the pure PbS nanoparticles. The nanoparticles were characterized by EDS analysis for the evaluation of their composition. The EDS pattern of PbS nanoparticles (sample no. 1) shows the presence of Pb and S peaks and no other impurities are observed in Fig. 8.

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Table 2 Characterization comparison of PbS nanoparticles with other works Method

Size

Morphology

Time

Reference

Ionic liquid

110 nm

Nanocubes

12 h

[30]

Hydrothermal

2 lm

Cage-like

12 h

[31]

Hydrothermal

35 nm

Star-like

90 min

[32]

Sonochemical

Width *80 nm

Nanobelts

4h

[33]

Microwave

Width *400 nm

Hexapod

4.5 min

[34]

Nanoparticles

5 min

Present work

Length up to several mm Length *1–1.5 lm Microwave

* 50 nm

PbS nanocrytallites have a unique property of luminescence characteristics, whose specific emission wavelengths mainly depend on the nature of the semiconductors, the physical dimensions, and the chemical environments. Figure 9 shows the PL emission spectrum of the as-synthesized PbS dispersed in ethanol. The sample (no. 1) kept at room temperature was excited with excitation wavelength of 556 nm using a xenon lamp. The PL spectrum presented in Fig. 9 shows a peak at 557 nm. SEM image in Fig. 3a shows that size of PbS nanoparticles is about 50 nm. The exciton Bohr radius of PbS is about 20 nm [28], much lower that the particle size. Thus no strong quantum confinement is expected from these nanoparticles and, hence, no blue-shift of the PL emission peak. For this particle size, the PL peak should be found in the infrared range, far from 557 nm [29]. Thus the peak at 557 nm can be attributed to the excitation wavelength of the xenon lamp. In Table 2 some of applied methods for the synthesis of PbS nanocrystals are compared. In comparison to other works, our method is more simple and commodious. In this work, the use of toxic chemicals and time-consuming methods were avoided. We applied various types of sulfur sources and solvents and obtained small nanoparticles in very short time (5 min). To the best of our knowledge, it is the first time that [Pb(O4C2)] is used as Pb source for the synthesis of PbS nanoparticles via a microwave method. This complex inhibits the agglomeration of metal cations and decreases the particle size of the PbS samples.

Conclusions The present study reports the preparation and characterization of PbS nanoparticles. The nanoparticles were obtained from the lead oxalate powder via cyclic microwave route and displayed a very strong luminescence around 500–600 nm at room temperature. It was shown that the sulfur source and solvent have an important influence on the size and the morphology of products. The reaction conditions in this route are very easy to maintain and control. This route may be extended to the fabrication of other metal sulfides with novel morphologies and properties. Acknowledgments Authors are grateful to the council of University of Kashan for their unending effort to provide financial support to undertake this work by Grant No (159271/123).

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