Synthesis and Characterization of Silver Selenide Nanoparticles via a

J Inorg Organomet Polym (2013) 23:357–364 DOI 10.1007/s10904-012-9784-7

Synthesis and Characterization of Silver Selenide Nanoparticles via a Facile Sonochemical Route Starting from a Novel Inorganic Precursor Maryam Jafari • Masoud Salavati-Niasari Fatemeh Mohandes

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Published online: 31 October 2012 Ó Springer Science+Business Media New York 2012

Abstract Silver selenide nanoparticles were synthesized by the reaction between silver benzoate and SeCl4 via a sonochemical method. The as-synthesized Ag2Se nanoparticles were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectra and energy-dispersive X-ray microanalysis. Facile preparation and separation were important features of this route. To the best of our knowledge, it is the first time that silver benzoate was used as silver precursor for the synthesis of silver selenide nanoparticles. Keywords Silver selenide Silver benzoate Ultrasound irradiation Nanoparticle

1 Introduction During the past two decades, semiconductor nanocrystals have attracted broad attention due to their unique shape and size dependent physical and chemical properties that differ drastically from their bulk counterparts [1]. Widespread attention has been paid to the preparation and characterization of metal selenides, due to their interesting properties and applications. Particularly, the applications and

M. Jafari M. Salavati-Niasari (&) F. Mohandes Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran e-mail: [email protected] M. Salavati-Niasari Institute of Nano Science and Nano Technology, University of Kashan, Kashan P.O. Box 87317-51167, Islamic Republic of Iran

investigations of silver selenide (Ag2Se) nanostructures have been increasing due to their rich behaviors in the structural and electronic properties [2–7]. Silver selenide is a narrow band-gap semiconductor. Its band gap energy is between 0.07 and 0.15 eV for the low temperature phase [8]. It is a conductor with high electronic and ionic mobility [9, 10]. The fabrication of metal chalcogenides has attracted much attention due to their usefulness as magneto-resistive [11]. Among these metal chalcogenide, electrical resistivity of Ag2Se is very high [12]. In addition, silver selenide has wide range of applications in solar cells, thermochromic materials, photosensitizers, IR detectors and magnetic field sensors [13–17]. The techniques that were applied for the fabrication of silver selenide nanostructures include combination of the precursors at high temperatures [18–20], hydrothermal method [15] microwave [21], and molecular precursor methods [22, 23]. The above processes usually require elevated temperature, inert atmosphere and long reaction time. A lowenergy approach is the precipitation of metal selenides from aqueous solution of the metal cation by using H2Se [24]. However, this method includes the use of a very toxic reagent like H2Se. In sonochemical processes, unusual mechanisms were occurred for generating high-energy with high temperatures and pressures [25]. Sonochemistry is obtained basically from three processes: the formation, growth, and implosive collapse of bubbles in liquids [26, 27]. Suslick and co-workers found different sonochemical reactions, such as synthesis of amorphous metal powders [28], intercalation into layered inorganic solids [29], and metal semi carbides [30]. Ultrasound will become a useful synthetic device for liquid–solid chemical reactions. Recently, the sonochemical method has become very attractive method for the preparation of nano-sized materials and

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J Inorg Organomet Polym (2013) 23:357–364

Table 1 The preparation conditions of Ag2Se nanoparticles Ultrasound power (W/cm2)

Temperature (°C)

Morphology

Yield (%)

5

45

25

Aggregated nanostructures

74.6

Silver benzoate

5

45

25

Nanospheres

80.8

Silver benzoate

5

55

25

Nanoparticles

77.0

4

Silver benzoate

5

65

25

Nanoparticles

84.6

5

Silver benzoate

5

55

10

Nanoparticles

77.0

6

Silver benzoate

5

55

45

Nanoparticles

79.2

7 8

Silver benzoate Silver benzoate

5 10

55 45

65 25

Nanoparticles Nanoparticles

76.1 80.0

9a

Silver benzoate

5

–

25

Aggregated nanostructures

73.1

Sample

Precursor

1

AgNO3

2 3

a

pH

Blank test

shown very rapid growth in its application to materials science due to its unique reaction effects. Furthermore, for a few years, our group has interested in the synthesis of different nanostructures by sonochemical method [31–35]. In this paper, we have attempted to develop a simple sonochemical route to synthesis silver selenide nanoparticles from new precursor. Up to now, there are no reports on the synthesis of Ag2Se by using coordination complexes as precursor. Thus, Ag2Se nanoparticles have been successfully produced via a sonochemical route by reaction between SeCl4 and silver benzoate, [Ag2(C6H5COO)2], in the presence of hydrazine without using any surfactant. In addition, the effects of different parameters, such as pH, reaction temperature, ultrasound power, and solvent on the morphology and purity of the products were investigated.

of silver benzoate was carried out with Pyris Diamond Perkin Elmer under air atmosphere at a heating rate of 10 °C/min from room temperature to 700 °C. Fourier

2 Experimental 2.1 Chemicals and Equipments All the reagents were of the commercial available purity. A multiwave ultrasonic generator (Sonicator 3000; Bandeline, MS 72, Germany), equipped with a converter/transducer and titanium oscillator (horn), 12.5 mm in diameter, operating at 20 kHz was used for the ultrasonic irradiation. All ultrasonication experiments were carried out at ultrasonic power between 100 and 110 mW measured by calorimetry [36]. Powder X-ray diffraction (XRD) patterns were collected from a diffractometer of Philips Company with X’PertPro monochromatized Cu Ka radiation ˚ ). Microscopic morphology of products was (k = 1.54 A visualized by a LEO 1455VP scanning electron microscope (SEM). The energy dispersive spectrometry (EDX) analysis was studied by XL30, Philips microscope. Transmission electron microscope (TEM) images were obtained on a LEO912-AB electron microscope with an accelerating voltage of 100 kV. The thermogravimetric analysis (TGA)

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Fig. 1 a TGA curve of silver benzoate precursor, FT-IR spectra of silver benzoate precursor (b), and Ag2Se nanoparticles (c)


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Fig. 2 SEM images of sample no. 1 (a, b), and sample no. 2 (c, d)

transform infrared spectra were performed using KBr pellets on FT-IR spectrometer (Magna-IR, 550 Nicolet) in the range 400–4,000 cm-1. 2.2 Synthesis of [Ag2(C6H5COO)2] Precursor [Ag2(C6H5COO)2] was synthesized according to the reported procedure [37]. In a typical experiment, 4 mmol (0.68 g) of AgNO3 was dissolved in 50 mL of distilled water. A stoichiometric amount of sodium benzoate (0.6 g) dissolved in an equal volume of distilled water was added dropwise into the above solution under magnetic stirring. The solution was stirred about 30 min and a white precipitate was obtained, isolated and washed with distilled water and ethanol several times to remove impurities, and then dried at 50 °C in vacuum. The as-synthesized silver benzoate was characterized by TGA and FT-IR. The yield percentage of silver benzoate was about 92 %. 2.3 Synthesis of Nano-Sized Ag2Se Nano-sized Ag2Se was prepared by reaction between [Ag2(C6H5COO)2] and SeCl4 with molar ratio of 1:1.

In a typical synthesis approach, 0.2 g (0.44 mmol) of [Ag2(C6H5COO)2] was dissolved in 100 mL of methanol (solution A). On the other hand, 3 mL of hydrazine hydrate (N2H4H2O, 100 %) was added drop-wise to the aqueous solution of 0.44 mmol (0.04 mL) SeCl4 in 50 mL of distilled water (solution B). Then the solution B was added to the solution A under strong magnetic stirring at room temperature. After stirring for 10 min, the as-obtained solution was irradiated with an ultrasonic horn for 30 min. After cooling to room temperature, the black precipitates were centrifuged, washed by distilled water and ethanol in sequence and dried in vacuum at 50 °C. Table 1 shows the preparation conditions of nano-sized Ag2Se in details.

3 Results and Discussion The dimeric structure of silver benzoate is determined by the formation of eight membered rings. Its structure involves two Ag atoms. The dimeric molecule is situated in a general position, but has a noncrystallographic centre of symmetry [37]. To determine the number of coordinated or crystallization water molecules of silver benzoate complex, TGA was carried out in air atmosphere. Figure 1a shows

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Fig. 3 SEM images of sample no. 3 (a, b), and sample no. 4 (c, d)

the TGA curve of [Ag2(C6H5COO)2] precursor. According to the TGA results, the weight loss observed at 276.9 °C was corresponding to silver benzoate decomposition to Ag2O. The TGA curve shows the total mass loss about 30 % (calcd. 40 %). Hence mass loss calculations showed that the final decomposition product was Ag2O. Besides, silver benzoate had no coordinated or crystallization water molecules. The IR spectra of silver benzoate and Ag2Se nanoparticles are shown in Fig. 1b, c, respectively. Since pure Ag2Se nanoparticle has no absorption peaks in the range of 4,000–400 cm-1, so Fig. 1c shows the product had no major IR-active peak. The morphology and structure of the products were investigated by SEM images. Figure 2a, b show SEM images of sample synthesized by AgNO3 after 30 min of sonication (sample no. 1). It is clear that aggregated nanostructures were formed by using AgNO3 as silver source. Figure 2c, d indicate that the morphology, size and agglomeration of the nanoparticles are greatly influenced by [Ag2(C6H5OO)2] as Ag source. Due to the dimeric structure of silver benzoate complex, high steric hindrance surrounds the Ag centre. The structure around Ag atoms plays the surfactant role [38].

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Figure 3 shows the SEM images of the Ag2Se samples prepared using different ultrasound powers. It can be observed that by increasing the ultrasound power from 45 W/cm2 (Fig. 2c, d) to 55 W/cm2 (Fig. 3a, b), particle size and agglomeration of nanoparticles decreased. In 65 W/cm2 (Fig. 3c, d), it is clear that the particles get agglomerated and the particle size distribution is not homogeneous. The morphology and size distribution of the product in the optimum power (55 W/cm2) were further studied by transmission electron microscopy (TEM). The TEM photographs of sample no. 3 are shown in Fig. 4a, b. The average particle size estimated by TEM images was about 50–60 nm. EDX technique was employed to investigate the chemical composition and purity of Ag2Se nanoparticles. The EDX spectrum presented in Fig. 5 confirmed the presence of Ag and Se elements in sample no. 3. The effect of reaction temperature on the morphology and shape of Ag2Se samples in the optimum ultrasound power (55 W/cm2) was investigated. For this purpose, the experiments were carried out at four different temperatures. It can be observed that with increasing temperature from room temperature (25 °C) (Fig. 2c, d) to 45 °C (Fig. 6a, b),


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Fig. 5 EDX spectrum of Ag2Se nanoparticles (sample no. 3)

Fig. 4 TEM images of sample no. 3

and to 65 °C (Fig. 6c, d), the particle size and agglomeration of the nanoparticles decreased. In addition, by decreasing temperature from 25 to 10 °C (Fig. 6e, f), the agglomeration of nanoparticles increased. It was found that low temperature causes a high viscosity, which makes the formation of the bubble more difficult. Hence, larger nanoparticles were obtained at lower temperature [39]. Figure 7a, b show the effect of pH on the morphology and size distribution of silver selenide samples. For the investigation of this parameter, the pH of reaction was adjusted to ten by adding aqueous solution of NaHCO3 0.1 M to the [Ag2(C6H5OO)2] solution. It was found that with increasing pH from five (Fig. 2b, c) to ten (Fig. 7a, b), smaller and very uniform nanoparticles with narrow size distribution were obtained. It is suggested that the bicarbonate anions encircle the Ag? ions; therefore they can act as surfactant in the reaction medium.

Figure 8a shows the XRD pattern of the sample prepared in the presence of distilled water as solvent. The sample involved metal silver (JCPDS card no. 87-0717) and Ag2Se (JCPDS card no. 24-1041) phases. As shown in Fig. 8b, in the presence of methanol as solvent, the peak of Ag disappeared and a pure Ag2Se phase formed. The diffraction peaks of the products in Fig. 8b can be indexed to orthorhombic phase Ag2Se (space group 19/m, JCPDS card 24-1041). No remarkable diffractions of other impurities such as selenium, silver or other compounds can be found in Fig. 8b, indicating that pure Ag2Se nanoparticles were synthesized. The sharp diffraction peaks indicate the good crystallinity of the nanoparticles and the peak broadening is due to the small particle size of Ag2Se nanoparticles. It has been found that during an aqueous sonochemical process, the elevated temperature and pressure inside the collapsing bubbles cause water to vaporize and further sonolysis into H and OH radicals [31]. Therefore, oxidation, reduction, dissolution, and decomposition reactions can take place by ultrasound irradiation [40]. As we are aware, N2H4H2O is as a reducing agent. Hydrazine hydrate is freely soluble in water, but because of the basic nature of N2H4, it is normally represented as N2H5? [41]. In the formation mechanism of Ag2Se nanoparticles, SeO32generated from the hydrolysis of SeCl4 could be reduced to Se0 by hydrazine hydrate. Further reduction of Se0 could be occurred to form Se2- by H2 produced during the sonication treatment [34]. At last, Ag? ions released form silver benzoate precursor react with Se2- ions to form Ag2Se nuclei. In the sonochemical formation of Ag2Se nanoparticles, some Ag? ions can react with excess hydrazin in the reaction medium, so Ag? ions are reduced to Ag0 by hydrazin. According to the available reports, organic liquids can support acoustic cavitations [42]. By decreasing the vapor pressure of solvent, the cavitation phenomenon increases. It was found that by using methanol as solvent, the formation of H2O2 during the sonochemical reactions increases. In this work, methanol could increase the value of H2O2 preparation, thus H2O2 participated in the Ag0

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Fig. 6 SEM images of sample no. 5 (a, b), sample no. 6 (c, d), and sample no. 7 (e, f)

Fig. 7 SEM images of sample no. 8 (a, b)

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To study the ultrasound irradiation effects on the morphology of Ag2Se, we prepare a sample at room temperature (25 °C) without using ultrasound irradiation. The preparation of blank test has been carried out under mechanical stirring conditions. SEM images of samples no. 9 are shown in Fig. 9a, b. It is clear that aggregated nanostructures were formed in the absence of sonocation, and it is difficult to measure the individual particle size. According to Table 1, it was found that by using silver benzoate as silver precursor in the presence of ultrasound irradiation, the yield of the products was higher than the yield of the product synthesized by using AgNO3 (sample no. 1). These results indicated that the advantages of silver benzoate as silver precursor. On the other hand, by comparing the yield of the products synthesized in the presence of ultrasound irradiation (samples 2–8) and the yield of the product formed in the absence of ultrasound irradiation (sample no. 9), it was found that the yield of Ag2Se synthesized by sonication was higher than that of Ag2Se obtained through blank test. Therefore, among the available new precursors to produce nano-sized materials [43–49], benzoate complexes are introduced to synthesize nanostructures with various grain size and morphology.

4 Conclusion

Fig. 8 XRD patterns of Ag2Se synthesized in a water as solvent, b methanol as solvent

oxidation reaction. So there were no Ag0 species in the reaction medium. It was found that the purity of Ag2Se nanoparticles depends on the ability of solvent to produce H2O2 during the sonication reaction.

In summary, Ag2Se nanoparticles were produced by using silver benzoate and selenium tetrachloride as starting agents in the presence of ultrasound irradiation. XRD and EDS results indicated that pure Ag2Se nanoparticles have been successfully obtained by this simple route. In this work, we attempted to produce Ag2Se nanoparticles without using any capping agent. Hence, we used silver benzoate as new precursor, which can act as surfactant in the reaction medium. This work is the only successful

Fig. 9 SEM images of sample no. 9 (a, b)

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sonochemical synthesis of silver selenide nanoparticles by utilizing silver benzoate. Acknowledgments Authors are grateful to the council of Iran National Science Foundation and University of Kashan for supporting this work by Grant No. 159271/38.

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