Sonochemical and hydrothermal synthesis of PbTe nanostructures with the aid of a novel capping agent

Accepted Manuscript Title: Sonochemical and hydrothermal synthesis of PbTe nanostructures with the aid of a novel capping agent Author: Shahla Ahmadian Fard-Fini Masoud Salavati-Niasari Fatemeh Mohandes PII: DOI: Reference:

S0025-5408(13)00598-9 http://dx.doi.org/doi:10.1016/j.materresbull.2013.07.010 MRB 6893

To appear in:

MRB

Received date: Revised date: Accepted date:

14-1-2013 2-7-2013 6-7-2013

Please cite this article as: S.A. Fard-Fini, M. Salavati-Niasari, F. Mohandes, Sonochemical and hydrothermal synthesis of PbTe nanostructures with the aid of a novel capping agent, Materials Research Bulletin (2013), http://dx.doi.org/10.1016/j.materresbull.2013.07.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Sonochemical and hydrothermal synthesis of PbTe nanostructures with the

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aid of a novel capping agent

Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, Kashan, P. O. Box. 87317-51167, I. R. Iran c

Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P. O. Box. 87317-51167, I. R. Iran

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b

Department of Chemistry, Payame Noor University, Tehran, P. O. Box. 19395-3697, I. R. Iran

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a

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Shahla Ahmadian Fard-Finia, Masoud Salavati-Niasari*b, c, Fatemeh Mohandesb

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* Corresponding author. Tel.: +98 361 5912383; Fax: +98 361 5552935;

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Abstract

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E-mail address: [email protected]

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In this work, a new Schiff-base compound derived from 1,8-diamino-3,6-dioxaoctane and 2-hydroxy-1naphthaldehyde marked as (2-HyNa)-(DaDo) was synthesized, characterized, and then used as capping agent for the preparation of PbTe nanostructures. To fabricate PbTe nanostructures, two different synthesis methods; hydrothermal and sonochemical routes, were applied. To further investigate, the effect of preparation parameters like reaction time and temperature in hydrothermal synthesis and sonication time in the presence of ultrasound irradiation on the morphology and purity of the final products was tested. The products were analyzed with the aid of SEM, TEM, XRD, FT-IR, and EDS. Based on the obtained results, it was found that pure cubic phased PbTe nanostructures have been obtained by hydrothermal and sonochemical approaches. Besides, SEM images showed that cubic-like and rod-like PbTe nanostructures have been formed by hydrothermal and sonochemical methods, respectively. Sonochemical synthesis of 1   

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PbTe nanostructures was favorable, because the synthesis time of sonochemical method was shorter than that of hydrothermal method.

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Keywords: A. Nanostructures; A. Chalcogenides; C. Nuclear magnetic resonance (NMR).

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1. Introduction

In the past decade, much attention has been focused on the preparation of low-dimensional materials such

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as nanorods, nanowires, nanobelts, and nanoparticles [1–5]. Synthesis of nano-sized lead telluride (PbTe) as a member of chalcogenides family has attracted significant attention due to its small band gap (0.31 eV at

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300 K) and larger Bohr excitation radius. PbTe nanostructures as semiconductors have been widely applied in many fields, such as infrared detectors, photo resistance, lasers, and thermoelectric materials [6–10]. Up to

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now, various methods have been explored to synthesize lead telluride nanostructures. Wan et al. synthesized

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PbTe nanocubes in hydrazine saturated alkaline solution by using Pb(NO3)2 and Te powder [11].

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Sonochemical synthesis of nanocrystalline PbTe with Pb as impurity has been reported by Li et al. after sonication for 8 h by using Pb(CH3COO)2.3H2O and Te powder [12]. PbTe nanowires with an average diameter of about 30 nm were produced by a hydrothermal process at 180 oC for 24 h using tellurium nanowires as templates and Pb(NO3)2 [13]. Flower-like PbTe nanostructures were synthesized using Pb(CH3COO)2.3H2O and Na2TeO3 as precursors under hydrothermal conditions at 240 oC for 24 h [14]. The alkaline hydrothermal synthesis of PbTe nanosheets by using Pb(NO3)2, Na2TeO3, and NaBH4 as starting materials and different surfactants was introduced by Zhu et al. [15]. Hierarchical superstructures of lead chalcogenides were prepared by microwave-assisted method with Pb(CH3COO)2.3H2O and Te powder in ethylene glycol [16]. Cubic-like nanostructures of PbTe have been obtained via solvothermal route at 150 oC for 12 h in various solvents including ethanol, acetone, and N,N-dimethylformamide (DMF) [17]. 2   

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It is well-known that the presence of capping agents during the formation of nanostructures has a great effect on the morphology of the final products. Therefore, much attention has been paid to the study of such supramolecular structures, which can act as both template and microreactor for the controllable growth of

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products, and it has been proved to be an effective structure director to control the morphology of nanomaterials [18, 19]. For example, sodium dodecylbenzene sulfonate (SDBS) [15], sodium dodecyl (SDS)

[20],

polyvinylpyrrolidone

(PVP)

[4],

polyethylene

glycol

(PEG)

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sulfate

[21],

and

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cetyltrimethylammonium bromide (CTAB) [22] have been applied as current ionic and polymeric capping agents.

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Here, a new Schiff-base compound derived from 1,8-diamino-3,6-dioxaoctane and 2-hydroxy-1naphthaldehyde marked as (2-HyNa)-(DaDo) was synthesized, characterized, and then used as capping agent

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for the synthesis of PbTe nanostructures. Lead nitrate, Te powder, N2H4.H2O, and NaOH as starting reagents

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and ethylene glycol as solvent were used in hydrothermal and sonochemical synthesis of PbTe

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nanostructures. The effect of reaction time and temperature in hydrothermal approach and sonication time in sonication treatment on the morphology and particle sized of the products have been studied. The as-

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produced PbTe nanostructures were characterized with the aid of XRD, SEM, TEM, EDS, and FT-IR.

2. Experimental 2.1.

Materials and characterization

All the reagents for the synthesis of the capping agent and PbTe nanostructures such as 1,8-diamino-3,6dioxaoctane, 2-hydroxy-1-naphthaldehyde, methanol, Pb(NO3)2, Te powder, ethylene glycol, hydrazine hydrate (N2H4.H2O), and NaOH were purchased from Merck Company (pro-analysis) and used without further purification. 3   

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The Fourier transform infrared spectra were performed using KBr pellets on FT-IR spectrometer (MagnaIR, 550 Nicolet) in the range of 400–4000 cm-1. The powder X-ray diffraction (XRD) patterns were collected from a diffractometer of Philips Company with X'PertPro monochromatized Cu Kα radiation (λ = 1.54 Å).

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Microscopic morphology of products was visualized by a LEO 1455VP scanning electron microscope (SEM). The energy dispersive spectrometry (EDS) analysis was studied by XL30, Philips microscope.

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Transmission electron microscope (TEM) images were obtained on a LEO912-AB transmission electron

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microscope with an accelerating voltage of 100 kV. 1HNMR spectrum of the (2-HyNa)-(DaDo) was

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recorded by BRUKER (400 MHz) in CDCl3.

2.2. Synthesis of the (2-HyNa)-(DaDo) as capping agent

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A typical procedure for the synthesis of the (2-HyNa)-(DaDo) compound is as follows: a solution

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containing 2 mol of 1,8-diamino-3,6-dioxaoctane in 50 mL of methanol was added dropwise into a solution

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involving 4 mol of 2-hydroxy-1-naphthaldehyde in 50 mL of methanol. The mixture was refluxed under magnetic stirring for 3 h to produce a yellow precipitate. The obtained yellow precipitate was filtered and

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washed with methanol several times, and finally dried at 50 oC in vacuum for 12 h. The chemical reaction for the preparation of the (2-HyNa)-(DaDo) is seen in Scheme 1. The as-obtained product was characterized by 1HNMR and FT-IR.

2.3.

Hydrothermal synthesis of PbTe nanostructures

In a typical experiment, Pb(NO3)2, Te powder, and the as-synthesized capping agent with molar ratio of 1:1:1 were put into a Teflon-lined stainless steel autoclave with a volume capacity of 100 mL. Then 0.02 g of NaOH, 3 mL of N2H4.H2O (85%), and 50 mL of ethylene glycol were added into the mixture. The

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autoclave was then sealed and maintained at 180 oC for 6 h. After that, the container was cooled to room temperature naturally. The black precipitates obtained were collected, washed with methanol and distilled water several times, and dried at 50 oC in vacuum for 12 h. The hydrothermal treatment was carried out at

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180 oC for 3, 9, 12, and 24 h. To investigate the effect of reaction temperature, the experiment was done at 160 and 200 oC for 6 h. The preparation conditions have been illustrated in Table 1. As shown in Table 1,

2.4.

Sonochemical synthesis of PbTe nanostructures

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The products were characterized by FT-IR, XRD, SEM, TEM, and EDS.

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the samples marked as H1-H7 were related to the PbTe nanostructures synthesized by hydrothermal method.

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Ultrasonic irradiation was accomplished with a high-intensity ultrasonic probe (Sonicator 3000; Bandeline,

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MS 72, Germany, Ti horn, 20 kHz, 60 Wcm-2) immersed directly in the reaction solution. In a typical

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process, 2 mmol of Te powder, 0.02 g of NaOH and 3 mL of N2H4.H2O (85%) were dissolved in 50 mL of ethylene glycol under magnetic stirring for 30 min. When Te powder was completely dissolved, the color of

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the reaction was purple. Then 2 mmol of Pb(NO3)2 dissolved in 50 mL of ethylene glycol was added in to the above solution dropwise in the presence of ultrasound irradiation. After sonication for 1 and 2 h, the asformed black precipitates were collected, washed with methanol and distilled water several times, dried at 50 o

C in vacuum for 12 h, and finally characterized by SEM and XRD.

3.

Results and discussion

The 1H NMR spectrum of the as-produced (2-HyNa)-(DaDo) in the range of 0–20 ppm and the expanded spectrum in the range of 7–9 ppm are shown in Fig. 1a and 1b, respectively. The 1H NMR (400 MHz,

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CDCl3) data for this compound is as follows: δ: 14.35 (s, 2H, OH); 8.657 (s, 2H, –CH=N–); 6.89–7.815 (m, 8H, aromatic); 3.633–3.708 (m, 12H, –CH2–). These results proved the formation of the Schiff-base compound derived from 1,8-diamino-3,6-dioxaoctane and 2-hydroxy-1-naphthaldehyde. The FT-IR

1

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spectrum of the as-prepared (2-HyNa)-(DaDo) was presented in Fig. 2a. The strong peak marked at 1633 cmis attributed to the v(C=N) as a characteristic band for the Schiff-base compound. All the absorption bands

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in the FT-IR spectrum of the (2-HyNa)-(DaDo) were attributed to the Schiff-base compounds [23, 24]. Fig.

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2b shows the FT-IR spectrum of the PbTe nanostructures synthesized by hydrothermal method at 200 oC for 6 h. Since the bond of Pb–Te is mainly electrovalent bond, the FT-IR spectrum of PbTe does not show

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strong bands associated with Pb–Se stretching and bending vibrations [25]. In Fig. 2b, the adsorption peaks at 2925 and 2852 cm-1 can be related to the v(–CH2–) due to the presence of the Schiff base on the surface of

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the product.

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SEM images of the PbTe synthesized via hydrothermal route at 180 oC for 6 h (sample H2) are seen in

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Figs. 3a and 3b. Synthesis of very fine nanostructures including nanorods has been took place under the as-

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mentioned conditions. The particles are agglomerated, and it is difficult to measure the individual particle size with the aid of SEM images. TEM images of the sample H2 were taken and presented in Figs. 4a and 4b. Appling hydrothermal process at 180 oC for 6 h led to the formation of PbTe nanorods with diameters ranging from 10–12 nm and lengths from 50–70 nm (Fig. 4). To study the effect of reaction time on the morphology of PbTe nanostructures, the experiment was carried out at different times at the same conditions. The autoclave containing the start materials was heated at 180 o

C for 3 (sample H1), 9 (sample H3), 12 (sample H4), 24 h (sample H5), respectively. SEM images of the

samples H1, H3, H4, and H5 were presented in Figs. 5a-d, respectively. Based on the SEM images in Figs. 5a, 5b, and 5c, it can be seen that by increasing the reaction time from 3 to 12 h, the grain size and aggregation of the formed nanoparticles increased. This phenomenon can be explained by an Ostwald 6   

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ripening growth mechanism where the larger particles grow at the expense of the smaller particles during the thermal treatment process [26]. When the reaction time was set for 24 at the same temperature, uniform

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cubic-like nanostructures with grain size of between 50–70 nm were formed (Fig. 5d). To investigate the effect of reaction temperature on the morphology of the products, the hydrothermal

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treatment was carried out at 160 and 200 oC at the same conditions for 6 h. SEM images of the PbTe nanostructures synthesized at 160 (sample H6) and 200 oC (sample H7) are seen in Figs. 6a and 6b,

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respectively. Although the morphology and grain sizes of the sample H6 prepared at 160 oC are not uniform,

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the presence of a small amount of cubic-like nanostructures is seen in its SEM image (Fig. 6a). When the temperature was increased to 200 oC, cubic-like nanostructures decorated by nanoparticles were formed (Fig.

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6b). With comparing the SEM images of the samples H6 (160 oC), sample H2 (180 oC), and sample H7 (200 C) at the same reaction time, it can be observed that morphology of the products includes uniform shapes

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including nanorods and nanocubes at high temperatures (Fig. 3 and Fig. 6).

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The XRD patterns of the samples H7, H2, and H6 prepared at 200, 180 and 160 oC are shown in Figs. 7a-c,

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respectively. The main reflection peaks in Figs. 7a-c can be indexed to the cubic PbTe with cell constants a = b = c = 6.4610 Å (JCPDS card No. 77–0246). Besides, in Figs. 7b and 7c, the weak reflection peak at 2θ = 34.19○ is attributed to the formation of PbCO3.Pb(OH)2 (JCPDS card No. 01–0687) as a by-product due to the presence of CO2 molecules in the autoclave. The sharp reflection peaks in the XRD patterns proves the high crystallinity of the products synthesized by hydrothermal method. To further study the purity and chemical composition of the products, EDS spectrum of the sample H7 formed at 160 oC for 6 h was taken and shown in Fig. 8. Fig. 8 indicates that the elements in the sample H7 are Pb and Te only (Si, and C signals are from the substrate). The EDS results gave a rough atomic ratio

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Pb:Te as nearly 1:1 (Pb: 46.84; Te: 53.16), confirming the purity of the products synthesized through hydrothermal route.

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Ultrasound has become an important tool in synthesis of novel nano-sized materials under ambient conditions in recent years [27–30]. The Ultrasound effects arise from acoustic cavitation, which is the

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formation, growth, and implosive collapse of bubbles in a liquid. The growth of the bubble occurs through the diffusion of solute vapor into the volume of the bubble, while the collapse of the bubble occurs when the

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bubble size reaches its maximum value. When solution is exposed to ultrasound irradiation, the bubbles are

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implosively collapsed by acoustic fields in the solution. According to hot spot theory, very high temperatures (>5000 K) are obtained upon the collapse of a bubble. Since this collapse occurs in less than a nanosecond,

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very high cooling rates (>1010 K/s) are also obtained [31, 32]. These extreme conditions can drive a variety of chemical reactions to fabricate nano-sized materials.

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In this work, besides hydrothermal approach, PbTe nanostructures have been synthesized with the aid of

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ultrasound irradiation. The starting reagents were exposed to ultrasound irradiation for 1 and 2 h at

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ultrasound intensity set at 60 Wcm-2. Figs. 9a and 9b show SEM images of the sonochemically synthesized products after irradiation for 1 h (sample S1) and 2 h (sample S2), respectively. As shown in Fig. 9a, morphology of the sample S1 includes rod-like structures composed of nanoparticles with particle size of between 90–95 nm. By increasing the sonication time from 1 to 2 h, the as-formed prepared rod-like structures disappeared, and PbTe nanoparticles with particle size of about 85 nm were obtained (Fig. 9b). It was found that the sonication time was a key factor in the morphological control of PbTe nanostructures. The increase in sonication time indicates that energy is continuously added to the reaction system, and this hinders the growth of PbTe nanoparticles.

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The XRD patterns of the sample S1 and S2 are shown in Figs. 10a and 10b, respectively. Most of the reflection peaks in Fig. 10a were attributed to cubic phased PbTe (JCPDS card No. 77–0246). The reflections marked as “ ” in Fig. 10a were assigned to the Te powder used as Te source. Fig. 10b shows the

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XRD pattern of the sample S2 obtained after sonication for 2 h. All the reflection peaks in this figure can be attributed to cubic phased PbTe (JCPDS card No. 77–0246). According to these results, it was found that

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pure PbTe nanostructures were formed after sonication for 2 h.

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Based on the previous reports, it was found that PbTe can be formed by two independent pathways,

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involving ionic and atomic processes. It was found that in the ionic process, Pb2+ could be reduced by Te2− to generate PbTe. Besides, a directly atomic process could proceed between the metal Pb reduced from Pb2+ by

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reducing agent and the metal Te [17, 33, 34]. It must be remembered that the formation of PbTe through both ionic and atomic processes results from red-ox reactions. The main steps for the formation of PbTe

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nanostructures by hydrothermal method are as follows:

According to the proposed formation mechanism of PbTe, the addition of NaOH accelerated the dissolution process of Te powder. The disproportional reaction of Te can produce TeO32− ions, which can reduce to Tea+12− ions by hydrazine hydrate. The Tea+12− ions with purple black color consist of Te and Te2−

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ions [17]. By reaction between Te2− and Pb2+ ions, PbTe was formed via an ionic process. In addition, Te powder can react with metal Pb reduced from Pb2+ by hydrazine hydrate through an atomic process.

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Based on the XRD results of the sonochemically synthesized nanostructures and our observation during the sonication reactions, we can propose an ionic process for the fabrication of PbTe nanostructures. As we

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know, the sonolysis of solvent molecules can be take place during the sonication reactions. In this work, ethylene glycol was applied as solvent because of solubility of the capping agent in it. When Te powder was

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dissolved in a mixture of NaOH, N2H4.H2O and ethylene glycol, the color of the solution changed purple

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black, indicating the formation of the Tea+12− ions composed of Te and Te2− ions [17]. The Te2− ions produced reacted with Pb2+ ions to form PbTe nuclei. When the sonication time was 1 h, the presence of Te

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was observed in the XRD pattern of the final product (Fig. 10a). By increasing the sonication time from 1 to 2 h, the presence of Te in the XRD pattern of the final product disappeared (Fig. 10b). These phenomena can

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be explained as follows. As we know, the sonolysis of solvent molecules can be take place during the

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sonication reactions. In this sonochemical formation mechanism, it was possible that H2 molecules could be

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formed by the sonolysis of ethylene glycol as an organic compound [4]. When the sonication reaction was done for long time (2 h), the quantity of the H2 molecules increased. By reaction between H2 molecules and the available Te powder, Te2– ions could be produced. At last, these Te2− ions combined with the remained Pb2+ ions to form PbTe crystals. Therefore, the presence of Te was not seen in the XRD pattern of the final product after sonication for 2 h (Fig. 10b). The main steps for the sonochemical formation of PbTe nanostructures are as follows:

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It is well-known that an ultrasound wave that is intense enough to produce cavitation can drive chemical reactions such as: oxidation, reduction, dissolution, and decomposition [35]. Successfully synthesis of the pure PbTe nanostructures in the presence of ultrasound irradiation can be introduced as a fast and efficient

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reduction reaction.

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To investigate the effect of the capping agent on the morphology of the products, the experiment was carried out without using the capping agent by hydrothermal method at 180 oC after heating for 24 h. This

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experiment was named as blank test. SEM images of the product obtained from the blank test are shown in

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Figs. 11a and 11b. According to the Fig. 11, it can be seen that the morphology of the product is very irregular. Particle-like, rod-like and cubic-like microstructures of PbTe have been formed in the absence of

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the capping agent. On the other hand, the formation of a small amount of PbTe nanoparticles with particle sizes less than 100 nm is observed in Fig. 11a. These results indicate that the presence of the capping agent

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is very favorable to produce uniform cubic-like PbTe nanostructures. This finding is proved by comparing

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same conditions.

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the SEM images of the products synthesized with (Fig. 5d) and without the capping agent (Fig. 11) at the

Conclusion

In summary, cubic-like and rod-like PbTe nanostructures were successfully fabricated by hydrothermal and sonochemical routes, respectively. A new Schiff-base compound derived from 1,8-diamino-3,6dioxaoctane and 2-hydroxy-1-naphthaldehyde was synthesized and then used as capping agent instead of the current surfactant molecules for the preparation of PbTe nanostructures. Pure cubic phased PbTe nanostructures without any impurities like Pb and Te have been obtained by hydrothermal and sonochemical approaches. It was found that by increasing sonication time from 1 h to 2 h, the formation of PbTe increased. 11   

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Very short reaction time in sonochemical synthesis was the advantage of this method compared to

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hydrothermal route.

Acknowledgements

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Authors are grateful to council of University of Kashan by Grant No (159271/83), and TEM section, SAIF,

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NEHU, Shillong, Meghalaya, India, for providing financial support to undertake this work.

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Fig. 1. (a) 1H NMR spectrum of the as-synthesized capping agent, (b) the expanded 1H NMR spectrum in the range of 7–9 ppm.

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Fig. 2. FT-IR spectra of (a) the as-produced capping agent, and (b) PbTe nanostructures from the sample H7.

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Fig. 3. SEM images of the sample H2 (a, b).

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ip t cr us an M d te Ac ce p Fig. 4. TEM images of the sample H2 (a, b).

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Fig. 5. SEM images of the samples (a) H1, (b) H3, (c) H4, and (d) H5.

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Fig. 6. SEM images of the samples (a) H6, and (b) H7.

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Fig. 7. XRD patterns of the samples (a) H7, (b) H2, and (c) H6.

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M

Fig. 8. EDS spectrum of the sample H7.

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ip t cr us an M d te Ac ce p

Fig. 9. SEM images of the samples (a) S1, and (b) S2.

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ip t cr us an M d te

Ac ce p

Fig. 10. XRD patterns of the samples (a) S2, and (b) S1.

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ip t cr us an M d te Ac ce p

Fig. 11. (a, b) SEM images of the product synthesized by hydrothermal method without using the capping agent (blank test).

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ip t cr us an M d

Ac ce p

te

Scheme 1. Chemical reaction for the preparation of the used capping agent.

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Table Time (h) 3 6 9 12 24 6 6 1 2

Products PbTe PbTe PbTe PbTe PbTe PbTe PbTe PbTe and Te PbTe

Morphology Nanoparticles Nanorods Nanorods Nanoparticles and nanocubes Nanocubes Irregular shapes and nanocubes Nanocubes decorated by nanoparticles Nanorods Nanoparticles

SEM image Fig. 5a Fig. 3a, b Fig. 5b Fig. 5c Fig. 5d Fig. 6a Fig. 6b Fig. 9a Fig. 9b

conditions for the synthesis of

PbTe

Ac ce p

te

d

M

an

us

nanostructures.

1.

Preparation

ip t

Temperature (oC) 180 180 180 180 180 160 200 27 27

cr

Sample H1 H2 H3 H4 H5 H6 H7 S1 S2

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ip t cr us an M

 

Ac ce p

te

d

Graphical Abstract

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Research highlights PbTe nanostructures were prepared with the aid of Schiff-base compound.

•

Sonochemical and hydrothermal methods were employed to fabricate PbTe nanostrucrues.

•

The effect of preparation parameters on the morphology of PbTe was investigated.

Ac ce p

te

d

M

an

us

cr

ip t

•

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