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Microelectronic Engineering 66 (2003) 46–52 www.elsevier.com / locate / mee

Synthesis of HgS and PbS nanocrystals in a polyol solvent by microwave heating Tao Ding, Jian-Rong Zhang, Su Long, Jun-Jie Zhu* Laboratory of Mesoscopic Materials Science, Department of Chemistry, Nanjing University, Nanjing 210093, PR China

Abstract A novel and quick method for the synthesis of PbS and HgS semiconductor nanoparticles using mercury acetate and lead acetate as source materials and sulfur powder employed as chalcogenide source by microwave heating in polyethylene glycol (PEG) solvent is presented in this paper. HgS and PbS nanoparticles were obtained with average sizes from 20 to 30 nm in PEG, which acts as both solvent and reducing agent. The products were characterized by powder X-ray analysis, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The probable mechanism was presented.  2003 Elsevier Science B.V. All rights reserved. Keywords: Microwave heating; Polyol solvent; Semiconductors

1. Introduction In recent years, sulfide semiconductors with nanometer size dimensions have been the focus of many researchers due to the quantum size effect exhibited by these nanosized semiconductors [1–3]. Mercury sulfide is a useful material and it can be widely used in ultrasonic transducers [4,5], electrostatic image materials [6] and photoelectric conversion devices [4–6]. Lead sulfide is also an attractive semiconducting material because of its variety applications in potential photonic material [7] and Pb 21 ion-selective sensor [8]. Nowadays, developments of techniques, such as the ‘Chemical Scissors’ route [9], electrodeposition in acidic medium [10], the ion-exchange colloidal method [11], and the ion beam method [12] have led to the preparation of lead and mercury sulfides. However, finding fast and energy-efficient methods to produce metal sulfides are new challenges to synthetic chemists and material scientists. The microwave-assisted route is yet another novel method to synthesize lead and mercury sulfides [13,14], and is a very rapidly developing area of research. It is well known that microwave is electromagnetic waves containing electric and magnetic field * Corresponding author. Tel.: 186-25-359-4976; fax: 186-25-331-7761. E-mail address: [email protected] (J.-J. Zhu). 0167-9317 / 03 / $ – see front matter  2003 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00023-6

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components. The electric field applies a force on charged particles as a result of which the charged particles start to migrate or rotate. Due to the movement of charged particles, further polarization of polar particles takes place. The concerted forces applied by the electric and magnetic components of microwave are rapidly changing in direction, which creates friction and collisions of the molecules. Claimed effects of microwave irradiation include thermal and non-thermal effects [15]. Compared with conventional methods, microwave synthesis has the advantages of short reaction time, small particle size, narrow particle size distribution and high purity. Thus, microwave irradiation as a heating method has found a number of applications in chemistry. In these years, there is a growing interest in the synthesis of nanosized semiconductors by microwave irradiation in polyol solvent, such as in ethylene glycol and glycerol [16,17]. Polyethylene glycol (PEG) as one of the polyols, combined with microwave irradiation, has also been widely employed as a reducing agent and solvent in organic synthesis [18,19] and as a phase transfer catalyst [20]. However, few studies have reported the preparation of inorganic nanomaterials by using PEG as a solvent. Liu and co-workers prepared TiO 2 nanoparticles by using a sol–gel method with inorganotitanates and polyethylene glycol (PEG) [21]. Zhu et al. prepared silver nanorods by an electrochemical technique from an aqueous solution of AgNO 3 in the presence of PEG [22]. In those studies discussed above, PEG was only used as a surface-modifying or capping agent. In this paper, we present a novel way to prepare HgS and PbS semiconductor nanoparticles in PEG solvent by a microwave-heating method, where PEG was employed as both a solvent and reducing agent. The power of the microwave and reaction time affected the purity of the final products. The products were characterized by powder X-ray analysis, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS). It was found to be a fast, convenient, mild, energy-efficient and environmentally friendly route to produce HgS and PbS nanoparticles.

2. Experimental

2.1. Materials and synthesis All the reagents used in the experiment were of analytical purity and were used without any purification. In a typical procedure, | 1.6 g lead acetate (Pb(Ac 2 ) ? 3H 2 O) and | 1.5 g mercury acetate (Hg(Ac) 2 ) were added to | 60 ml polyethylene glycol (PEG-200), respectively. Then | 0.2 g sulfur powders were introduced into the two PEG solvents. The amount of the entire reagent was optimized. The two mixtures were placed in the microwave-refluxed system and the reaction was performed under ambient air for 20 min. The microwave oven followed a working cycle of 9 s on and 21 s off (30% power). After the reaction finished, the black precipitates were centrifuged, washed with distilled water and absolute acetone, and dried in air. The final products were collected for characterization

2.2. Instrumentation A microwave oven with 650 W power (Sanle General Electric Corp. Nanjing, China) was used in this experiment. A refluxing system was connected with the microwave oven. The X-ray diffraction (XRD) patterns were recorded on a Shimadzu XD-3A X-ray diffractometer (CuK a radiation,

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l 5 0.15418 nm). The transmission electron microscopy (TEM) images were obtained by employing a Jeol JEM-200CX instrument, working at 200 KV accelerating voltage. The X-ray photoelectron spectra (XPS) measurements were performed on an Escalab MKII instrument, using non-monochromatized MgK a X-ray as the excitation source.

3. Results

3.1. XRD, TEM and XPS studies The XRD patterns (Fig. 1) show that the products obtained are the cubic phase HgS and PbS. The peaks are corresponding to khkll values of (111), (200), (220), (311) and (222), which match well with literature patterns (JCPDF card No. 6-0261 and 5-0592). The average sizes of the PbS and HgS nanoparticles are estimated by the Debye–Scherrer formula to be 20 nm and 30 nm, respectively. The XPS was employed to investigate the composition and purity of the prepared PbS nanoparticles (Fig. 2). The C1s peak lies at 294.55 eV, which should be correctly shifted to 284.6 eV. All the other peaks are corrected accordingly. Fig. 3a and b show the high-resolution XPS spectra of Pb (4f) and S (2p), respectively. The two strong peaks at 146.50 eV and 169.75 eV correspond to Pb (4f) and S (2p), respectively. No peaks of impurities are detected, indicating the high purity of the products. The peak areas of the Pb and S cores are measured and yield a ratio of Pb to S as 42:58, which shows that the surface of the products is rich in sulfur. HgS nanoparticles were also investigated by XPS and the ratio of Hg to S as 58:42 was given. The TEM observation for the as-prepared PbS and HgS nanoparticles is shown in Fig. 3a and b. It can be clearly seen that most particles are spherical. Although the majorities of the particles are in aggregation, there are still some separated particles. The average sizes of HgS and PbS particles are in the range of 30 | 35 nm and 15 | 20 nm, respectively. They are in good agreement with those estimated by the Debye–Scherrer equation from the XRD patterns.

Fig. 1. The XRD patterns of the (A) PbS, (B) HgS nanoparticles prepared in PEG solvent.

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Fig. 2. The high-resolution XPS spectra taken for the Pb and S region of PbS (a) Pb (4f); (b) S (2p).

4. Discussion In the preparation of HgS and PbS, it was found the microwave power and the reaction time would influence the formation of HgS and PbS. The reaction was carried out in different power and time. On one hand, the reaction could not fully proceed with low power and short reaction time. On the other hand, due to the insolubility of reactants in PEG, excessive high power could lead to the agglomeration of the reactants, therefore, pure final products could hardly be obtained. Prolonging the reaction time to 30 min did not change the morphology and size of the final products. The probable reaction mechanism could be described as follows:

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Fig. 3. TEM images of the PbS and HgS nanoparticles prepared in PEG solvent: (a) PbS; (b) HgS.

Hg 21 , Pb 21

PEG as reducing agent

→

microwave irradiation

Hg or Pb 1 S

Hg, Pb

In PEG solvent

→

microwave irradiation

HgS or PbS

(1) (2)

In the first step, Hg 21 and Pb 21 were reduced to metal Hg and Pb as the intermediate of the whole

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reaction. The process is known as the polyol process and temperature is a dominant factor in affecting the reactivity [23]. There are two main modes of operation. The first occurring in the liquid phase is a coupling between the oscillating electric field (2.45310 9 Hz) and the permanent dipole moment of the molecule resulting in molecular rotations, which bring rapid volumetric heating of the liquid phase. In the second mode, metallic particles produced in the polyol reaction are also good acceptors of microwave irradiation and cause rapid heating of these particles. The two heating modes provide enough high temperature to guarantee the reaction to be fully carried out. In addition to PEG, many other polyols can also play reducing capacity at high temperatures [24]. In the second step, metal particles were directly combined with S powder under microwave irradiation, leading to the formation of the final product. With microwave irradiation in polar solvents, high and uniform heating could be provided. Thus high temperature could be reached and temperature gradients could be avoided, providing a uniform environment for the nucleation [25]. Then, uniform HgS and PbS nanosized particles could be formed.

5. Conclusion In summary, cubic HgS and PbS semiconductor nanoparticles were synthesized by a microwave heating method in PEG solvent. It was found to be a quick and efficient route. Further studies may extend the method for the preparation of the other semiconductor metal sulfides.

Acknowledgements This work is supported by the National Natural Science Foundation of China (Nos. 50072006 and 90206037) and JiangSu New Technique Program of China (BG 2001093).

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