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Materials Science and Engineering B 131 (2006) 200–202

A novel electrochemical preparation of PbS nanoparticles Yu Jun Yang ∗ Zhuzhou Institute of Technology, Zhuzhou, 412008 Hunan, PR China Received 3 December 2004; accepted 19 April 2006

Abstract A simple one-step anodic sonoelectrochemical method to synthesize PbS nanoparticles has been developed. With the lead foil as the sacrificing anode, Pb(II) was anodically dissolved from the lead electrode into the aqueous solution of sodium sulfide, supporting electrolyte (potassium nitrate) and capping agent (PVA) at a constant potential, and then the produced Pb(II) reacted with the sulfide anion to form PbS nanoparticles under ultrasonic irradiation. The effects of the applied potential, capping agent and ultrasound in the formation of PbS nanoparticles are discussed, and the results suggest that the anodic sonoelectrochemical method may be a general and convenient way to prepare metal sulfide nanoparticles. © 2006 Elsevier B.V. All rights reserved. Keywords: Electrochemistry; Nanoparticles; Lead sulfide; Anodic electrolysis; Ultrasonic irradiation

1. Introduction Semiconductor nanocrystals have electronic properties between those of molecular entities and macrocrystalline solids and there has been much interest in the synthesis and characterization of sulfide semiconductor nanocrystals in the past decade. Various new methods for the preparation of metal sulfide nanoparticles such as ultraviolet irradiation [1], X-ray irradiation [2], solvothermal method [3] and microwave assisted heating [4] have been reported recently. The combination of sonochemistry and electrochemistry is a new technique used to generate novel materials. Silver nanoparticles [5], nanowire [6] and nanorods [7] were prepared by reducing the Ag+ at the cathode surface from AgNO3 aqueous solution in an ultrasonic bath. Metal selenides nanoparticles have also been successfully synthesized with sonoelectrochemical method, which combines the existing cathodic electrodeposition techniques of metal selenides and ultrasonic irradiation. CdSe and PbSe nanoparticles were fabricated by reductively co-depositing Cd/Pb and selenium at negative potential on the cathode which also acted as the ultrasound emitter from the aqueous solution of CdSO4 /Pb(Ac)2 and Na2 SeSO3 [8,9]. Although metal sulfide thin films can be electrodeposited on the cathode surface [10,11], however, until now no research has

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been done on the electrochemical preparation of semiconductor sulfide nanomaterials. It is due to the fact that the cathodic electrodeposition of semiconductor sulfide was usually carried out in an acidic bath (pH 3.00–4.00) containing metals ion and sodium thiosulfate (Na2 S2 O3 ). Under ultrasonic irradiation, pure lead sulfide nanoparticles cannot be produced because it is inevitable for the colloidal sulfur to form in the acidic solution. In this report, we take a different approach to electrochemically synthesize metal sulfide nanoparticles. With the lead foil used as the sacrificing anode, PVA capped lead sulfide nanoparticles were successfully synthesized in the sodium sulfide aqueous solution under ultrasonic irradiation. Our further study suggests that this novel anodic sonoelectrochemical method may be a general and convenient way to prepare other metal sulfide nanoparticles. 2. Experimental All the chemical reagents except lead foil were of pure grade. The experiment was performed in a one-compartment cell. Electrolysis was carried out with a PGSTAT 30 (Eco. Chemie, Netherlands) electrochemical system. The reference electrode was Ag/AgCl (saturated KCl) and the counter electrode was a platinum grid. A 3 cm2 lead foil (AR, Ajax Chemicals, Australia) was used as sacrificing anode. PVA of 2.5 g was dissolved into 250 mL 0.04 M KNO3 and 0.04 M Na2 S solution. Working, reference and counter electrodes were immersed into the prepared solution. The distance between working electrode and

Y.J. Yang / Materials Science and Engineering B 131 (2006) 200–202

counter electrode was kept at 5 cm. High purity nitrogen gas was bubbled into the solution to remove the dissolved oxygen from the system. The electrolysis cells were put into an operating ultrasonic bath (ZQ25-2A Subminiature Ultrasonic Cleaning Machine, Gang Xing Ultrasonic Co. Ltd., China) and the electrolysis was carried out in the presence of an ultrasonic field (25 kHz, 100 W) under N2 atmosphere. The temperature during the reactions was controlled at ca. 25 ◦ C. PbS nanonanoparticles were prepared via electrochemical oxidation with controlledpotential at 0.8, 1.0 and 1.2 V at ambient temperature. Black colloid solution was obtained after 15 min of electrolysis. Centrifuge the solution and wash the precipitate with distilled water and absolute ethanol for several times. The black powder was dried in vacuum oven at room temperature for 4 h. The products were characterized by using ultraviolet spectroscopy (UV), powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). Powder X-ray diffraction data were collected using a Siemens X-ray diffractometer at a scanning rate of 4◦ /min in the 2θ range from 10 to 90◦ , with graphite monochromatized Cu K␣ radiation (λ = 0.15418 nm). TEM experiments were carried out employing Philips CM 10 transmission electron microscope, using an accelerating voltage of 100 kV. The samples used for TEM were prepared by dispersing some products in ethanol followed by ultrasonic vibration for 30 min. One drop of the suspension was placed on a copper net, which was dried carefully in vacuum oven. Absorption spectrum was recorded using a Shimadzu UV–vis 265 spectrophotometer in the wavelength range of 300–900 nm at room temperature. 3. Results and discussion During the experiment, black product was formed immediately once upon the beginning of electrolysis in the ultrasonic bath. This indicates that the chemical reaction rate of the reactive system is very fast at room temperature (25 ◦ C). It can be concluded from the experimental phenomenon that the nucleation rate is far excess the growth rate of the particles. So this reactive mechanism favors the formation of nanoparticles. The UV–vis absorption spectrum of the as-prepared PbS nanoparticles is shown in Fig. 1. It can be seen that the UV–vis absorption edge is in the 370 nm, and shows a very significant blue shift from the bulk PbS crystals [12]. This is an indication of quantum confinement, because the average size of PbS nanoparticles is smaller than the excitonic Bohr radius of the bulk PbS (ca. 18 nm). The applied potential is an important factor affecting the reaction. The higher the applied potential, the higher the anodic current is. And a higher current will definitely leads to a faster dissolution rate of Pb2+ from the lead electrode surface. As shown in Fig. 1, with constant reaction time (15 min), the absorption peak increased as the potential applied on the working electrode was increased while no obvious peak shift can be observed. However, the large amount of hydrogen gas evolved when the potential higher than 0.8 V was applied on the working electrode. The pH value of the solution dropped very quickly and caused the evolution of toxic H2 S. So 0.8 V is the optimum potential in this experiment.

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Fig. 1. UV–vis absorption spectrum of PbS nanoparticles synthesized in 0.04 M KNO3 , 0.04 M Na2 S and 1 wt.% PVA aqueous solution with 0.8, 1.0 and 1.2 V applied on the lead electrode under 25 kHz ultrasonic irradiation.

The purity and composition of the product were further analyzed using powder X-ray diffraction. The XRD pattern of the as-prepared PbS nanoparticles (Fig. 2) shows the presence of broad peaks corresponding to the cubic crystal structure. The positions and intensities of the peaks are in good agreement with the JCPDS values for cubic phase PbS. The broadening of the peaks indicates the nanocrystallite nature of the product. The dimensions and morphologies of the PbS nanoparticles are shown in the TEM image (Fig. 3). It is apparent that the as-prepared PbS nanoparticles present spherical morphologies, although some very large particles exist. The average size of these nanoparticles is 9 nm, which is less than the excitonic Bohr radius of the bulk PbS (ca. 18 nm). The average size of PbS nanoparticles calculated from the measurements of nanoparticles on the TEM image is consistent with the result calculated for the half-width of diffraction peaks using the Scherrer’s formula.

Fig. 2. The powder XRD patterns of PbS nanoparticles synthesized in 0.04 M KNO3 , 0.04 M Na2 S and 1 wt.% PVA aqueous solution with 0.8 V applied on the lead electrode under 25 kHz ultrasonic irradiation.

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of PbS nanoparticles. In the absence of PVA, PbS aggregated into micron-sized particles. Lower concentration of PVA cannot prevent the aggregation of the PbS nanoparticles. However, as indicated by the XRD study of the products, a mixture of PbS and metal lead nanoparticles was obtained when the concentration of PVA was higher than 1.5 wt.%. 4. Conclusion We have demonstrated a novel sonoelectrochemical method for the preparation of nanoparticles of PbS. With lead electrode as sacrificing anode and the application of a positive potential, PVA capped PbS nanoparticles with average size of 9 nm were produced in the aqueous solution of sodium sulfide under ultrasonic irradiation. This method is also applicable for the preparation of other sulfide semiconductor nanoparticles such as CdS and ZnS. References

Fig. 3. The TEM images of PbS nanoparticles synthesized in 0.04 M KNO3 , 0.04 M Na2 S and 1 wt.% PVA aqueous solution with 0.8 V applied on the lead electrode under 25 kHz ultrasonic irradiation.

The basis of this novel sonoelectrochemical technique to synthesize PbS nanoparticles is massive nucleation using high current density electrolysis, followed by the removal of the deposit from the electrode by the ultrasonic pulse. The electrolysis of lead produces Pb2+ in the ultrasonic bath, which immediately reacts with the S2− . The high-density burst of ultrasound removes the PbS particles from the anode, clean the electrode surface and replenishes the double layer with S2− by stirring the solution [9]. If ultrasonic irradiation was not employed, it was observed that PbS was deposited on the lead electrode surface. As a result, PbS nanoparticles cannot be obtained. The concentration of the PVA is another factor affecting the formation

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