Room Temperature Synthesis of Lead Sulfide Nanoparticles
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DOI 10.1515/mgmc-2012-0036 Main Group Met. Chem. 2012; 35(5-6): 173–178 Gholamreza Nabiyouni* , Emad Moghimi , Kambiz Hedayati and Rouhollah Jalajerdi Room temperature synthesis of lead sulfide nanoparticles Abstract: Lead sulfide (PbS) nanoparticles were synthe- properties of materials strongly depend on particle sizes sized at room temperature via a simple chemical reaction. (Ji et al., 2003; Ni et al., 2006; Yousefi et al., 2011; Salavati- In this work, thiophenol was used as the capping agent Niasari and Ghanbari, 2012). PbS is an important semicon- and sodium sulfide was used as a sulfur source. The prod- ductor material because of its exciton Bohr radius which ucts were characterized using X-ray diffraction, scanning is 18 nm at room temperature. It is well known that, if the electron microscopy, transmission electron microscopy, average crystal size of a typical semiconductor is smaller ultraviolet-visible, photoluminescence and Fourier trans- than the Bohr radius, having a small band gap and a large form infrared spectroscopy. Electron microscopy studies exciton Bohr radius makes it as an interesting system for reveal that synthesis of PbS nanoparticles using no cap- studying the effect of size confinement. PbS nanoparticles ping agent produces PbS nanoparticles with relatively may also be useful in electroluminescent devices such as large size, whereas adding thiophenol leads to the pro- light-emitting diodes and optical switches (Kuang et al., duction of nano-sized PbS particles. We found that elec- 2003; Wang et al., 2003; Chakraborty and Moulik, 2005; tronic absorption spectra as well as particle sizes depend Lu et al., 2006; Zhang et al., 2006a,b; Zhou et al., 2006). In on the used capping agents. Compared with bulk PbS, this work, we introduce an easy chemical route to synthe- the absorption spectra of the obtained PbS nanoparticles size PbS nanoparticles with different particle sizes, simply exhibit a blue shift, which can be attributed to the quan- using thiophenol as a capping agent. The PbS nanoparti- tum confinement of charge carriers in the nanoparticles. cle size is controlled by the concentration of thiophenol. Keywords: lead sulfide; nanoparticles; thiophenol. Results * Corresponding author: Gholamreza Nabiyouni, Faculty of Science, Department of Physics, Arak University, Arak 38156-8-8349, Iran, X-ray diffraction (XRD) patterns of as-synthesized PbS e-mail: [email protected] with 1 m of thiophenol and a standard peak list are shown Emad Moghimi: Department of Physics, Islamic Azad University, in Figure 1 A and B, respectively. The patterns were indexed Karaj Branch, Karaj, Iran Kambiz Hedayati: Faculty of Science, Department of Physics, Arak as pure cubic phases (galena), which are very close to the University, Arak 38156-8-8349, Iran literature value (JCPDS No. 02 – 1431). The crystallite size of Rouhollah Jalajerdi: Young Researchers Club, Arak Branch, Islamic PbS was calculated from the XRD pattern using the Scherrer Azad University, Arak, Iran formula. 0.9λ D= (1) βθcos Introduction where λ is the X-ray wavelength ( = 1.54 Å ), θ is the Bragg Nanostructured materials have attracted extraordinary diffraction angle and β is the full width at half maximum research interest owing to their fundamental significance (FWHM) of the peak appearing at the diffraction angles for addressing some basic issues of the quantum confine- (Nabiyouni et al., 2012). The crystallite size estimated from ment effect. It is expected that the third-order nonlinear the Scherrer formula was about 20 nm. optical response of lead sulfide (PbS) nanocrystals is 1000 A scanning electron microscopy (SEM) image of the times larger than that of cadmium selenide. Thus PbS synthesized bulk PbS (without using thiophenol) is shown nanocrystals may be considered as a promising candi- in Figure 2 A. SEM images of PbS with 0.05, 0.1, 0.5 and date for photonic and optical switching devices (Salavati- 1 m of thiophenol are shown in Figure 2 B – E, respectively. Niasari et al., 2009, 2010, 2012; Ghanbari et al., 2012; Nabi- The images show that the PbS nanocrystals are spheri- youni et al., 2012). Many physical and chemical properties, cal in shape with an average diameter of < 100 nm which such as melting point, color and band gap, and electronic decreases with increasing thiophenol concentration. 174 G. Nabiyouni et al.: Synthesis of PbS nanoparticles A 800 (200) (220) (111) (311) 400 (420) Intensity (222) (422) (331) (511) (400) (440) 0 20 30 40 50 60 70 80 90 100 2θ (°) B 100 Ref. Pattern: Galena (111) (200) (220) 50 Intensity (311) (420) (222) (400) (331) (422) (511) (440) (531) 0 30 40 50 60 70 80 90 100 2θ (°) Figure 1 XRD pattern of the (A) obtained PbS with 1 m of thiophenol (B) standard peak list. Transmission electron microscopy (TEM) images of the is observed (Yousefi et al., 2011). It can also be seen that PbS nanoparticles with 0.5 and 1 m of thiophenol are the peak values of PbS nanoparticles (synthesized with shown in Figure 3 A and B, respectively. The average par- 0.5 and 1 m of thiophenol) are smaller than those observed ticle size, estimated from TEM images, was about 20 nm, from the absorption spectra of the sample synthesized which is in agreement with the XRD results. The purity of with concentrations of 0.05 and 0.1 m of thiophenol, indi- PbS nanoparticles was also confirmed by energy-disper- cating that the latter have a smaller size than the former, sive X-ray spectroscopy (EDX) analysis ( Figure 4 ), where in agreement with our previous work (Nabiyouni et al., the La , Lb lines of lead and the Ka and Kb lines of sulfur are 2012). obviously observed. Photoluminescence (PL) measurement of the PbS It is well known that changing the semiconduc- nanoparticles obtained with 1 m of thiophenol at room tor particle size leads to a shift in semiconductor energy temperature with an excitation wavelength of 362 nm is levels. Thus materials with different particle sizes exhibit shown in Figure 6 . The PL spectrum consisted of a strong different band gaps. Usually, a decrease in particle size peak at 421 nm that can be ascribed to a high level of causes an increase in the band gap and a blue shift occurs transition in PbS semiconductor crystallites. It has been (Salavati-Niasari et al., 2009). Figure 5 A – D gives the UV- reported that this kind of band edge luminescence arises visible electronic absorption spectra of the nanoparticles from the recombination of excitons and/or shallowly with 0.05, 0.1, 0.5 and 1 m of thiophenol, respectively. As trapped electron-hole pairs (Salavati-Niasari et al., 2012). can be seen from the Figure, adding thiophenol to the The Fourier transform infrared (FT-IR) of the PbS solution (as the capping agent) leads to a decrease in PbS nano particles obtained with 0.05, 0.1, 0.5 and 1 m of thio- particle size and results in the quantum confinement of phenol are shown in Figure 7 A – D, respectively. FT-IR charge carriers in the nanoparticles, and a large blue shift spectroscopy shows the presence of small amounts of (compared with the bulk PbS, band gap: 0.4 eV, 3020 nm) thiophenol adsorbed on the PbS surface. G. Nabiyouni et al.: Synthesis of PbS nanoparticles 175 A 200 nm EHT=10.00 kV WD=9 mm Mag=20.00 KX Signal A=SE1 Institue for Colorants, Paints and Coatings (ICPC) BC 1 μm EHT=15.00 kV WD=5 mm Mag=10.00 KX Signal A=SE1 300 nm EHT=10.00 kV WD=5 mm Mag=50.00 KX Signal A=SE1 Institue for Colorants, Paints and Coatings (ICPC) Institue for Colorants, Paints and Coatings (ICPC) DE 200 nm EHT=10.00 kV WD=7 mm Mag=25.00 KX Signal A=SE1 200 nm EHT=10.00 kV WD=5 mm Mag=20.00 KX Signal A=SE1 Institue for Color Science and Technology Institue for Colorants, Paints and Coatings (ICPC) Figure 2 SEM images of the PbS nanocrystals synthesized (A) without thiophenol and with (B) 0.05, (C) 0.1, (D) 0.5 and (E) 1 m of thiophenol. Absorption peaks at 1440 and 1600 cm -1 are attrib- Conclusions uted to the stretching vibration of aromatic ring, and a peak at 3025 cm -1 is responsible for the aromatic C H Lead sulfide nanoparticles were synthesized at room bond, and an absorption peak at 1106 cm-1 is due to the temperature via a simple chemical reaction. In this syn- C O bond. thesis, thiophenol was used as the capping agent and 176 G. Nabiyouni et al.: Synthesis of PbS nanoparticles A 50 nm B Figure 5 UV-Visible absorption spectra of the PbS nanocrystals synthesized with (A) 0.05, (B) 0.1, (C) 0.5 and (D) 1 m of thiophenol. sodium sulfide was used as a sulfur source. SEM images show that, without using a capping agent, bulk PbS is 50 nm obtained, whereas adding thiophenol leads to the produc- tion of nanoparticles. Compared with bulk PbS, which has a band gap of 3020 nm, absorption of the obtained PbS Figure 3 TEM images of the PbS nanoparticles synthesized with nanostructures exhibits a large blue shift, which is attrib- (A) 0.5 and (B) 1 m of thiophenol. uted to the quantum confinement of charge carriers in the nanostructures. The average particle size, estimated from the TEM images, was about 20 nm. Intensity (arb. unit) 400450 500 Wavelength (nm) Figure 4 EDX pattern of the PbS nanocrystals synthesized with 1 m Figure 6 Room temperature photoluminescence spectrum of the of thiophenol. obtained PbS with 1 m of thiophenol.