8050
J. Phys. Chem. C 2009, 113, 8050–8057
Structural and Optical Properties of PbS Nanoparticles Synthesized by the Radiolytic Method A. H. Souici,†,‡ N. Keghouche,*,‡ J. A. Delaire,§ H. Remita,† A. Etcheberry,| and M. Mostafavi*,† Laboratoire de Chimie Physique, UMR CNRS/UPS 8000, Bat. 349, UniVersite´ Paris-Sud, 91405 Orsay Cedex, France, Laboratoire Microstructures et De´fauts dans les Mate´riaux, UniVersite´ Mentouri-Constantine, Route de Ain El Bey, 25010 Constantine, Algeria, Laboratoire de Photophysique et Photochimie Supramole´culaires et Macromole´culaires, ENS Cachan CNRS, 61 aVenue du Pre´sident Wilson 94235 CACHAN, France, Institut LaVoisier de Versailles, UniVersite´ Versailles Saint-Quentin-en-YVelines, UMR 8180-CNRS, 45 aVenue des Etats-Unis, 78035 Versailles Cedex, France ReceiVed: December 17, 2008; ReVised Manuscript ReceiVed: March 26, 2009
Lead sulfide nanoparticles with diameter in the range of 9-45 nm were synthesized by the radiolytic method in aqueous solutions containing Pb2+ and thiol. PbS nanoparticles were identified by HRTEM, SAED, and XPS techniques. The irradiation dose plays a crucial role to control the size of the nanoparticles and consequently to modify their optical properties. These properties were studied by measuring the absorption and emission bands for each irradiation dose. For irradiation doses higher than 2 kGy, the PbS nanoparticles exhibit an emission band within the ultraviolet range (300-550 nm). This band shifts from 360 to 380 nm. The excitation of PbS nanoparticles shows that they display three emission bands in the near-infrared region around 900, 980, and 1100 nm. In addition, it is shown that the unusual emission bands of PbS nanoparticles are due to the presence of complexes of lead by thiol on the surface of the nanoparticles. Introduction Because of their specific properties and large potential applicationsinnanotechnology,biology,medicine,andoptoelectronics,1-4 nanosized materials have attracted increasing attention of the scientists. Semiconductor nanoparticles, or so-called quantum dots, have specific optical and electronic properties because of their quantum size and dielectric confinement effects.5-7 These properties are tunable by varying the size and the morphology of the particles. Nowadays, the challenge is to control these two factors in order to reach the best conditions, permitting the synthesis of adequate materials for advanced technologies. Among semiconductors, lead sulfide PbS has been used in several areas such as light-emitting diodes, infrared detectors, optic fibers, infrared lasers, solar energy panels, window coatings, and environment as Pb2+ sensors.8,9 This large panel of applications is due to its interesting physical properties. In fact, PbS has a large Bohr exciton radius (18 nm), small effective electron and hole masses (m*e ) m*h ) 0.085 me), a large optical dielectric constant (ε∞ ) 17.2), and an infrared direct band gap in the bulk state (0.41 eV at 298 K), which corresponds to an absorption onset at 3024 nm.10 Nanoparticles with sizes larger than 10 nm present almost the same optical properties as the bulk.10 * To whom correspondence should be addressed. For M.M.: E-mail,
[email protected]; phone, +33 1 69 15 78 87; fax, +33 1 69 15 30 55. For N.K.: E-mail,
[email protected]; phone, +213 31 81 88 72; fax, +213 31 81 80 83. † Laboratoire de Chimie Physique. ‡ Laboratoire Microstructures et De´fauts dans les Mate´riaux. § Laboratoire de Photophysique et Photochimie Supramole´culaires et Macromole´culaires. | Institut Lavoisier de Versailles.
In the near-infrared, PbS has good photoconductive properties. Moreover, it exhibits a third-order nonlinear optical property11 and its band structure is complicated by a large relativistic splitting.12 Theoretical considerations have been held with the aim to explain the size dependence of the optical properties. According to the above-mentioned PbS characteristics, the hyperbolic band model is more suitable than the effective mass approximation one. It holds particularly for the sizes lower than 10 nm,8 and is in a good agreement with experimental data. Theoretical investigations, based on functional density theory (DFT), were also performed in the literature in order to study the size dependence of the band gaps for ultrasmall clusters (PbS)n)1-9 and their geometrical structure.13 The electronic structure of spherical PbS nanoparticles was studied using a semiempirical tight-binding method after taking into account the spin-orbit effects.14 The particularly narrow band gap of PbS gives the possibility to tune the optical absorption in a large domain by reducing the size to the nanometric scale. A large blue-shift due to the effect of the quantum confinement on the charge carriers is expected if the experimental route allows controlling the particle size. Several methods have been used to grow lead sulfide nanoparticles in different environments such as zeolites, glasses, polymers, inverse micelles, or in colloidal state. Small nanoparticles (2.7-4 nm) have been synthesized in the presence of thioglycerol (TGL) and dithioglycerol (DTG).15,16 These nanoparticles present an absorption band edge located around 900 nm and emitting light in the near-infrared range. Patel et al.17 have already reported that when poly(vinyl alcohol) PVA is used as stabilizer agent, 4 nm sized PbS nanoparticles are synthesized. They present absorption bands at 390, 580, and around 1200 nm. In the presence of DNA or gelatin as stabilizer, these absorption bands are less important and present broader
10.1021/jp811133b CCC: $40.75 2009 American Chemical Society Published on Web 04/17/2009
PbS Nanoparticles Synthesized by the Radiolytic Method shoulders.17 The PbS nanoparticles formed in the presence of a capping agent as poly(vinyl-pyrrolidone) PVP, polystyrene (PS), and poly(methyl-methacrylate) PMMA have similar absorption spectra corresponding to that of indirect band gap semiconductors with no detectable light emission at room temperature. In polycrystalline PbS, the sensitizing centers have been identified at 0.16 and 0.32 eV below the conduction band for the PbS nanoparticles of 5 nm. They have been also attributed to oxidizing sites on PbS and they depend on the growth mechanism.17 Recently, it has been found that photosensitizing centers in PbS colloidal quantum dots lie around 0.1, 0.2, and 0.3 eV below the conduction band.18 As an alternative way, the radiolytic process has been proven to be an adequate tool to synthesize monodispersed and sizecontrolled semiconductor nanoparticles. The particle size can be tailored by controlling the irradiation dose. In this way, nanosized CdS and ZnS particles have been synthesized in the presence of mercaptoethanol (RSH).19-21 The present study is devoted first to examine the radiation induced synthesis of lead sulfide nanosemiconductors and second to investigate in detail their optical and structural properties in relation with the irradiation dose and the surface state. Here, we report that in controlling the conditions, the PbS nanoparticles presenting the bulk absorption band in IR could present also other emission and absorption bands in the UV-visible range. Experimental Section Semiconductor nanoparticules were synthesized by irradiation of a solution containing lead(II) perchlorate hydrate (Pb (ClO4)2 · 3H2O) and thiol (mercaptoethanol, HOCH2CH2SH), noted RSH. The latter plays both the role of sulfur atom source and stabilizing agent. The used solutions are slightly acidic (pH ≈ 4), and in some cases an amount of HClO4 is added to reduce the pH of the solutions. The chemicals were of highest quality, purchased from Sigma-Aldrich. Deionized water with resistivity 18.2 MΩ cm was used in all experiments. Before irradiation of the samples, the solutions were deaerated by flushing them with nitrogen. The steady state irradiations were carried out using a 60Co γ source with a dose rate of 2.2 kGy h-1. X-ray photoelectron spectroscopy (XPS) measurements were completed by a Thermo Electron ESCALAB 220i-XL equipped with a nonmonochromatic or a monochromatic X-ray Al KR line for excitation. The photoelectrons were detected perpendicularly to the holder, and the constant analyzer energy mode was used with pass (bandwidth) energy of 20 eV. The absorption and photoluminescence spectra of the samples have been registered with a Hewlett-Packard 8453A nearinfrared UV-visible spectrophotometer and a Horiba-JobinYvon Spex Fluorolog fluorimeter, respectively. For the analysis of near-infrared fluorescence, the Spex Fluorolog was equipped with a specific InGaAs photodetector cooled at 77 K, and the emission spectra were corrected with the response curve of the detector. The estimation of the size and the observation of the shape of the nanoparticles were achieved by transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) on JEOL 100 CXII and 200 apparatuses. The size was estimated from a histogram, established from the TEM images. Selected area electron diffraction technique (SAED) was used to determine the crystallographic structure of the nanoparticles. For TEM and SAED experiments, the samples were prepared by deposition, followed by drying of droplets of irradiated solutions onto copper grids coated with an ultrathin amorphous carbon layer.
J. Phys. Chem. C, Vol. 113, No. 19, 2009 8051 Principle of the Radiation Induced Synthesis of PbS Nanoparticles. The radiation induced formation of CdS and ZnS semiconductor nanoparticles in the presence of thiol has been previously investigated.19-21 The formation of PbS nanoparticles follows a close mechanism. The irradiation of a solution containing a polar solvent leads to the generation of transient species that react with the solute present in the solution. The most reducing species among them is the aqueous electron e-aq; it reacts with thiol to form SH- following the reactions (reaction 1, see ref 22; reaction 2, see ref 19; reaction 3, see ref 23): • HOCH2CH2SH + eaq f CH2CH2OH + HS
k1 )
1.2 × 1010 M-1s-1 •
•
HOCH2CH2SH + OH f SCH2CH2OH + H2O
k2 )
6.8 × 109 M-1 s-1 2+
Pb
•
+ OH f Pb
3+
-
+ OH
-1 -1
k3 < 2 × 10 M 8
(1)
s
(2) (3)
Note that in acidic solution, reaction 1 can also occur with H• atoms instead of hydrated electrons with a rate constant of 4 × 108 M-1 s-1. When comparing the rate constants (k2 to k3) and taking into account the concentration ratio [RSH]/[Pb2+], the reaction of thiol with OH• radicals is favored and also the formation of Pb3+ ions, although the (reaction 3) is negligible. In this way, according to the following (reaction 4), the lead sulfide monomer appears when Pb2+ ions react with hydrogenosulfide anion (HS-):
Pb2+ + HS- f PbS + H+
(4)
Finally, the PbS monomers coalesce to form nanoparticles.
nPbS f (PbS)n
(5)
The growth process and the final size of PbS nanoparticles after irradiation could be controlled by the concentration ratio R of thiol and lead ions (R ) [RSH]/[Pb2+]), the dose, the dose rate, and the presence of complexes of Pb2+ with thiol (noted RSH-Pb2+ for the mononuclear complex). The thiyl radical RS• produced via reaction 2 dimerizes to form RSSR. According to the above reactions, by considering then only hydrated electron forms HS- through reaction 1, the radiolytic formation yield of PbS nanoparticles in these conditions is expected to be that of the hydrated electron and is around G ) 2.7 × 10-7 mol J-1. Results and Discussion Structural Characterization. In the aim to study the dose effect, a series of solutions containing 2.5 × 10-3 M Pb(ClO4)2 and 2.5 × 10-2 M RSH (concentrations ratio [RHS]/[Pb2+] ) 10, with pH ) 3.8, without acid) were irradiated at several doses and with the dose rate of 2.2 kGy h-1. Figure 1 shows TEM and HRTEM images of the nanoparticles obtained after irradiation at 0.2, 0.6, 1, 3, and 8 kGy. It reveals clearly that the size of the nanoparticles and their density increase with the increasing the dose (Figure 1a-e). For low doses, the particles evolve from homogeneous repartition and narrow size distribution (0.2 kGy) to collar shape particles repartition (0.6 kGy). The average particle size ranges from around 9-11 nm for 0.2 kGy to 14-18 nm for 0.4 and 0.6 kGy. The size distribution is presented by the histograms (inset in each image of Figure 1). The increase of the dose above 1 kGy leads to a regular evolution of the particle size from 22 to 28 nm then 40 nm for 1, 3, and 8 kGy, respectively. The solutions are very stable during at least a few weeks. But for the doses higher than 10 kGy, the particles
8052
J. Phys. Chem. C, Vol. 113, No. 19, 2009
Souici et al.
Figure 1. Transmission electron microscopy images of PbS nanoparticles at different irradiation doses. (a,b,c,d,e) correspond to 0.2, 0.6, 1, 3, and 8 kGy, respectively, solution without acid, pH ≈ 4. The size histogram is reported inset for each sample. (f) HRTEM image of PbS nanoparticles obtained at 8 kGy. Images (g) and (h) correspond to 1 and 3 kGy respectively, solution with acid (HClO4).
agglomerate just after irradiation (cf. optical section). This agglomeration leads to the formation of large particles which are not stabilized by thiol. According to previously published work, a variation of the diffraction pattern with the shape of the nanoparticles (sphere,
cube, rod,...) was observed.24,25 Figure 2 reports the patterns for different doses 0.6, 1, 3, and 8 kGy. For 8 kGy, the HRTEM micrograph presents the diffraction fringes. The lattice spacing is in good agreement with that of the (111) direction (Figure 1f). The selected area electron diffraction (SAED) pattern
PbS Nanoparticles Synthesized by the Radiolytic Method
J. Phys. Chem. C, Vol. 113, No. 19, 2009 8053
Figure 2. Selected area electron diffraction pattern (SAED) of PbS nanoparticles obtained at different irradiation doses. (a,b,c,d) correspond to doses 0.6, 1, 3, and 8 kGy, respectively.
TABLE 1: Lattice Spacing of PbS Nanoparticles Estimated from the Electron Diffraction Data lattice spacing from electron diffraction data (Å)
lattice spacing of PbS (fcc) from JCPDSa data (Å)
hkla
3.434 2.946 2.109 1.782 1.712 1.465 1.361 1.330 1.197 1.158 1.057 0.926 0.909 0.857
3.429 2.690 2.099 1.790 1.714 1.484 1.362 1.327 1.212 1.142 1.049 0.938 0.905 0.856
111 200 220 311 222 400 331 420 422 511 440 620 533 444
a JCPDSsInternational Centre for Diffraction Data; file no. 05-0592.
examination confirms that PbS nanoparticles in the cubic rocksalt structure (NaCl) are generated under γ irradiation (Figure 2). These patterns and HRTEM images show that the number of reflection plans (spots) increases with the dose. It indicates a better crystallization of the nanoparticles that evolve to polycrystalline ones. The SAED pattern, shown in Figure 2b, reveals a preferential growth of nanoparticles along the [111] direction. It shows a reinforcement of the intensity of the spots corresponding to the planes (111), (200), (220), and (222). The lattice parameter obtained from electron diffraction data for all doses are reported in Table 1. The estimation of this parameter gives a mean value of 0.5929 nm. This one is slightly lower than that of JCPDS file no. 05-0592 (0.5936 nm)26 for the bulk state. XPS spectra obtained after analysis of the sample irradiated under 2 kGy are reported (Figure 3a,b). The peaks at 138.2 and 143.1 eV (Figure 3a) correspond to the binding energies of Pb
Figure 3. High resolution XPS spectra for core levels of PbS nanoparticles obtained after γ irradiation (dose 2 kGy). Solution prepared in the same conditions as in Figure 1. (a) Signal of Pb 4f and (b) signal of S 2p.
4f5/2 and Pb 4f7/2 core levels. Only one kind of contribution is present, demonstrating that only one chemical phase is detected. Figure 3b reveals the photoelectron spectrum of S2p of the sample. It also shows a specific background contribution that is associated to the coating support. The corresponding energies
8054
J. Phys. Chem. C, Vol. 113, No. 19, 2009
Souici et al.
Figure 4. (a) Absorption spectra of Pb2+, thiol (RSH) and solution before irradiation, solution containing (2.5 × 10-2 M) thiol (HOCH2CH2SH) and (2.5 × 10-3 M) lead(II) perchlorate hydrate Pb(ClO4)2. (b) Absorption spectra of solutions at different pH adjusted with HClO4 acid, spectra from 1 to 7 correspond to 0, 10-3, 2 × 10-3, 3 × 10-3, 4 × 10-3, 5 × 10-3, and 10-2 M HClO4, respectively.
of S 2p3/2 and S 2p1/2 are 162.3 and 161.5 eV, respectively. Again the energy distribution of the peak is typical of only one chemical environment. The quantification of peaks of Pb4f and S2p gives a corrected atomic ratio as: %Pb4f/%S2p) 0.8581 depending on the background subtraction on S2p level. This range of estimated values supports the presence of a well-defined PbS phase. Moreover, the energy position of the peaks obtained in this work are in good agreement with those reported in the literature for PbS.27,29 XPS spectra and SAED patterns confirm that the studied samples are composed of PbS and that Pbn are not formed under these conditions. These structural characterizations do not exclude the presence of complex of lead ions with thiol. When the solutions are irradiated at lower pH, the sizes of the PbS particles are much larger (Figure 1g,h). The size of the nanoparticles obtained for an irradiation dose as low as 1 kGy is around 100 nm and leads to very large precipitated particles with diameters larger than 1 µm for doses of 3 kGy or higher. Optical Properties. For further characterization of the PbS nanoparticles, we conducted the absorption measurements from 200 to 3200 nm. First of all, it is important to note that usually the thiol form a complex with the Pb2+ ions. In fact, the UV-visible absorption spectra of the nanoparticles generated under irradiation are measured also from 200 to 800 nm. Figure 4a shows the absorption spectra of thiol and Pb2+ before mixture in solution and that of the solution before irradiation. The strong absorption band observed at 208 nm is attributed to Pb2+ ions.30 After mixing of Pb2+ ions with thiol, a complex absorbing around 272 nm is immediately formed. After changing the pH
Figure 5. Absorption spectra of PbS nanoparticles obtained after irradiation for different doses. Solution containing (2.5 × 10-2 M) thiol (HOCH2CH2SH) and (2.5 × 10-3 M) lead(II) perchlorate hydrate Pb(ClO4)2. pH ≈ 4. (a) Absorption spectra of solutions containing PbS nanoparticles deposited on sapphire as a substrate after removing water for irradiation doses of 0, 0.1, 1, and 3 kGy. Note that the intensity depends on the thickness of the sample. (b) UV-visible absorption spectra of the samples after irradiation in solution.
of the solution from 4 to 1.6 by adding acid, a decrease of absorption band, located at 272 nm, is observed (Figure 4b). This observation is in agreement with those reported in the literature with other thiols.31,32 According to the size of the nanoparticles observed by TEM microscopy reported in the previous section, their optical properties need the investigation in the near-infrared. Indeed, as the solutions absorb in IR, we removed water from the solutions containing PbS nanoparticles irradiated at 0.1, 1, and 3 kGy. They were deposited on sapphire used as substrate, and their absorption bands were measured from 200 to 3200 nm. The main absorption band in the IR corresponds to the lowest excitonic features, close to the one of the bulk. The absorption bands of the different samples have a band gap with an energy around 0.40 eV (Figure 5a). Nevertheless, we observed also a weak shift of the absorption maximum from 2835, 2855 to 2870 nm for doses 0.1, 1, and 3 kGy, respectively. This band with energy around 0.43 eV is due to the band-edge exitonic features known for the PbS particles displaying bulk properties. For higher irradiation doses, we found similar results. These observations are in agreement with the size of PbS nanoparticles reported in previous section. In fact, the size of the particles obtained at different doses is larger than 8 nm and those particles present almost bulk
PbS Nanoparticles Synthesized by the Radiolytic Method
J. Phys. Chem. C, Vol. 113, No. 19, 2009 8055
Figure 6. (a) Photoluminescence excitation spectra (PLE) and (b) photoluminescence spectra (PL) of PbS nanoparticles obtained after irradiation for different irradiation doses, solution as in Figure 5. For PL spectra, the excitation wavelength is 240 nm and the PLE are obtained with emission at 380 nm.
properties. Nevertheless, these particles absorb also in the UV-visible range. The ultraviolet absorption band of solutions at pH 4 after irradiation at doses 0.2, 0.6, 1, 2, 3, 4, 6, 8, 10, 12, and 15 kGy are also reported (Figure 5b). Up to 0.6 kGy, the absorption band increases with irradiation dose in the UV domain. With increasing the irradiation dose from 0.6 up to 10 kGy, the induced clusters absorb also in visible and near-infrared regions, indicating the formation of larger sizes. At 12 kGy, the intensity of the absorption band decreases slightly and, at 15 kGy, the agglomeration of the nanoparticles occurs, resulting in very low absorption intensity. For the particles obtained with 15 kGy, we observe a very broad band due to light scattering. If we assume that at around 10 kGy, almost all Pb2+ complexes are transformed into PbS nanoparticles, this transformation corresponds to a radiolytic yield of around 2.5 × 10-7 mol J-1, which is consistent with the mechanism reported above and in agreement with the radiolytic yield of hydrated electron. Assuming this radiolytic yield, the extinction coefficient of species formed at 270 nm is estimated to be around 1.6 × 104 L mol-1 cm-1 per monomer of PbS for the nanoparticles of the solution irradiated at a dose lower than 10 kGy. The emission properties of PbS nanoparticles formed in aqueous solution and in the presence of thiol were also investigated after different irradiation doses. Figure 6 shows the photoluminescence (PL) and the photoluminescence excitation (PLE) spectra of the PbS nanoparticles obtained with excitation wavelength at 240 nm and for emission at 380 nm respectively for the solution irradiated at pH 4. The first curve of this figure shows that there is almost no emission from the solution before irradiation. The photoluminescence is very low for the sample irradiated less than 0.6 kGy, i.e., for the particles smaller than 18 nm. The sample irradiated at 1 kGy displays an emission band within the ultraviolet region (300-450 nm). At 3 kGy, this one shows a maximum fluorescence around 360 nm, which shifts slightly toward 380 nm for 10 kGy. The intensity of the emission band decreases for the doses higher than 10 kGy. This intensity is reduced by 25% at 12 kGy, whereas at 15 kGy, the reduction is dramatic, about 95% (Figure 6). For this sample, when we add an amount of nonirradiated solution to the solution containing the large nanoparticles obtained at 15 kGy, the intensity of the emission band increases up to 4 times (Figure 7). Moreover, when the initial pH of the solution is fixed to 1.6 (by adding an acid before irradiation), the intensity of the emission band becomes much lower compared to the solutions at pH 4 and it decreases for doses higher than 2 kGy (Figure 8).
Figure 7. Photoluminescence spectra (PL) of solution before irradiation (a) and (b) solution irradiated at 15 kGy. (c,d,e) correspond to the addition to solution (b) of 0.1, 0.2, and 0.3 mL of nonirradiated solution, respectively.
Figure 8. Photoluminescence spectra (PL) before and after irradiation at different doses of solution containing (2.5 × 10-2 M) thiol (HOCH2CH2SH), (2.5 × 10-3 M) lead(II) perchlorate hydrate Pb(ClO4)2 and (10-2 M) HClO4 acid. pH ≈ 1.6.
The evolution of the emission spectra with irradiation dose was also followed after excitation with wavelengths of lower energy, i.e., at 700 and 900 nm. The emission band of the different samples in the near-infrared region is shown in Figure 9a,b. When the excitation is 700 nm, three fluorescence bands with maxima around 900, 1000, and 1080 nm appear. However, when the excitation is 900 nm, a fluorescence band around 1100
8056
J. Phys. Chem. C, Vol. 113, No. 19, 2009
Figure 9. Near-infrared photoluminescence spectra of PbS nanoparticles obtained after irradiation for two different excitation wavelengths for doses 0, 3, 6, and 10 kGy: (a) λex ) 700 nm, (b) λex ) 900 nm.
nm occurs (Figure 9b). The near-infrared emission bands are associated with an increase of the intensity when the dose increases up to 10 kGy. The photoluminescence excitation spectra (recording for an emission wavelength fixed at 380 nm) show the specific absorption bands of PbS nanoparticles. These bands reveal that the emission band around 380 nm is originated from the nanoparticles absorbing at around 280 nm. Their Stokes shift is around 100 nm. The change of the optical properties of the different samples is not drastic, and only a small shift of emission is observed. This can be explained by the fact that size of the nanoparticles vary from 8 to 45 nm and the bulk properties dominate above 10 nm. Indeed, theoretical approaches (effective mass approximation and hyperbolic band model) suggest that the gap of the nanoparticles is close to that of the bulk state.9 The light emission from the small nanoparticles results generally from the surface radiative electron hole recombination. In the case of radiation induced PbS nanoparticles, thiol radiolysis products, lead counterion (ClO4-), and the impurities could be adsorbed on the nanoparticles surface. The emission characteristics are controlled by different parameters as the ligand nature, the surface to volume ratio, the shape, and the quantum size effect. It is well-known that the PbS nanoparticles synthesized in the presence of capping agent and embedded in different media generally exhibits a photoluminescence in both the visible (600-800 nm) and near-infrared (1100-1500 nm) regions.33,34 Tunable photoluminescence in the near-infrared region was observed for hybrid composites of PbS embedded in sulfonated polystyrene matrices.35 Recently, it was shown that the corresponding absorption bands for the PbS nanopar-
Souici et al. ticles, with size greater than 6 nm, are located in the nearinfrared region.36 Nevertheless, in the present study, the nanoparticles larger than 10 nm emit light in the UV. This unusual result can be understood by considering three facts: first, the emission band intensity decreases drastically when almost all lead complexes are transformed to PbS particles. Second, when all Pb2+ are transformed to PbS, by adding a certain amount of nonirradiated solution of thiol-lead complex, the increase of the emission band is observed. Finally, when the initial pH of the solution is acidic, the emission intensity becomes much weaker. The difference between the solutions at pH 1.6 and 4 is related to the amount of Pb2+ complexed by thiol. By decreasing the pH to 1.6, the quantity of the lead complex decreases as it is shown in Figure 4b. Hence, it is clear that the lead complexes RSH-Pb2+ at the surface of the nanoparticles is necessary for the light emission even for the nanoparticles larger than 10 nm. Note that when the pH is lower than 5, a mononuclear complex noted RSH-Pb2+ is formed between thiol and Pb2+. For low irradiation doses, the concentration of particles is weak and the emission band is weak. Hence, by increasing the dose up to 10 kGy, the emission band intensity increases. However, at higher doses, when there is no more RSH-Pb2+ complex in the solution and if it contains nanoparticles, the emission band becomes very weak. At low pH (1.6), similar observations were found for the emission band. Therefore, it can be concluded that the presence of thiol-lead complex at the surface of the nanoparticles is responsible for the emission. The emission bands can be attributed to the defects states on the colloids. Recently, it has been reported that in the case of CdSe quantum dots, the treatment of the particles with elemental sulfur leads to a fluorescence enhancement and an excellent stability.37 Conclusion In the present work, semiconductor PbS nanoparticles were studied. It has been shown that the radiolytic method is a powerful one for controlling their size. Lead sulfide nanoparticles of tailored size in the range of 8-45 nm can be obtained using γ irradiation for different doses of solutions containing Pb2+ with thiol. Contrary to the naked PbS, which absorbs and emits light only in the IR around 3000 nm, these nanoparticles exhibit an emission band in the UV-visible and in the near IR. It is interesting to note that the RSH-Pb2+ complex does not only play the role of the stabilizer, but it is also responsible for these optical properties. Acknowledgment. The present work was partially supported by French-Algerian cooperation, Program Agreement CMEP no. 04 MDU 616, and by both research funds, Ministry of Higher Education and Scientific Research of Algeria and A. Mira University of Be´jaia. We thank Prof. J. Belloni, coordinator of CMEP program, for her deep interest in this work, and we are grateful to M. Lourseau, A. Demarque, and J. Vigneron for their technical and XPS assistance. We acknowledge Patricia Beaunier, Laboratoire de Re´activite´ de Surfaces, Universite´ Paris VI, for TEM, HRTEM observations, and SAED pattern. References and Notes (1) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (2) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H. J.; Bawendi, M. G. Science. 2000, 290, 314. (3) Gurin, V. S. J. Cryst. Growth 1998, 191, 161.
PbS Nanoparticles Synthesized by the Radiolytic Method (4) Jiang, Y.; Wu, Y.; Xie, B.; Yuan, S. W.; Liu, X. M.; Qian, Y. T. J. Cryst. Growth 2001, 231, 248. (5) Voit, J. Rep. Prog. Phys. 1994, 58, 977. (6) Lieber, C. M.; Wu, X. L. Acc. Chem. Res. 1991, 24, 170. (7) Dai, H.; Lieber, C. M. Annu. ReV. Phys. Chem. 1993, 44, 237. (8) Ni, Y.; Liu, H.; Wang, F.; Liang, Y.; Hong, J.; Ma, X.; Xu, Z. Cryst. Res. Technol. 2004, 39, 200. (9) Peterson, J. J.; Krauss, T. D. Nano Lett. 2006, 6, 510. (10) Wang, Y.; Suna, A.; Mahler, W.; Kasowski, R. J. Chem. Phys. 1987, 87, 7315. (11) Wang, Y. Acc. Chem. Res. 1991, 24, 133. (12) Lin, P. J.; Kleinman, L. Phys. ReV. 1966, 142, 478. (13) Zeng, H.; Schelly, Z. A.; Ueno-Noto, K.; Marynick, D. S. J. Phys. Chem. A 2005, 109, 1616. (14) Kane, R. S.; Cohen, R. E.; Silbey, R. J. Phys. Chem. 1996, 100, 7928. (15) Bakueva, L.; Gorelikov, I.; Musikhin, S.; Zhao, X. S.; Sargent, E. H.; Kumacheva, E. AdV. Mater. 2004, 16, 926. (16) Zhao, X.; Gorelikov, I.; Musikhin, S.; Cauchi, S.; Sukhovatkin, V.; Sargent, E. H.; Kumacheva, E. Langmuir 2005, 21, 1086. (17) Patel, A. A.; Wu, F.; Zhang, J. Z.; Torres-Martinez, C. L.; Mehra, R. K.; Yang, Y.; Risbud, S. H. J. Phys. Chem. B 2000, 104, 11598. (18) Konstantatos, G.; Sargent, E. H. Appl. Phys. Lett. 2007, 91, 173505. (19) Mostafavi, M.; Lin, Y. P.; Pernot, P.; Belloni, J. Radiat. Phys. Chem. 2000, 59, 49. (20) Souici, A. H.; Keghouche, N.; Delaire, J. A.; Remita, H.; Mostafavi, M. Chem. Phys. Lett. 2006, 422, 25. (21) Swayambunathan, V.; Hayes, D.; Schmidt, K. H.; Liao, Y. X.; Meisel, D. J. Am. Chem. Soc. 1990, 112, 3831.
J. Phys. Chem. C, Vol. 113, No. 19, 2009 8057 (22) Jayson, G. G.; Stirling, D. A.; Swallow, A. J. Int. J. Radiat. Biol. 1971, 19, 143. (23) Sukhov, N. L.; Ershov, B. G. High Energy Chem. 1982, 16, 395. (24) Wang, S.; Yang, S. Langmuir 2000, 16, 389. (25) Hao, E.; Yang, B.; Yu, S.; Gao, M.; Shan, J. Chem. Mater. 1997, 9, 1598. (26) JCPDS Files-ICDD-1997 PCPDFWin; International Centre for Diffraction Data: Newtown Square, PA, 1997. (27) Jiang, P.; Liu, Z.; Cai, S. Langmuir 2002, 18, 4495. (28) Lobo, A.; Mo¨lle, T.; Nagel, M.; Borchert, H.; Hickey, S. G.; Weller, H. J. Phys. Chem. B 2005, 109, 17422. (29) Bakshi, M. S.; Thakur, P.; Sachar, S.; Kaur, G.; Banipal, T. S.; Possmayer, F.; Peterson, N. O.; Mandeep, S. B.; Pankaj, T.; Shweta, S.; Gurpreet, K.; Tarlok, S. B.; Fred, P.; Nils, O. P. J. Phys. Chem. C 2007, 111, 18087. (30) Henglein, A.; Janata, E.; Fojtik, A. J. Phys. Chem. 1992, 96, 4734. (31) De Brabander, H. F.; Tombeux, J. J.; Van Poucke, L. C. J. Coord. Chem. 1974, 4, 87. (32) Nenadovic, M. T.; Comer, M. I.; Vasic, V.; Micic, O. I. J. Phys. Chem. 1990, 94, 6390. (33) Chen, S.; Truax, L. A.; Sommers, J. M. Chem. Mater. 2000, 12, 3864. (34) Bukner, S. W.; Konold, R. L.; Jelliss, P. A. Chem. Phys. Lett. 2004, 394, 400. (35) Lim, W. P.; Low, H. Y.; Chin, W. S. J. Phys. Chem. B 2004, 108, 13093. (36) Joshi, S.; Sen, S.; Ocampo, P. C.; Swati., J.; Sabyaachi, S.; Patricia, C. O J. Phys. Chem. C 2007, 111, 4105. (37) Rene-Boisneuf, L.; Scaiano, J. C. Chem. Mater. 2008, 20, 6638.
JP811133B
