8992
J. Phys. Chem. 1994,98,8992-8997
Size Variation of PbS Particles in High-Refractive-IndexNanocomposites Tasoula Kyprianidou-Leodidou, Walter Caseri, and Ulrich W. Suter’ Eidgenossische Technische Hochschule, Institute f i r Polymere, E TH-Zentrum, CH-8092 Zurich, Switzerland Received: March 7, 1994;In Final Form: May 23, 1994” The mean size of colloidal lead sulfide particles in nanocomposites with organic materials was varied from 4 to 80 nm by addition of sodium dodecyl sulfate or acetic acid. The refractive index and absorption coefficients of the resulting nanocomposites were measured at 632.8 and 1295 nm. The refractive index of the resulting nanocomposites was in the range 1.7-3.9 and belongs, therefore, to the highest values reported for such systems. The refractive indices and absorption coefficients of the nanocomposite decrease continuously when the particle diameters are below ca. 25 nm. For particles with average diameters above ca. 25 nm,the extrapolated refractive index of the P b S particles is in good agreement with that of bulk PbS (4.3), but the values drop for smaller particle diameters, e.g., to 2 for mean particle diameters of 4 nm.
Introduction Composites can show physical properties that either are not found in conventional materials or are considerably changed with respect to their components.1.2 The properties of such a composite material depend not only on the properties of each component but also on the morphology of each phase and the interfacial properties.3 Depending on their special characteristics, composite materials are classified in different categories such as nanostructured materials (disorder introduced in a perfect crystal by incorporating a high density of lattice defect^),^ organoceramics (macromolecules molecularly dispersed in an inorganic crystalline phase),5 and nanocomposites with colloidal particles dispersed in another, usually polymer phase. Our work focuses on the third class. It has been shown that nanocomposites of polymers and inorganic materials may exhibit optical properties that cannot be obtained with pure polymers alone,@ For instance, the introduction of PbS in a polymer matrix can increase the refractive index to values of 2.5-3.0 at 632.8 nm,6 rendering the nanocomposite suitable material for optical applications such as the manufacture of improved efficiency solar cells.9 I t has also been shown that the refractive index in PbS-gelatine nanocomposites increases linearly with the PbS volume fraction in the experimentally available range 0-50% v/v PbS.7 However, the extrapolation to 100% PbS gives a refractive index of 3.47 compared to 4.3 for bulk PbS.IO This difference might be due to a refractive index dependence on the particle size. It is a well-known fact that optical, electrochemical, or catalytic properties of semiconductor particles strongly depend on the particle size and can be dramatically different from the related properties of the bulk material. This is due to a quantization effect, and the particles showing this effect are often designated as Q particles. The quantization effect can be observed, depending on the material, for particles as large as 25 nm, a size that corresponds to that of an exciton (electron-hole combination) in the macrocrystalline material.l1-l4 Characteristic examples of size quantization effects are the changes in absorption spectra of semiconductor particles with changing size: A decrease in size results in a “blue shift” in the absorption spectrum.15-19 For semiconductor materials, having rather high values of the bandgap energy, quantum size effects begin a t small particle sizes, e.g., 6 nm in the case of CdS;I7 for particle size below 2.2 nm, CdS becomes colorless.17 For PbS, this sizeeffect can be observed for crystallites as large as 18 nm, containing ca. 10’ atoms;20in terms of color, a decrease in the crystallite size will show a progressive change from black PbS (bulk) to yellow-brown PbSZ1 *Abstract published in Advance ACS Abstracts, July 15, 1994.
A very interesting example of color variation with particle size decrease is observed in the preparation of Cd3Pz nanocrystals: As the size decreases to approximately 1.5 nm, the color changes from black (corresponds to bulk material) to red, orange, yellow, and finally ~ h i t e . ~ ~ , 2 3 Refractive indices are also significantly affected by size quantization. A theoretical treatment of the optical properties of semiconductor crystallites by Schmitt-Rink et al. predicts rapid changes in the absorption coefficient and refractive index as a function of wavelength near the excitonic t r a n ~ i t i o n .Although ~~ many experimental studies exist on the effect of size quantization on absorption coefficients, the studies on refractive index variations are limited. A characteristic example is the observation by lightscattering studies on colloidal PbS that the refractive indices of particles of 20 and 40 nm at low wavelengths (366 and 405 nm) are similar to the bulk values, while the refractive indices at 436 and 546 nm are smaller but close to the bulk value.25 For certain technological applications of nanocomposite materials, the study of the particle size variation effect on the optical properties for a certain wavelength range is important because it will provide the optimum material with the optimum optical and mechanical properties. Colloidal semiconductor particles of different sizes can be produced by reactions that have been carried out under a variety of conditions such as in nonaqueous media,26-27reversed mic e l l e ~ vesicles,30 , ~ ~ ~ ~ Langmuir-Blodgett ~ films,3I p o l y m e r ~ , 3 ~ . ~ ~ and porous crystalline zeolites.34 In aqueous media, stabilizing agents are used to vary the particle size. Polyphosphates and thiols are the most commonly used stabilizing agents. Polyphosphates of an average chain length of 15 phosphate units stabilize Q particles in ~ o l u t i o n .The ~ ~ polyphosphate ~~~ chains are thought to be strongly bound to the metal ions on the surface of the particles, causing a decrease in the growth rate because of inhibition of active growth sites and repulsion and steric hindrance between neighboring particles. Thiols terminate the growth of colloidal particles probably by attaching to the surface of the particles.35-37 Herein, we report on our experimental approach to thevariation of PbS particle size in nanocomposites and examine the effect of PbS size variation on the optical properties of the produced nanocomposi tes . Experimental Section Preparation of the Nanocomposites. A hydrogen sulfide solution was prepared by bubbling hydrogen sulfide gas (Pangas; Luzern, Switzerland) for approximately 20 min into a gas wash bottle that contained deoxygenated water (the excess of the gas was absorbed by a sodium hydroxide solution). Theconcentration
0022-365419412098-8992%04.50/0 0 1994 American Chemical Society
PbS Particles in High-Refractive-Index Nanocomposites of the resulting hydrogen sulfide solution (between 0.04 and 0.08 M) was determined by the following back-titration procedure: I mL of a 1 M solution of sodium hydroxide was added to 10 mL of the hydrogen sulfide solution. The resulting solution was titrated with a 1 M hydrogen chloride solution with thymolphthalein as indicator until the color of the solution changed from blue to colorless. In a typical experiment, 20 mL of a 0.5% w/w solution of poly(ethy1ene oxide) (Aldrich, average molecular weight 5 X 10') was mixed with 4 mL of a 0.5 M solution of lead acetate and a certain amount of either a sodium dodecyl sulfate solution (2g/L)oraceticacid. Themixturewaswellstirred byamagnetic stirrer,and thecorrespondingisomolaramountofhydrogensulfide solution, i.e., the amount required for PbS formation, was added witha pipet. Immediatelya chewing-gum-like precipitate formed. This precipitate was collected, filtered, and dried under vacuum (100 Torr) at 40 OC for a day. After drying, the material was ground in a mortar and then shaped into a cylindrical pellet of 13-mmdiameter with a hydraulic press. The maximum pressure applied was 125 bar. Chemical Analysisof theNanwomposites. Elemental analysis for C and H was performed by the microanalytical service of the Laboratorium fiir Organische Cbemie of the ETH Ziirich. For the lead content, a small amount of the nanocomposite was treated with an excess of concentrated hydrogen chloride solution. Hydrogen sulfide evolved, and the black composite material turned white, presumingly due to the formation of lead chloride. The sample was subsequently dried, weighted, and dissolvedin 200mLofa 0.1 N solutionofnitricacid. The solutions were analyzed for their lead content by atomic emission spectroscopy (ICP-AES). Physicochemical Analysis of the Nanocomposites. X-ray powder diffraction analysis was carried out on a Siemens D5000 diffractometer, with the use of Cu Ka radiation (A = 1.5406 A, from a 1.2-kW source. Ellipsometric measurements were obtained on a Plasmos SD 2300 ellipsometer at 632.8 and 1295 nm. Refractive indices were measured at five different spots, and five measurements were performed at each spot. The error limits of the single refractive index values are of the order of *0.05. Particle size distributions were determined from transmission electron micrographs of the nanocomposite samples by the following procedure: Small amounts of nanocompositematerial were embedded in a resin (EponfAraldite) and hardened for a t least 24 h at 60 "C. The hardened resin specimens were cut to athicknessof IOOnmwithadiamondknifeona Reichertultracut microtome. The small pieces were collected on copper sieves and examined by a Phillips EM301 microscope. TEM pictures were taken with an acceleration voltage of 80 kV on Agfa Scientia 23D 56 cut film. f-potential measurements were performed with a r-sizer 3 (Malvern). Results and Discussion
Preparation of PbS Nanocomposites with Different Mean ParticleSizes. At first,nanocomposites ofPbS and poly(etby1ene oxide) (PEO) were prepared, based on earlier work.6 by addition ofa H~SsolutiontoasolutioncontainingPb(OAc)l(leadacetate) and PEO. The precipitate that immediately forms consists of PbS, PEO, and water. After a drying and pressing procedure (see Experimental Section), flat nanocomposite pellets were obtained, as needed for refractive index measurements by ellipsometry. The fraction of PEO in the nanocomposite can be calculated from microanalysis.6 Here, a value of 3.4% w/w was obtained, corresponding to 18.0% v/v, based on a density of 1.2 g/cm3 for PE018 and of 7.5 g/cml for PbS.39 In earlier reports, 9.0% w/w had been found;6 the difference is probably due to the diffeent
The Journal of Physical Chemistry, Vol. 98. No. 36, 1994 8993
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Figure 1. TEM pictures ofdiflerentPbS/PEO nanocomposiles prepared withvaryingamountroladditives. (a) With 83,3%v/vCH1COOH. (b) Wilh8.3%v/vCH1COOH. (c) With4.2%v/vCH,COOH. (d) Without additives. (e) SDS/Pb'+ = 0.0034. (0SDS/Pb*+ = 0.0104. ( 9 ) SOS/ Pbz+ = 0.260. (h) SDS/Pb*+ = 0.347.
techniqueofmixing the reactants for the nanocompositesynthesis. It should also be noted that the results for the organic content obtained by microelemental analysis agree well with the values obtained from atomic emission analysis for lead. A characteristic transmission electron micrograph of the composite material is shown in Figure Id. The corresponding size distribution, giving a 29-nm mean particle diameter, is presented in Figure 2b. Attempts to vary the particle sizes by changing the concentration of the reactants did not produce size variations in the desired range. Therefore, other methods were tried to influence the particle size. f-potential measurements of colloidal PbS particles, prepared under the conditions of our experiments but in the absence of polymer,showtbat tbePbScrystals haveapositivesurfacecharge. As a consequence, we speculated that the addition of an anionic surfactant, such assodiumdodecyl sulfate (SDS), would decrease the mean particle size. Addition of SDS in various SDS/PL?+ mole ratios yielded particles smaller than the 29 nm obtained in the absence of SDS (Figure le-h, Table I). The surfactant concentrations in all experiments were lower than the critical micelle concentration
Kyprianidou-Leodidou et al.
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TABLE 1: Mean Particle Diameters in Nanocomposites from PbS and Poly(ethy1ene oxide) with Acetic Acid or Sodium Dodecvl Sulfate (SDSP mean diameter ( h t d dev) PbS cryst, nm prep method addn of 83.3% v/v CH3COOH 80(*3 5 ) addn of 8.3% v/v CH3COOH 58(*20) addn of 4.2% v/v CH3COOH 40(* 12) without additives 29(*11) SDS/Pb2+ = 0.0034 24(*8) SDS/Pbz+ = 0.0104 18(*6) 23(*9) SDS/Pb2+ = 0.017 SDS/Pb2+ = 0.034 23(+9) 13(+6) SDS/Pb2+ = 0.051 11(*4) SDS/Pb2+ = 0.104 15(*5) SDS/Pb2+ = 0.173 lO(14) SDS/Pb2+ = 0.208 SDS/Pb2+ = 0.250 8(*3) 7(*2) SDS/Pb2+ = 0.260 SDS/PbZ+= 0.347 4W) Indicated limits are standard deviations. neglected a t p H 2.5-6, the PbS seed formation reaction can be represented by the following equilibria:
01
+ HS- e PbHSPbHS- e PbS + H+ Pb2+ + H2S e PbS + 2H+ Pb2+
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Figure 2. Size distributions of PbS particles in various PbS/PEO nanocomposites. (a, top) With 83.3% v/v CH3COOH (cf. Figure la).
(b, middle) Without additives (cf. Figure Id). (c, bottom) With SDS, SDS/Pb2+ = 0.347 (cf. Figure l h ) . (cmc) of SDS (0.0081 M or 2.33 g/L40), thus ensuring that the surfactant was molecularly dispersed. The nanocomposite with the smallest mean particle diameter of 4 nm was prepared with a SDS/Pb2+ mole ratio of 0.347. A particle size distribution for a SDS/PbZ+ mole ratio of 0.347 is presented in Figure 2c. The effect of SDS on the particle size can be explained by the following mechanism; a clouding upon addition of SDS to a solution of Pb(OAc)2 indicates the formation of Pb(03SC12H23)~ (eq 1). Pb2+
+ 2Cl,H,,0SO
