pubs.acs.org/Langmuir © 2010 American Chemical Society
Stabilization of Lead Sulfide Nanoparticles by Polyamines in Aqueous Solutions. A Structural Study of the Dispersions Elena Koupanou,† Silvia Ahualli,‡,§ Otto Glatter,‡ Angel Delgado,§ Frank Krumeich,^ and Epameinondas Leontidis*,† † Department of Chemistry, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus, ‡Department of Chemistry, Karl-Franzens-University, Graz, Austria, §Department of Applied Physics, University of Granada, Granada, Spain, and ^Laboratory of Inorganic Chemistry, ETH Z€ urich, HCI-H111, 8093 Z€ urich, Switzerland
Received August 6, 2010. Revised Manuscript Received September 22, 2010 Lead sulfide (PbS) nanoparticles have been synthesized in aqueous solutions by a reaction between inorganic lead salts and sodium sulfide and stabilized using the cationic polyelectrolytes branched poly(ethylenimine) (PEI), poly(allylamine hydrochloride) (PAH), and poly(diallyldimethylammonium chloride) (PDDA). The structures of the polyamine-stabilized nanoparticle dispersions were examined in detail using UV-vis spectroscopy, small-angle X-ray scattering (SAXS), static and dynamic electrophoretic mobility measurements, and transmission electron microscopy (TEM). Considerable differences were found between the stabilizing efficiencies of these polyelectrolytes, which cannot be attributed to their charge densities or their persistence lengths. Small monodisperse nanoparticles of PbS with a tight stabilizing shell were consistently found only when PEI was used as a stabilizer even at high pH values, although its charge density is then very low. The excellence of PEI as a stabilizer is mainly due to the extensive branching of the chains and the presence of uncharged secondary and tertiary amine groups, which apparently serve as good anchoring points at the nanoparticle surfaces. None of the polyelectrolytes examined here provide long-term protection of the nanoparticles toward oxidation by air, showing that a need for more complex multipurpose stabilizers exists for aqueous PbS dispersions.
1. Introduction Lead sulfide (PbS) is an important direct band gap semiconductor. The optical band gap of bulk PbS is equal to 0.41 eV at 298 K, and its Bohr exciton radius is 18 nm, a high value due to the high dielectric constant and the small effective mass of electrons and holes.1-4 PbS nanoparticles therefore exhibit strong sizequantization effects even at a relatively large size, easily attainable by simple synthetic procedures. The band gap of PbS can be manipulated in the range of almost 2 eV quite easily by changing the particle size.3-5 PbS fluorescence is often in the near-infrared region,6,7 which is very useful for telecommunication and biological imaging applications.8-13 PbS particles have been proposed as ideal materials for IR detectors8,10 and as photosensitizers.11 The recent observation of multiexciton generation in PbS (and *To whom correspondence should be addressed: Tel þ357 22 892767, Fax þ357 22 892801, e-mail
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also PbSe and PbTe) nanoparticles14,15 increased the scientific interest in these materials since their use may improve the efficiencies of future generations of solar cells and other photovoltaic devices.16-19 Given the technological significance of PbS nanoparticles, their synthesis methods have proliferated in the past 30 years, with emphasis placed on size control, monodispersity, or shape variation. PbS synthesis can take place in organic5-8,12,19-22 or aqueous23-27 media, in zeolites,28,29 glasses,30 reversed micelles and microemulsions,31-33 in surfactant34-39and (14) Ellingson, R. J.; Beard, M. C.; Johnson, J. C.; Yu, P.; Micic, O. I.; Nozik, A. J.; Shabaev, A.; Efros, A. L. Nano Lett. 2005, 5, 865. (15) Sukhovatkin, V.; Hinds, S.; Brzozowski, L.; Sargent, E. H. Science 2009, 324, 1542. (16) McDonald, S. A.; Konstantatos, G.; Zhang, S.; Cyr, P. W.; Klem, E. J. D.; Levina, L.; Sargent, E. H. Nature Mater. 2005, 4, 138. (17) Klein, E. J.; MacNeil, D. D.; Cyr, P. W.; Levina, L.; Sargent, E. H. Appl. Phys. Lett. 2007, 90, 183113. (18) Ma, W.; Luther, J. M.; Zheng, H.; Wu, Y.; Alivisatos, P. A. Nano Lett. 2009, 9, 1699. (19) Noone, K. M.; Strein, E.; Anderson, N. C.; Wu, P.-T.; Jenekhe, S. A.; Ginger, D. S. Nano Lett. 2010, 10, 2635. (20) Chen, S. W.; Truax, L. A.; Sommers, J. M. Chem. Mater. 2000, 12, 3864. (21) Hines, M.; Scholes, G. D. Adv. Mater. 2003, 15, 1844. (22) Gurin, V. S. J. Cryst. Growth 1998, 191, 161. (23) Chiu, G.; Meehan, E. J. J. Colloid Interface Sci. 1974, 49, 160. (24) Murphy Wilhelmy, D.; Matijevic, E. Colloids Surf. 1985, 16, 1. (25) Levina, L.; Sukhovatkin, V.; Musikhin, S.; Cauchi, S.; Nisman, R.; BazzetJones, D. P.; Sargent, E. H. Adv. Mater. 2005, 17, 1854. (26) Zhao, X. S.; Gorelikov, I.; Musikhin, S.; Cauchi, S.; Sukhovatkin, V.; Sargent, E. H.; Kumacheva, E. Langmuir 2005, 21, 1086. (27) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065. (28) Wang, Y.; Herron, N. J. Phys. Chem. 1987, 91, 257. (29) Moller, K.; Bein, T.; Herron, N.; Mahler, W.; Wang, Y. Inorg. Chem. 1989, 28, 2914. (30) Okuno, T.; Lipovskii, A. A.; Ogawa, T.; Amagai, I.; Masumoto, Y. J. Lumin. 2000, 87-89, 491. (31) Ward, A. J. I.; O’Sullivan, E. C.; Rang, J. C.; Nedeljikovic, J.; Patel, R. C. J. Colloid Interface Sci. 1993, 161, 316. (32) Eastoe, J.; Cox, R. A. Colloids Surf., A 1995, 101, 63.
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polymer-surfactant solutions,40,41 and in polymers.1,42-45 Excluding some older aqueous procedures, based on sulfur sources such as hydrogen sulfide gas or thioacetamide,23,24 optimal size and polydispersity control is currently provided in organic solvents either at high temperatures, using variants of the TOP/TOPO method,21 or closer to room temperature with oleylamine as a coordinating solvent.46-48 In many applications, however, and certainly in all biological applications, the nanoparticles must be water-dispersible. This necessitates ligand exchange steps to render hydrophobic nanoparticles water-dispersible.12,49,50 Aqueous synthesis processes that provide good control of size and polydispersity are thus highly desirable. Quite surprisingly, it has been found that several stabilizers, which provide excellent stabilization of cadmium chalcogenide nanoparticles in aqueous solutions, do not work or perform only marginally in the case of PbS. It is also quite difficult to protect PbS nanoparticles from oxidation even for a few days. Oxidation proceeds quite fast in most cases, eventually leading to lead sulfate through a range of intermediates.51-54 Among the rather few working aqueous stabilizers of PbS dispersions there are water-soluble polymers, such as poly(vinyl alcohol) (PVA)1,55,56 and poly(vinylpyrrolidone) (PVP),1,57,58 polyelectrolytes like poly(ethylenimine) (PEI),59,60 poly(stryrenesulfonate) (PSS),57 and poly(acrylic acid),61 nucleotides (33) Wang, S.; Yang, S. Langmuir 2000, 16, 389. (34) Zhang, C.; Kang, Z.; Shen, E.; Wang, E.; Gao, L.; Luo, F.; Tian, C.; Wang, C.; Lan, Y.; Li, J.; Cao, X. J. Phys. Chem. B 2006, 110, 184. (35) Dong, L.; Chu, Y.; Liu, Y.; Li, M.; Yang, F.; Li, L. J. Colloid Interface Sci. 2006, 301, 503. (36) Wan, J.; Chen, X.; Wang, Z.; Yu, W.; Qian, Y. Mater. Chem. Phys. 2004, 88, 217. (37) Bakshi, M. S.; Thakur, P.; Sachar, S.; Kaur, G.; Banipal, T. S.; Possmayer, F.; Petersen, N. O. J. Phys. Chem. C 2007, 111, 18087. (38) Zhao, N.; Qi, L. Adv. Mater. 2006, 18, 359. (39) Zhou, G.; L€u, M.; Xiu, Z.; Wang, S.; Zhang, H.; Zhou, Y.; Wang, S. J. Phys. Chem. B 2006, 110, 6543. (40) Leontidis, E.; Kyprianidou-Leodidou, T.; Caseri, W.; Robyr, P.; Krumeich, F.; Kyriacou, K. C. J. Phys. Chem. B 2001, 105, 4133. (41) Leontidis, E.; Orphanou., M.; Kyprianidou-Leodidou, T.; Krumeich, F.; Caseri, W. Nano Lett. 2003, 3, 569. (42) Qiao, Z.; Xie, Y.; Chen, M.; Xu, J.; Zhu, Y.; Qian, Y. Chem. Phys. Lett. 2000, 321, 504. (43) Zheng, Z.; Wang, S.; Yang, S. Chem. Mater. 1999, 11, 3365. (44) Zhou, Y.; Itoh, H.; Uemura, K.; Naka, Y.; Chujo, Y. Langmuir 2002, 18, 5287. (45) Wang, J. Y.; Chen, W.; Liu, A. H.; Lu, G.; Zhang, G.; Zhang, J. H.; Yang, B. J. Am. Chem. Soc. 2002, 124, 13358. (46) Joo, J.; Na, H. B.; Yu, T.; Yu, J. H.; Kim, Y. W.; Wu, F.; Zhang, J. Z.; Hyeon, T. J. Am. Chem. Soc. 2003, 125, 11100. (47) Liu, J.; Yu, H.; Wu, Z.; Wang, W.; Peng, J.; Cao, Y. Nanotechnology 2008, 19, 345602. (48) Choi, S.-H.; An, K.; Kim, E.-G.; Yu, J. H.; Kim, J. H.; Hyeon, T. Adv. Funct. Mater. 2009, 19, 1645. (49) Lin, W.; Fritz, K.; Guerin, G.; Bardajee, G. R.; Hinds, S.; Sukhovatkin, V.; Sargent, E. H.; Scholes, G. D.; Winnik, M. A. Langmuir 2008, 24, 8215. (50) Hinds, S.; Myrskog, S.; Levina, L.; Koleilat, G.; Yang, J.; Kelley, S. O.; Sargent, E. H. J. Am. Chem. Soc. 2007, 129, 7218. (51) Fornasiero, D.; Fengsheng, L.; Ralston, J. J. Colloid Interface Sci. 1994, 164, 345. (52) Hsieh, Y. H.; Huang, C. P. J. Colloid Interface Sci. 1989, 131, 537. (53) Zingg, D. S.; Hercules, D. M. J. Phys. Chem. 1978, 82, 1992. (54) Shapter, J. G.; Brooker, M. H.; Skinner, W. M. Int. J. Miner. Process. 2000, 60, 199. (55) Nenadovic, M. T.; Comor, M. I.; Vasic, V.; Micic, O. I. J. Phys. Chem. 1990, 94, 6390. (56) Asunskis, D. J.; Hanley, L. Surf. Sci. 2007, 601, 4648. (57) Bakshi, M. S.; Kaur, G.; Possmayer, F.; Petersen, N. O. J. Phys. Chem. C 2008, 112, 4948. (58) Dong, L.; Chu, Y.; Zhuo, Y.; Zhang, W. Nanotechnology 2009, 20, 125301. (59) Hirasawa, I.; Mikami, T.; Katayama, A.; Sakuma, T. Chem. Eng. Technol. 2006, 29, 212. (60) Lioudakis, E.; Koupanou, E.; Kanari, C.; Leontidis, E.; Othonos, A. J. Appl. Phys. 2008, 103, 083511. (61) Li, C.; Zhao, Y.; Li, F.; Shi, Z.; Feng, S. Chem. Mater. 2010, 22, 1901. (62) Hinds, S.; Taft, J.; Sukhovatkin, V.; Dooley, C. J.; Roy, M. D.; MacNeil, D. D.; Sargent, E. H.; Kelley, S. O. J. Am. Chem. Soc. 2006, 128, 64.
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(GTP, ATP, CTP, and UTP),62,63 and polynucleotides, such as DNA1,25,63 and RNA,63,64 and a few small molecules, such as cysteine,65 hydroxythiophenol,66 dihydrolipoic acid,67 and a combination of thioglycerol and dithioglycol.13,26,68-70 As far as we know, only the two last systems, and also DNA,25 have been reported to provide long-term oxidative protection of PbS nanoparticle dispersions, for reasons that have not been fully understood to date. In only few cases have the actual dispersion structures been probed in detail, beyond the usual provision of TEM images of the particles. In the present work we explore cationic polyelectrolytes (polyamines) as “colloidal” stabilizers for aqueous PbS dispersions, even though they may not be expected to protect the particles from oxidation. The reasons for this choice are as follows: (a) The surface of large PbS particles is negatively charged over a broad range of conditions and even down to quite low pH values.71 We made the hypothesis that, although nanoparticle surfaces may not be identical to those of large particles, their inherent surface charge may also be negative; hence, a cationic stabilizer might interact quite strongly with the particles through Coulomb interactions. (b) We are interested in the assembly of PbS particles into films using the layer-by-layer (LBL) deposition method.27,60,72-74 This method relies on the partial neutralization of layers of charged species that are sequentially deposited, and its most successful variants are based on alternate polyelectrolyte adsorption. Polyamines have been used in the past for the stabilization of semiconductor nanoparticles, albeit less frequently than one would expect. In the late 1990s a considerable body of work addressed the stabilization of CdS nanoparticles by linear and branched poly(ethylenimine) and polyamine dendrimers.75-79 PEI was also used by some of the present authors for the short-term stabilization of PbS nanoparticles and their transfer to surfaces with the LBL method.60 Other polyelectrolytes, often used in LBL deposition, such as poly(allylamine hydrochloride) (PAH)80,81 and poly(diallyldimethylammonium chloride) (often abbreviated PDADMAC, in what follows PDDA),74,82-84 have not been examined as PbS stabilizers to the best of our knowledge. Here we focus on the stabilization of PbS nanoparticles in water by PEI, PAH, and PDDA because they have some distinct (63) Berti, L.; Burley, G. A. Nature Nanotechnol. 2008, 3, 81. (64) Kumar, A.; Jakhmola, A. Langmuir 2007, 23, 2915. (65) Xiang, J.; Cao, H.; Wu, Q.; Zhang, S.; Zhang, X. Cryst. Growth Des. 2008, 8, 3935. (66) Torimoto, T.; Uchida, H.; Sakata, T.; Mori, H.; Yoneyama, H. J. Am. Chem. Soc. 1993, 115, 1874. (67) Deng, D.; Zhang, W.; Chen, X.; Liu, F.; Zhang, J.; Gu, Y.; Hong, J. Eur. J. Ionrg. Chem. 2009, 3440. (68) Cornacchio, A. L. P.; Jones, N. D. J. Mater. Chem. 2006, 16, 1171. (69) Zhao, X.; Gorelikov, I.; Musikhin, S.; Cauchi, S.; Sukhovatkin, V.; Sargent, E. H.; Kumacheva, E. Langmuir 2005, 21, 1086. (70) Zhao, X. S.; Xu, S. Y.; Liang, L. Y. J. Mater. Sci. 2007, 42, 4265. (71) Bebie, J.; Schoonen, M. A. A.; Fuhrmann, M.; Strongin, D. R. Geochim. Cosmochim. Acta 1998, 62, 633. (72) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210-211, 831. (73) Decher, G. Science 1997, 277, 1232. (74) Fendler, J. H. Chem. Mater. 1996, 8, 1616. (75) Mao, J.; Yao, J. N.; Wang, L. N.; Liu, W. S. J. Colloid Interface Sci. 2008, 319, 353. (76) Sooklal, K.; Hanus, L. H.; Ploehn, H. J.; Murphy, C. J. Adv. Mater. 1998, 10, 1083. (77) Huang, J.; Sooklal, K.; Murphy, C. J.; Ploehn, H. J. Chem. Mater. 1999, 11, 3595. (78) Hanus, L. H.; Sooklal, K.; Murphy, C. J.; Ploehn, H. J. Langmuir 2000, 16, 2621. (79) Qi, L.; C€olfen, H.; Antonietti, M. Nano Lett. 2001, 1, 61. (80) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309. (81) Riegler, H.; Essler, F. Langmuir 2002, 18, 6694. (82) Dutta, A. K.; Ho, T.; Zhang, L.; Stroeve, P. Chem. Mater. 2000, 12, 1042. (83) Schlennof, J. B.; Dubas, S. T. Macromolecules 2001, 34, 592. (84) Von Klitzing, R. Phys. Chem. Chem. Phys. 2006, 8, 5012.
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Figure 1. Chemical structure of the polyamines (a) branched PEI, (b) PAH, and (c) PDDA.
structural differences (see Figure 1). Branched PEI is a polybase containing primary, secondary, and tertiary amine groups in an approximate ratio 1:2:1. The complex branching topology creates nonequivalent nitrogen centers, with a broad range of pKa values. At basic pH values PEI is weakly protonated, while at a pH value of 5.0 about half of its amino groups are protonated and the polymer has a high charge density.85-87 Similarly to PEI, PAH is strongly charged at close to neutral pH and very weakly at basic pH values.81,86 Its persistence length increases by a factor of 2 by decreasing pH from 10 to 6,88 but in general the polymer is less rigid than PDDA, its “intrinsic” persistence length being roughly 1 nm.89 The significant difference between PAH and branched PEI is mostly due to the branching structure of the latter. Finally, PDDA is a strong cationic polyelectrolyte. Its quaternary amine structure is insensitive to pH changes. It is also a rather rigid polymer with significant persistence length even when the sidegroup charges are screened in a high ionic strength medium.90 The variability of structural and electrical properties of these stabilizers will allow us to test the nature of the stabilization process and to examine specific polymer-particle interactions, which play a major role in these systems. The stabilization of a colloidal dispersion is the result of a fine balance of time, energy, and length scales. In this work we focus on the latter, since we are using a fast reaction scheme, and envision the picture of Figure 2 for a particle dispersion stabilized with polyelectrolyte molecules. There are several length scales associated with the polymer chains (radius of gyration, Rg, persistence length, Lp, and distance between consecutive charges, δ), the particles (size, a, Bohr exciton radius, aB), the polymer-particle pairs (thickness of polymer corona on the particles, t, density of anchoring points of the chains on the particle surfaces, ξ), the solution (Debye length, κ-1), and the dispersion as a whole (critical nucleus size, R*, diffusion lengths, interparticle distance, rinterp). While it is not possible to examine the importance of all these characteristic scales exhaustively, an important goal of the present work is to navigate through this maize of length scales in order to clarify: (a) if the persistence length and linear charge density of the (85) Lindquist, G. M.; Stratton, R. A. J. Colloid Interface Sci. 1976, 55, 45. (86) Suh, J.; Paik, H.-J.; Hwang, B. K. Bioorg. Chem. 1994, 22, 318. (87) Griffiths, P. C.; Paul, A.; Stilbs, P.; Petterson, E. Macromolecules 2005, 38, 3539. (88) Jachimska, B.; Jasinski, T.; Warszynski, P.; Adamczyk, Z. Colloids Surf., A 2010, 355, 7. (89) Von Klitzing, R.; Wong, J. E.; Jaeger, W.; Steitz, R. Curr. Opin. Colloid Interface Sci. 2004, 9, 158. (90) Marcelo, G.; Tarazona, M. P.; Saiz, E. Polymer 2005, 46, 2584.
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Figure 2. Schematic representation of several length scales involved in the stabilization of PbS nanoparticles in polyelectrolyte solutions. The charges on the polymer backbone are depicted as red circles and the ions in solution as blue circles. The characteristic lengths and their symbols are discussed in the text.
polyelectrolyte play important roles in the stabilization process, (b) if the polymer “architecture” (branching, topology, bulkiness of side groups, etc.) is important or not, (c) if the polyelectrolytes provide effective protection from oxidation as well, and (d) what is the range of PbS particle sizes stabilized by these polymers and what is the overall solution structure? The work outline is as follows: We first determine the charge density of the stabilizers at the pH values that were used in the experiments using a polyelectrolyte titration method.91,92 After obtaining “stability maps” for the polymer-PbS systems (i.e., stabilizer concentration vs particle concentration graphs showing for which compositions stable systems are obtained), we examine the effect of pH, charge density, and ionic strength on the stabilization efficiency. Besides pH variation experiments, the effect of charge density and persistence length is also assessed by using various random copolymers of diallyldimethylammonium chloride (DDAC) with N-methyl-N-vinylacetamide (NMVA). The sizes of the primary nanoparticles in solution are estimated through TEM and Tauc plot analysis of the UV-vis spectra of the dispersions,93,94 while the dispersion structure is probed in greater detail using small-angle X-ray scattering (SAXS) and static and dynamic electrophoretic measurements. Finally, the measurement of UV spectra of stable dispersions as a function of time probes the resistance (or lack thereof) of these systems to oxidation and also the propensity of oxidation vs aggregation as a means of “destroying” the PbS dispersions. The result is a reasonably complete picture of PbS stabilization with cationic polyelectrolytes, providing useful and hitherto unavailable insights into these systems and guidelines for particle deposition on solid surfaces with the layer-by-layer self-assembly method.
2. Experimental Section 2.1. Chemicals and Reagents. Lead(II) nitrate (Pb(NO3)2) and sodium sulfide (Na2S 3 9H2O) were obtained from Sigma-Aldrich. (91) (92) 2687. (93) (94)
Kam., S.; Gregory, J. Colloids Surf., A 1999, 159, 165. Tanaka, H.; Sakamoto, Y. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, Tauc, J.; Grigorovici, R.; Vancu, A. Phys. Status Solidi 1966, 15, 627. Collins, R. W.; Huang, C. Y. Phys. Rev. B 1986, 34, 2910.
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Article Poly(allylamine hydrochloride) (PAH; MW 56 000), poly(diallyldimethylammonium chloride) (PDDA; medium molecular weight (100 000-200 000) a solution of 20 wt % in water), and sodium poly(vinyl sulfonate) (SPVS; MW 2000-4000, a solution of 25% in water) were also obtained from Sigma-Aldrich. Poly(ethylenimine) (PEI; MW 60 000, a solution of 50 wt % in water) and hexadecyltrimethylammonium bromide (CTAB, 99þ%) were obtained from Acros Organics. Random copolymers of diallyldimethylammonium chloride (DDAC) with N-methyl-N-vinylacetamide (NMVA) were synthesized in the laboratory of Prof. Andre Laschewsky in the Fraunhofer Institute f€ ur Angewandte Polymerforschung, Potsdam, Germany, using established literature methods.95,96 The amount of the DDAC monomer in these copolymers changes from 14 to 89% (14, 25, 33, 53, 65, 74, and 89 mol % DDAC). All chemicals were used without any further purification. The ultrapure water for all the experiments was obtained from a Sartorius Arium 611 reverse osmosis unit and had a resistivity of 18.2 MΩ cm.
2.2. Synthesis of PbS Nanoparticles Stabilized with Cationic Polyelectrolytes. In a typical experiment 2.5 mL of an aqueous solution containing various concentrations of lead(II) nitrate was added to 5 mL of aqueous polyelectrolyte solutions of various concentrations (0.0025-0.5 wt %). Then 2.5 mL of a stoichiometric amount of a sodium sulfide solution (Pb2þ:S2-= 1:1) was added fast upon stirring. The color of the solution immediately changed to brown or orange-brown. The synthesis was always carried out at room temperature. We consider as “stable” those dispersions that were free of any precipitates 24 h after their preparation. This turned out to be a good criterion in the vast majority of cases, since the stability was almost always prolonged for several days. The UV-vis spectra of all stable dispersions were taken after the synthesis and in certain cases over a time span of a few days to assess the long-term oxidative stability of the solutions. The pH effect on the stabilizing ability of PEI was studied as follows. Solutions with 1 mM PbS nanoparticle concentrations were made in the presence of 0.1 wt % PEI at different pH values. The pH was varied between 1 and 9.5 prior to Na2S addition, using drops of concentrated solutions of either HCl or NaOH. The pH was checked again after the nanoparticle formation, and the final pH value is reported here. We generally found that the interaction between the lead salt and the polyamines does not lead to visible changes in the solution properties, some clouding being detectable only at pH values smaller than 2. Likewise, we did not detect any visible changes in the solution upon interaction of the HS- anion with the polyamines. We assume that in these solutions the strong interaction between Pb2þ and S2- dominates.
2.3. Determination of Charge Density of Cationic Polyelectrolytes. The charge density of cationic polyelectrolytes was determined by colloid titrations between the anionic polyelectrolyte sodium poly(vinyl sulfonate) (SPVS) and the cationic polyelectrolytes at different pH values,91 using o-toluidine blue (o-Tb) as the titration indicator. The SPVS solution was standardized initially with a solution of cetyltrimethylammonium bromide (CTAB). 50 mL of water, 3 mL of a 1 mM CTAB solution, and 3 drops of o-Tb (0.01 wt % aqueous) were added to a beaker. This solution was titrated with a 0.1% wt solution of the anionic polyelectrolyte SPVS, until the color of the solution changed from blue to violet. For a more accurate determination of the equivalent point the titration was followed by UV-vis spectrophotometry. SPVS was subsequently used in titrations of polyamine solutions. These were formulated by mixing 50 mL of water, 13 mL of a 0.1-1.0 wt % polyamine solution, and 3 drops of o-Tb (0.01 wt %). To study the pH effect on the charge density of PEI and PAH, drops of concentrated solutions of HCl or NaOH were added to the polyelectrolyte solution, until the required pH value was reached. The pH was monitored throughout the titration. (95) Dautzenberg, H.; G€ornitz, E.; Jaeger, W. Macromol. Chem. Phys. 1998, 199, 1561. (96) Ruppelt, D.; K€otz, J.; Jaeger, W.; Friberg, S.; Mackay, R. A. Langmuir 1997, 13, 3316.
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2.4. Characterization of Dispersions. 2.4.1. UV-vis Spectroscopy. UV-vis spectra of PbS dispersions were obtained on a Shimadzu UV-1601 UV-vis spectrophotometer or a Shimadzu UV-3600 UV-vis-NIR spectrophotometer. The spectra were transformed according to the Tauc-Lorentz analysis,93,94 which leads to the following equation for a direct band gap semiconductor: ðAhνÞ2 ¼ βðhν - Eg Þ
ð1Þ
where A is the absorbance, β a constant, and ν the frequency. A tangent to the (Ahν)2 vs hν plot at the limit A f 0 (absorbance cutoff) provides the particle band gap, Eg. 2.4.2. Small-Angle X-ray Scattering (SAXS). The smallangle X-ray scattering (SAXS) equipment is composed of a SAXSess camera (Anton Paar, Austria), attached to a PW 3830/10 X-ray generator (Philips) with a sealed-tube anode (Cu KR wavelength of 0.154 nm). The generator was operated at 40 kV and 50 mA. The SAXSess camera is equipped with a focusing multilayer optics and a block collimator for an intense and monochromatic primary beam with low background.97 A semitransparent beam stop enables a measurement of an attenuated primary beam for the exact definition of the zero scattering vector and transmission correction. The samples were enclosed into vacuum-tight, reusable 1 mm quartz capillary to attain exactly the same scattering volume and background contribution. The sample temperature was controlled with a thermostated sample holder unit (TCS 120, Anton Paar). The 2D scattering pattern was recorded with a CCD detector and integrated into a one-dimensional scattering function with SAXSQuant software from Anton Paar, Graz, Austria. The CCD camera from Princeton Instruments, which is a division of Roper Scientific Inc. (Trenton, NJ), is equipped with a PI-SCX fused 1:1 fiber-optic taper. The CCD camera features a 2048 2048 array with a pixel size of 24 24 μm (chip size: 50 50 mm) at a sample-detector distance of 311 mm. All 2D scattering patterns were converted to one-dimensional scattering curves as a function of the magnitude of the scattering vector q = (4π/λ) sin(θ/2), where θ is the total scattering angle. All scattering patterns were transmission-corrected by setting the attenuated scattering intensity at q = 0 to unity and correcting for the scattering of the sample cell and the solvent. In order to get the scattering patterns on absolute scale, water was used as a secondary standard.98 Samples were equilibrated at the desired temperature of 25 C for 10 min before each measurement. The samples were exposed to X-rays (three times) for 15 min, and the integrated scattering profiles were averaged. The scattering patterns were evaluated using the indirect Fourier transformation (IFT) method.99 Because of the low concentration of particles in the samples, particle interactions could be neglected. The IFT technique includes the correction of instrumental broadening effects; the scattering curves can be interpreted in real space, where they are represented by the pair distance distribution function (PDDF). The PDDF is the Fourier transform of the angle-dependent scattering intensity. It is the convolution square of the electron density distribution and represents a histogram of the distances inside the particle weighted with the electron density differences to the solvent. It becomes zero at the maximum particle dimension.100 2.4.3. Electrophoretic Measurements. Static electrophoretic measurements were performed on a Malvern Zetasizer 2000. Dynamic mobility measurements were performed for frequencies ranging between 1 and 18 MHz in an Acoustosizer II (Colloidal Dynamics). This instrument is based on the ESA (electrokinetic sonic amplitude) technique,101,102 in which an alternating electric (97) Bergmann, A.; Orthaber, D.; Scherf, G.; Glatter, O. J. Appl. Crystallogr. 2000, 33, 869. (98) Orthaber, D.; Bergmann, A.; Glatter, O. J. Appl. Crystallogr. 2000, 33, 218. (99) Glatter, O. J. Appl. Crystallogr. 1977, 10, 415. (100) Glatter, O. J. Appl. Crystallogr. 1979, 12, 166. (101) O’Brien, R. W. J. Fluid Mech. 1990, 212, 81.
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Koupanou et al. field is applied to the suspension and the amplitude and phase of the acoustic wave generated is measured and transformed into a dynamic mobility frequency spectrum, ue(ω). Determinations were repeated at least 15 times for each sample, and the average spectrum was used for further calculations. The model elaborated by Ahualli et al.103 was used to fit the mobility spectra. This is a modification of existing cell models accounting for the fact that particles consist of a rigid core and a soft polyelectrolyte layer. The fitting was carried out using the core size and charge as input parameters. For that purpose, HRTEM micrographs and standard dc microelectrophoresis results were used. The fitting consisted in varying the charge and thickness of the coating layer to minimize the squared differences between calculated and measured mobility data (both the real and imaginary parts were used) at each frequency. 2.4.4. Transmission Electron Microscopy (TEM). (S)TEM investigations were performed on a Tecnai F30 microscope (FEI; field emission cathode, operated at 300 kV, point resolution ∼2 A˚). STEM images were recorded with a high-angle annular dark field detector (HAADF) so that the particles appear with bright contrast. In each case some droplets of the suspensions were deposited on a carbon foil supported on a Cu grid. Measurements were performed immediately after solvent evaporation. Some measurements were performed on a JEOL-1010A instrument, with an acceleration voltage of 80 kV.
3. Results and Discussion 3.1. Charge Density of the Polyelectrolytes Used as Stabilizers. Polyelectrolytes stabilize colloidal particles through steric and electrostatic interactions.104 The charge density of the polyelectrolytes is thought to be crucial for the stabilization process, not only because it creates the necessary repulsion between the particles through adsorption but also because it determines the persistence length of the polymeric backbone. High charge densities lead to large persistence lengths.105,106 The chains then become stiff and therefore less capable of wrapping around a growing particle and stabilizing it. PDDA is a highly charged polymer, but the charge density of PEI and PAH depends strongly on pH. It is therefore necessary to have a good estimate of their charge density at the range of pH values used in the stabilization experiments. There is no simple method to obtain polyamine charge densities routinely and accurately. Results of direct acid-base titrations are not easy to interpret because the polymers contain many nitrogen centers, which are not equivalent.86 In addition, acid-base titrations always dilute the polyamine solutions, leading to different ionic strengths, and their results depend on polyamine concentration.86 Finally, the results of acid-base titrations do not usually agree with those of electrophoresis, NMR, and other methods.85-87 We have tried various optical methods using absorbing or fluorescent dyes,92 and we found that a colloid titration method developed in Japan some decades ago91,92 is accurate enough for the present purposes. In this method the polybase solution is titrated with a sodium poly(vinyl sulfonate) (SPVS) solution, and the cationic dye o-toluidine blue, which forms strong complexes with SPVS, is used as an indicator. We used the sodium salt of poly(vinyl sulfonate), instead of the potassium salt used in the original publication,92 since it was recommended to us that SPVS leads to (102) Hunter, R. J. Colloids Surf., A 1998, 141, 37. (103) Ahualli, S.; Jimenez, M. L.; Carrique, F.; Delgado, A. V. Langmuir 2009, 25, 1986. (104) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1983. (105) Stoll, S.; Chodanowski, P. Macromolecules 2002, 35, 9556. (106) Ulrich, S.; Seijo, M.; Jaguecir, A.; Stoll, S. J. Phys. Chem. B 2006, 110, 20954.
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Article Table 1. Experimental Charge Densities of the Polyelectrolytes Used in This Worka polyelectrolyte
experimental charge density (mequiv/g)
theoretical maximum charge density (mequiv/g)
SPVS 7.30 ( 0.30 7.7 PDDA 6.60 ( 0.20 6.2 PEI at pH 5 15.40 ( 0.30 23.3b PEI at pH 10 0.73 ( 0.12 PAH at pH 5 8.40 ( 0.30 10.7 PAH at pH 10 0.43 ( 0.02 a The theoretical charge densities refer to a fully charged state of a 100% pure polymer. b Based on a -CH2CH2NH- average monomer unit.
more reproducible titrations (Gregory, J. personal communication). The charge density of SPVS was originally determined with the same titration method using hexadecyltrimethylammonium bromide (CTAB) as the cationic species. It was found close to the theoretically expected value (Table 1). SPVS was subsequently used to titrate the polyamine solutions. All titrations were followed using UV-vis spectroscopy. The titration of PEI with SPVS at pH = 10 is presented in Figure 3. In all cases the end points of the titrations were clearly established. The results of all titrations are summarized in Table 1. The charge density of PDDA was also found close to but slightly higher than the theoretical value, illustrating the limits of accuracy of this method. The charge densities of PEI and PAH decrease sharply at high pH values. At pH = 10, a value at which several PbS stabilization experiments were carried out, both PEI and PAH are very weakly charged, in good agreement with the literature.85-87 3.2. Stability Maps: Which Is the Best Stabilizer? Effect of Charge Density. Figure 4a-c shows stability maps for PbS dispersions using PEI, PAH, and PDDA as stabilizers. In these and all subsequent figures a “PbS concentration” implies the total concentration of Pb2þ or S2- ions added to the solution. The stabilizing efficiency of the three polymers follows the order PEI>PAH>PDDA. This sequence reflects the interactions of the polyelectrolytes with the nanoparticles and must be rationalized in terms of the actual stabilization mechanism. PEI is clearly a better stabilizer than the two other cationic polyelectrolytes, since at the same concentration it can stabilize higher particle concentrations. Interestingly, the stability map of PEI is very weakly pH-sensitive at a pH range between 10 and 5 (results not shown), even though the charge density increases dramatically as the pH is lowered (see Table 1). We will return to this point later, but we should mention that the same applies to PAH. The stable regimes at pH values of 6 and 10 are very similar, indicating that the charge density has a minor effect on the stabilization efficiency at this pH range. PDDA is the worst stabilizer. This was anticipated since the polymer is quite stiff, with a persistence length of more than 3-4 nm measured by light scattering.90,95 To directly test the effect of charge density on the stabilizing efficiency of PDDA, we have opted to use a series of random copolymers of its monomer (DDAC) with N-methylN-vinylacetamide (NMVA). The chemical structure of these copolymers is presented in Figure 5. To our surprise, the stability map for PbS nanoparticles with any of these copolymers is essentially identical to that of Figure 4c, obtained for the fully charged PDDA! The conclusion is that the charge density and the persistence length of the polyelectrolytes play a much less crucial role in the stabilization mechanism than we had anticipated. A further proof of this fact was found when we added 1 M NaCl in systems that were practically at the limit of stability, as indicated by the stability maps of Figure 4. In all cases the addition of salt did not lead to DOI: 10.1021/la1031366
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Figure 3. (a) UV-vis spectra of PEI solutions at pH = 10 containing the dye o-toluidine blue, after additions of various volumes of a 0.1% SPVS solution. (b) Absorbance of the previous solutions at 635 nm as a function of the volume of SPVS solution added. The straight line is a guide to the eye in the region of the end point.
Figure 4. Stability maps for PbS dispersions stabilized with (a) PEI at pH = 10, (b) PAH at pH = 6 and pH = 10, and (c) PDDA at pH = 6. Stable systems are shown with open circles and unstable systems with filled circles. The solid lines separate the stable and unstable regions of the maps, and they are drawn as a guide to the eye. In the case of PAH, the solid line is for pH = 6 and the dotted line for pH = 10.
precipitation 3 days after the preparation of the solutions. All this evidence implies that the stabilization of the PbS nanoparticles is not achieved mainly through electrostatic interactions of the polyelectrolytes with the particle surfaces, and it is not strictly based on electrostatic repulsions between chains adsorbed on separate particles. There remains an interesting issue, pointed out by one of the reviewers of this article. Both PAH and PDDA contain chloride as a counterion. Chloride could certainly create complexes with lead ions and affect the PbS crystallization process in various other ways (e.g., by adsorbing on PbS crystal faces). To clarify this issue, one should attempt to exchange chloride with nitrate as PAH and PDDA counterion and re-examine the stability maps, but we have not done this here. We assumed that chloride will play a minor role, but this is not entirely obvious and further work needs to be done on this point. 16914 DOI: 10.1021/la1031366
3.3. UV-vis Spectroscopy of PbS Dispersions. The Tauc-Lorentz approach93,94 allows the approximate detection of the direct band gap of semiconductors absorbing in the UV-vis range. From the band gap one can then compute the average size of the particles in the dispersion, by using appropriate methods. The problem of this approach is that one needs very accurate UV-vis spectra to correctly detect the absorbance cutoff, which is difficult to locate for dilute dispersions. In addition, for particle band gaps below about 1.0 eV, their determination from the UV-vis spectra of aqueous solutions is practically impossible, since there are several water adsorption peaks in this spectral region,107,108 which cannot be completely removed by using appropriate references in the presence of the particles and hinder a clear identification of the absorbance onset. (107) Curcio, J. A.; Petty, C. C. J. Opt. Soc. Am. 1951, 41, 302.
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Figure 5. Chemical structure for the random copolymers of DDAC and NMVA.
The relation between band gap and particle size for PbS has been thoroughly discussed in the literature.3-5,109 We have opted to use the “hyperbolic band model” (HBM) of Wang et al.,109 which is more accurate for small particle sizes and results in the following equation: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2π2 p2 Eg, ¥ ð2Þ Eg ¼ Eg, ¥ 2 þ m R2 Here Eg is the experimental band gap of the nanoparticles, Eg,¥ is the macroscopic band gap of PbS (ca. 0.41 eV1-4), R is the particle radius, and m* is the effective mass, which for PbS has been found equal to 0.085me (me is the rest mass of the electron).109 In Figure 6 we plot the experimentally obtained band gap as a function of stabilizer concentration (for a fixed PbS concentration) and as a function of PbS concentration for a fixed polyelectrolyte concentration. The pH was equal to 6 in the cases of PAH and PDDA and equal to 10 in the case of PEI, these being the “natural” pH values of the dispersions, obtained immediately upon mixing their components. From the large values of the band gaps (relative to bulk) it can be seen that the particle sizes are in the range of a few nanometers in all solutions. It can also be seen that the band gap is very little affected by the stabilizer concentration at a fixed PbS concentration. The slight decrease of the band gap observed in Figure 6a at very low stabilizer concentrations is in fact within the experimental error ((0.1 eV) of the band gap as obtained from a Tauc plot, and we do not wish to assign it any special significance. On the contrary, the band gap is quite affected by PbS concentration for fixed polyelectrolyte concentration. We speculate that this effect is at least partly related to the competition between nucleation rates in these suspensions, which increase with Pb2þ concentration, and the rates of polymer adsorption on the surfaces of the growing crystals. For PbS concentrations larger than 0.2 mM the band gap of the nanoparticles in the PAH and PDDA systems becomes smaller than 1.2 eV and cannot be accurately determined from Tauc plots. The results of Figure 6b are transformed into the corresponding plot of particle radius vs PbS concentration (Figure 7), with the radius calculated from eq 2. In the systems of Figure 6 the particle diameters are predicted to be in the range of 3-5 nm, but larger particles are obtained for higher PbS concentrations, a fact which is verified below using TEM and SAXS. The emerging picture is that the size of the nanoparticles stabilized by the polyamines follows the order PEI
