Preparation and Characterization of Oriented PbS Crystalline

This has indeed received considerable attention.20-28 A further elaboration is to arrange ... Following mixing in a separation funnel, 50 mL of diethy...
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Langmuir 2000, 16, 389-397

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Preparation and Characterization of Oriented PbS Crystalline Nanorods in Polymer Films Suhua Wang and Shihe Yang* Department of Chemistry, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Received June 17, 1999. In Final Form: September 8, 1999 We report on the synthesis and characterization of rod-shaped PbS nanocrystals dispersed in a polymer film using a functionalized lead(II) salt of the surfactant anion AOT-, Pb(AOT)2, as the precursor. Essentially all the PbS nanocrystallites prepared using this method have been oriented with their (100) lattice planes parallel to the substrate surface. While the PbS nanorods with oriented lattice planes were produced in the polymer film, only cubic or spherical PbS nanoparticles were obtained when the preparation was carried out in solution. Possible mechanisms are briefly discussed for the growth of the rod-shaped PbS nanocrystals with the preferred orientation in the polymer film.

Introduction Research on nanoclusters has evolved into an important interdisciplinary field of materials science in recent years.1-5 The challenge has been and remains to be the control of the size, size distribution, morphology, and organization of nanoclusters.6-16 Template-mediated mineralization has been used to control the particle size, size distribution, and morphology using polymers, micelles, zeolites, colloidal assemblies, and so forth. The organization of these spherical or pseudospherical nanocrystals into two- and three-dimensional superlattices has also been achieved.16-19 To fully exploit the application potential of nanocrystals as novel optical and electronic * To whom correspondence should be addressed. E-mail: [email protected]. (1) Nanostructured Materials; Shalaev, V. M., Moskovits, M., Eds.; ACS Symposium Series 679; American Chemical Society: Washington, DC, 1997. (2) Alivisatos, A. P. Endeavour 1997, 21, 56. (3) Krauss, T. D.; Wise, F. W. Phys. Rev. B 1997, 55, 9860. (4) Kang, I.; Wise, F. W. J. Opt. Soc. Am. B: Opt. Phys. 1997, 14, 1632. (5) Ogawa, S.; Hu, K.; Fan, F. R.; Bard, A. J. J. Phys. Chem. B 1997, 101, 5707. (6) Machol, J. L.; Wise, F. W.; Patel, R.; Tanner, D. B. Physica A 1994, 207, 427. (7) Zhou, H. S.; Sarahara, H.; Honma, I.; Komiyama, H. Chem. Mater. 1994, 6, 1534. (8) Zeng, Z. H.; Wang, S. H.; Yang, S. H. Chem. Mater. 1999, 11, 3365. (9) Meldrum, F. C.; Flath, J.; Knoll, W. Langmuir 1997, 13, 2033. (10) Eastoe, J.; Cox, R. A. Colloids Surf., A: Physicochem. Eng. Aspects 1995, 101, 63. (11) Wang, Y.; Suna, A.; Mahler, W.; Kasowski, R. J. Chem. Phys. 1987, 87, 7315. (12) Kane, R. S.; Cohen, R. E.; Silbey, R. Chem. Mater. 1996, 8, 1919. (13) Mukherjee, M.; Datta, A.; Chakravorty, D. Appl. Phys. Lett. 1994, 64, 1159. (14) Zhao, X. K.; Xu, S. Q.; Fendler, J. H. Langmuir 1991, 7, 520. (15) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (16) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Adv. Mater. 1996, 8, 428. Harfenist, S. A.; Wang, Z. L.; Alvarez, M. M.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. 1996, 100, 13904. (17) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335. (18) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690. (19) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978.

materials, it is important to grow nanocrystals in one dimension, forming nanowires. This has indeed received considerable attention.20-28 A further elaboration is to arrange these nanocrystallites in a suitable matrix in such a way that specific lattice planes of the nanocrystallites all point to the same direction. Recently, oriented crystallization of nanoparticles was observed using the amphiphilic monolayers as the templates for crystallization.14,29,30 Here we report our results on the growth of rodlike semiconductor PbS nanocrystals and the preferential orientation of these nanocrystals with their (100) lattice planes parallel to the substrate surface. This onedimensional structure is remarkable considering that the PbS nanocrystal has a highly symmetric rock-salt structure. It provides a unique avenue to fabricate nanowires with application prospects in electronic, optical, and magnetic devices. In this work, we attempt to use a polymer matrix, which acts as both a stabilizer and a robust template, for the nanocrystal growth. Poly(vinyl butyral) was chosen in this study because a uniform and optical transparent film can be readily prepared from this material. In addition, the relatively low Tg of this polymer allows for the facile diffusion of the gaseous reactants and the one-dimensional growth of the PbS nanocrystals. Because gas-solid reactions are stereospecific,31 the application of these reactions to the colloidal assemblies of a functionalized surfactant is a promising avenue for preparing oriented (20) Dai, H.; Wong, E. W.; Lu, Y. Z.; Fan, S.; Lieber, C. M. Nature 1995, 375, 769. (21) Pileni, M. P.; Gulik-Krzywicki, T.; Tanori, J.; Filankembo, A.; Dedieu, J. C. Langmuir 1998, 14, 7359. (22) Wang, N.; Tang, Y. H.; Zhang, Y. F.; Yu, D. P.; Lee. C. S.; Bello, I.; Lee, S. T. Chem. Phys. Lett. 1998, 283, 368. (23) Wang, W. K.; Geng, Y.; Qian, Y. T.; Ji, M. R.; Liu, X. M. Adv. Mater. 1998, 10, 1479. (24) Yazawa, M.; Koguchi, M.; Hiruma, K. Adv. Mater. 1993, 5, 577. (25) Glezie, P.; Schouler, M. C.; Gradelle, P.; Caillet, M. J. Mater. Sci. 1994, 29, 1576. (26) Braun, P. V.; Osenar, P.; Stupp, S. I. Nature 1996, 380, 325. (27) Yang, J. P.; Meldrum, F. C.; Fendler, J. H. J. Phys. Chem. 1995, 99, 5500. (28) Trentler, T. J.; Hickman, K. M.; Goel, S. C.; Viano, A. M.; Gibbons, P. C.; Buhro, W. E. Science 1995, 270, 1791. (29) Zhao, X. K.; Mccormick, L. Appl. Phys. Lett. 1992, 61, 849. (30) Zhao, X. K.; Yang, J.; McCormick, L. D.; Fendler, J. H. J. Phys. Chem. 1992, 96, 9933. (31) Isshike, M.; Endo, T.; Masumoto, K.; Usui, Y. J. Electrochem. Soc. 1990, 137, 2697.

10.1021/la990780t CCC: $19.00 © 2000 American Chemical Society Published on Web 11/04/1999

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nanocrystals. This paper gives a full account of our work on the directional growth of rod-shaped PbS nanoparticles in polymer films, including the systematic study on the critical conditions for forming the PbS nanorods and controlling the nanocrystal lattice orientations relative to the substrates. In particular, we obtained orientated PbS nanorods with a high degree of lattice orientation and a large aspect ratio in a narrow size range. We also present some preliminary experimental results which may shed light on the mechanism of the PbS nanorod formation and the preferred lattice plane orientation of the PbS nanocrystals on the substrates. Experimental Section Chemicals and Reagents. Lead bis(2-ethylhexyl)sulfosuccinate, Pb(AOT)2, the precursor of PbS nanorods in the present experiments was synthesized and purified following the procedure given in ref 10. Briefly, two solutions were prepared: one is a 125 mL solution of 0.5 mol dm-3 sodium AOT (Aldrich) in absolute ethanol, and the other is a 125 mL aqueous solution of 1.5 mol dm-3 lead(II) nitrate (Aldrich 99%). Following mixing in a separation funnel, 50 mL of diethyl ether was added. After the mixture was left in the separation funnel for 30 min, phase separation occurred, with Pb(AOT)2 in the upper organic layer and the nitrate salts in the lower aqueous layer. The lower layer was discarded, and the upper layer was retained and washed at least six times with water. The Pb(AOT)2 solution was then rotary evaporated, and a white powder was collected and stored for later use. H2S gas was prepared in air by the reaction of Na2S‚ 9H2O (Farco) with an aqueous solution of 3 M HCl, and the prepared mixture of air and H2S was used directly. The volume concentration of the H2S was ∼10%. All other chemicals were used as purchased without further purification. Solution and Film Preparations. The poly(vinyl butyral) (PVB, Aldrich, MW 36 000, 18% hydroxyl and 0-1.5% acetate by weight) stock solution was prepared by dissolving PVB in chloroform (Fisher Scientific) with a concentration of 0.05 g/mL. Each PVB chain contains ∼147 hydroxyl groups. Quantitative Pb(AOT)2 was dissolved in chloroform, and the final concentration of Pb(AOT)2 was calculated to be 0.043 g/mL. In this concentration regime, we believe that the Pb(AOT)2 molecules could selfassemble into an ordered structure in chloroform. Solutions with various concentrations (expressed as the ratio of Pb(AOT)2 to PVB) were obtained by diluting the Pb(AOT)2 solution with the PVB stock solution. The assemblies of Pb(AOT)2 interact with the polymer chains through van der Waals and hydrophobic forces, and the ordered structure of Pb(AOT)2 is believed to be stabilized by the polymer chains.32 Glass slides were used as solid substrates, and before use, they were treated with chromic acid and acetone successively and dried in air at 80 °C. The PVB film impregnated with Pb(AOT)2 was prepared on the substrate surface by dip casting and then dried in a vacuum for 24 h at room temperature. Formation of PbS Nanoparticles. Upon exposure to H2S gas, the Pb(AOT)2-loaded PVB film turned from colorless to yellow brown, brown, and dark brown at different concentrations of Pb(AOT)2, indicating the formation of PbS particles. The sample prepared in this way is designated as film A. For the sake of comparison, PbS nanoparticles were prepared in another experiment by passing H2S to the Pb(AOT)2-PVB solution. The Pb(AOT)2-PVB solution turned from colorless to yellow brown when it was exposed to the H2S gas. The colored colloid solution was very stable, and no precipitation took place for several months, presumably because the PbS nanoparticles were stabilized by the surfactant molecules and the polymer chains. A film was prepared by dip casting the colloid solution onto a glass substrate, and this film is designated as film B. Prior to any measurements, both film A and film B were kept in a dry air flow for 12 h and subsequently in a vacuum for 12 h at room temperature to remove all the residual H2S gas trapped in the film. Characterization. Powder X-ray diffraction (XRD) measurements were carried out in a Phillips PW1830 instrument with (32) Myers, D. Surfactant Science and Technology; VCH Publishers: New York, 1996.

Figure 1. (a) TEM image of the PbS nanorods formed in PVB polymer film. (b) Selected area electron diffraction pattern of the rodlike PbS nanoclusters. a 1.54 Å Cu KR rotating anode point source. The source was operated at 40 kV and 40 mA, and the Kβ radiation was eliminated using a nickel filter. These films were transferred to amorphous carbon-coated copper grids for transmission electron microscopic (TEM) investigation. TEM experiments were performed with Joel 100CX II and Joel 2010 instruments, with an operating voltage of 100 kV and 200 kV, respectively. For the optical measurements, these films were transferred to quartz cells or KBr wafers. UV-vis absorption spectra of the PbS nanoclusters were acquired on a Milton Roy Spectronic 300 spectrometer, and FTIR transmission spectra of the films were obtained using a Perkin-Elmer 16 PC spectrometer.

Results and Discussion TEM Observation: Morphology, Structure, and Size of Particles. A typical TEM image of the PbS samples prepared in the PVB film is shown in Figure 1a. The corresponding selected area electron diffraction pattern is shown in Figure 1b. It is striking that essentially all the PbS species are of the rod shape on this magnification scale. These PbS rods were formed at room temperature with the ratio Pb(AOT)2/PVB being 3.34 × 10-4 mol/g (molar ratio: 12.0) and the H2S gas exposure time being 12 h. Although some much smaller PbS particles (3-5 nm) with roughly spherical shape are also present in the TEM image, it is clear that their volume is small compared with that of the rods. Moreover, we believe that at least some of these ultrafine particles might be formed during the TEM sample preparation. Because the PbS rods were

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Figure 2. High-resolution TEM image of a PbS nanorod prepared in PVB film. The fringe spacing (more clear in the original TEM picture) is estimated to be 2.87 Å, which is close to d200 of rock-salt PbS.

trapped in the PVB polymer, they are difficult to be imaged directly using transmission electron microscopy. We therefore used chloroform to dissolve the polymer matrix. The bubble-like features seen in the TEM image shown in Figure 1a are the chloroform-induced polymer/surfactant assemblies. During this dissolution process, the PbS

nanorods may suffer from severe strain, which leads to fracture and breakage to smaller particles. High-resolution TEM images of these nanoparticles show that these nanoparticles have irregular shapes, as expected from the breakage of larger PbS rods. The fringe spacings were observed, which are consistent with the (200) and (111)

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Table 1. Lattice Spacings (Å) of Spherical and Rod-Shaped PbS Nanocrystals Estimated from the Electron Diffraction Data electron diffraction sphere shape rod shape 3.37 2.97 2.10 1.80 1.73 1.48 b 1.32 1.21

b 2.97 2.09 b b 1.50 b 1.33 b

ref a Galenac

hkl a

3.429 2.969 2.099 1.790 1.714 1.484 1.362 1.327 1.212

111 200 220 311 222 400 331 420 422

a JCPDS-International Center for Diffraction Data. b Rings are too weak to be determined. c Galena: PbS(fcc).

lattice spacings of PbS. Because the thinner and longer rods are more likely to be broken, the actual average aspect ratio of the PbS nanorods in the polymer film may be much larger than that estimated from the TEM image. Selected-area electron diffraction patterns of the PbS nanorods all show similar features. As shown in Figure 1b, the electron diffraction pattern of the PbS rods reveals only a few lattice planes. The identified lattice spacings are closely matched to those of the cubic fcc structure of PbS, as listed in Table 1. It is interesting that, for all the PbS nanorods, only (200), (220), (400), and (420) diffraction features are notable. This shows that the crystallite PbS nanorod is oriented with its (200) plane parallel to the substrate, as further demonstrated below by XRD studies. Shown in Figure 2 is a high-resolution TEM image of a PbS nanorod. Well-defined fringes are revealed, running all the way along the rod direction. The fact that the fringe pattern can be resolved by transmission electron microscopy indicates that the PbS rods are somewhat thinner than the width. Therefore, these one-dimensional PbS nanocrystals should be more correctly called PbS nanostrips. The fringe spacing is estimated to be 0.287 nm, which is close to the (200) lattice spacing of PbS. As is shown by electron diffraction and will be shown below by XRD, the (200) lattice planes of the PbS nanorods are oriented parallel to the substrate, which indicates that the preferred particle growth is in the (200) direction. This growth pattern was observed by Weller et al. for PbS needles in amphiphilic block copolymer micelles.33 However, when the PbS particles were formed in solution with the same Pb/PVB ratio as in the film preparation (Figure 1a), no rod-shaped particles were formed, and instead, PbS nanoparticles of spherical shape were the only form produced, as revealed from the TEM image shown in Figure 3a. The corresponding selected area electron diffraction pattern is shown in Figure 3b. Analogous to the electron diffraction pattern in Figure 1b for the PbS rods, the electron diffraction pattern in Figure 3b for the spherical PbS particles shows a cubic fcc structure. However, more lattice spacings are observed, presumably because the PbS spherical particles prepared in solution are randomly oriented. The estimated lattice d spacings are shown in Table 1. Clearly, the lattice constants obtained from the electron diffraction data (Table 1) are consistent with those for the face-centeredcubic rock-salt PbS crystal. It was found that not only the average particle size but also the average aspect ratio (the ratio of length to width) of the PbS nanorods was affected by the lead(II) concentration and the detailed preparation procedures. When (33) Schneider, T.; Haase, M.; Kornowski, A.; Naused, S.; Weller, H. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 1654.

Figure 3. (a) TEM image of the PbS nanocrystals prepared in solution. (b) Selected area electron diffraction patterns of the spherical PbS nanoclusters.

the PbS nanorods were formed with the molar ratio Pb(II)/PVB being 12.0, they had a relatively large aspect ratio and a narrow size distribution. On the basis of the TEM image shown in Figure 1a, the PbS rods are 2.2 ( 0.2 µm in length and 105 ( 30 nm in width. The average aspect ratio is ∼20. Experimental results revealed that the necessary concentration for forming PbS rods is within a narrow range of the Pb(II)/PVB molar ratios, 7.6-35 (Table 2). Only round objects were observed from transmission electron microscopy when the concentration of Pb(AOT)2 was beyond the above range. As will be discussed below, these round objects are likely to have a threedimensional disk shape. Moreover, the change in the aspect ratios of the PbS nanorods within the narrow concentration range appears to be gradual. This is demonstrated in Figure 4, which shows TEM images of the PbS nanorods at two different Pb(II)/PVB ratios: one is lower and the other is higher than the ratio which gives the optimum PbS nanorods (Figure 1a). At both the lower

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Table 2. Pb(II) Concentration Dependence of the Shape and Size of the PbS Nanoparticles Prepared in PVB Film Pb/PVB (Pb/OH) (molar ratio)

particle shape

particle size

1.80 (0.012) 7.53 (0.051) 12.0 (0.082) 19.2 (0.130) 35.2 (0.239)

spheres spheres rods rods disks

1-10 nm 5-20 nm 2.2 ( 0.2 µm × 0.1 ( 0.03 µm 0.9 ( 0.2 µm × 0.12 ( 0.03 µm 20-200 nm

and higher Pb(II)/PVB ratios, the aspect ratios are much smaller than that shown in Figure 1a although the lower Pb(II)/PVB ratio yields PbS nanorods with a smaller width. We also discovered that, to form the PbS nanorods, it is important to dilute the surfactant solution with the PVB solution to reach the desired Pb(II) concentration. If, however, the desired Pb(II) concentration was attained by dissolving the solid surfactant, Pb(AOT)2, in the PVB stock solution, no PbS nanorods were observed whatsoever. This may be explained by the formation and stabilization of micelle structures. Within a certain concentration range, the surfactant molecules are expected to self-assemble into a micelle structure in the absence of other species, and this structure can be stabilized by the polymer chains when mixed with the PVB solution during the process of dilution. But things are different when the solid surfactant Pb(AOT)2 is dissolved in the PVB solution. In this case, the polymer chains will interact initially with the surfactant molecules before the surfactant molecules can assemble themselves into the micelle structure. This demonstrates the role of the polymer molecules as a stabilizer, which is important for the formation of PbS nanorods in later stages. XRD Measurements: Preferred Lattice Plane Orientation of PbS Rods in the PVB Film. Figure 5a shows the XRD pattern of the PbS nanorods prepared in the PVB film. It is remarkable that, from Figure 5a, only the (200) and (400) diffraction peaks are present for the PbS samples prepared in the PVB film. In other words, the intensity distribution of all the observed X-ray diffraction peaks deviates drastically from what is characteristic of bulk PbS (galena) powders. It appears that the PbS nanorods dispersed in the PVB film are single crystals with an fcc structure and that they are oriented in such a way that their (100) planes are parallel to the substrate surface. In contrast, the XRD pattern of the PbS nanoparticles prepared in the CHCl3 solution (Figure 5b) is nearly identical to the typical XRD pattern of powder PbS (galena) (Figure 5c). This shows that the PbS nanoparticles which were prepared in solution and subsequently transferred onto a glass substrate have random orientations, in marked contrast to those synthesized in the PVB film. This is understandable because the PbS nanoparticles are unlikely to have a preferred orientation in the solution phase, and they would remain so after being transferred onto the glass substrate. Effects of the Pb(II) Concentration on the Degree of the PbS Lattice Plane Orientation. We examined the effects of the Pb(II) concentration relative to PVB on the degree of the PbS nanocrystal orientation in the PVB film. Figure 6 plots the intensity ratio (I200/I111) of the first two peaks in the XRD pattern against the concentration of Pb(II). This intensity ratio is proportional to the degree of the (100) plane orientation of the PbS nanorods parallel to the substrate surface. As can be seen in Figure 6, in general, the ratio increases as the concentration of Pb(AOT)2 increases. A large intensity ratio (I200/I111) is correlated to a high degree of the (100) lattice plane orientation and vice versa. This demonstrates that the

Figure 4. TEM images of PbS nanorods prepared in PVB film at a Pb(II)/PVB molar ratio of (a) 10.0 and (b) 19.2.

degree of the preferred (100) crystal plane orientation of the PbS nanorods prepared in the PVB polymer film increases as the concentration of Pb(AOT)2 increases. For samples with higher Pb(II) concentrations, the intensity of the (111) diffraction peak was too weak to be measured accurately. In other words, all the PbS nanorods were orientated with their (100) planes parallel to the substrate surface given a sufficiently high Pb(II) concentration. On the other hand, for samples with lower Pb(II) concentrations, the intensity of all the diffraction peaks was too weak to be determined correctly. For comparison, in the

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Figure 5. X-ray diffraction patterns of (a) PbS nanorods prepared in PVB film, (b) PbS nanocrystals prepared in solution, and (c) a powder PbS sample.

Figure 7. UV-vis spectra of the PbS nanocrystals synthesized in PVB film (a) and in solution (b). The inset plots the spectrum of part b as a function of the photon energy instead of wavelength. The absorption edge is estimated to be ∼1.57 eV.

Figure 6. XRD patterns of PbS nanocrystals synthesized in PVB film containing different concentrations of Pb(II). The molar ratios Pb(II)/PVB are (a) 12.0 and (b) 9.18, respectively. The inset shows the dependence of the X-ray diffraction intensity ratio (I200/I111) on the concentration ratio Pb(II)/PVB.

featureless spectrum. On the other hand, the spectrum of the PbS nanocrystals prepared in solution (Figure 7b) shows an absorption onset of 750 nm with three salient shoulders around 600, 400, and 300 nm. These exciton peaks were attributed to 1se-1sh, 1se-1ph, and 1pe-1ph transitions, respectively.36 This is a clear indication of quantum confinement, since the average particle size of the PbS nanocrystals (∼10 nm) is much smaller than the exciton Bohr radius of bulk PbS (∼18 nm). One of the reasons for the absence of exciton absorption peaks in the spectrum of PbS nanorods is that the nanorods are confined only in two directions and the third dimension is extended. This may wash out the exciton absorption peaks. In fact, even the widths of the PbS nanorods are much larger than the Bohr radius of bulk PbS. An alternative explanation is related to the surface structure of the nanoparticles.37,38 The appearance of these three peaks implies that the excitons are not perturbed or are less perturbed by the surface-trapped electron-hole pairs. In the solution synthesis, as the PbS nanoparticles grow, AOT- may diffuse away, leaving the HO groups of PVB to be attached to the nanoparticle surface.39 It follows that the exciton peaks are observed as in PVA-capped PbS nanoparticles. However, in the polymer film, the diffusion of AOT- is hindered, and its SO3- groups may stick to the PbS nanoparticle surface. The strong electronhole trapping effect of the ionic group may also render the exciton absorption peaks unobservable. For the PbS nanocrystals prepared in solution, a band gap of 1.57 eV was estimated from the UV-vis absorption spectra (see the inset of Figure 7). The particle size was calculated to be ∼3.6 nm according to the hyperbolic model11 ∆E ) [Eg2 + 2p2Eg(π/R)2/m*]1/2. In the above equation, ∆E is the band gap of PbS nanocrystals, Eg ) 0.41 eV for PbS, m*/me ) 0.085 (m* is the actual electron mass in the PbS nanocrystals and me is the free electron mass), and R is the radius of the nanoparticle. The PbS particle size estimated from the UV-vis absorption onset is slightly smaller than that obtained from the TEM observation and the XRD measurements. This is under-

solution, the change of Pb(II) concentration did not result in any change in the intensity ratios of different diffraction peaks of the PbS nanocrystals. Instead, the only change was the increase of the PbS particle size as the Pb(II) concentration increased. UV-Vis Absorption Spectroscopy. It is well established that the absorption edge and luminescence cutoff are red-shifted when the size of semiconductor nanoparticles increases.34 For PbS nanocrystals, Cohen et al. found that the UV-vis absorption edge shows a marked blueshift from that of the bulk PbS crystals.35 In the present work, we used UV-vis spectroscopy to obtain some information about the size as well as the surface states of the PbS nanoclusters. The UV-vis absorption spectra of the PbS nanocrystals prepared both in films and in solutions are shown in Figure 7. Clearly, the spectra display a dramatic deviation (blue-shift) from that of the bulk PbS. The UV-vis absorption of the PbS nanorods (Figure 7a) starts at around 800 nm and increases smoothly with decreasing wavelength, giving rise to a (34) See, for example: Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (35) Kane, R. S.; Cohen, R. E.; Silbey, R. Chem. Mater. 1996, 8, 1919.

(36) Machol, J. L.; Wise, F. W.; Patel, R. C.; Tanner, D. B. Phys. Rev. B 1993, 48, 2819. (37) Wang, Y.; Herron, N. J. Phys. Chem. 1991, 95, 525. (38) Gao, M.; Yang, Y.; Yang, B.; Shen, J.; Ai, X. J. Chem. Soc., Faraday Trans. 1995, 91, 4121. (39) Nenadovic, M. T.; Comor, M. I.; Vasic, V.; Micic, O. I. J. Phys. Chem. 1990, 94, 6390.

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Figure 8. Dependence of the UV-vis absorption spectra of the PbS nanocrystals formed in solution on the time elapsed after H2S exposure.

standable considering the crudeness of the hyperbolic model we adopted. Further information on the different surface structures of the PbS nanocrystals prepared in PVB film and in solution was obtained by taking the UV-vis spectra as a function of the time elapsed after H2S exposure. For the PbS nanorods prepared in PVB film, the spectra did not change much with time due to the restricted motion of the molecules in the polymer film and therefore the limited change in surface states. However, the spectra of the PbS nanocrystals prepared in solution changed significantly with time, as shown in Figure 8; namely, the exciton absorption peaks became more and more pronounced with the elapse of time after the H2S exposure. This is likely to be due to the different surface states (passivation) of the PbS nanocrystals prepared in PVB film and in solution.40 In the solution preparation, the surface was probably capped by the SO3- groups immediately after the formation of PbS nanoparticles. Due to the polarization of the charged capping groups, the trapped electron density is reduced significantly, leading to the disappearance of the exciton peak.40,41 However, thermodynamic forces may drive the surfactant molecules out of the nanoparticle surface and trap the PbS nanoparticles in the pocket of the entanglement of the polymer chains. The driving force may be derived from the fact that the freed surfactant molecules can form more stable micelles. The surface contact between the PbS nanoparticles and the PVB polymer molecules is likely through the OH groups, which stabilize the PbS nanoparticles via dipole-dipole interactions. The appearance of the exciton absorption and the slow increase of the intensity against time elapsed showed the reduction of the surface coverage of SO3- on the PbS nanaoparticles. In the film, however, the surfactant molecules and polymer chains cannot move as freely as in the solution. So the surface of PbS particles is expected to be capped by SO3-, and therefore, no change in the absorption spectrum can be observed. FTIR Transmission Spectra. Fourier transform infrared (FTIR) absorption spectroscopy is another tool we used to characterize the PbS nanocrystals synthesized in PVB film. Figure 9 presents the FTIR transmission spectra for PVB films without and with the loading of (40) Guo, L.; Al, X. C.; Ibrahim, K.; Liu, F. Q.; Zou, Y. H.; Li, Q. S.; Zhu, H. S. Nolinear Opt. 1998, 19, 239. (41) Gao, M. Y.; Yang, Y.; Yang, B.; Shen, J. C. J. Chem. Soc., Faraday Trans. 1995, 91, 4121.

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Figure 9. FTIR spectra of pure PVB film (a), PVB films containing different concentrations of Pb(AOT)2 before exposure to the H2S gas (b, c, and d), and PVB films containing different concentrations of Pb(AOT)2 after exposure to H2S gas for 10 min (e, f, and g). The Pb(II)/PVB molar ratios are 1.8 (b and e), 12.0 (c and f), and 35.2 (d and g), respectively.

Pb(II) and PbS nanoclusters. When the PVB film was loaded with Pb(AOT)2 at different concentrations, the FTIR spectra changed from the pure PVB spectrum (Figure 9a) to that shown in Figure 9d. The main spectral features are all similar except that the intensity of the peak at 1736 cm-1 exhibits an obvious increase as the Pb(II) concentration increases. This peak at 1736 cm-1 is assigned to the carbonyl asymmetric stretching vibration in both the polymer PVB and the Pb(AOT)2 molecules. On the other hand, the out-of-plane twist of the O-H located at 758 cm-1 decreases as the concentration of Pb(II) increases because the OH groups form hydrogen bonds with the COO- groups of the AOT molecules. This combination reduced the intensity ratio of the peak at 758 cm-1 to that at 1736 cm-1. Upon exposure to H2S gas, these FTIR spectra changed from b, c, and d to e, f, and g, respectively. An evident change is the intensity of the peak at 758 cm-1, which gradually decreased and finally disappeared (Figure 9g) as the Pb(AOT)2 concentration increased to a certain extent. We attribute this to the release of the AOT molecules, which form hydrogen bonds with the OH groups of PVB, perhaps through their sulfonic groups SO3-. At high concentrations of Pb(II), all the OH groups were titrated by AOT molecules, forming hydrogenbonded complexes, and this broadened the peak at ∼758 cm-1 and in turn reduced the intensity of this peak. Discussion We have described the characterization of the PbS nanorods and spherical nanocrystals synthesized in PVB film and solution, respectively. The results showed that PbS can be grown into one-dimensional rods with a preferred (100) orientation parallel to the substrate surface, whereas only spherical PbS nanocrystals can be obtained in solution. An immediate question is why such a highly symmetric cubic crystal can grow in one dimension with such a large aspect ratio. It is likely that the structure of the precursor Pb(AOT)2 for synthesizing PbS nanocrystals dispersed in the polymer PVB film plays a crucial role in growing the nanorods and defining the crystal plane orientations. In the solution, because the surfactant concentration is believed to be above the cmc, Pb(AOT)2 molecules presumably self-assemble into a micelle structure, which is further stabilized by interactions with the

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Figure 10. Schematic showing the formation of PbS nanorods in PVB film.

Figure 11. XRD patterns of PVB films containing different concentrations of Pb(AOT)2 before exposure to the H2S gas. The inset shows the XRD patterns of the PVB films (a) before and (b) after exposure to the H2S gas for 15 min. The molar ratios Pb(II)/PVB are (a) 35.2, (b) 12.0, and (c) 1.80, respectively.

PVB polymer chains through van der Waals forces and hydrophobic effects.18 This is schematically shown in Figure 10a. The Pb(II)-containing ionic domains will be enclosed inside the micelles. Clearly, when H2S gas is infused into this solution, spherical PbS nanoparticles will be formed, as observed in our experiments. However, during the process of transferring these species to a glass substrate by dip casting, the micelle structure is believed to change to a lamella structure due to the mechanical shear and the rapid evaporation of the solvent. Evidence for the lamella structure on the glass substrate is provided by the XRD pattern of the film. Figure 11 shows the XRD data for the PVB films containing different concentrations of Pb(AOT)2. As seen from Figure 11, there is no diffraction peak in the XRD pattern of pure PVB film. It is only when the PVB was loaded with Pb(AOT)2 that diffraction peaks started to appear. It is reasonable to attribute these diffraction peaks to the presence of ordered structures of Pb(AOT)2 in the PVB film. Furthermore, the XRD pattern reveals that the Pb(AOT)2/ PVB polymer composite displays a layer structure with a nearest interplane distance of ∼22.0 Å, as calculated from the series of diffraction peaks. A proposed layer structure is schematically illustrated in Figure 10b. With

a parafin structure for the AOT- assemblage, the smallest distance between the layers is estimated to be ∼23.9 Å. This figure is in fairly good agreement with the XRD result. In this layered structure, Pb(II) ions were arranged in two dimensions parallel to the substrate. As the H2S molecules diffused into the layered structure, they reacted with the Pb(II) ions to produce PbS nanoclusters, with the original Pb(II) planes being developed into the (100) lattice plane of the PbS nanorods. In addition, the diffusion of H2S into the hydrophilic domain and the formation of the PbS nucleation center are bound to modify the original layered structure. The likelihood is that a curvature develops and the layered structure curls up, starting to form one-dimensional structures. PVB may assist the rolling-up process by lowering the free energy barrier for the structural transformation. The growth of PbS then proceeds in one dimension. This is supported by the disappearance of the original diffraction peaks after exposure to the H2S gas (Figure 11, inset b). The layered precursor structure and the curvature development are considered to be the crucial elements for forming the crystalline PbS nanorods with the (100) lattice planes parallel to the substrate surface (Figure 10c). It is believed that the curvature of the layered structure restricts the lateral growth of the PbS nanocrystals, whereas the amount of Pb(II) in each domain determines their thickness. When the Pb(II)/PVB ratio is too low, each domain may be too small to form even the layered precursor structure, as shown by the XRD data in Figure 11. On the other hand, when the Pb(II)/PVB ratio is too high, each domain would be too large to curl up effectively. In this situation, the isotropic growth in the hydrophilic layer results in the formation of crystalline PbS disks, as revealed from TEM observations. Indeed, the (100) lattice plane of the PbS nanoclusters becomes even better oriented as the Pb(II)/PVB ratio increases (see Figure 6). As described above, the UV-vis absorption spectra as a function of the time after H2S exposure revealed some differences in the stability of the nanocrystals against aggregation between the PbS nanorods prepared in PVB film and the solution-prepared spherical PbS nanocrystals. For the film-gas reaction, the particle size was affected only by the H2S exposure time. When the H2S exposure time was lengthened, the absorption edge shifted to a longer wavelength, indicating the continuous growth of the PbS nanorods. However, when the films were removed from the H2S atmosphere, no further red-shift took place in the absorption spectra, indicating that the particle

Oriented PbS Crystalline Nanorods in Polymer Films

growth is not simply due to aggregation of existing small particles. So the exposure time to the H2S gas can be used to control the size of the PbS nanoclusters formed in the PVB films. In the solution-phase preparation, the absorption edge of the PbS nanocrystals was also found to shift to longer wavelength as the exposure time increased. But when the solution was removed from the H2S atmosphere, the absorption edge kept on shifting to longer wavelengths with the elapse of time. This indicates that the aggregation of existing small particles might be an important process to form large particles in the solution phase. Therefore, the exposure time to H2S alone cannot control precisely the particle size of the PbS nanoclusters in the solution phase. This is understandable because small particles are in constant motion, and when they encounter each other, aggregation or coalescence may take place if their capping molecules can undergo dynamic exchange with the molecules in the solution. On the other hand, once formed, the PbS nanorods cannot move freely in the polymer film, and aggregation or coalescence of the PbS rods is therefore unlikely. Summary and Conclusions In summary, we have developed a synthetic route for growing novel one-dimensional PbS nanocrystals (nano-

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rods) using a combination of surfactant and polymer matrix as the template. These nanocrystals have a preferred (100) lattice plane orientation parallel to the substrate surface. We have investigated some important experimental parameters which can be used to control the PbS nanocrystal size, shape, surface states, and crystal plane orientation. These nanoclusters have a relatively large aspect ratio, and most of them have a preferred crystal plane orientation relative to the substrate surface. For comparison, solution preparation of PbS nanocrystals was carried out under similar conditions, and only spherical particles were obtained. This provided some clue for understanding the mechanism of the PbS nanorod growth in the PVB films. Given the generality of this approach, we hope to extend our synthetic method to the preparation of other one-dimensional semiconductor nanosized materials with preferential crystallite plane orientations. Acknowledgment. This work is supported by an RGC grant administered by the UGC of Hong Kong. The authors would like to thank Yan Zhang for her assistance in sample characterization and MCPF of HKUST for providing the facilities. LA990780T