CRYSTAL GROWTH & DESIGN
PbS Cubes with Pyramidal Pits: An Example of Etching Growth Yanglong Hou,*,†,‡ Hiroshi Kondoh,‡ and Toshiaki Ohta‡,§ Department of AdVanced Materials and Nanotechnology, College of Engineering, Peking UniVersity, Beijing 100871, China, Department of Chemistry, School of Science, The UniVersity of Tokyo, Tokyo 113-0033, Japan, and SR Center, Ritsumeikan UniVersity, Shiga 525-8577, Japan
2009 VOL. 9, NO. 7 3119–3123
ReceiVed September 10, 2008; ReVised Manuscript ReceiVed March 21, 2009
ABSTRACT: In this paper, we present the synthesis of PbS cubes with a hierarchical pyramidal pit on each face by a facile template-free solvothermal approach in the presence of cetyltrimethyl ammonium bromide (CTAB). In addition to anisotropic PbS cubes, we obtained an amount of PbS tapes. The growth mechanism of the cubes with pyramidal pits is discussed; the formation of the PbS hierarchal cross-sections should be attributed to the etching force of the base in the system. These structures might be model systems for understanding the growth process of anisotropic crystals and for fabricating optical devices. Introduction The physical and chemical properties of nanocrystals strongly depend not only on the chemical compositions but on particle sizes, surface chemistry, crystalline structures, and the presence of defects.1-4 The ability to tune these parameters provides an opportunity to investigate and understand the relationship between chemical structures and physical properties. On the other hand, the architectural control of nanoblocks with welldefined morphologies is indispensable to the success of the bottom-up approach for access to devices. Recently, there have been significant efforts in identifying routes to control the shape and morphology of nanoscale materials.5,6 The solution phase method has been widely used to produce nanoscale materials with controllable size and dimensionality.7-10 In this process, the use of stabilizers commonly affects the growth kinetic and surface energy of different crystalline facets, leading to the anisotropic growth of nanocrystals such as nanorods, nanobelts, nanopolyhedrons, and so on. Although numerous examples have been reported, shape control of nanoscale materials has been difficult to achieve and turns out to be a great challenge. Nanoscale lead chalcogenides have been an intensive research interest because of their potential applications in sensors, lasers, solar cells, infrared detectors, thermoelectric cooling materials, etc.11,12 As a family member of lead chalcogenides, PbS is an important semiconductor (π-π) with a small band gap (0.41 eV) and an exciton of large Bohr radius (18 nm).13 Bakueva et al. reported that PbS nanoparticles might provide significant applications in biological labeling and integrated optical fiber communication because of its nonlinear optical property.14 Various morphologies of PbS nanocrystals can be synthesized, for example, nanorods, nanowires,15 nanostars,16,17 and nanodendrics.18 Except of these anisotropic shapes, hollow nanostructures have attracted much attention in recent years because of their potentials to be carriers.19-21 It was interesting that the nickel and gold pyramidal microstructures were fabricated using surface-modified pyramidal pits patterned by either conventional photolithography or soft lithography.21 In the fabrication process, the funnelform patterned pits as templates are a prerequisite to obtaining pyramidal structures. Herein, we report the preparation of novel PbS cubes with a pyramidal pit on each face, base * Corresponding author. E-mail:
[email protected]. † Peking University. ‡ The University of Tokyo. § Ritsumeikan University.
dimensions of 2-5 µm, wall thickness of ∼500 nm, by a facile template-free solvothermal approach in the water/glycerol solution of cationic surfactant. These interesting structures might find some applications in optical devices and be used as a model for fabricating the pyramidal pit structures and understanding the growth process of crystals. Experimental Section In a typical procedure, an aqueous solution (70 mL) was first prepared by dissolving Pb(NO3)2 · 6H2O (50 mM), NaOH (2 M), glycerol (63 mL), and cetyltrimethyl ammonium bromide (CTAB) (15 mM) in distilled water. Subsequently, Na2S2O3 (50 mM) was added to this solution, of which should be alkalescent, facilitating the reaction of lead nitrate and sodium thiosulfate to form PbS. After vigorous stirring of 30 min, the mixture was transferred to a stainless Teflonlined autoclave with the capacity of 100 mL. The autoclave was maintained at 160 °C for 24 h. After the reaction, the autoclave was allowed to cool to room temperature. A silver gray solid product was deposited on the bottom of Teflon cup, indicating the formation of lead sulfide. The resulting sample was collected, rinsed with distilled water and ethanol several times to remove any alkaline and surfactants that remained in the final product, and then dried in a vacuum oven at 60 °C for 4 h. The structural information on the samples was collected by powder X-ray diffraction (XRD) method. Diffraction patterns of intensity versus 2θ were recorded with a Rigaku mini diffractometer equipped with a Cu KR radiation source (λ ) 0.15418 nm) from 20 ° to 70 ° with a scanning rate of 0.2 °/min. The morphology of the sample was characterized by a JEOL 6700F scanning electron microscope (SEM). The chemical composition of the products was recorded by energy dispersive spectroscopy (EDS) equipped in SEM. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained with a Hitachi HF-2000 transmission electron microscope.
Results and Discussion The synthesis of PbS cubes with a pyramidal pit on each face was carried out by a reaction of lead nitrate and sodium thiosulfate in the mixture of water and glycerol, in which cationic surfactant, such as cetyltrimethyl ammonium bromide (CTAB), was used as stabilizer. The morphology and chemical structure of the products were characterized by scanning electronic microscope (SEM) and X-ray diffraction (XRD) technology. The as-synthesized sample was composed of the cubes (∼20%), the tapes (∼70%), and a small amount of particles (see Figure S1 in the Supporting Information). Figure 1 shows the typical SEM images of the cubes with different
10.1021/cg801013t CCC: $40.75 2009 American Chemical Society Published on Web 05/07/2009
3120
Crystal Growth & Design, Vol. 9, No. 7, 2009
Hou et al.
Figure 1. SEM images of the PbS cubes with pyramidal pits, prepared by the reaction of lead nitrate with sodium thiosulfate in the presence of CTAB at 160 °C for 24 h; the molar ratio of Pb2+:CTAB is 3.3. (A, D) Side view, (B) top view, and (C) a fraction of a cube, indicating the angles between the cross-sections of the pyramid.
views. The cubes have dimensional sizes of 2-5 µm and cubic morphologies with pyramidal pits. It is worth noting that each pyramidal pit exhibits a hierarchical surface, as “stairs”, and the center of the cube is hollow cubic structure. It might be due to the hierarchical “etching” of the strong base (sodium hydroxide) in the solution, as illustrated in the discussion next. One can see from the top view (Figure 1 B) that the crosssection of the cube looks like a “tunnel with stairs”. In a fragment shown in Figure 1C, it can be seen that the top angle of the triangle is 76°, whereas each bottom one is 52°. More cubes with pyramidal pits are shown in Figure 1D. Energydispersive X-ray spectroscopy of as-synthesized sample indicated the molar ratio of Pb:S is 51:49 (see Figure S2 in the Supporting Information). The X-ray diffraction (XRD) pattern of the as-synthesized product is shown in Figure 2. All diffraction peaks of the sample are matched with those of a typical cubic PbS with the lattice constant a ) 5.9348 Å (JCPDS 5-592). The intensity ratio of (200) to (111) is about 2.31, suggesting the formation of anisotropic nanostructures.17 In addition to anisotropic cubes, PbS tapes were obtained in the one-pot reaction, which might result from the influence of various cationic vesicle (CTAB) and salt microstructures in the aqueous solutions.16 A typical TEM image of the PbS tape is shown in Figure 3. The tape of PbS is more than 10 µm in length, about 2 µm in width, and ∼100 nm in thickness. The HRTEM image (Figure 3B) of PbS tape in Figure 3A exhibits a lattice spacing of 0.293 nm, which is in good agreement with that of the (200) plane of cubic PbS. The inset is a fast Fourier
Figure 2. XRD pattern of the product.
translation (FFT) pattern, indicating the well-crystalline feature of the sample. At the present, it is still a challenge to synthesize monodisperse PbS cubes with pyramidal pits in the solution. To study the influence of reaction conditions on the product morphology, we tested some parameters such as reaction temperature and surfactants. The synthesis should be carried out at elevated temperature to trigger the reaction of lead salt and sulfide source. Note that the temperature strongly affects the morphology of the products. Reaction temperature below 110 °C did not lead to the formation of any cubes, whereas temperature higher than 190 °C promoted the production of cubic cracks rather than uniform cubes. The mild condition (160
PbS Cubes with Pyramidal Pits
Crystal Growth & Design, Vol. 9, No. 7, 2009 3121
Figure 3. TEM and HRTEM images of PbS tape.
Figure 4. Influence of the CATB concentration on the morphology of the products. The molar ratios of Pb2+:CTAB are (a, b) 2, (c) 3.5, and (d) 7, respectively. The concentration of Pb2+ is kept the same (50 mM) and the reactions were performed at 160 °C for 24 h.
°C) facilitates the formation of cubes. Prolonged reaction time increased particle size, but more fragments of the cubes appeared in the product. Sunagwa proposed a qualitative model of the growth kinetic of PbS crystals. 22 Decreasing the supersaturation favors the formation of concave structure and forms thermodynamically stable cubic morphology. In the present work, the formation of cubic PbS crystals with pyramidal pits is also ascribed to the competition between CTAB and the base in the system.17 The concentration of CTAB tunes the surface stability of the intermediates. From Figure 4, it can be seen that the
CTAB concentration (images A and B in Figure 4) was set as higher and part of the cube surfaces were etched into “bowels”, whereas lower CTAB concentration was applied (Figure 4D), overetching occurred, leading to the formation of irregular shapes after 24 h of reaction. The mild concentration of CTAB promotes the balance between “freezing” and etching of cubic faces, facilitating the formation of cubes with pyramidal pits (Figure 4C). These results indicate that CTAB plays the key role in controlling the morphology of the products. For comparison, we examined the influence of polyvinyl pyrrolidone
3122
Crystal Growth & Design, Vol. 9, No. 7, 2009
Hou et al.
Figure 5. (A-D) Evolution of PbS cubes from semisolid cube to “non-center” body-center cubes. The schematic diagram shows that the cubes with pyramidal pits were formed by a two-step process: the cubes were formed initially as intermediate morphology, and the base etching process consequently occurred, leading to the formation of pyramidal pits.
(PVP) and anionic surfactants such as sodium dodecylbenzenesulfonate (SDBS) and sodium dodecylsulfate (SDS) on the product morphology; however, only larger particles with irregular anisotropic shapes were obtained in the three cases as described above. In addition, the combination of H2O and glycerol is also beneficial to obtain this kind of PbS cubes, which should be related to the viscous factors of the solvents. It is important to understand the formation mechanism of anisotropic nanostructures for developing the synthetic approach of nanostructures with controllable morphologies. To discuss the formation mechanism of the cube with pyramidal pits, we analyzed the intermediates of PbS cubes by SEM. As shown in Figure 5, it can be seen that the cubes was formed at the initial stage of the reaction. With increase in reaction time, the etching process was activated at the faces of cubes by the base, as schematically shown in the inset of Figure 5. On the basis of the shape evolution of the intermediates, we propose that the PbS cubes with pyramidal pits are fabricated by a two-step process, i.e., intact cubes are formed at first, and then etched by the base to cubes with pyramidal pits. It is well-known that the intrinsic surface energy of the cubic PbS {111} face is higher than that of the {100} face;16 as a result, it tends to bring about a slight increase in the growth rate of the {111} face relative to the {100} face, and results in the formation of PbS cubes as an intermediate morphology in the initial stage of the whole reaction. However, the cationic surfactants (CTAB) have a relative weak interaction on the {100} facets, which are predominated by both Pb and S, leading to uncharged state of
{100} facets. Upon continuous heating at the elevated temperature in the process, the etching ability of the base overcomes the stabilization of CTAB to etch the {100} faces, leading to the gradual formation of pyramidal pits on the faces of PbS cubes toward the interior along the axis. This process is similar to that occurring in Cu2O,23 ZnO,24 and Fe2O3,25 which were etched from the some facets of the precursors. However, the exact mechanism of the morphological control of PbS crystals by cationic surfactants is worth further investigation. In the present samples, there are three kinds of PbS shapes including cubes, tapes, and particles. The next challenge would be to find a synthetic route to produce cubes with pyramidal pits selectively. The synthesis of PbS nanocrystals with controllable morphologies is under way. Conclusions In summary, unique PbS cubes with a pyramidal pit on each face have been synthesize by a facile solvothermal reaction, which involves a reaction of Pb(NO3)2 with Na2S2O3 in a mixed solvent of water and glycerol, combined with CTAB stabilization. In addition to PbS cubes, an amount of PbS long tapes was produced. The formation of cubes with pyramidal pits should be helpful to understanding the controllable nuclear and growth process of anisotropic crystals. Because of their interesting unique structures, the PbS cubes with pyramidal pits have the potential to be used as building blocks in the fabrication of artificial materials, carriers, and devices.
PbS Cubes with Pyramidal Pits
Acknowledgment. This work was supported in part by the Japan Society of the Promotion of Science (JSPS), the 21st Century COE Program and Nanotechnology Support Project from the Ministry of Education, Culture, Sports, Science and Technology (MEXT, Japan), Beijing New Star Project of Science and Technology (2008B02), and the Start-up Fund of Distinguished Young Scholar at Peking University.
Crystal Growth & Design, Vol. 9, No. 7, 2009 3123
(12)
(13)
Supporting Information Available: A typical SEM image of the as-synthesized PbS tapes and cubes with pyramidal pits (Figure S1); energy-dispersive X-ray spectroscopy of as-synthesized PbS cubes (Figure S2) (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
(14) (15)
References (1) (a) Milliron, D. J.; Hughes, S. M.; Cui, Y.; Manna, L.; Li, J.; Wang, L.-W.; Alivisatos, A. P. Nature 2004, 430, 190–195. (b) Wang, D.; Lieber, C. M. Nat. Mater. 2003, 2, 355. (2) Dick, K. A.; Deppert, K.; Larsson, M. W.; Mårtensson, T.; Seifert, W.; Wallenberg, L. R.; Samuelson, L. Nat. Mater. 2004, 3, 380. (3) Kim, F.; Connor, S.; Song, H.; Kuykendall, T.; Yang, P. Angew. Chem., Int. Ed. 2004, 43, 3673. (4) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. J. Am. Chem. Soc. 2002, 124, 8673. (5) Jun, Y. -W.; Choi, J. -S.; Cheon, J. Angew. Chem., Int. Ed. 2006, 45, 3414. (6) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (7) Jin, R.; Cao, Y. C.; Hao, E.; Me´traux, G. S.; Schatz, G. C.; Mirkin, C. A. Nature 2003, 425, 487. (8) (a) Hou, Y.; Kondoh, H.; Che, R.; Takeguchi, M.; Ohta, T. Small 2006, 2, 235. (b) Hou, Y.; Hiroshi, K.; Shimojo, M.; Kogure, T; Ohta, T. J. Phys. Chem. B 2005, 109, 19094. (9) Maillard, M.; Giorgio, S.; Pileni, M.-P. AdV. Mater. 2002, 14, 1084. (10) Dumestre, F.; Chaudret, B.; Amiens, C.; Fromen, M. -C.; Casanove, M. -J.; Renaud, P.; Zurcher, P. Angew. Chem., Int. Ed. 2002, 41, 4286. (11) (a) Lu, W.; Gao, P.; Jian, W. B.; Wang, Z. L.; Fang, J. J. Am. Chem. Soc. 2004, 126, 14816. (b) Schaller, R. D.; Petruska, M. A.; Klimov,
(16)
(17) (18)
(19) (20) (21) (22) (23) (24) (25)
V. I. J. Phys. Chem. B 2003, 107, 13765. (c) Wehrenberg, B. L.; Wang, C.; Guyot-Sionnest, P. J. Phys. Chem. B. 2002, 106, 10634. (a) Bierman, M. J.; Lau, Y. H. A.; Kvit, A. V.; Schmitt, A. L.; Jin, S. Science 2008, 320, 1020. (b) Kowshik, M.; Vogel, W.; Urban, J.; Kulkarni, S. K.; Paknikar, K. M. Microb. AdV. Mater. 2002, 14, 815. (c) Nanda, K. K.; Sahu, S. N. AdV. Mater. 2001, 13, 280. (d) Zhou, Y.; Itoh, H.; Uemura, T.; Naka, K.; Chujo, Y. Langmuir 2002, 18, 5287. Lim, W. P.; Low, H. Y.; Chin, W. S. J. Phys. Chem. B 2004, 108, 13093. (b) Kuang, D.; Xu, A.; Fang, Y.; Liu, H.; Frommen, C.; Fenske, D AdV. Mater. 2003, 15, 1747. Bakueva, L.; Gorelikov, I.; Musikhin, S.; Sheng, X.; Sargent, E. H.; Kumacheva, E. AdV. Mater. 2004, 16, 926. (a) Gao, F.; Lu, Q.; Liu, X.; Yan, Y.; Zhao, D. Nano Lett. 2001, 1, 743. (b) Wang, S.; Yang, S. Langmuir 2000, 16, 389. (c) Leontidis, E.; Orphanou, M.; Kyprianidou-Leodidou, T.; Krumeich, F.; Caseri, W. Nano Lett. 2003, 3, 569. (a) Lee, S. -M.; Jun, Y.-W.; Cho, S.-N.; Cheon, J. J. Am. Chem. Soc. 2002, 124, 11244. (b) Jun, Y.-W.; Lee, J.-H.; Choi, J. S.; Cheon, J. J. Phys. Chem. B 2005, 109, 14795. (a) Zhao, N.; Qi, L. AdV. Mater. 2006, 18, 359. (b) Peng, Z.; Jiang, Y.; Song, Y.; Wang, C.; Zhang, H. Chem. Mater. 2008, 20, 3153. (a) Ma, Y.; Qi, L.; Ma, J.; Cheng, H. Cryst. Growth Des. 2004, 4, 351. (b) Wang, D.; Yu, D.; Shao, M.; Liu, X.; Yu, W.; Qian, Y. J. Cryst. Growth 2003, 257, 384. Sun, Y.; Xia, Y. Science 2003, 298, 2176. Yang, H. G.; Zeng, H. Angew. Chem., Int. Ed. 2004, 43, 5930. Xu, Q.; Tonks, I; Fuerstman, M. J.; Christopher Love, J.; Whitesides, G. M. Nano Lett. 2004, 4, 2509. Sunagawa, I. Bull. Mine´r 1981, 104, 81. Lu, C.; Qi, L.; Yang, J.; Wang, X.; Zhang, D.; Xie, J.; Ma, J. AdV. Mater. 2005, 17, 2562. Li, F.; Ding, Y.; Gao, P.; Xin, X.; Wang, Z. L. Angew. Chem., Int. Ed. 2004, 43, 5238. Jia, C.-J.; Sun, L.-D.; Yan, Z.-G.; You, L.-P.; Luo, F.; Han, X.-D.; Pang, Y.-C.; Zhang, Z.; Yan, C.-H. Angew. Chem., Int. Ed. 2005, 44, 4328.
CG801013T
