Shape-Controlled Preparation of PbS with Various Dendritic

Jul 1, 2009 - PbS fishbone-like architectures were synthesized by using a ... synthesized PbS fishbone-like structure was found to indicate strong ...
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Shape-Controlled Preparation of PbS with Various Dendritic Hierarchical Structures with the Assistance of L-Methionine Shuzhen Liu,† Shenglin Xiong,*,‡ Keyan Bao,† Jie Cao,† and Yitai Qian*,† Department of Chemistry and Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei, Anhui, 230026 China and College of Material Science & Engineering, Nanjing UniVersity of Aeronautics and Astronautics, Nanjing, 210016 (PR China) ReceiVed: NoVember 27, 2008; ReVised Manuscript ReceiVed: February 6, 2009

PbS fishbone-like architectures were synthesized by using a biomolecule (L-methionine)-assisted approach in a mixture solvent made of ethanolamine (EA) and distilled water. Additionally, other PbS homogeneous morphologies (e.g., hexapod-like, coralloid, and dendritic) were obtained through altering the experimental parameters, such as the molar ratio of the reactants and the volume ratio of the mixture solvent. It is noteworthy that our experiments were envionmentally friendly, because no disgusting scent (H2S) appeared in our experiments, which could be hardly avoided in other previous reports. The as-prepared PbS products were examined by using XRD, FESEM, TEM, HRTEM, SAED, and PL. Furthermore, a possible growth mechanism (an initial nucleating stage and a subsequent growth stage) was proposed to explain the formation of dendritic architectures on the basis of TEM observations, XRD, and FTIR analyses. Up to now, this is the first case of the direct growth of novel PbS with various hierarchical nanostructures with L-methionine’s assistance. The synthesized PbS fishbone-like structure was found to indicate strong photoluminescence in the blue-light range. 1. Introduction Semiconductor nanocrystals have been extensively explored in recent years because of the possibility to tune their properties by controlling their sizes,1 shapes,2 surface morphologies,3,4 and self-assembly.5,6 Of them, metal sulfide nanomaterials have generated strong interest.7-9 As an important IV-VI semiconductor, lead sulfide (PbS) has attracted much attention for many years due to its strong absorption cross-section, strong quantum confinement of both electrons and holes,10 and a tunable band gap from the nearinfrared (NIR) to the visible spectral region.11,12 Therefore, nanoscaled PbS has shown some novel and excellent optical and electronic properties, such as IR photodetectors, photovoltaics, electroluminescence, photoluminescence, thermal and biological images, and display devices.13-15 In addition, it has been expected that the third-order nonlinear optical response of PbS nanocrystals is 30 times larger than that of GaAs and 1000 times larger than that of CdSe, which makes PbS nanocrystals a promising candidate for photonic and optical swiching device applications.13 So far, different morphologies of PbS nanocrystals have been achieved by various methods. Most prominent among them include cubes,20,21,36-38,42 spheres,22,41 tubes,28 rod-like shapes,49 wires,23,24,29 truncated octahedrons,25 multipods,48,51 flowerlike shapes,26,38 dendritic,18,27,48 star-shaped,31,41 and pagodalike shapes43 structures. Various synthetic approaches have been reported for the fabrication of PbS nanoparticles, such as reverse micelles,33 microwave analysis,35,39 colloidial solutions,32 solvothermal analysis,34,50 surfactant-assisted,20,41,42 and self-assembly.30,48,49 * To whom correspondence should be addressed. E-mail: xsl8291@ ustc.edu.cn; [email protected]. Tel: 86-551-3603204. Fax: 86-5513607402. † University of Technology of China. ‡ Nanjing University of Aeronautics and Astronautics, Nanjing.

Biomolecules, as life’s basic building blocks, have special structures and fascinating self-assembling functions. Thus, they have been exploited as structure-directing agents or reactant sources. For example, Alivisatos et al. have demonstrated that DNA is useful in the assembly of nanoparticles into 2D or 3D structures.17,40 Komarneni et al. have used Glutathione (GSH) as both assembling molecules and the sulfur source to synthesize snowflake-like Bi2S3 nanorods under microwave irradiation.18 Starch has also been introduced to serve as the reducing and morphology-directing agent for the preparation of tellurium nanowires.19 Recently, Xie’s group have synthesized the flowerlike Bi2S3,44 the porous spongy-like Ni3S2,45 and network-like MnS nanostructures46 by the assistance of L-cysteine(HSCH2CH(NH2)COOH) that has a special structure (for example, a mercapto (-SH)) and fascinating self-assembling functions. Our group has prepared Sb2S3 nanowires,39 CdS nanospheres,40 and PbS flower-like structures.37 Therefore, biomolecule-assisted synthesis has been proven to be a novel, environmentally friendly, and promising method in the preparation of various nanomaterials owing to its convenience and strong function in morphology control. Being inspired by the fact that L-methionine contains a sulfur atom, we believe that this biomolecule should also be used as a sulfur source to prepare sulphide. At the same time, it contains several functional groups, which may exert similar polymorph selection, oriented nucleation of crystals, and morphology modulating effect occurring in biomineralization, we achieved the fishbonelike architecture of PbS through an L-methionine-assisted strategy at 200 °C for 24 h with a 2:1 molar ratio of Pb(NO3)2 to L-methionine in a water-to-EA binary solution (Vwater/VEA ) 2:1). To the best of our knowledge, this novel approach of L-methionine-assisted synthesis has never been adopted. The formation mechanism study suggests that an initial nucleating stage and a subsequent growth stage during the growth procedure are responsible for the formation of such architecture.

10.1021/jp8104437 CCC: $40.75  2009 American Chemical Society Published on Web 07/01/2009

Shape-Controlled Preparation of PbS

Figure 1. XRD pattern of the as-obtained dentritic PbS microcystals.

Further study shows that different morphologies of the final products can be controlled effectively by simply adjusting the reaction conditions, for example, when the molar ratio of Pb(NO3)2 and L-methionine was 1:5 and 1:3 while keeping other experimental parameters unchanged, hexapod-like and coralloid morphologies were obtained, respectively. 2. Experimental Section Chemicals. All chemical reagents in analytical grade were purchased from the Shanghai Chemical Company and were directly used without any further purification. Systhesis. In a typical procedure, 0.666 g of Pb(NO3)2 (2 mmol) and 0.597 g of L-methionine (4 mmol) were separately dissolved in 15 mL of distilled water. Then, the two solutions were mixed under stirring. Subsequently, anhydrous ethanolamine (15 mL) was added into the solution. After being stirred magnatically for 10 min, the solution was transferred to a 60 mL stainless steel Teflon-lined autoclave. Finally, the autoclave was sealed and heated at 200 °C for 24 h. After the autoclave was cooled to room temperature naturally, a black precipitate was collected by centrifugation and washed with distilled water and absolute alcohol several times. Then the final products were dried under vacuum at 50 °C for 6 h. Characterization. The X-ray diffraction (XRD) analysis was performed with a Japanese Rigaku D/max-γA rotating-anode X-ray diffractometer equipped with graphite-monochromatized Cu KR radiation (λ ) 1.54178 Å). X-ray photoelectron spectroscopy (XPS) measurements were performed on a VGESCALAB MKII X-ray photoelectron spectrometer with a Mg KR ) 1253.6 eV excitation source. Field emission scanning electron microscopic (FESEM) images were collected on a JEOL JSM6700F SEM. Transmission electron microscopic (TEM) images were measured on a Hitachi model H-800 instrument at an accelerating voltage of 200 kV. High-resolution TEM (HRTEM) image and selected-area electron diffraction (SAED) pattern were performed with a JEOL-2010 TEM at an acceleration voltage of 200 kV. Photoluminescence (PL) measurements were carried out on a Perkin-Elmer LS-55 luminescence spectrometer using a pulsed Xe lamp. 3. Results and Discussion Figure 1 shows the X-ray diffraction (XRD) patterns of the as-prepared PbS. The diffraction patterns distinctly indicate the fine crystallinity of the obtained samples. The reflection peaks of the different products can be indexed as cubic PbS with a lattice constant of a ) 5.921 Å, which is in good agreement with the literature values (JCPDS Card No. 78-1897, a ) 5.914 Å). No peaks of impurities were detected, revealing the high purity of the as-synthesized products.

J. Phys. Chem. C, Vol. 113, No. 30, 2009 13003 The morphology of PbS products obtained was investigated by using FESEM, TEM and HRTEM. Figure 2 demonstrates PbS microstructures synthesized at 200 °C for 24 h with a 2:1 molar ratio of Pb(NO3)2 to L-methionine in a water-to-EA binary solution (Vwater/VEA ) 2:1). Figure 2a exhibits a panoramic FESEM image of the product, which presents a dendritic structure. The product consists of almost all of such structures, giving the information that high yield and good uniformity can be readily achieved through this approach. Figure 2b exhibits SEM image of an individual architecture, whose average length is about 20 µm. Further observation shows that each dendrite consists of a long central backbone and secondary branches (images b and c in Figure 2), which demonstrates the branches are rod-shaped, good symmetry and self-similarity characteristic of fractals. Further structural characterization of PbS dendrites was carried out by TEM measurements. A typical TEM image of PbS dendritic microstructure is shown in Figure 2d, which reveals a clear and well-defined dendritic fractal structure with branches paralleling each other. In addition, the crystallinity of the products was examined by high-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED). Figure 2e exhibits a HRTEM image taken from the labeled area in Figure 2d. The lattice spacing of 0.348 nm between each adjacent plane is in accordance with the spacing of the {111} crystal planes of cubic PbS. The SAED (Figure 2f) taken from the branch tip clearly displays only cubic diffraction spots pattern, which indicates that the branch, as the building unit of the PbS dendritic microstructures, is in a single crystalline state. Surprisingly, when keeping other experimental parameters unchanged (Vwater/VEA ) 2:1 at 200 °C for 24 h) but only changing the molar ratio of Pb(NO3)2 and L-methionine to 1:5, the resulting product was composed of hexapod-like structures. Figure 3a displays a FESEM panorama image of the morphology, from which one can see that the length of each pod is about 6 µm. Figure 3b is a FESEM image of one pod of the configuration, which demonstrates each pod is of pencil shape and the tip is thinner than the stalk. The TEM image is shown in Figure 3c, further testifying to a hexapod-like morphology. When the Pb2+-to-L-methionine molar ratio was changed to 1:3, perfect coralloid configurations with each branch length of about 2 µm were obtained,and a FESEM panorama image is shown in Figure 3d. To observe the substructure of the morpholgy in detail, a FESEM image of an individual coral is shown in Figure 3e, which indicates that the length of one branch in an coral is approximately 4 µm. In order to scrutinize the structure of the branch, the high-magnification FESEM image of a branch is listed in Figure 3e, revealing that each branch of the morphology is self-assembled along the stem. Compared to the dendritic structures shown in Figure 2, this structure is even more dendritic but less symmetric with shorter stems (Figure 3e). The TEM image in Figure 3f further confirms the fact. The volume ratio of the mixed solvents can also influence the morphologies of the products. When the volume ratio of ethanolamine (EA) and water was changed to 1:3, and Pb2+/Lmethionine ) 1:2, with reaction for 24 h at 200 °C, the morphology of the products was obviously different from the structure of the above-mentioned products in Figure 2. From the low-magnification SEM image (Figure 4a), one can see that the product is still made up of a well-defined dendritic structure. However, detailed study from Figure 4b indicates that the stalks of the dendrites are shorter (the length of the stalk is about 4 µm) and the branches are aligned more compactly on the stems. However, when the volume ratio of the mixed solvents was

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Figure 2. FESEM and HRTEM images of the product prepared at 200 °C, Pb2+/L-methionine ) 1:2, Vwater/VEA ) 2:1, with reaction for 24 h. (a-c) FESEM images, (d) TEM image, (e) HRTEM image taken from the area labeled by arrow in panel d, and (f) the corresponding SAED pattern.

Figure 3. FESEM and TEM images of the products prepared at various molar ratio of Pb(NO3)2 and L-methionine: (a-c) 1:5 and (d-f) 1:3. Vwater/VEA ) 2:1, with reaction at 200 °C for 24 h.

Figure 4. FESEM images of the products prepared at different volume ratio of EA and water: (a-b) 1:3 and (c-d) 1:1. Pb2+/L-methionine ) 2:1, with reaction at 200 °C for 24 h.

Figure 5. FESEM (a-c) and TEM (d) images of the product prepared with reaction at 220 °C for 24 h, Pb2+/L-methionine ) 2:1, Vwater/VEA ) 2:1.

increased to 1:1, the resultant products showed a groovelike characteristic (see Figure 4c). The leaves in each side of the central trunk arrange in the same plane, parallel to each other and perpendicular to the central trunk (Figure 4d). Detailed observation further demonstrates that the closer it is to the tip of the trunk, the smaller the leaves become. Interestingly, when the reaction temperature was increased to 220 °C, the resultant products showed cross-shaped characteristics (see Figure 5). From Figure 5b (FESEM image of individual cross), one can see that the length of each branch of

individual crosss is about 4 µm, and the four branches are similar in shape. The high-magnification FESEM image in Figure 5c shows that each branch has a central trunk and many protuberances arranged in a same plane along two sides of a trunk, parallel to each other and perpendicular to the central trunk. Although the exact mechanism for the L-methionine-assisted formation of PbS dendritic architectures is still under investigation, the interaction between Pb2+ and methionine is undoubtedly significant. The function groups in the methionine molecule such as -NH2, -COOH, and -S- have a strong tendency to interact

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Figure 6. TEM (a) 0.5 h, (b) 1 h, (c) 4 h, (d) 6 h, (e) 12 h, and FESEM images (f) 24 h of the fishbone-like products collected at different growth stages.

Figure 7. XRD patterns of the intermediates collected after the reaction has proceeded for (a) 0.5 h, (b) 1 h, (c) 4 h, (d) 6 h, (e) 12 h, and (f) 24 h.

with inorganic cations, which has been confirmed by Burford and co-workers on the basis of mass spectrometry.56 When mixing the L-methionine solution and the Pb(NO3)2 solution, Pb2+ can interact with L-methionine molecules to form precursor complexes. Importantly, the methionine molecule contains a C-S-C function group, which is different from the H-S-C in the L-cysteine molecule, that is to say, the stability of the S atom in the methionine molecule is higher than that of in the L-cysteine molecule. In the experiment, it is found that when the reaction temperature is at 180 °C, even the reaction time was prolonged to 48 h, and no products were obtained. The reactions taking place in the system can be described as follows:

Pb2+ + L-methionine f [Pb(L-methionine)n]2+

(1)

[Pb(L-methionine)n]2+ f PbS

(2)

To understand the growth mechanism of the fishbone-like structure, we’d followed the nucleation and growth steps and investigated the intermediate stages involved in the growth process by TEM, FESEM, XRD, and FTIR, which are shown in Figures 6-8, respectively. A white suspension was scrutinized to form after the mixture solution had been aged at 200 °C for 0.5 h. The suspension has been confirmed by XRD and FTIR spectra (as shown in Figures 7a and 8a, respectively) to be the precursor formed by L-methionine and Pb(NO3)2. The peaks in Figure 8a at 1738 and 1636 cm-1 are attributed to the

Figure 8. FTIR spectra of the intermediates collected after the reaction has proceeded for (a) 0.5 h, (b) 4 h, (c) 12 h, and (d) 24 h.

characteristic peaks of the I and II bands for amino acids, from the diagnostic vibrations of acylamino (sCdOsNH2s) I and II bonding. The peaks around 3550 cm-1 are the characteristic peaks of the amido (-NH2), the peaks at ca. 2422 cm-1 are believed to be the -OH signal, and the peaks around 1406 cm-1 are the signal of -CN bonding, while the peaks about 686 cm-1 are referred to be the particular peaks of the -SC. Figure 6a shows a TEM image of the product obtained at this stage, turning out to be a microrod-like morphology, but there are a few of such structures. With the reaction aging at 200 °C for 1 h, the mircorod-like structures became more and they were aggregated (see Figure 6b). When the reaction time was prolonged to 4 h, the dendritic morphology came into existence disorderly and the microrod-like architectures almost disappeared (see Figure 6c). When the reaction duration was 6 h, the configuration of the products changed and the all microrodlike architectures disappeared. As shown in Figure 6d, a majority of dendritic structures existed in the products. While the reaction time was prolonged to 12 h, the fishbone-like configurations came out (see Figure 6e). With the reaction time lasting for 24 h, all of the products had been pure PbS (see Figure 7f) and become fishbone-like aichitectures (see Figure 6f). The FTIR spectra (Figure 8) of the intermediates obtained at the time intervals of 0.5, 1, 4, 6, 12, and 24 h confirm that the precursor of the Pb2+ and L-methionine complex shows a gradual decomposition exhausting as the reaction proceeds. Pure PbS can be obtained after solvothermal treatment for 24 h. Therefore, the above investigation suggests that the growth process of fishbone-like PbS crystals includes two stages: an initial nucleating stage and a subsequent growth stage. The investigation of the intermediate distinctly indicates that the biomolecule, L-methionine, plays a key role. On the one hand,

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Pb2+ reacts with L-methionine molecules to form complex rods (Figure 6a). With the reaction time increased, the coordinate bonds between the combined sulfide group and Pb2+ rupture because of the high reaction temperature, that is to say, PbS units are obtained which serve as the seeds for the subsequent nucleation and growth of PbS crystals at the expense of the precursor of [Pb(L-methionine)n]2+ microrods (see Figure 6b-c). Worth the whistle, the IR peaks of -SC and the amino acid become weak, demonstrating that the ligands are gradually released and consumed in the reaction system (see Figure 8b-c). It is well-known that surface energies with different crystallographic planes are usually different, and a general sequence can be illuminated as γ{111} < γ{100} < γ{110}.57 Furthermore, the shape of such an fcc crystallite as PbS is mainly determined by the ratio (R) between the growth rates along the {100} and {111} directions, for instance, an octahedron bounded by the most stable {111} planes will be formed when R g 1.73, and perfect cubes bounded by the less-stable {100} planes will result if R is reduced to 0.58.57 The growth rates on different facets are controlled by the surface energy. In our case, because the interaction between L-methionine molecules and high-energy crystallography planes {110} is stronger than other low-energy planes of the nuclei, the {110} direction growth is then prohibited. Additionally, the solvent ethanolamine (EA) might adsorb onto the special facets of the incipient PbS nuclei, which not only prevents the particles from agglomeration but also affects the growth of these planes. Accordingly, although the intrinsic surface energy of cubic PbS {111} facets that contain Pb or S only is higher than its {100} facets, which contain mixed Pb/S,58 yet the synergetic effect of both L-methionine and EA mentioned above gradually changes the rate ratio and induces a large R value, then results in the formation of the fishboneshaped microcrystal as shown in Figure 2 and Figure 6c-d. In other words, the synergetic effect of both L-methionine and EA benefits L-methionine molecules being adsorbed on the {100} faces not the {111} faces; thus, the growth of the {100} faces is inhibited. In the FTIR spectra of the products after reacting for 12 h, we found that the signal of -NH2 and -SC disappeared and the amino acid signal became very weak, which suggests that during the second nucleation stage, the concentration of the L-methionine and other ligands decreased drastically. As the L-methionine in our experiments is always preferentially adsorbed by the {100} facets and blocks their growth, it is reasonable to deduce that the distinctly decrease of L-methionine brings forth the inhibition on {100} facets. So PbS seeds grow gradually along {111} and become fishbone-shaped dendrites, which is also testified by HRTEM. However, the L-methionineassisted binary solution system is quite complicated and further work to better understand the formation of certain PbS microstructures is still under way. Figure 9 shows a typical PL spectrum (λexc ) 360 nm) of PbS fishbone-shaped structures that was measured using a Perkin-Elmer LS-55 luminescence spectrometer with an excitation slit width of 5 nm and an emission slit width of 5 nm. A strong peak centered at a wavelength of 423 nm is observed. Emission bands at 423 nm are usually related to the transition of electrons from the conduction band edge to holes, tapped at interstitial Pb2+ sites. As reported previously,55 the emission at about 423 nm presents a Stokes shift compared to the absorption band edge (246 nm) in the UV-vis absorption spectra. There is another possibility that the observed peaks may correspond to transitions into high-energy bands rather than excitonic transitions.15 Moreover, scattering effects also need be taken into account since the particles are comparable in size to the

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Figure 9. Room-temperature photoluminescence spectrum of PbS fishbone-like structures.

wavelength of light in the present case.15 Indeed, the origin of the observed emission peaks in the blue-light bands is still far from well understood, and more detailed investigations are needed to elucidate the interesting optical properties of the fishbone-shaped PbS microcrystals. 4. Conclusion In summary, various dendritic architectures of PbS such as fishbone-like, cross-shaped, coralloid, and hexapod-like configurations were successfully obtained by using a simple L-methionine-assisted technique in a mixed solution made of ethanolamine (EA) and distilled water. Furthermore, the shape of PbS microcrystals can be easily modulated by varing the reaction conditions, such as the molar ratio of the reactants, the volume ratio of the mixed solvents, the reaction temperature. The process of an initial nucleating stage and a subsequent growth stage for the formation of PbS dendritic morphologies is carefully studied and discussed through TEM observations, XRD and FTIR analyses. The PL emission at about 423 nm can be attributed to transitions between states in excitons that are trapped on nanocrystal surfaces. This simple, environmentally benign, and inexpensive biological route can also be extended to the preparation of other metal chalcogenides (e.g., Sb2S3, Bi2S3, CdS, CuS, NiS, MnS, and CoS) micro- or nanostructures. Acknowledgment. The financial support of this work, by the China Postdoctoral Science Foundation (200801236), the K. C. Wong Education Foundation of Chinese Academy of Sciences, National Natural Science Foundation of China (No. 20431020) and the 973 Project of China (No. 2005CB623601), is gratefully acknowledged. References and Notes (1) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (2) Milliron, D. J.; Hughes, S. M.; Cui, Y.; Manna, L.; Li, J.; Wang, L. W.; Alivisatos, A. P. Nature 2004, 430, 190. (3) Chang, T-W. F.; Musikhin, S.; Bakueva, L.; Levina, L.; Hines, M. A.; Cyr, P. W.; Sargent, E. H. Appl. Phys. Lett. 2004, 84, 4295. (4) Chan, W. C. W.; Shuming, N. Science 1998, 281, 2016. (5) Warner, J. H.; Watt, A. A. R.; Tilley, R. D. Nanotechnology 2005, 16, 2381. (6) Patla, I.; Acharya, S.; Zeiri, L.; Israilachvili, J.; Efrima, S.; Golan, Y. Nano Lett. 2007, 7, 1459. (7) (a) Hu, J.; Odom, T. W.; Liber, C. M. Acc. Chem. Res. 1999, 32, 435. (b) Shipway, A. N.; Katz, E.; Willner, I. Chem. Phys. Chem. 2000, 1, 18. (c) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (d) Wang, M. S.; Cha, J. N.; Choi, K. S.; Deming, T. J.; Stucky, G. D. Nano

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