Fabrication of Symmetric Hierarchical Hollow PbS Microcrystals via a

Particularly, star-shaped and high-quality star-shaped dendrite nanocrystals ..... different length symmetry hierarchical morphologies; a few particle...
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J. Phys. Chem. B 2006, 110, 22400-22406

Fabrication of Symmetric Hierarchical Hollow PbS Microcrystals via a Facile Solvothermal Process Pingtang Zhao, Jinmin Wang, Guoe Cheng, and Kaixun Huang* Department of Chemistry, Huazhong UniVersity of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China ReceiVed: July 12, 2006; In Final Form: September 13, 2006

Symmetric hierarchical hollow PbS structures consisting of nanowalls were successfully fabricated by a facile solvothermal process in ethylenediamine at 120 °C for 12 h, employing lead acetate trihydrate and dithizone as precursors; the thickness of the nanowalls is about 80 nm. No surfactants or other templates were used in the process. The synthesized product was characterized by X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), electron diffraction (ED), ultraviolet-visible spectrometer (UV-vis), near-infrared absorption spectroscopy (near-IR), and fluorescence spectrophotometer. The effect of the reaction conditions on the size and morphologies of PbS structures was investigated. The results show that the temperatures, solvent, and sulfur sources are crucial factors on the morphologies and sizes of the symmetric hierarchical hollow PbS microcrystals. A possible growth mechanism of hierarchical hollow PbS structures is presented. UV-vis absorption spectrum holds a weak peak at 253 nm; the near-infrared absorption spectrum of PbS microcrystals has the two absorption peaks centered at 9613 cm-1 (1040 nm) and 6771 cm-1 (1477 nm), showing a blue shift compared with the bulk PbS (∼3020 nm). And the fluorescence spectrum of PbS microcrystals consists of an emission peak with a maximum at 305 nm. These PbS microcrystals may have potential applications in the fundamental study of nanostructures as well as fabricating nanodevices.

1. Introduction The properties of semiconductor nanostructured materials depend not only on their chemical composition but also on their shape and size.1,2 The exploration of various shape-controlling synthetic methods and studies on their unusual properties will inevitably drive the progress in nanotechnology. Moreover, the architectural control of nanosized material with well-defined shapes is crucial to develop it as building blocks in constructing future nanoscale electronic and optoelectronic devices using the so-called “bottom-up” approach.3,4 Therefore, many studies on shape-controlled synthesis of semiconductor nanomaterials with various morphologies have been reported, such as the helical structure of zinc oxide consisting of a superlattice-structured nanobelts,5 zigzag-shaped SnO2,6 and ZnS architecture made of three intersecting ribbons that form a dart-shaped tricrystal nanostructure,7 but the shape-control synthesis has been difficult to achieve and will be a great challenge in the future.8 It is well-known that lead sulfide is an important π-π semiconductor material with a rather small bulk band gap (0.41 eV at 300 K), a large excitation Bohr radius of 18 nm, and its quantum size confinement effect even if it has relatively larger particles or crystallites.9 It has been widely studied in many applied fields such as IR photodetectors, photovoltaics, solar absorbers, electroluminescence, photoluminescence, thermal and biological images, and mode-locking in lasers.10-17 Recently, an exceptional third-order nonlinear optical property of PbS nanoparticles has been found, which may be useful in optical devices such as light-emitting diodes and the high-speed optical * Corresponding author. Telephone: +86-27-87543133. Fax: +86-2787543632. E-mail: [email protected] (k.huang).

switch.18 Consequently, obtaining PbS nanocrystals with different morphologies is potentially meaningful in finding novel applications. Up to now, various shapes of PbS nano- and microcrystals, such as nanorods,19 nanaowires,20 star-shaped PbS nanocrystals with six symmetric horns along the 〈100〉 direction,21 ellipse to parallelogram nanocrystals of PbS connected by nanorods,22 nanotubes,23,24 dendrites consisting of nanorods,8 hierarchical star-shaped PbS microcrystals with eight symmetric arms along the 〈111〉 directions,25 cloverlike microcrystals,26 flowershaped,27 nanorod bundles,28 nanobelts,29 hollow nanospheres,30 and nanowire (average diameter, 30 nm) orthogonal arrays and networks,31 have been prepared through various routes, including hydrothermal and solvothermal method, microwave-assisted heating, ultrasonic irradiation, electrodeposition, templateassisted, self-assembly, thermolysis of a single-source precursor in ligating solvents, and atmospheric pressure chemical vapor deposition (APCVD) method. Particularly, star-shaped and highquality star-shaped dendrite nanocrystals were obtained at low temperature by a simple solution route recently.32,33 Although numerous examples have been reported, most methods used surfactants, which are not washed completely, and finally affect their properties and applications. Solvothermal synthesis is an important technology for the preparation of nanostructures at low temperature. Under the pressure generated by solvothermal reactions, the as-prepared nanostructures are well-crystallized, and for water-sensitive reactions, solvothermal reaction can fully avoid the presence of water, so this technology was extensively applied to nanostructure preparation.34 In this paper, we report the fabrication

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and characterization of the symmetric hierarchical hollow lead sulfide nanostructures by a facile solvothermal reaction between lead acetate trihydrate and dithizone (C13H12N4S) in ethylenediamine at 120 °C; none of the surfactants or other templates was used in the system. To the best of our knowledge, this kind of symmetry hierarchical hollow architecture has not been reported previously for lead sulfide materials. The temperature can be used as the additional means to adjust the morphology size; the effects of solvent and sulfur sources on the morphologies of the final PbS crystal are studied. The formation mechanisms of such novel hierarchical architectures are discussed. These PbS nanostructures may have great potential application in the fundamental study of nanostructures as well as fabricating nanodevices based on these nanostructures. 2. Experimental Section 2.1. Synthesis. All the chemicals were of analytical grade and purchased from Shanghai Chemical Reagent Co. In a typical procedure, equivalent molar amounts (1.4 mmol) of Pb(Ac)2‚ 3H2O and dithizone (C13H12N4S) were dissolved into ethylenediamine (70 mL), the solution was continuously stirred for a half-hour until a clear purple solution, and then the resulting solution was sealed into an 80 mL Teflon-lined stainless steel autoclave and heated at 120 °C for 12 h in an electric oven. The autoclave was cooled to room temperature naturally when the reaction time was finished. The black product was collected by centrifugation at 9000 rpm for 10 min and washed with distilled water and absolute ethanol several times to remove the excessive reactants and byproducts, followed by drying in a vacuum at 60 °C for 6 h. The black powders were collected for characterization. The experiments were repeated two times in the same condition; the same morphology of PbS nanostructures was gotten. To study the crystal growth mechanism and evolution process, synthesis experiments at different reaction times were also performed with other reaction conditions unchanged. Moreover, the effects of the growth conditions, such as the temperature, solvent, and sulfur source, on the sizes and morphologies of PbS nanostructures were investigated. 2.2. Measurements of Photoluminescence Quantum Yield. The photoluminescence spectra of the PbS microcrystals and anthracene were measured under the same setting of a JASCO FP6500 fluorescence spectrophotometer. The scanning step of the spectrophotometer was set as 1 nm. The optical density (OD) at the excitation wavelength of anthracene and the PbS microcrystal sample was set to the same value. The OD at either the exciton absorption peak of the PbS microcrystals or the absorption peak of anthracene was below 0.05. The photoluminescence quantum yield (QY) of the PbS microcrystals was finally obtained by comparing the integrated photoluminescence intensities of the microcrystals and the corresponding anthracene. The photoluminescence QY values of the anthracene (0.30) were provided by the vendor. 2.3. Characterization. Scanning electron microscopy (SEM) images were obtained on a FEI Sirion 200 field emission scanning electron microscope (FESEM), with energy-dispersive X-ray spectroscopy (EDS) attached to FESEM. The X-ray diffraction (XRD) pattern of the products was recorded by employing a PANalytical B. V. (Philips) χ′ Pert PRO XRD with Cu KR radiation at a scanning rate of 0.02° s-1 in a 2θ range of 10-70°. FT-IR spectrum was measured on a Bruker EQUINOX55 FT-IR spectrophotometer. A small amount of products was dispersed in ethanol by ultrasonic treatment for 10 min; then, one drop of the resulting solution was placed onto a carbon-coated copper grid and dried at room temperature for

Figure 1. (a) X-ray diffraction of symmetric hierarchical hollow PbS structures; (b) FT-IR spectrum of PbS crystals; (c) EDS of PbS crystals.

high-resolution transmission electron microscope (HRTEM) visualization. The electron diffraction (ED) patterns and HRTEM images were carried out on a JEM-2010FEF transmission electron microscopy (TEM) at an acceleration voltage of 200 kV. TEM images were observed on a Tecnai G220 transmission electron microscope at an acceleration voltage of 200 kV. UVvis absorption spectrum was recorded on Lambda Bio 40 UVvis spectrometer. The near-infrared absorption spectrum was carried out on VERTEX70 FT-IR spectrophotometer. Photoluminescence spectra were measured on a JASCO FP6500 fluorescence spectrophotometer. All the measurements were carried out at room temperature. 3. Results and Discussion 3.1. Structural and Composition Analysis. The phase and purity of the sample was confirmed by the X-ray diffraction patterns. A typical XRD pattern of as-synthesized symmetric hierarchical hollow PbS structures at 120 °C is shown in Figure 1a. All the diffraction peaks can be indexed to face-centeredcubic (fcc) rock-salt structured PbS with a lattice constant a ) 5.935 Å, which is in good agreement with the standard data

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Figure 2. SEM images of the PbS microcrystals fabricated by solvothermal process at 120 °C for 12 h. The concentrations of Pb(Ac)2‚3H2O and H2DZ are both 0.02 mol/L. (a) Low-magnification SEM image; (b) higher magnification SEM image of several symmetric hierarchical hollow PbS structures; (c) higher magnification SEM image of an individual PbS symmetry hierarchical hollow structure.

Figure 3. (a) SEM image of the tunnel entrylike structure; (b) SEM image of hollow rectangle pyramid without bottom; (c) SEM image of hollow tetragonal pyramid without bottom; (d) SEM image of cubes with multicavity faces.

Figure 4. (a) TEM image of an individual crystal; (b-d) High-resolution TEM images of PbS microcrystals taken from marked parts 1-3, respectively. The insets are the fast Fourier transform (FFT) pattern of high-resolution TEM images. (e,h) Electron diffraction (ED) patterns recorded with the electron beam perpendicular to the bottom cross-section (e) and one nanowall (f) of an individual novel PbS hierarchical hollow structure.

from JCPDS card No. 5-592 (a ) 5.936 Å). No impurity phase can be detected. The strong and sharp diffraction peaks indicated that the as-obtained products are well-crystalline. N-H, C-H bonds and other peaks are not observed in the FT-IR spectrum of PbS nanostructures (Figure 1b), indicating that the product has no organic impurities. Spectrum of EDS from a PbS crystal is shown in Figure 1c, where a C peak comes from the organic glue used for fixing the crystal. EDS analysis demonstrates that the crystal consists of Pb and S. Moreover, according to the quantitative analysis of EDS, the molar ratio of Pb to S is about 1.065:1, which is almost consistent with stoichiometric PbS. 3.2. Morphologies of PbS Microcrystals. Figure 2 shows SEM images of PbS microcrystals obtained under the typical condition with dithizone and lead acetate trihydrate as precursors in ethylenediamine at 120 °C for 12 h. A low-magnification image is shown in Figure 2a, clearly exhibiting that the assynthesized products are well-defined symmetric hierarchical hollow structures. For clarifying the structures, the highmagnification images are shown in Figure 2b,c, which more clearly indicates the product is the three-dimensional (3D) symmetric hierarchical hollow structures with a symmetric

plane. Each hierarchical structure is constructed of the nanowalls with thickness of about 80 nm; the adjacent walls are upright, and the inner surfaces of the nanowalls are not smooth and hold a lot of parallel strias. The hierarchical structure in middle is bigger than its two ends. The length of the nanostructures is several micrometers. More interesting, the tunnel entrylike structure (Figure 3a), a hollow rectangle pyramid without bottom (Figure 3b), a hollow tetragonal pyramid without bottom (Figure 3c), and cubes with multicavity faces are observed (Figure 3d) under this experimental condition. More details of the nanostructures were investigated by highresolution transmission electron microscope (HRTEM) and electron diffraction ED. Figure 4a shows the TEM image of the nanostructure. Parts b-d of Figure 4 are HRTEM images of the marked parts 1-3 in Figure 4a. The HRTEM images indicate that two sets of lattice distances of 0.201, 0.293 nm are almost in accordance with {220} and {200} lattice distances of fcc PbS crystal, respectively. The results show that HRTEM images taken from different areas of an individual PbS microcrystal are almost identical (Figure 4b-d). From the HRTEM

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Figure 5. Influence of sulfur ion sources and solvent on the morphology. (a) SEM image of PbS nanosheets as-obtained using ethanolamine as solvent instead of ethylenediamine; (b) low-magnification SEM image of the PbS microcrystals employing thiourea (NH2CSNH2) as sulfur source in place of dithizone; (c) SEM images of several PbS microcrystals in different growth stages; (d-f) high-magnification SEM images of PbS microcrystals in different growth stages; (g) SEM image of PbS nanoparticles using thioacetamide as sulfur source in place of dithizone.

images, the preferential growth direction of the PbS nanostructure could be indexed as along 〈100〉 directions. The fast Fourier transform (FFT) patterns also indicate that 〈100〉 directions are preferred growth directions. Every HRTEM image exhibits clear lattice planes, which show a perfect single crystal. The ED pattern (Figure. 4e) is recorded with the electron beam perpendicular to the bottom cross-section of an individual PbS hierarchical hollow nanostructure (Figure 4a), which could be indexed to the diffraction pattern of the [001] zone axis of the fcc rock-salt structured PbS; it indicates that the bottom cross-section face is (001). Figure 4f is the ED pattern taken from the Figure 4a with the electron beam perpendicular to the nanowall of one PbS hierarchical nanostructure, the zone axis is [011] of the face-centered-cubic rock-salt structured PbS, indicating that the nanowall belong to the (011) face. The symmetric ED patterns also indicate that the PbS symmetric hierarchical hollow nanostructure is perfectly single crystalline. The result is consistent with the results from XRD and the HRTEM analysis. 3.3. Influence of Solvent and Sulfur Ion Sources. It is believed that the physical and chemical properties of the solvent can influence the solubility, reactivity, and diffusion behavior of the regents and the intermediate.35 In this synthesis system, when ethanolamine was used as solvent in place of ethylenediamine, irregular nanosheets with an average thickness of 30 nm were obtained (Figure 5a). It reveals that ethylenediamine is a key factor on forming PbS symmetry hierarchical hollow structure. To further understand the role of ethylenediamine in this preparation, other bifunctional amines with different chain lengths 1,4-butylamine was used for this reaction; no PbS product was obtained due to lead acetate no dissolution in 1,4butylamine. During the step of nanoseeds growing, ethylenediamine molecules can cap in a cis configuration onto the facets of the crystals. Other diamines with longer chain length and ethanolamine with one amino will not readily form a stable cis configuration due to the higher entropy factor. Ethylenediamine molecule could also cap in a monodendate manner onto either the Pb or the S atom. In this manner, one of the -NH2 groups will be extending out and may thus attach onto other growing nanocrystals. Since the interatomic distances match rather well, this will thus facilitate oriented attachment of the nanocrystals onto the growing crystals.36 To explore the influence of different sulfur sources on the shape of PbS crystals, we employed thiourea and thioacetamide instead of dithizone as the sulfur source under the same other

conditions. Figure 5b shows SEM image of the as-prepared products. Figure 5b exhibits that the major products are cubic PbS microcrystals with multicavity faces and eight horns; the side length is about 2 µm. SEM images (Figure 5c and Figure 5d-f) of PbS microcrystals in different growth stages clearly indicated that nuclei grew along eight equivalent 〈111〉 and six equivalent 〈100〉 directions and formed these cubic microcrystals with multicavity faces. When dithizone was replaced by thioacetamide, nanoparticles were obtained (Figure 5g) because of the fast releasing speed of S2-. These results imply that dithizone plays an important role in the formation process of symmetric hierarchical hollow PbS microcrystal. 3.4. Time-Dependent Experiments and Formation Mechanism. To study the growth mechanism of the symmetric hierarchical hollow PbS microcrystal, time-dependent experiments were carried out for 2, 2.5, 3, 4, and 8 h at 120 °C under the same other reaction conditions. No precipitate was formed before 2 h. When the reaction time was prolonged up to 2.5 h, a small amount of nanoparticles was obtained (Figure 6a). When the reaction time is up to 3 h, facet crystals were formed (Figure 6b); the high-magnification TEM image of an individual crystal (Figure 6c) shows that the crystal is symmetric and holds the tendency of growing perpendicularly along two 〈100〉 directions; the TEM image of a growing PbS microcrystal (Figure 6d) shows that the PbS microcrystal is formed by the oriented attachment of nanoparticles. Figure 6e implies that two PbS crystals growing along two 〈100〉 directions perpendicularly can take place at an oriented attachment due to the higher energy at the tips, and it is possible that the crystal nucleus grows along eight equivalent 〈111〉 and six equivalent 〈100〉 directions with a special (R) value (Figure 6f). When the reaction time was increased up to 4 h, symmetric hollow structures and a few faceted crystals were obtained (Figure 6g). Further increasing the reaction time, the amount of precipitate was increased. The products after 8 h are shown in Figure 6h, which indicates that the products are different length symmetry hierarchical morphologies; a few particles still exist. Those results suggest that the growth process of PbS microcrystal is from small nanoparticles to the final symmetric hierarchical hollow structures, and the nucleating and growth is a synchronous process. Dithizone (H2DZ) is a good complexing agent, which can form M(HDz)2 with metal ion under basic conditions. The reaction in the present system is dependent on the formation of lead-dithizone complexes followed by thermal decomposition of these complexes to get the final product. The chemical

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Figure 6. TEM and SEM images of the samples synthesized in ethylenediamine at 120 °C, under different reaction time. The concentrations of Pb(Ac)2 and dithizone are both 0.02 mol/L. (a) TEM image of as-prepared PbS nanocrystals after 2.5 h reaction; (b) TEM image of as-prepared PbS crystals after 3 h reaction; (c) high-magnification TEM image of an individual symmetric PbS crystal with the tendency of growing perpendicularly along two 〈100〉 directions; (d) TEM image of a growing PbS nanostructure by the oriented attachment of nanoparticles; (e) TEM image of two PbS crystals growing perpendicularly along two 〈100〉 directions and taking place an oriented attachment; (f) TEM image of a PbS crystal growing along eight equivalent 〈111〉 and six equivalent 〈100〉 directions with the special R value; (g) SEM image of as-prepared products after 4 h reaction; (h) SEM image of as-prepared products after 8 h reaction.

Figure 7. Formation process of the symmetric hierarchical hollow PbS structures.

reaction that took place in the system can be described as follows:

Pb2+ + C13H12N4S f Pb(C13H12N4S)2 f PbS Dithizone metal complexes can prevent metal ions and sulfur ions from fast releasing; thus, it is favorable for the preferentially growth of the final product, PbS microcrystal.37 Present study is consistent with the following conclusion: a higher monomer concentration favors 1D-growth, and a lower monomer concentration favors 3D-growth.38 It is well-known that the process of crystal growth can be divided into two stages: an initial nucleating stage and a subsequent crystal growth stage. Under a particular growth condition, the crystalline phase during nucleation and also the growth rate difference between the surfaces of the crystal determine the overall nanostructure.1 Surface energies associated with different crystallographic planes are usually different, and a general sequence may hold γ{111} < γ{100} < γ{110}; the shape of an fcc crystal is mainly determined by the ratio (R) between the growth rates along the 〈100〉 and 〈111〉 directions.39 But the intrinsic surface energy of the {111} faces of cubic PbS, containing Pb or S only, is higher than that of the {100} faces, which contain mixed Pb/S.40 On the basis of the above analyses and the experiment results, we propose the following growth mechanism for the hierarchical architectures: at fist, nanoparticles were formed through decomposition of lead-dithizone complexes. Because ethylenediamine complexes with lead of high-energy crystallography planes {110} tighter than other low-energy planes of the nuclei, the 〈110〉 direction growth is inhibited. In the subsequent step, these nuclei preferentially grew along 〈100〉 and 〈111〉 directions with a larger R value, forming {110} planes with inner strias through layer dislocation; the intrinsic fcc structure and the special solvothermal reaction conditions result in symmetrically growing perpendicularly along 〈100〉 directions and tending to close for the tendency of forming lower energy face {100}. Finally, the symmetric hollow structure is formed. The second

nucleation can occur at the tops of the symmetric hollow structure right-angle parts due to the higher energy of the tops, then, growing along 〈100〉 and 〈111〉 directions again, forming next layer symmetric hollow structures. The existence of second nucleation was reported by Kar and Chaudhur.41At the different stages, the nucleation and growth speed are continuously changed, and their changes are different. At the initial stage, nucleation and growth are all fast, forming a smaller symmetric hollow structure; at the middle stage, because the nucleation speed decreases and the nucleation speed is lower than the growth speed, the biggest structure is formed; however, at the last stage, because the speeds of the growth and nucleation all are reduced markedly due to the concentration of precursors decreasing drastically, smaller symmetric hollow nanostructures formed again. In this way, the symmetric hierarchical hollow structure with a larger middle and smaller ends are formed. The formation process of the symmetric hierarchical hollow PbS microcrystal is shown in Figure 7. On the other hand, the growing nanostructure along two 〈100〉 directions perpendicularly could take place an oriented attachment at the beginning occasionally due to the higher energy at the tips (Figure 6e), then, growing again and forming a few hollow tetragonal or rectangle pyramids without bottoms and the tunnel entrylike nanostructures. A few nuclei grow along eight equivalent 〈111〉 and six equivalent 〈100〉 directions with the special R value (Figure 6f), and a few cubes with multicavity faces could be formed. 3.5. Influence of Reaction Temperature. The influence of reaction temperature on the formation of PbS microcrystal was investigated at different reaction temperatures. No precipitate was obtained when the temperature was below 100 °C, indicating that the nucleation reaction did not occur. When the temperature was increased to 100 °C, the black precipitate was obtained. Figure 8 exhibits the low- and high-magnification SEM images of the samples prepared at different temperatures. Parts a and b of Figure 8 show that shorter (∼2 µm) well-fined symmetry hollow structures were formed and the inner surface

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Figure 8. SEM images of the PbS crystals obtained under various initial reaction temperatures prepared by solvothermal method for 12 h. The concentrations of Pb(Ac)2‚3H2O and H2DZ are both 0.02 mol/L. (a) Low-magnification SEM images of PbS nanostructures prepared at 100 °C; (b) SEM image of an individual PbS crystal formed at 100 °C; (c) SEM image of PbS crystals prepared at 160 °C.

Figure 9. (a) UV-vis absorption spectrum of as-prepared PbS microcrystals at 120 °C for 12 h; (b) Near-infrared absorption spectrum of PbS crystals prepared at 120 °C for 12 h.

has a lot of patterns. Figure 8c shows that the PbS hierarchical hollow structures were formed at a relatively higher temperature (160 °C). All the results indicate that the temperature plays an important role on the fabrication of the special symmetric hierarchical structures. 3.6. Optical Properties of PbS Nanostructures. Some basal optical property examinations were carried out. The UV-vis absorption spectrum of as-prepared PbS nanostructures at 120 °C for 12 h is shown in Figure 9a, which exhibits a weak peak at 253 nm. Figure 9b describes its near-infrared absorption spectrum. Two absorption peaks were observed; the weaker peak is centered at 9613 cm-1 (1040 nm), and the stronger peak is centered at 6771 cm-1 (1477 nm). Three peaks in UV-vis and near-infrared regions all show a blue shift compared with the bulk PbS (∼3020 nm). Zhao et al.32 also observed this interesting phenomenon from their synthesized PbS nanostars and speculated that position-dependent quantum-size effects exist for the relatively large but highly faceted nanocrystals (stars and octahedrons) with regular shapes and thin tips, resulting in several dramatically blue-shifted excitonic absorptions. In present studies, because these optical properties are rarely observed for other semiconductors with smaller Bohr radius,32 the large Bohr radius of PbS (18 nm) largely contributes to the observed optical properties; it is possible that the three peaks may all correspond to transitions into high-energy bands rather than excitonic transitions. However, there is another possibility that the weakest peak at 253 nm is probably caused by the crystal defects, the weaker peak at 9613 cm-1 maybe corresponds to transition into high-energy band, and another peak at 6771 cm-1 is an excitonic transition. Figure 10 describes the room-temperature photoluminescence spectra of the as-obtained PbS crystals. An emission peak at about 305 nm (4.07 eV) was observed with an excitation wavelength of 265 nm. The photoluminescence quantum yield of PbS crystals prepared after different reaction time (8, 12, and 20 h) is 0.21, 0.19, and 0.10, respectively. Hu et al.42 reported the fluorescence spectroscopy hollow spheres PbS structure, which is similar to ours. They suggested that it was a consequence of intermolecular exciton

Figure 10. Photoluminescence spectra of PbS microcrystals prepared at 120 °C for different reaction time: (a) 8, (b) 12, and (c) 20 h. An inset is the exciting spectrum.

interactions, but it is possible that the newly observed peak at 305 nm results from crystals defects, such as sulfur vacancies or lead interstitials in the obtained PbS microcrystals; the PbS crystals are more perfect with increasing reaction time, so the intensity of the peaks decreases, as displayed in Figure 10. But then, the origin of the observed optical properties is still far from well-understood, and more detailed investigations are needed. 4. Conclusion A facile solvothermal method for large-scale synthesis PbS symmetric hierarchical hollow structures was prepared by employing lead sulfide trihydrate and dithizone as precursors in ethylenediamine at 120 °C. The results show that the temperature, solvent, and sulfur sources are crucial factors on the morphology and size of the PbS symmetry hierarchical hollow structures. Through the analysis of field emission scanning electron microscopy, high-resolution transmission electron microscopy, and electron diffraction, the growth mechanism for the novel structures is put forward, ethylenediamine more tightly capped high-energy crystallography planes

22406 J. Phys. Chem. B, Vol. 110, No. 45, 2006 {110} of nuclei, inhibiting the 〈110〉 directions growth, and these nuclei grew along 〈100〉 and 〈111〉 directions, forming {110} planes. The second nucleation can easily occur at the top of the symmetry hollow structure right-angle due to the higher energy of the tops, then, growing along 〈100〉 and 〈111〉 directions again, forming symmetry hierarchical hollow structures. The UV-vis and near-infrared absorption spectra of PbS crystals have three absorption peaks centered at 253, 1040, and 1477 nm, showing a blue shift compared with the bulk PbS (∼3020 nm). The photoluminescence spectrum of the novel PbS nanostructures consists of an emission peak with a maximum at 305 nm. These PbS crystals may have great potential applications in the fundamental study of nanostructures and nanodevices. In addition, this approach is expected to be employed for the control-shaped synthesis of other semiconductor nanomaterials. But the origin of the observed optical properties is still far from well-understood, and more detailed investigations are needed. Acknowledgment. We thank the faculty from the Analysis and Test Center of Huazhong University of Science and Technology and Senior engineer Dongshan Zhao from the Center for Electron of Microscopy of Wuhan University for technical assistance on characterization. References and Notes (1) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, H. Q. AdV. Mater. 2003, 15, 353. (2) Xiao, Z. L.; Han, C. Y.; Kwok, W. K.; Wang, H. H.; Welp, U.; Wang, J.; Crabtree, G. W. J. Am. Chem. Soc. 2004, 126, 2316. (3) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (4) Duan, X.; Huang, Y.; Cui, Y.; Wang, J.; Lieber, C. M. Nature (London) 2001, 409, 66. (5) Gao, P. X.; Ding, Y.; Mai, W. J.; Hughes, W. L.; Lao, C. S.; Wang, Z. L. Science (Washington, D. C.) 2005, 309, 1700. (6) Duan, J. H.; Yang, S. G.; Liu, H. W.; Gong, J. F.; Huang, H. B.; Zhao, X. N.; Zhang, R.; Du, Y. W. J. Am. Chem. Soc. 2005, 127, 6180. (7) Fan, X.; Meng, X. M.; Zhang, X. H.; Shi, W. S.; Zhang, W. J.; Zapien, J. A.; Lee, C. S.; Lee, S. T. Angew. Chem., Int. Ed. 2006, 45, 2568. (8) Kuang, D. B.; Xu, A. W.; Fang, Y. P.; Liu, H. Q. AdV. Mater. 2003, 15, 1747. (9) Dutta, A. K.; Ho, T.; Zhang, L.; Stroeve, P. Chem. Mater. 2000, 12, 1042. (10) Mcdonald, S. A.; Konstantatos, G.; S. Zhang, G.; Cyr, P. W.; Klem, E. J. D.; Levina, L.; Sargent, E. H. Nat. Mater. 2005, 4, 138. (11) 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.

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