A Reverse Cation-Exchange Route to Hollow PbSe Nanospheres

May 4, 2006 - Graduate School of the Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, ... designing and preparing special building blocks ...
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J. Phys. Chem. B 2006, 110, 9785-9790

9785

A Reverse Cation-Exchange Route to Hollow PbSe Nanospheres Evolving from Se/Ag2Se Core/Shell Colloids Wei Zhu,†,‡ Wenzhong Wang,*,† and Jianlin Shi† State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, P. R. China, and Graduate School of the Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, P. R. China ReceiVed: January 16, 2006; In Final Form: March 30, 2006

A reverse cation-exchange approach for the synthesis of hollow PbSe nanospheres is successfully established. This route involves a new strategy of a stepwise, in-situ template-based evolution from spherical amorphous Se colloids to Se/Ag2Se core/shell colloids, then to hollow PbSe nanospheres. Se colloids are prepared as the initial product by utilizing the chelation of ethylenediamine to bulk Se. They are converted into Se/Ag2Se core/shell colloids through the reaction with Ag+ in ethylene glycol. During the conversion from Ag2Se shell to PbSe shell, a small amount of tributylphosphine is crucial as the capping agent. The characterization results, including XRD, SEM, TEM, HRTEM, and EDX, reveal that hollow PbSe nanospheres with polycrystalline and cubic structure are prepared. The corresponding optical band gap is calculated to be 0.56 eV. This conformation is potentially beneficial to the improvement concerning the applications of PbSe nanostructures.

Introduction For nanomaterials, the morphological retrofit has always been one of the focusing subjects for the affinitive relationship among the shape, the property, and the subsequent opportunity of designing and preparing special building blocks in nanodevice fabrications.1-3 Compared with other structured nanoparticles (NPs), hollow nanospheres are anticipated to exhibit enhanced or novel functionalities due to their higher surface area and the capability of forming composite structures by embedding specific particles in the interiors. The breadth of the subject material has been demonstrated by the fact that it covers and intertwines with the research of catalysis,4 drug delivery,5 bioencapsulation,6 plasmonics,7 superparamagnetism,8 etc. For example, Suslick demonstrated that catalytic activity of hollow MoS2 nanospheres for hydrodesulfurization of triophene is substantially superior to both MoS2 NPs and conventional microsized powders.4 By encapsulating iron oxides nanoparticles in the interiors, Xia synthesized superparamagnetic colloids with controllable surfaces.9 Therefore, the successful extension of this conformation into the existing important materials is of significance for the fundamental research and prospective applications. As a narrow-band semiconductor, lead selenide (PbSe) is one of the most attractive selenides used for IR detectors, photographic plates, selective and photovoltaic absorbers, and so on.10 The sensitivity of its band gap to the dimension also makes it useful to investigate the nanosized effects specially. Over the past few years, much effort has been devoted to the preparation of nanoscale PbSe in solution system. For example, Zhu and co-workers illustrated a sonochemical method for the fabrications of spherical and rectangular PbSe NPs.11 Murray et al. * Corresponding author. Phone: +86-21-5241-5295. Fax: +86-21-52413122. E-mail: [email protected]. † State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.

reported the first synthesis of highly monodisperse and sizecontrolled PbSe colloidal nanocrystals (NCs),12 and then the investigations relevant to their interband and intraband optical properties were followed.13 Kuno et al. described the productions of both straight and branched PbSe nanowires, where Au/Bi core/shell NPs were introduced as seeds, through a seeded solution approach.14 Owing to PbSe crystallographic texture (cubic, Fm3m), however, kinetic control is futile for the evolution to the artificial hollow nanospheres. In comparison, potential advantages of hollow PbSe nanospheres include the possibility of higher efficiency and an increased flexibility by which the resulting spheres can incorporate with other functional NPs. As a consequence to the fundamental interest and prospective applications progress, the realization of this structure becomes naturally attractive, whereas the concerned production has never been reported to date. Presently, an improved in-situ template method involving a cation-exchange reaction is illustrated as an efficient way to “duplicate” nanostructures without altering the morphology. Large solubility product (Ksp) difference is a key reference for the direct replacement between two kinds of cations. The NCs, in the solution containing appropriate precursor, can spontaneously convert into another kind of more stable NCs with much smaller Ksp. For example, several cations including Cu+, Ag+, Sb3+, Bi3+ have been used to replace Zn2+ in ZnS nanotubes to produce sulfides with preservation of the tubular shape.15 This approach opens up a new access to design and prepare nanostructures that is difficult to achieve through other general methods. On the other hand, this brings another challenge. When the Ksp of goal product is larger than that of the precursor that used as the template, how to make the reverse process proceed is uncertain. To facilitate the reverse reaction, generally, the stronger bond between cation and anion in primal NCs should be impaired or substituted through special management. Introduction of an organic coupling agent is substantiated in some cases as an effective strategy. The ligand could complex with target cations of template and form nearly inert cation-organic

10.1021/jp060305l CCC: $33.50 © 2006 American Chemical Society Published on Web 05/04/2006

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Figure 1. XRD patterns of (a) Ag2Se shells, obtained after the removal of Se core in the core/shell Se/Ag2Se colloids by hydrazine; (b) hollow PbSe nanospheres.

ions, such as tributylphosphine (TBP) to Ag+ 16,17 and 12crown-4 to Li+.18 Then this allows another kind of cation to associate with the anions backbone. These explorations significantly increase the scope of the methodological application in the field of nanostructures preparation. Herein, the current work further advances our understanding about the cation-exchange reaction in the appliance of complicated structures, by describing a synthetic approach through which one can achieve hollow PbSe nanospheres. Briefly, hollow PbSe nanospheres have been successfully prepared via a reverse cation-exchange approach. This method is primarily concerned with the production of amorphous (a-) Se spheres based on dissolution-reprecipitation of bulk Se in ethylenediamine, and then the formation of Se/Ag2Se core/shell structure through the reaction of a-Se with insufficient Ag+ in ethylene glycol (EG), subsequently the conversion from orthorhombic Ag2Se (Ksp ) 2.0 × 10-64) shell to cubic PbSe (Ksp ) 7.94 × 10-43) shell via a reverse cation-exchange process. Experimental Section In a typical procedure, 18 mg bulk gray Se was first dissolved into 10 mL ethylenediamine to form a yellowish solution after being stirred for about 4 h at room temperature. Then, this solution was gradually added into 10 mL deionized water under agitation, which resulted in a brick-red opaque, indicating the formation of a-Se that mainly existed as spherical colloids.19,20 The temperature of the mixture also rose as a response to the exothermic reaction. After the mixture was quenched with ice water for about 5 min, a-Se was readily collected by centrifugation, washed by deionized water for several times and dried at 3 °C. The preparation of the Se/Ag2Se core/shell structure was conducted by re-dispersing 6 mg a-Se colloids homogeneously in 5 mL EG, then introducing them into 15 mL EG solution of AgNO3. The mole ratio of Ag+ and Se was set as 3:2. This reaction was allowed to proceed at 3 °C for 4 h and the suspension turned from red to brown gently. Subsequently, the suspension was diluted with 30 mL H2O and the colloids were separated by centrifugation, washed with ethanol and dried in air. To prepare hollow PbSe nanospheres, these as-obtained core/ shell colloids were suspended in 20 mL methanol that containing 0.5 g poly(vinylpyrrolidone) (PVP) and 1 g Pb(CH3COO)2‚

Figure 2. (a) SEM image of spherical a-Se colloids, the inset is a TEM micrograph of an individual colloid; (b) TEM of the Se/Ag2Se core/shell colloids, where the thickness of the shell is measured to be 20 nm; (c) TEM of Ag2Se shells, the inset is a SEM image of an individual shell.

3H2O (the molar ratio of Pb2+:Ag+ was about 23:1). After 0.05 mL TBP was added in, this suspension was kept at 55 °C in a reflux system for 15 h to ensure the entire reaction. Then the product was collected by centrifugation and washed three times with methanol. After the as-prepared products were immersed in 5 M hydrazine for 10 min, hollow PbSe nanospheres were obtained. The X-ray diffraction (XRD) patterns were recorded with a Japan Rigaku Rotaflex diffractometer equipped with a rotating anode and using Cu KR radiation over the range of 20° e 2θ e 80°. The scanning electron microscope (SEM) characterizations were performed on a JEOL JSM-6700F field emission scanning electron microscope. The transmission electron microscope (TEM) analyses were performed by a JEOL JEM2100F field emission electron microscope. The optical diffuse reflectance spectra were conducted on a UV-Vis-NIR scanning spectrophotometer (UV-3101PC, Shimadzu) using an integrating sphere accessory.

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Figure 3. The images of hollow PbSe nanospheres prepared through the reverse cation-exchange process. (a) TEM micrograph at low magnification; (b) TEM micrograph at high magnification; (c) TEM of an individual PbSe shell, the inset is the corresponding SAED pattern; (d) HRTEM image corresponding to the local region of the shell; (e) SEM image of the hollow PbSe nanospheres.

Results and Discussion The compositional conversion of the shell structure from Ag2Se to PbSe, resulting from the cation-exchange reaction, is first confirmed by the use of wide-angle X-ray diffraction (XRD). Figure 1 shows the XRD patterns of both the template Ag2Se shell and hollow PbSe nanospheres. In Figure 1a all the diffraction peaks are in good agreement with the corresponding literature data of orthorhombic Ag2Se, with lattice parameters a ) 4.333 Å, b ) 7.062 Å, and c ) 7.764 Å (JCPDS 24-1041). These Ag2Se crystals would be transformed into PbSe during the cation-exchange process. Figure 1b displays the typical XRD pattern of the as-prepared PbSe nanospheres. As represented in the micrograph, the diffraction peaks can be indexed as pure cubic phase of PbSe with lattice parameters a ) 6.124 Å (JCPDS 06-0354). The evident intensity of the peaks reveals the fine crystallinity of the final product. In comparison to Figure 1a, moreover, the peaks from Ag2Se are not detected, implying the efficient and complete replacement of Ag+ by Pb2+.

Utilizing the chelation of ethylenediamine to bulk Se, in our synthesis, spherical a-Se colloids are first synthesized through a dissolution-reprecipitation procedure. Then they are converted into Se/Ag2Se core/shell colloids, which are used as the in-situ template in the following cation-exchange reaction. Figure 2 reveals the electronic microscopic images of the stepwise products, including a-Se colloids (Figure 2a), Se/Ag2Se core/ shell colloids (Figure 2b), and Ag2Se shells (Figure 2c), to demonstrate the whole evolution process of the in-situ template. From the SEM image shown in Figure 2a, it is obvious that the initial product (a-Se) consists of spherical colloids with diameters in range of 150∼300 nm. Due to the low glass transition temperature of a-Se (32 °C),21 a small amount of assemblies also form through the limbic attachment between the adjacent colloids (as marked by the arrow). TEM of an individual colloid, which represents the round characteristic of the sample, is inserted. Via the reaction with insufficient Ag+ in EG, these colloids are coated by thin Ag2Se layers and Se/Ag2Se core/

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Figure 4. EDX spectra of (a) Ag2Se shells; (b) PbSe shells.

shell structures form. Figure 2b is the typical TEM micrograph of the complicated colloids, in which a deep contrast along the colloid edges is observed. The thickness of the surrounding Ag2Se shell is estimated to be approximately 20 nm. These Ag2Se layers in the in-situ template would be converted into PbSe shells in the following cation-exchange step. To closely examine the template conformation, Figure 2c displays the TEM of the Ag2Se shells obtained after the removal of Se core with hydrazine. The obvious contrast implies the formation of the shell structure. And as shown in the SEM image (inset of Figure 2c), which reveals an individual Ag2Se particle containing a gap in the surface, the hollow characteristic is apparent. For the conversion to hollow PbSe nanospheres, the Se/Ag2Se core/shell colloids (Figure 2b) are used as the in-situ template instead of the Ag2Se shells (Figure 2c). The reservation of Se core is expected as substrate to prevent the collapse of the shell structure during the cation-exchange reaction. Figure 3 represents the morphologies of the final PbSe product. As shown in Figure 3a and b, hollow PbSe nanospheres with diameters in range of 150∼300 nm are achieved. Both the TEM images show a uniform shell thickness without collapse of the shell structure during the cation-exchange preparation. The thickness of the shell is characterized to be around 20 nm, which is in accordance with the value of the initial Ag2Se layer in the template. Clearer structure is displayed in Figure 3c that illustrates an individual hollow PbSe nanosphere. The corresponding select area electron diffraction (SAED) pattern (inset of Figure 3c) approves that the PbSe sphere is polycrystalline. Further investigations of the shell structure come from the high-resolution transmission electron microscope (HRTEM) analyses (Figure 3d). The legible crystal lattice implies the crystal perfection on the shell. And the observed interplanar spacing marked representatively in Figure 3d is about 0.353 nm, corresponding to the (111) plane of cubic PbSe. Figure 3e is the typical SEM image that displays the exterior morphology of the PbSe spheres. As depicted in the micrograph, most of the PbSe nanospheres have a compact and sealed surface. The similarities in morphology and dimensions of hollow Ag2Se and PbSe nanospheres demonstrate the success of the reverse cation-exchange approach in duplicating nanostructures. To further confirm the componential transformation of the hollow nanospheres, energy dispersion X-ray (EDX) spectra of the template shell and the final product are respectively recorded in Figure 4. Figure 4a displays that the chemical component of the template shell consists of Ag and Se. After the cation-

Figure 5. Diffuse reflectance spectra of (a) Se/Ag2Se core/shell colloids and (b) hollow PbSe nanospheres, the insets are the corresponding curves plotted as normalized (F(R)*hν)2 versus hν (eV).

exchange reaction, as shown in Figure 4b, the peaks from Pb and Se are observed and Ag is not detected in the sample. The atomic ratio of Pb and Se is computationally 1:1. Both these two results are in well accordance with the XRD analyses mentioned above. Figure 5a and b show the optical diffuse reflection spectra of both the Se/Ag2Se core/shell colloids and the hollow PbSe nanospheres in order to resolve the excitonic or interband (valence-conduction band) transitions, which allows us to calculate their band gaps. The spectral envelopes are clearly the summations of a number of subspectra. The estimates of the optical band gap (Eg) can be obtained using the following equation (eq 1) for a semiconductor:

R(V) ) A(h′V/2 - Eg)m/2

(1)

where A is a constant, h′ ) h/2π, R is the absorption coefficient, and m is equal to 1 for a direct allowed transition. Since R is proportional to F(R), the Kubelka-Munk function, the energy intercept of a plot of (F(R)*hν)2 versus hν yields Eg for a direct allowed transition when the linear region is extrapolated to zero ordinate.22 From the spectra (insets of a and b), the band gaps of the Se/Ag2Se colloids and PbSe nanospheres are calculated to be 0.69 and 0.56 eV respectively. Both the values of the band gap energies are larger than those of the reported values for bulk materials (0.07∼0.15 eV for Ag2Se and 0.29 eV for PbSe).23,24 The increase in the band gaps of the as-prepared

Reverse Cation-Exchange Route to Hollow PbSeNanospheres

J. Phys. Chem. B, Vol. 110, No. 20, 2006 9789 process about the hollow PbSe nanospheres is schematically illustrated in Figure 6. And it’s worth noting that, to facilitate the entire conversion from Ag2Se to PbSe, large excess amounts of Pb2+ is necessary. When the mole ratio of Pb2+ to Ag+ is adjusted to 10, even after 30 h of reaction, Ag2Se peaks are also detected in the XRD pattern besides PbSe. Conclusion

Figure 6. Schematic illustration of the whole evolution process about the hollow PbSe nanospheres.

materials are indicative of size quantization effects that lead to a series of discrete states in the conduction and valence bands.25-27 Within the stepwise evolution process, a-Se colloids are prepared as the starting template via the chelation of ethylenediamine to bulk gray Se. Ethylenediamine could dissolve the gray Se powder to produce a homogeneous solution based on the formation of complex polyanions.28-30 However, the high metastability of the complex ions makes the system labile and extraordinary sensitive to the environment in the solution. When the solution is mixed with deionized water, the equilibrium is destroyed and a large quantity of Se molecules forms. Spherical a-Se colloids are of preference resulting from the rapid aggregation of the molecules. And the average diameter of the colloids is determined by the amount of the as-prepared Se molecules that depends on the initial concentration of Se in the ethylenediamine solution, as the other experimental parameters are kept unchanged. Subsequently, these colloids are converted into the Se/Ag2Se core/shell structures by storing the EG suspension of a-Se and insufficient Ag+ for the desired time. EG is a solvent possessing reducibility.31,32 In the dispersion, the Ag+ cations are reduced by EG to Ag atoms that further react with a-Se to generate spherical colloids consisting of a-Se cores and Ag2Se shells as seen in eq 2: EG

Se

Ag+ 98 Ag 98 Ag2Se

(2)

Low environmental temperature (herein, 3 °C) is favorable for slowering the reaction so as to ensure Ag2Se layer with relatively uniform thickness. In the following step, the core/shell colloids serve as the in-situ template to synthesize hollow PbSe spheres. The reservation of Se cores is expected to preserve the shell structure during the conversion from Ag2Se to PbSe. TBP is a necessary coupling agent in the reverse cation-exchange process. In the methanolic solution, TBP can bind to Ag+ on the surface of Ag2Se shells to produce stable complexes in the form of TBPnAg+ (n ) 2, 3, 4)33,34 that are nearly immune to the Se2anions backbone. Once these TBP capped Ag+ cations form, Pb2+ cations are able to associate with Se2- to generate PbSe NCs. This reaction will stop until Ag2Se NCs are consumed up. As a result, the composition of the shell is transformed from Ag2Se to PbSe. Then, when the sample is immersed into hydrazine, the hydrazine penetrates the polycrystalline PbSe shell and dissolves the residual Se core, leading to the achievement of hollow PbSe nanospheres. The whole evolution

In summary, we have prepared pure hollow PbSe nanospheres with a cubic structure through a reverse cation-exchange approach, in which Se/Ag2Se core/shell colloids are used as the in-situ template. The dimensions of the as-obtained PbSe spheres, including diameter and thickness, are similar to those of the initial Ag2Se layer in the template. In view of the extensive applications of PbSe such as IR and photography, this conformation is anticipated to enhance the efficiency or introduce novel functionality. Furthermore, the current work demonstrates the availability of the reverse cation-exchange approach in the field of duplicating nanostructures, especially for some particular morphologies. This strategy is also suggested to be practicable to other functional selenides or tellurides. Acknowledgment. Financial support from Chinese Academy of Sciences and Shanghai Institute of Ceramics under the program for Recruiting Outstanding Overseas Chinese (Hundred Talents Program) is gratefully acknowledged. References and Notes (1) Matijevic, E. Langmuir 1994, 10, 8. (2) Jeong, U.; Wang, Y.; Ibisate, M.; Xia, Y. AdV. Funct. Mater. 2005, 15, 1907. (3) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (4) Dhas, N. A.; Suslick, K. S. J. Am. Chem. Soc. 2005, 127, 2368. (5) Bergbreiter, D. E. Angew. Chem., Int., Ed. 1999, 38, 2870. (6) Marinakos, S. M.; Anderson, M. F.; Ryan, J. A.; Martin, L. D.; Feldheim, D. L. J. Phys. Chem. B 2001, 105, 8872. (7) Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Chem. Phys. Lett. 1998, 288, 243. (8) Vasquez, Y.; Sra, A. K.; Schaak, R. E. J. Am. Chem. Soc. 2005, 127, 12504. (9) Jeong, U.; Herricks, T.; Shahar, E.; Xia, Y. J. Am. Chem. Soc. 2005, 127, 1098. (10) Mulik, R. N.; Rotti, C. B.; More, B. M.; Sutrave, D. S.; Shahane, G. S.; Garadkar, K. M.; Deshmukh, L. P.; Hankare, P. P. Indian J. Pure Appl. Phys. 1996, 34, 903. (11) Zhu, J.; Wang, H.; Xu, S.; Chen, H. Langmuir 2002, 18, 3306. (12) Murray, C. B.; Sun, S. H.; Gaschler, W.; et al.; IBM J. Res. DeV. 2001, 45, 47. (13) Wehrenberg, B. L.; Wang, C. Sionnest, P. G. J. Phys. Chem. B 2002, 106, 10634. (14) Hull, K. L.; Grebinski, J. W.; Kosel, T. H.; Kuno, M. Chem. Mater. 2005, 17, 4416. (15) Dloczik, L.; Ko¨nenkamp, R. Nano Lett. 2003, 3, 651. (16) Son, D. H.; Hughes, S. M.; Yin, Y.; Alivisatos, A. P. Science 2004, 306, 1009. (17) Jeong, U.; Kim, J.; Xia, Y.; Li, Z. Nano Lett. 2005, 5, 937. (18) Song, J. H.; Messer, B.; Wu, Y.; Kind, H.; Yang, P. J. Am. Chem. Soc. 2001, 123, 9714. (19) Gates, B.; Yin, Y.; Xia, Y. J. Am. Chem. Soc. 2000, 122, 12582. (20) Watillon, A.; Dauchot, J. J. Colloid Interface Sci. 1967, 27, 507. (21) Zingaro, R. A.; Cooper, W. C. Selenium 1974, Van Nostrand Reinhold: New York. (22) Luca, V.; Djajanti, S.; Howe, R. F. J. Phys. Chem. B 1998, 102, 10650. (23) Dalven, R.; Gill, R. Phys. ReV. 1967, 159, 645. (24) Streltov, E. A.; Osipovich, N. P.; Ivashkevich, L. S.; Lyakhov, A. S.; Sviridov, V. V. Electrochim. Acta 1998, 43, 869. (25) Pejova, B.; Najdoski, M.; Grozdanov, I.; Dey, S. K. Mater. Lett. 2000, 43, 269. (26) Gorer, S.; Hodes, G. J. Phys. Chem. 1994, 98, 5338. (27) Gorer, S.; Yaron, A. A.; Hodes, G. J. Phys. Chem. 1995, 99, 16442.

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