DOI: 10.1002/asia.201500191
Communication
Nanoparticles
Formation of Metal Selenide and Metal–Selenium Nanoparticles using Distinct Reactivity between Selenium and Noble Metals Se Ho Park,[a] Ji Yong Choi,[b] Young Hwan Lee,[a] Joon T. Park,*[a] and Hyunjoon Song*[b] Abstract: Small Se nanoparticles with a diameter of 20 nm were generated by the reduction of selenium chloride with NaBH4 at ¢10 8C. The reaction with Ag at 60 8C yielded stable Ag2Se nanoparticles, which subsequently were transformed into M–Se nanoparticles (M = Cd, Zn, Pb) through cation exchange reactions with corresponding ions. The reaction with Pt formed Pt layers that were evenly coated on the surface of the Se nanoparticles, and the dissolution of the Se cores with hydrazine generated uniform Pt hollow nanoparticles. The reaction with Au generated tiny Au clusters on the Se surface, and eventually formed acorn-shaped Au–Se nanoparticles through heat treatment. These results indicate that small Se nanoparticles with diameters of 20 nm can be used as a versatile platform for the synthesis of metal selenide and metal–selenium hybrid nanoparticles with complex structures.
Selenium (Se) is an attractive material with intriguing physical and chemical properties.[1] It has photoconductive features with a high refractive index,[2] which are useful for application in photodetectors and sensors.[3] Selenium is also important as an antioxidant in biological environments.[4] It also exhibits a relatively low melting temperature of 217 8C, and it is readily dissolved in CS2 and hydrazine. Furthermore, Se has a glass transition temperature at approximately 31 8C, and it forms an amorphous solid at room temperature, which transforms into trigonal selenium at higher temperatures.[5] Its high reactivity with other chemicals enables Se to be a versatile template for various monodisperse colloids.[6, 7] Xia et al. pioneered the use of Se as a platform of complex colloidal structures.[8–10] The sur-
face of Se colloids formed Ag2Se through the reaction with Ag,[11] and it generated uniform Se@MSe (M = Zn, Cd, and Pd) by cationic exchange reactions with conservation of the original spherical morphology.[12, 13] The amorphous Se spheres were converted into trigonal Se nanowires either by heating[14] or through sonication.[15] These nanowires also reacted with Ag to yield Ag2Se nanowires in a similar manner,[11] and finally led to the transformation into CdSe nanowires by the exchange reaction with Cd2 + .[12] These Se-based colloids and nanowires have been employed in photonic crystals[8–13] and electrical switching devices.[16] Although Se colloids are versatile in forming various Sebased semiconductors, the resulting particle sizes are very large (up to 300 nm),[9] owing to their low glass transition temperature. Through changing the ratio of the Se precursor and hydrazine, the size of the Se colloids changed from 90 nm to 420 nm, which remained limited in the sub-micron range. Several approaches have also been developed for the synthesis of nanometer scale Se particles, but the Se particle diameters were larger than 100 nm with wide size distributions.[17–19] The synthesis of Se nanostructures using biological reagents was also reported to provide bio-conjugated materials.[20, 21] In the present study, small Se nanoparticles with a size range of 20 nm are demonstrated as a platform for various metal and semiconductor nanostructures (Scheme 1). Low temperature conditions with the addition of an active reductant generated unstable Se nanoparticles with diameters of 20 nm. The reaction of Se and Ag formed stable Ag2Se nanoparticles, which could be converted into CdSe, ZnSe, and PbSe nanoparticles by cationic exchange. The addition of the Pt precursor coated the Se surface with thin Pt layers, and the disso-
[a] S. H. Park, Y. H. Lee, Prof. J. T. Park Department of Chemistry Korea Advanced Institute of Science and Technology Daejeon, 305-701 (Korea) E-mail: [email protected] [b] J. Y. Choi, Prof. H. Song Department of Chemistry Korea Advanced Institute of Science and Technology Center for Nanomaterials and Chemical Reactions Institute for Basic Science Daejeon, 305-701 (Korea) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201500191. Chem. Asian J. 2015, 10, 1452 – 1456
Scheme 1. Distinct reactivity between Se nanoparticles and noble metals to form metal–selenium hybrid structures.
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Communication lution of the Se cores yielded uniform Pt hollow spheres. In contrast, through the reaction with the Au precursor, small Au nuclei were attached to the Se surface, and the Au atoms migrated to form particles with acorn-like structures through a high temperature treatment. This distinct reactivity of Se nanoparticles to noble metals can provide a general platform to synthesize complex structures in multiple components. For the synthesis of large amorphous Se colloids, selenious acid and hydrazine were treated with ethylene glycol at temperatures of 15–20 8C for 1 h.[8, 9] To reduce the particle size, low temperature conditions were investigated, but the reduction rates were significantly slower. Instead, using selenium chloride (SeCl4) as a Se precursor and sodium borohydride (NaBH4) as a strong reductant, the reduction occurred at low temperatures of ¢10 8C. The addition of poly(vinyl pyrrolidone) (PVP) stabilized the structure after the formation of the Se nanoparticles. Under these conditions, small amorphous Se nanoparticles were formed with an average diameter of 25 nm, and these were relatively stable at ¢10 8C. However, these particles agglomerated immediately and formed large spheres at room temperature, and Se nanowires were grown after 1 h. Eventually, the spheres were converted into Se nanowires (Figure S1, in the Supporting Information).[9, 14] To make a stable structure based on the Se nanoparticles, Ag ions were added at an elevated temperature. It is known that Ag ions are reduced under the polyol process conditions, where ethylene glycol behaves as both a solvent and a reductant. Then, Ag directly reacts with Se to form a stable Ag2Se structure.[11] Ag2Se is a narrow-band gap semiconductor with high electronic and ionic mobilities,[22] and it is particularly useful for thermoelectric materials owing to its high electrical conductivity with low thermal conductivity.[23] The transmission electron microscopy (TEM) image in Figure 1 A demonstrates that the particles are polygonal with an average diameter of 20 4 nm. The high resolution TEM (HRTEM) image (inset and Figure S3 A, in the Supporting Information) has continuous lattice fringe images over the entire region of an individual particle; this indicates that the particles are single crystalline. The average distance between neighboring fringe images is 0.27 nm, corresponding to the distance between the {112} planes of orthorhombic Ag2Se. The energy dispersive X-ray absorption (EDX) data demonstrated that the atomic percentage
of the nanoparticles was 70.5 % for Ag and 29.5 % for Se, which is close to the 2:1 ratio of Ag and Se in Ag2Se. The five distinguishable peaks in the X-ray diffraction (XRD) data (Figure 1 B) are assignable to the reflections of the orthorhombic Ag2Se (JCPDS No. 24-1041).[24] The crystalline domain size is 20 nm, which was calculated from the peak width using the Debye–Scherrer equation. This matches the diameters observed in the TEM image, which also confirms the single-crystalline nature of the Ag2Se nanoparticles. Cationic exchange reactions were applied to the Ag2Se nanoparticles.[12, 13] The benefit of these reactions is that the morphology of the starting materials remains unchanged during the process.[25, 26] For the conversion into CdSe, the replacement of Ag + by Cd2 + is known to be non-spontaneous, due to the large solubility difference between Ag2Se (2.0 Õ 10¢22 mol L¢1) and CdSe (2.5 Õ 10¢18 mol L¢1).[27] Alivisatos et al. added tributylphosphine (TBP) to increase the solubility of Ag ions in the Ag2Se, which facilitated the exchange reaction with Cd2 + . As a solvent, methanol was crucial in stabilizing the intermediate complexes during the reaction.[25] The exchange reaction with Cd2 + at 50 8C did not change the morphology of the original particles. Figure 2 A presents the spherical particles with an average diameter of 20 4 nm. However, in the HRTEM image, the distance between the adjacent lattice fringe images changed to 0.35 nm (Figure 2 A inset and see Fig-
Figure 1. (A) TEM and (inset) HRTEM images, and (B) XRD spectrum of the Ag2Se nanoparticles prepared through the reaction of the Ag precursor and Se nanoparticles.
Figure 2. (A,C,E) TEM and (insets) HRTEM images, and (B,D,F) XRD spectra of (A,B) CdSe, (C,D) ZnSe, and (E,F) PbSe nanoparticles prepared by cationic exchange reactions from Ag2Se nanoparticles.
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Communication ure S3B, in the Supporting Information), corresponding to the distance of either the {002} planes of the wurtzite or the {111} planes of hexagonal CdSe. The EDX analysis data confirmed that the Cd and Se elements have an atomic ratio of 54:46. The XRD data had completely changed from the original pattern, and the peaks were assigned to the diffractions of the mixture of hexagonal wurzite (JCPDS No. 08-0459) and zinc blende CdSe (JCPDS No. 19-0191; Figure 2 B).[28, 29] These analyses indicate that the original Ag2Se was completely converted into the CdSe nanoparticles without any change in morphology. Similar exchange reactions were conducted with Zn2 + and Pb2 + ions. Owing to their high solubilities (ZnSe, 1.9 Õ 10¢13 mol L¢1; PbSe, 3.2 Õ 10¢20 mol L¢1)[27, 30] compared with that of Ag2Se, the addition of TBP to the reaction mixture was required. The reaction temperature (60 8C) was higher than that of Ag + because the lattice structures of ZnSe and PdSe differed to that of Ag2Se. For Cd2 + , the anion sublattice in CdSe had a topotactic relationship with that of Ag2Se; this led to the reaction at a lower temperature (50 8C).[13] Figure 2 C demonstrates that the resulting ZnSe particles have a polygonal shape with an average diameter of 20 4 nm. The HRTEM image presents a single-crystalline feature with a distance between the neighboring fringe images of 0.33 nm, which is in good agreement with the distance of the {002} planes of the hexagonal ZnSe (Figure 2 C inset, and see Figure S3C, in the Supporting Information). The XRD pattern corresponds to the mixture of hexagonal wurzite ZnSe (JCPDS No. 80-0008; Figure 2 D). The atomic percentages of Zn and Se were measured to be 49 % and 51 %, respectively, using the EDX analysis, which ensured the 1:1 stoichiometric ratio of ZnSe. The PbSe nanoparticles were also prepared through a similar pathway with an average diameter of 20 1 nm, and the XRD pattern was assigned to be cubic PbSe (Figure 2 E, F, and Figure S3 D, in the Supporting Information). It should be noted that all particles were single crystalline with similar spherical morphologies with diameters of 20 nm in all distinct compositions, including Ag2Se, CdSe, ZnSe, and PdSe. For semiconducting materials, numerous studies have been undertaken for the synthesis of small nanoparticles below 10 nm or large colloidal structures over 100 nm in size.[8–13, 17–19] However, the synthesis of small nanoparticles with diameters of few tens of nanometers has been rarely reported, although this scale is significant for the formation of hybrid materials with complex structures. In this range, the physical properties are similar to those in the bulk form, but the chemical properties differ due to the relatively high surface areas and small diffusion lengths from the surface. For instance, when large Se spheres with diameters of 300 nm were used, Ag reacted with Se at the surface only, and it formed a Se@Ag2Se core-shell structure, in which the average thickness of the Ag2Se shells was 25 nm.[11] Subsequent cationic exchange reactions yielded Se@MSe core-shell colloids (M = Cd, Zn, Pb).[12, 13] In contrast, the original diameter of the Se nanoparticles was 20 nm in the present experiment; thus, the entire Se structure completely reacted with metal ions and formed single-crystalline nanoparticles. Chem. Asian J. 2015, 10, 1452 – 1456
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Another method to stabilize the Se nanoparticles is surface coating with Pt layers.[31] Additional NaBH4 and Pt precursor were sequentially added to the Se dispersion, and the reaction was allowed to proceed for 5 h at 60 8C. The dispersion color changed from orange to dark brown, thus suggesting that the Pt atoms were directly deposited on the Se surface. Figure 3 A
Figure 3. TEM and (insets) HRTEM images of (A) Se@Pt nanoparticles and (B) Pt hollow nanoparticles after dissolving the Se cores. The bars in the insets represent 20 nm.
illustrates that the particles are uniform in size with an average diameter of 23 5 nm. A dark thin layer covers the periphery of an individual nanoparticle in the HRTEM image (Figure 3 A inset). In the EDX spectrum, Pt peaks are distinguishably detected as well as a strong Se peak. To additionally confirm the Pt layer formation on the Se surface, the particles were treated with hydrazine to dissolve the Se cores. After dissolution, uniform Pt hollow nanoparticles were afforded in a high yield (Figure 3 B). The HRTEM image demonstrated that the shell thickness of the Pt layer was 2 nm (Figure 3 B inset), and the layer was sufficiently continuous on the surface to maintain the spherical morphology. The EDX data demonstrates that only a Pt peak remains in the absence of Se signals, which indicates the complete removal of Se. With the addition of the Au precursor and NaBH4 to the Se nanoparticle dispersion at 60 8C, stable particles were generated, as in the case of Pt (Figure 4 A).[7] However, instead of the formation of even layers, some discontinuous fringe images were observed on the surface of the Se nanoparticles in the HRTEM image (Figure 4 A inset). The particles shrunk in size
Figure 4. TEM and (insets) HRTEM images of (A) Au-on-Se nanoparticles and (B) acorn-like Au¢Se nanoparticles by thermal treatment. The bars in the insets represent 10 nm.
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Communication due to the prolonged irradiation of the electron beam, which also revealed that Au could not effectively cover the nanoparticle surface. The EDX analysis estimated that the atomic ratios of Au and Se were 12.7 % and 87.3 %, respectively. To ensure the existence of the Au clusters, the particles lying on the TEM grid were annealed at 140 8C for 3 h. Then, the anisotropic acorn-shaped Au–Se nanoparticles were obtained in a high yield (Figure 4 B). The HRTEM image of an individual nanoparticle demonstrated that the distance of the neighboring lattice fringe image in the dark domain corresponds to the distance of the Au {111} planes and that, in the bright domain, it matches the distance of the Se {310} planes (Figure 4 B inset). The EDX data demonstrated that the atomic ratio of Se decreased to 73.1 %, presumably due to the sublimation of the Se atoms during the heat treatment. It is assumed that the Au clusters attached to the Se surface migrated and collapsed to form nanoparticles by the Ostwald ripening process under high temperature conditions. In summary, small Se nanoparticles with the size range of 20 nm were used as a platform for the preparation of metal selenides and metal–selenium nanoparticles. Noble metal precursors such as Ag, Pt, and Au were added to the dispersion at 60 8C for the stabilization of the Se spheres. Ag reacted easily with Se and this yielded single-crystalline Ag2Se nanoparticles, which were converted to CdSe, ZnSe, and PbSe nanoparticles through cationic exchange reactions with the corresponding metal ions in the presence of TBP. Pt was evenly deposited on the Se nanoparticle surface. Au formed Au clusters on the Se surface at low temperatures and converted into the acornshaped Au–Se nanostructures in the high temperature treatment. In this present study, various metal selenides and metal– selenium nanoparticles were successfully yielded in the size range of 20 nm using the distinct reactivity of noble metals with Se, which would be useful for potential applications. As an example, the Pt hollow nanoparticles in Figure 3 B were tested as a catalyst for methanol oxidation (Figure S5, in the Supporting Information). The oxidation current of the hollow nanoparticles exhibited an 80 % increase compared to that of the Se@Pt nanoparticles, due to the intrinsic high surface area of the hollow structure. The use of the Se nanoparticles as a versatile platform would also be particularly beneficial for metal–semiconductor hybrid materials,[32] similarly as in the case of metal–metal sulfide heterostructures,[33–35] which have significant potential in electronic, photochemical, and antibacterial applications.
Experimental Section Synthesis of Se nanoparticles and the reactions with Ag, Pt, and Au precursors SeCl4 (57 mg, 0.26 mmol) and poly(vinyl pyrrolidone) (PVP, Mw = 55 000, 1.4 g, 13 mmol based on the repeating unit) were dissolved in ethylene glycol (EG, 40 mL). This solution was added to a sodium borohydride (NaBH4, 98 mg, 26 mmol) solution in EG (40 mL) at ¢10 8C. The reaction was allowed to stir at ¢10 8C for 3 h. For the synthesis of Ag2Se nanoparticles, an additional PVP (1.2 g, 11 mmol) solution in EG (40 mL) was added. Then, the reacChem. Asian J. 2015, 10, 1452 – 1456
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tion vessel was quickly transferred into an oil bath set at 60 8C. A AgNO3 (23 mg, 0.14 mmol) solution in EG (1.5 mL) was added dropwise over 20 min. During the addition, the solution color turned from light red to black. The reaction mixture was allowed to proceed for 5 h at 60 8C. The final product was collected by centrifugation and washed three times with water and ethanol to remove excess PVP. For the synthesis of Se@Pt nanoparticles, a H2PtCl6 (57 mg, 0.14 mmol) solution in EG (1.5 mL) was added dropwise over 20 min, otherwise the reaction procedure was identical to the synthesis of Ag2Se nanoparticles. For the reaction with the Au precursor, a HAuCl4 (48 mg, 0.14 mmol) solution in EG (1.5 mL) was added dropwise over 20 min, otherwise the reaction procedure was identical to the synthesis of Ag2Se nanoparticles. After the drop-casting of the Au-on-Se nanoparticle sample on a TEM grid, the particles were heated at 140 8C in a vacuum oven for 3 h.
Cationic exchange reactions for MSe nanoparticles (M = Cd, Zn, and Pb) After the centrifugation, the Ag2Se nanoparticles were dispersed in methanol (10 mL). For the synthesis of CdSe nanoparticles, Cd(NO3)2·4 H2O (50 mg, 0.16 mmol) was dissolved in a mixture of water (5.0 mL) and methanol (5.0 mL). PVP (0.60 g, 5.4 mmol) was added to the Ag2Se dispersion and the reaction mixture was heated to 50 8C. The Cd precursor solution (5.0 mL) was added, and tributhylphosphine (TBP, 50 mL) was injected dropwise to the mixture. The color of the reaction mixture changed into dark brown within 20 min. After 20 min, another portion of the Cd precursor solution (5.0 mL) was added, and the reaction mixture was allowed to proceed for 2 h at 50 8C. The final product was collected by centrifugation and washed three times with methanol. For the synthesis of ZnSe nanoparticles, Zn(NO3)·6 H2O (120 mg, 0.51 mmol) was dissolved in a mixture of water (5.0 mL) and methanol (5.0 mL), otherwise the reaction procedure was identical. During the reaction with the Ag2Se dispersion and the Zn precursor solution in the presence of TBP, the color of the reaction mixture changed into dark yellow within 20 min. For the synthesis of PbSe nanoparticles, Pb(NO3) (140 mg, 0.51 mmol) was dissolved in a mixture of water (5.0 mL) and methanol (5.0 mL), otherwise the reaction procedure was identical. During the reaction with the Ag2Se dispersion and the Pb precursor solution in the presence of TBP, the color of the reaction mixture changed into dark gray instantaneously.
Characterization Transmission electron microscopy (TEM) images were obtained on a Carl Zeiss EM912 FE-TEM operated at 120 kV at Korea Basic Science Institute (KBSI). High-resolution TEM (HRTEM) images were obtained using a Philips F20 Tecnai FE-TEM operated at 200 kV at KAIST. The samples were prepared by placing a few drops of the colloidal solutions on copper grids coated with a formvar/carbon film (Ted Pella, Inc., 300 mesh, 63 mm grid hole size). X-ray diffraction (XRD) patterns were measured on a Rigaku D/Max-RC (12 kW) diffractometer using a CuKa radiation source.
Acknowledgements This work was supported by IBS-R004-D1, and the National Research Foundation of Korea (NRF) funded by the Korea Government (MSIP) (2012-005624, R11-2007-050-00000-0).
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Communication Keywords: ion exchange · nanoparticles · noble metals · selenium [1] L. I. Berger, Semiconductor Materials, CRC Press, Boca Raton, FL, 1997. [2] Selenium (Eds: R. A. Zingaro, W. C. Cooper), Van Nostrand Reinhold, New York, 1974. [3] D. M. Chizhikov, V. P. Shchastlivyi, Selenium and Selenides, Collet’s, London, 1968. [4] J. T. Rotruck, A. L. Pope, H. E. Ganther, A. B. Swanson, D. G. Hafeman, W. G. Hokestra, Science 1973, 179, 588 – 590. [5] B. Gates, B. Mayers, B. Cattle, Y. Xia, Adv. Funct. Mater. 2002, 12, 219 – 227. [6] B. Gates, Y. Wu, Y. Yin, P. Yang, Y. Xia, J. Am. Chem. Soc. 2001, 123, 11500 – 11501. [7] L. Yang, Y. Shen, A. Xie, J. Liang, J. Zhu, L. Chen, Eur. J. Inorg. Chem. 2007, 1128 – 1134. [8] U. Jeong, T. Herricks, E. Shahar, Y. Xia, J. Am. Chem. Soc. 2005, 127, 1098 – 1099. [9] U. Jeong, Y. Xia, Adv. Mater. 2005, 17, 102 – 106. [10] U. Jeong, P. H. C. Camargo, Y. H. Lee, Y. Xia, J. Mater. Chem. 2006, 16, 3893 – 3897. [11] U. Jeong, Y. Xia, Angew. Chem. Int. Ed. 2005, 44, 3099 – 3103; Angew. Chem. 2005, 117, 3159 – 3163. [12] U. Jeong, J.-K. Kim, Y. Xia, Z.-Y. Li, Nano Lett. 2005, 5, 937 – 942. [13] P. H. C. Camargo, Y. H. Lee, U. Jeong, Z. Zou, Y. Xia, Langmuir 2007, 23, 2985 – 2992. [14] B. Gates, Y. Yin, Y. Xia, J. Am. Chem. Soc. 2000, 122, 12582 – 12583. [15] B. Gates, B. Mayers, A. Grossman, Y. Xia, Adv. Mater. 2002, 14, 1749 – 1752. [16] D. T. Schoen, C. Xie, Y. Cui, J. Am. Chem. Soc. 2007, 129, 4116 – 4117. [17] Y. Zhu, Y. Qian, H. Huang, M. Zhang, Mater. Lett. 1996, 28, 119 – 122. [18] T. W. Smith, R. A. Cheatham, Macromolecules 1980, 13, 1203 – 1207.
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[19] T. C. Franklin, W. K. Adeniyl, R. Nnodimele, J. Electrochem. Soc. 1990, 137, 480 – 484. [20] X. Gao, J. Zhang, L. Zhang, Adv. Mater. 2002, 14, 290 – 293. [21] P. Huang, L. Bao, D. Yang, G. Gao, J. Lin, Z. Li, C. Zhang, D. Cui, Chem. Asian J. 2011, 6, 1156 – 1162. [22] M. C. Santhosh Kumar, B. Pradeep, Bull. Mater. Sci. 2002, 25, 407 – 411. [23] M. Ferhat, J. Nagao, J. Appl. Phys. 2000, 88, 813 – 816. [24] R. Harpeness, O. Palchik, A. Gedanken, V. Palchik, S. Amiel, M. A. Slifkin, A. M. Weiss, Chem. Mater. 2002, 14, 2094 – 2102. [25] D. H. Son, S. M. Hughes, Y. Yin, A. P. Alivisatos, Science 2004, 306, 1009 – 1012. [26] B. J. Beberwyck, Y. Surendranath, A. P. Alivisatos, J. Phys. Chem. C 2013, 117, 19759 – 19770. [27] H. L. Clever, M. E. Derrick, S. A. Johnson, J. Phys. Chem. Ref. Data 1992, 21, 941 – 1004. [28] H.-B. Kim, D.-J. Jang, CrystEngComm 2012, 14, 6946 – 6951. [29] Z. Deng, L. Cao, F. Tang, B. Zou, J. Phys. Chem. B 2005, 109, 16671 – 16675. [30] H. L. Clever, F. J. Johnston, J. Phys. Chem. Ref. Data 1980, 9, 751 – 784. [31] B. Mayers, X. Jiang, D. Sunderland, B. Cattle, Y. Xia, J. Am. Chem. Soc. 1993, 115, 8706 – 8715. [32] R. Costi, A. E. Saunders, U. Banin, Angew. Chem. Int. Ed. 2010, 49, 4878 – 4897; Angew. Chem. 2010, 122, 4996 – 5016. [33] L. Wu, B. Quan, Y. Liu, R. Song, Z. Tang, ACS Nano 2011, 5, 2224 – 2230. [34] J. Yang, Y. Zhou, S. Zheng, X. Liu, X. Qiu, Z. Tang, R. Song, Y. He, C. W. Ahn, J. W. Kim, Chem. Mater. 2009, 21, 3177 – 3182. [35] X. Ma, Y. Zhao, X. Jiang, W. Liu, S. Liu, Z. Tang, ChemPhysChem 2012, 13, 2531 – 2535. Manuscript received: February 26, 2015 Revised: April 1, 2015 Accepted article published: April 16, 2015 Final article published: April 27, 2015
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Communication
Nanoparticles
Formation of Metal Selenide and Metal–Selenium Nanoparticles using Distinct Reactivity between Selenium and Noble Metals Se Ho Park,[a] Ji Yong Choi,[b] Young Hwan Lee,[a] Joon T. Park,*[a] and Hyunjoon Song*[b] Abstract: Small Se nanoparticles with a diameter of 20 nm were generated by the reduction of selenium chloride with NaBH4 at ¢10 8C. The reaction with Ag at 60 8C yielded stable Ag2Se nanoparticles, which subsequently were transformed into M–Se nanoparticles (M = Cd, Zn, Pb) through cation exchange reactions with corresponding ions. The reaction with Pt formed Pt layers that were evenly coated on the surface of the Se nanoparticles, and the dissolution of the Se cores with hydrazine generated uniform Pt hollow nanoparticles. The reaction with Au generated tiny Au clusters on the Se surface, and eventually formed acorn-shaped Au–Se nanoparticles through heat treatment. These results indicate that small Se nanoparticles with diameters of 20 nm can be used as a versatile platform for the synthesis of metal selenide and metal–selenium hybrid nanoparticles with complex structures.
Selenium (Se) is an attractive material with intriguing physical and chemical properties.[1] It has photoconductive features with a high refractive index,[2] which are useful for application in photodetectors and sensors.[3] Selenium is also important as an antioxidant in biological environments.[4] It also exhibits a relatively low melting temperature of 217 8C, and it is readily dissolved in CS2 and hydrazine. Furthermore, Se has a glass transition temperature at approximately 31 8C, and it forms an amorphous solid at room temperature, which transforms into trigonal selenium at higher temperatures.[5] Its high reactivity with other chemicals enables Se to be a versatile template for various monodisperse colloids.[6, 7] Xia et al. pioneered the use of Se as a platform of complex colloidal structures.[8–10] The sur-
face of Se colloids formed Ag2Se through the reaction with Ag,[11] and it generated uniform Se@MSe (M = Zn, Cd, and Pd) by cationic exchange reactions with conservation of the original spherical morphology.[12, 13] The amorphous Se spheres were converted into trigonal Se nanowires either by heating[14] or through sonication.[15] These nanowires also reacted with Ag to yield Ag2Se nanowires in a similar manner,[11] and finally led to the transformation into CdSe nanowires by the exchange reaction with Cd2 + .[12] These Se-based colloids and nanowires have been employed in photonic crystals[8–13] and electrical switching devices.[16] Although Se colloids are versatile in forming various Sebased semiconductors, the resulting particle sizes are very large (up to 300 nm),[9] owing to their low glass transition temperature. Through changing the ratio of the Se precursor and hydrazine, the size of the Se colloids changed from 90 nm to 420 nm, which remained limited in the sub-micron range. Several approaches have also been developed for the synthesis of nanometer scale Se particles, but the Se particle diameters were larger than 100 nm with wide size distributions.[17–19] The synthesis of Se nanostructures using biological reagents was also reported to provide bio-conjugated materials.[20, 21] In the present study, small Se nanoparticles with a size range of 20 nm are demonstrated as a platform for various metal and semiconductor nanostructures (Scheme 1). Low temperature conditions with the addition of an active reductant generated unstable Se nanoparticles with diameters of 20 nm. The reaction of Se and Ag formed stable Ag2Se nanoparticles, which could be converted into CdSe, ZnSe, and PbSe nanoparticles by cationic exchange. The addition of the Pt precursor coated the Se surface with thin Pt layers, and the disso-
[a] S. H. Park, Y. H. Lee, Prof. J. T. Park Department of Chemistry Korea Advanced Institute of Science and Technology Daejeon, 305-701 (Korea) E-mail: [email protected] [b] J. Y. Choi, Prof. H. Song Department of Chemistry Korea Advanced Institute of Science and Technology Center for Nanomaterials and Chemical Reactions Institute for Basic Science Daejeon, 305-701 (Korea) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201500191. Chem. Asian J. 2015, 10, 1452 – 1456
Scheme 1. Distinct reactivity between Se nanoparticles and noble metals to form metal–selenium hybrid structures.
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Communication lution of the Se cores yielded uniform Pt hollow spheres. In contrast, through the reaction with the Au precursor, small Au nuclei were attached to the Se surface, and the Au atoms migrated to form particles with acorn-like structures through a high temperature treatment. This distinct reactivity of Se nanoparticles to noble metals can provide a general platform to synthesize complex structures in multiple components. For the synthesis of large amorphous Se colloids, selenious acid and hydrazine were treated with ethylene glycol at temperatures of 15–20 8C for 1 h.[8, 9] To reduce the particle size, low temperature conditions were investigated, but the reduction rates were significantly slower. Instead, using selenium chloride (SeCl4) as a Se precursor and sodium borohydride (NaBH4) as a strong reductant, the reduction occurred at low temperatures of ¢10 8C. The addition of poly(vinyl pyrrolidone) (PVP) stabilized the structure after the formation of the Se nanoparticles. Under these conditions, small amorphous Se nanoparticles were formed with an average diameter of 25 nm, and these were relatively stable at ¢10 8C. However, these particles agglomerated immediately and formed large spheres at room temperature, and Se nanowires were grown after 1 h. Eventually, the spheres were converted into Se nanowires (Figure S1, in the Supporting Information).[9, 14] To make a stable structure based on the Se nanoparticles, Ag ions were added at an elevated temperature. It is known that Ag ions are reduced under the polyol process conditions, where ethylene glycol behaves as both a solvent and a reductant. Then, Ag directly reacts with Se to form a stable Ag2Se structure.[11] Ag2Se is a narrow-band gap semiconductor with high electronic and ionic mobilities,[22] and it is particularly useful for thermoelectric materials owing to its high electrical conductivity with low thermal conductivity.[23] The transmission electron microscopy (TEM) image in Figure 1 A demonstrates that the particles are polygonal with an average diameter of 20 4 nm. The high resolution TEM (HRTEM) image (inset and Figure S3 A, in the Supporting Information) has continuous lattice fringe images over the entire region of an individual particle; this indicates that the particles are single crystalline. The average distance between neighboring fringe images is 0.27 nm, corresponding to the distance between the {112} planes of orthorhombic Ag2Se. The energy dispersive X-ray absorption (EDX) data demonstrated that the atomic percentage
of the nanoparticles was 70.5 % for Ag and 29.5 % for Se, which is close to the 2:1 ratio of Ag and Se in Ag2Se. The five distinguishable peaks in the X-ray diffraction (XRD) data (Figure 1 B) are assignable to the reflections of the orthorhombic Ag2Se (JCPDS No. 24-1041).[24] The crystalline domain size is 20 nm, which was calculated from the peak width using the Debye–Scherrer equation. This matches the diameters observed in the TEM image, which also confirms the single-crystalline nature of the Ag2Se nanoparticles. Cationic exchange reactions were applied to the Ag2Se nanoparticles.[12, 13] The benefit of these reactions is that the morphology of the starting materials remains unchanged during the process.[25, 26] For the conversion into CdSe, the replacement of Ag + by Cd2 + is known to be non-spontaneous, due to the large solubility difference between Ag2Se (2.0 Õ 10¢22 mol L¢1) and CdSe (2.5 Õ 10¢18 mol L¢1).[27] Alivisatos et al. added tributylphosphine (TBP) to increase the solubility of Ag ions in the Ag2Se, which facilitated the exchange reaction with Cd2 + . As a solvent, methanol was crucial in stabilizing the intermediate complexes during the reaction.[25] The exchange reaction with Cd2 + at 50 8C did not change the morphology of the original particles. Figure 2 A presents the spherical particles with an average diameter of 20 4 nm. However, in the HRTEM image, the distance between the adjacent lattice fringe images changed to 0.35 nm (Figure 2 A inset and see Fig-
Figure 1. (A) TEM and (inset) HRTEM images, and (B) XRD spectrum of the Ag2Se nanoparticles prepared through the reaction of the Ag precursor and Se nanoparticles.
Figure 2. (A,C,E) TEM and (insets) HRTEM images, and (B,D,F) XRD spectra of (A,B) CdSe, (C,D) ZnSe, and (E,F) PbSe nanoparticles prepared by cationic exchange reactions from Ag2Se nanoparticles.
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Communication ure S3B, in the Supporting Information), corresponding to the distance of either the {002} planes of the wurtzite or the {111} planes of hexagonal CdSe. The EDX analysis data confirmed that the Cd and Se elements have an atomic ratio of 54:46. The XRD data had completely changed from the original pattern, and the peaks were assigned to the diffractions of the mixture of hexagonal wurzite (JCPDS No. 08-0459) and zinc blende CdSe (JCPDS No. 19-0191; Figure 2 B).[28, 29] These analyses indicate that the original Ag2Se was completely converted into the CdSe nanoparticles without any change in morphology. Similar exchange reactions were conducted with Zn2 + and Pb2 + ions. Owing to their high solubilities (ZnSe, 1.9 Õ 10¢13 mol L¢1; PbSe, 3.2 Õ 10¢20 mol L¢1)[27, 30] compared with that of Ag2Se, the addition of TBP to the reaction mixture was required. The reaction temperature (60 8C) was higher than that of Ag + because the lattice structures of ZnSe and PdSe differed to that of Ag2Se. For Cd2 + , the anion sublattice in CdSe had a topotactic relationship with that of Ag2Se; this led to the reaction at a lower temperature (50 8C).[13] Figure 2 C demonstrates that the resulting ZnSe particles have a polygonal shape with an average diameter of 20 4 nm. The HRTEM image presents a single-crystalline feature with a distance between the neighboring fringe images of 0.33 nm, which is in good agreement with the distance of the {002} planes of the hexagonal ZnSe (Figure 2 C inset, and see Figure S3C, in the Supporting Information). The XRD pattern corresponds to the mixture of hexagonal wurzite ZnSe (JCPDS No. 80-0008; Figure 2 D). The atomic percentages of Zn and Se were measured to be 49 % and 51 %, respectively, using the EDX analysis, which ensured the 1:1 stoichiometric ratio of ZnSe. The PbSe nanoparticles were also prepared through a similar pathway with an average diameter of 20 1 nm, and the XRD pattern was assigned to be cubic PbSe (Figure 2 E, F, and Figure S3 D, in the Supporting Information). It should be noted that all particles were single crystalline with similar spherical morphologies with diameters of 20 nm in all distinct compositions, including Ag2Se, CdSe, ZnSe, and PdSe. For semiconducting materials, numerous studies have been undertaken for the synthesis of small nanoparticles below 10 nm or large colloidal structures over 100 nm in size.[8–13, 17–19] However, the synthesis of small nanoparticles with diameters of few tens of nanometers has been rarely reported, although this scale is significant for the formation of hybrid materials with complex structures. In this range, the physical properties are similar to those in the bulk form, but the chemical properties differ due to the relatively high surface areas and small diffusion lengths from the surface. For instance, when large Se spheres with diameters of 300 nm were used, Ag reacted with Se at the surface only, and it formed a Se@Ag2Se core-shell structure, in which the average thickness of the Ag2Se shells was 25 nm.[11] Subsequent cationic exchange reactions yielded Se@MSe core-shell colloids (M = Cd, Zn, Pb).[12, 13] In contrast, the original diameter of the Se nanoparticles was 20 nm in the present experiment; thus, the entire Se structure completely reacted with metal ions and formed single-crystalline nanoparticles. Chem. Asian J. 2015, 10, 1452 – 1456
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Another method to stabilize the Se nanoparticles is surface coating with Pt layers.[31] Additional NaBH4 and Pt precursor were sequentially added to the Se dispersion, and the reaction was allowed to proceed for 5 h at 60 8C. The dispersion color changed from orange to dark brown, thus suggesting that the Pt atoms were directly deposited on the Se surface. Figure 3 A
Figure 3. TEM and (insets) HRTEM images of (A) Se@Pt nanoparticles and (B) Pt hollow nanoparticles after dissolving the Se cores. The bars in the insets represent 20 nm.
illustrates that the particles are uniform in size with an average diameter of 23 5 nm. A dark thin layer covers the periphery of an individual nanoparticle in the HRTEM image (Figure 3 A inset). In the EDX spectrum, Pt peaks are distinguishably detected as well as a strong Se peak. To additionally confirm the Pt layer formation on the Se surface, the particles were treated with hydrazine to dissolve the Se cores. After dissolution, uniform Pt hollow nanoparticles were afforded in a high yield (Figure 3 B). The HRTEM image demonstrated that the shell thickness of the Pt layer was 2 nm (Figure 3 B inset), and the layer was sufficiently continuous on the surface to maintain the spherical morphology. The EDX data demonstrates that only a Pt peak remains in the absence of Se signals, which indicates the complete removal of Se. With the addition of the Au precursor and NaBH4 to the Se nanoparticle dispersion at 60 8C, stable particles were generated, as in the case of Pt (Figure 4 A).[7] However, instead of the formation of even layers, some discontinuous fringe images were observed on the surface of the Se nanoparticles in the HRTEM image (Figure 4 A inset). The particles shrunk in size
Figure 4. TEM and (insets) HRTEM images of (A) Au-on-Se nanoparticles and (B) acorn-like Au¢Se nanoparticles by thermal treatment. The bars in the insets represent 10 nm.
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Communication due to the prolonged irradiation of the electron beam, which also revealed that Au could not effectively cover the nanoparticle surface. The EDX analysis estimated that the atomic ratios of Au and Se were 12.7 % and 87.3 %, respectively. To ensure the existence of the Au clusters, the particles lying on the TEM grid were annealed at 140 8C for 3 h. Then, the anisotropic acorn-shaped Au–Se nanoparticles were obtained in a high yield (Figure 4 B). The HRTEM image of an individual nanoparticle demonstrated that the distance of the neighboring lattice fringe image in the dark domain corresponds to the distance of the Au {111} planes and that, in the bright domain, it matches the distance of the Se {310} planes (Figure 4 B inset). The EDX data demonstrated that the atomic ratio of Se decreased to 73.1 %, presumably due to the sublimation of the Se atoms during the heat treatment. It is assumed that the Au clusters attached to the Se surface migrated and collapsed to form nanoparticles by the Ostwald ripening process under high temperature conditions. In summary, small Se nanoparticles with the size range of 20 nm were used as a platform for the preparation of metal selenides and metal–selenium nanoparticles. Noble metal precursors such as Ag, Pt, and Au were added to the dispersion at 60 8C for the stabilization of the Se spheres. Ag reacted easily with Se and this yielded single-crystalline Ag2Se nanoparticles, which were converted to CdSe, ZnSe, and PbSe nanoparticles through cationic exchange reactions with the corresponding metal ions in the presence of TBP. Pt was evenly deposited on the Se nanoparticle surface. Au formed Au clusters on the Se surface at low temperatures and converted into the acornshaped Au–Se nanostructures in the high temperature treatment. In this present study, various metal selenides and metal– selenium nanoparticles were successfully yielded in the size range of 20 nm using the distinct reactivity of noble metals with Se, which would be useful for potential applications. As an example, the Pt hollow nanoparticles in Figure 3 B were tested as a catalyst for methanol oxidation (Figure S5, in the Supporting Information). The oxidation current of the hollow nanoparticles exhibited an 80 % increase compared to that of the Se@Pt nanoparticles, due to the intrinsic high surface area of the hollow structure. The use of the Se nanoparticles as a versatile platform would also be particularly beneficial for metal–semiconductor hybrid materials,[32] similarly as in the case of metal–metal sulfide heterostructures,[33–35] which have significant potential in electronic, photochemical, and antibacterial applications.
Experimental Section Synthesis of Se nanoparticles and the reactions with Ag, Pt, and Au precursors SeCl4 (57 mg, 0.26 mmol) and poly(vinyl pyrrolidone) (PVP, Mw = 55 000, 1.4 g, 13 mmol based on the repeating unit) were dissolved in ethylene glycol (EG, 40 mL). This solution was added to a sodium borohydride (NaBH4, 98 mg, 26 mmol) solution in EG (40 mL) at ¢10 8C. The reaction was allowed to stir at ¢10 8C for 3 h. For the synthesis of Ag2Se nanoparticles, an additional PVP (1.2 g, 11 mmol) solution in EG (40 mL) was added. Then, the reacChem. Asian J. 2015, 10, 1452 – 1456
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tion vessel was quickly transferred into an oil bath set at 60 8C. A AgNO3 (23 mg, 0.14 mmol) solution in EG (1.5 mL) was added dropwise over 20 min. During the addition, the solution color turned from light red to black. The reaction mixture was allowed to proceed for 5 h at 60 8C. The final product was collected by centrifugation and washed three times with water and ethanol to remove excess PVP. For the synthesis of Se@Pt nanoparticles, a H2PtCl6 (57 mg, 0.14 mmol) solution in EG (1.5 mL) was added dropwise over 20 min, otherwise the reaction procedure was identical to the synthesis of Ag2Se nanoparticles. For the reaction with the Au precursor, a HAuCl4 (48 mg, 0.14 mmol) solution in EG (1.5 mL) was added dropwise over 20 min, otherwise the reaction procedure was identical to the synthesis of Ag2Se nanoparticles. After the drop-casting of the Au-on-Se nanoparticle sample on a TEM grid, the particles were heated at 140 8C in a vacuum oven for 3 h.
Cationic exchange reactions for MSe nanoparticles (M = Cd, Zn, and Pb) After the centrifugation, the Ag2Se nanoparticles were dispersed in methanol (10 mL). For the synthesis of CdSe nanoparticles, Cd(NO3)2·4 H2O (50 mg, 0.16 mmol) was dissolved in a mixture of water (5.0 mL) and methanol (5.0 mL). PVP (0.60 g, 5.4 mmol) was added to the Ag2Se dispersion and the reaction mixture was heated to 50 8C. The Cd precursor solution (5.0 mL) was added, and tributhylphosphine (TBP, 50 mL) was injected dropwise to the mixture. The color of the reaction mixture changed into dark brown within 20 min. After 20 min, another portion of the Cd precursor solution (5.0 mL) was added, and the reaction mixture was allowed to proceed for 2 h at 50 8C. The final product was collected by centrifugation and washed three times with methanol. For the synthesis of ZnSe nanoparticles, Zn(NO3)·6 H2O (120 mg, 0.51 mmol) was dissolved in a mixture of water (5.0 mL) and methanol (5.0 mL), otherwise the reaction procedure was identical. During the reaction with the Ag2Se dispersion and the Zn precursor solution in the presence of TBP, the color of the reaction mixture changed into dark yellow within 20 min. For the synthesis of PbSe nanoparticles, Pb(NO3) (140 mg, 0.51 mmol) was dissolved in a mixture of water (5.0 mL) and methanol (5.0 mL), otherwise the reaction procedure was identical. During the reaction with the Ag2Se dispersion and the Pb precursor solution in the presence of TBP, the color of the reaction mixture changed into dark gray instantaneously.
Characterization Transmission electron microscopy (TEM) images were obtained on a Carl Zeiss EM912 FE-TEM operated at 120 kV at Korea Basic Science Institute (KBSI). High-resolution TEM (HRTEM) images were obtained using a Philips F20 Tecnai FE-TEM operated at 200 kV at KAIST. The samples were prepared by placing a few drops of the colloidal solutions on copper grids coated with a formvar/carbon film (Ted Pella, Inc., 300 mesh, 63 mm grid hole size). X-ray diffraction (XRD) patterns were measured on a Rigaku D/Max-RC (12 kW) diffractometer using a CuKa radiation source.
Acknowledgements This work was supported by IBS-R004-D1, and the National Research Foundation of Korea (NRF) funded by the Korea Government (MSIP) (2012-005624, R11-2007-050-00000-0).
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