FULL PAPER DOI: 10.1002/asia.201402729
Converting Ag2S CdS and Ag2S ZnS into Ag CdS and Ag ZnS Nanoheterostructures by Selective Extraction of Sulfur Jiangcong Zhou, Feng Huang,* Ju Xu, and Yuansheng Wang*[a]
Abstract: A mild three-step solution strategy is developed to prepare Ag MS (M = Zn, Cd) nanoheterostructures composed of MS nanorods with silver tips. First, Ag2S MS heterostructures are synthesized by following a solution–liquid–solid mechanism with Ag2S nanoparticles as catalysts, then the Ag2S sections of the heterostructures are converted into silver nanoparticles
by selective extraction of sulfur. Notably, for the prepared Ag CdS heterostructures, the localized surface plasmon resonance of silver remarkably inKeywords: luminescence · nanostructures · photochemistry · semiconductors · surface plasmon resonance
tion of metal islands on semiconductor nanocrystals,[7] and metal nanoparticle seeded epitaxial growth of nano-semiconductors.[8] However, it is difficult to control the deposit locations of metal particles on a semiconductor surface through the former route, whereas accurate control of the exposed faces for metal seeds to minimize lattice mismatch between two nanophases is required for the latter route; these drawbacks limit their wide applications. Therefore, exploiting a new alternative route for preparing metal–semiconductor nanoheterostructures is still highly desired. Herein, a three-step strategy is developed to synthesize Ag MS (M = Zn, Cd) nanoheterostructures in solution. First, Ag2S MS heterostructures are synthesized through a solution–liquid–solid (SLS) mechanism[9, 10] with Ag2S as a catalyst, then selective extraction of sulfur to convert Ag2S catalysts into silver nanoparticles is carried out, yielding the desired Ag MS hybrid nanostructures. Importantly, for the synthesized Ag CdS heterostructures, in which the localized surface plasmon resonance (LSPR) absorption of silver overlaps well with the interband absorption of CdS, a remarkable enhancement in CdS photoluminescence (PL) is observed. The strategy reported herein would be useful for fabricating other metal–semiconductor hybrid nanostructures with desirable performances.
Introduction In the past decade, combining semiconductors and metals into a single nanostructure, for integrating their diverse functions and even generating novel properties, has attracted great attention, owing to their potential applications in fields such as photovoltaics, electronic devices, photocatalysis, and luminescence.[1, 2] For example, Joshi et al. succeeded in fabricating Ag CdS nanocomposites with enhanced luminescence;[3] Pradhan et al. constructed the Au Bi2S3 nanoheterostructure with advanced photocatalysis performance;[4] and Belcher et al. developed Ag@TiO2 core–shell nanoparticles for application in a high-performance photovoltage device.[5] Of the investigated metal–semiconductor hybrid nanostructures, metal-tipped semiconductor nanorods are of particular interest, in which the metal tips can provide anchor points for electrical connections, manifest light absorption, and generate charge separation at semiconductor– metal junctions.[6] To date, two synthetic routes have been used to prepare metal–semiconductor hybrid nanostructures: direct deposi-
[a] J. Zhou, Dr. F. Huang, J. Xu, Prof. Y. Wang State Key Laboratory of Structural Chemistry Fujian Institute of Research on the Structure of Matter Chinese Academy of Sciences University of Chinese Academy of Sciences Fuzhou, Fujian, 350002 (P.R. China) Fax: (+ 86) 591-83751402 . E-mail: [email protected] [email protected]
Results and Discussion The preparation of Ag CdS heterostructures is taken as an example to demonstrate the three-step solution strategy. First, monodisperse Ag2S nanospheres were synthesized by following the method reported by Xu and Zhu.[10c] As revealed by the XRD and TEM characterization results shown in Figure 1 a and b, the products are pure monoclinic
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402729.
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tensifies the photoluminescence of CdS by enhancing the excitation light absorption, which is beneficial for potential applications of CdS nanoparticles in the fields of biolabeling, light-emitting diodes, and so forth. The strategy reported herein would be useful for designing and fabricating other metal– semiconductor hybrid nanostructures with desirable performances.
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Figure 2. a) TEM micrograph of the Ag CdS heterostructures; inset: HRTEM image of an individual Ag CdS heterostructure. FFT patterns taken from Ag (b) and CdS regions (c) shown in the inset of a). d) XRD pattern of the Ag CdS nanocrystals; the bars on the bottom represent standard data of cubic Ag (JCPDS no. 65-2871) and hexagonal CdS (JCPDS no. 41-1049).
Figure 1. a) TEM micrograph of the Ag2S nanocrystals; inset: high-resolution (HR) TEM image of an individual Ag2S nanocrystal. b) XRD pattern of the Ag2S nanocrystals; the bars on the bottom represent standard data of monoclinic Ag2S (JCPDS no. 75-1061). c) TEM micrograph of the Ag2S CdS heterostructures; inset: HRTEM image of an Ag2S CdS heterostructure. Fast Fourier transform (FFT) patterns taken from Ag2S (d) and CdS (e) sections shown in the inset of c). f) XRD pattern of the Ag2S CdS heterostructures; the bars on the bottom represent standard data of hexagonal CdS (JCPDS no. 41-1049).
phase Ag2S (JCPDS no. 75-1061) with a mean grain size of 7.6 nm. Second, the Ag2S-tipped CdS nanorods were prepared by following the SLS mechanism with presynthesized Ag2S nanocrystals as catalysts.[10c] As revealed by the results in Figure 1 c–f, the prepared heterostructures comprise monoclinic Ag2S tips and hexagonally structured CdS rods. In the Ag2S CdS heterostructure, CdS nanorods grow along the [001] direction on the (121) surface of Ag2S particles to form a (002)CdS/ACHTUNGRE(121)Ag2S heterojunction. Finally, the Ag2S CdS heterostructures were reheated in a mixed solvent of tri-n-octylphosphine (TOP) and oleylamine at 160 8C for 15 min. Interestingly, the Ag2S tips convert into silver nanoparticles, whereas the CdS nanorods remain unchanged in shape, phase structure, and orientation, as revealed by XRD and TEM characterization results shown in Figure 2 a–d. Detailed HRTEM and FFT analyses demonstrate that, in each Ag CdS heterostructure, a (201)Ag/ACHTUNGRE(002)CdS heterojunction is generated between the silver tip and CdS rod, whereas the (111) lattices of silver consecutively connect with the (101) ones of CdS at the Ag CdS interface. It is supposed that, in this step, TOP acts as a selective reducing agent to extract sulfur from the Ag2S tips of Ag2S CdS heterostructures to leave Ag CdS heterostructures. In comparative experiments under the same conditions, after TOP treatment, the Ag2S nanospheres were completely reduced to silver nanocrystals, whereas the CdS nanorods, which were prepared without the assistance of Ag2S catalyst, exhibited no notable change, as demonstrated
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Figure 3. a) TEM micrograph of the Ag ZnS heterostructures; inset: HRTEM image of an individual Ag ZnS heterostructure. b) XRD pattern of the Ag ZnS heterostructures; the bars on the bottom represent standard data of hexagonal ZnS (JCPDS no. 36-1450). c) Schematic illustration of the three-step strategy to prepare Ag MS (M = Zn, Cd) heterostructures.
in Figure S1 in the Supporting Information. These results further confirm the selective reducing role of TOP. By using a similar strategy, Ag2S ZnS heterostructures were also prepared and then successfully converted into Ag ZnS heterostructures, as shown in Figure 3 a and b and Figure S2 in the Supporting Information. The whole preparation process for the Ag MS (M = Cd, Zn) heterostructures is illustrated in Figure 3 c. Notably, this strategy proceeds under mild conditions: all three steps were performed at relatively low temperatures ( 160 8C). To investigate the effect of silver formation on the luminescence of CdS, PL spectra of pure CdS nanorods, and Ag2S CdS and Ag CdS heterostructures, were measured, as presented in Figure 4 a; these results demonstrate that both
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the curve of Ih/Ip versus excitation wavelength shown in Figure 4 b, the luminescent enhancement factor varies with the excitation wavelength, and reaches a maximum of about 4 when the excitation wavelength matches the LSPR peak of the silver nanocrystals at l 420 nm. Figure 4 c and d display the luminescence decay curves of these samples. Notably, both emissions at l = 470 and 600 nm of the Ag CdS heterostructures exhibit a somewhat shorter lifetime than that of the pure CdS nanorods; this implies a slight increase in the radiative rate of CdS,[12] which indicates the existence of a weak emission enhancement effect in the Ag CdS heterostructures. As for the prepared Ag ZnS Figure 4. a) PL spectra of the pure CdS nanorods, and Ag2S CdS and Ag CdS heterostructures, excited at l = heterostructures, no lumines420 nm; inset: photographs of Ag CdS heterostructures and pure CdS nanorods under UV (l = 365 nm) irradicent enhancement was obation. b) Enhancement factor (Ih/Ip) versus the excitation wavelength accompanied by the absorption spectrum of silver nanoparticles. Luminescence decay curves of the pure CdS nanorods (c) and Ag CdS heterostrucserved; this can be explained by tures (d), monitored at l = 470 and 600 nm, respectively, are also shown. the Ag LSPR peak not matching with the interband absorption of ZnS. On the contrary, the luminescence intensity of Ag ZnS is weaker than that the interband transition emission at l 470 nm and the of pure ZnS, as presented in Figure S5 in the Supporting Indefect emission at l 600 nm of CdS in the Ag CdS heteroformation; this is attributed to the quenching effect of the structures exhibit a remarkable enhancement compared formed Ag ZnS heterojunctions.[13] with those in the pure CdS nanorods and Ag2S CdS heterostructures. It is well known that there are mainly two mechanisms that govern the luminescent enhancement effect in semiconConclusion ductor–metal heterostructures:[1a] one is the excitation enhancement related to the intensified excitations in the presA three-step solution strategy, which comprised the syntheence of the LSPR electric field, and the other is the emissis of Ag2S nanocatalysts, heterogrowth of MS (M = Zn, Cd) sion enhancement ascribed to the increased radiative rate nanorods on Ag2S surfaces through the SLS mechanism, and induced by coupling of the LSPR field with the transition conversion of Ag2S into silver through selective extraction dipole moment of the semiconductor. Herein, because the of sulfur, was developed to synthesize Ag MS nanoheterosinterband absorption of CdS overlaps well with the LSPR tructures. Importantly, the LSPR effect of silver in the Ag absorption of silver, as shown in Figure S3 in the Supporting CdS heterostructures remarkably intensified the CdS lumiInformation, the observed luminescent enhancement effect nescence by promoting excitation light harvesting. The stratis mainly attributed to the excitation enhancement mechaegy reported herein would be useful for fabricating other nism. metal–semiconductor hybrid nanostructures with desirable To reveal the relationship between the luminescent enperformances. hancement effect and excitation wavelength, PL spectra of the pure CdS nanorods and the Ag CdS heterostructures under various excitation wavelengths were measured, as preExperimental Section sented in Figure S4 in the Supporting Information. The enhancement factor of CdS luminescence in the Ag CdS hetMaterials erostructures is denoted by Ih/Ip,[11] in which Ip and Ih are the AgNO3, sulfur powder, cadmium acetate, zinc acetate, TOP, cyclohexane, integral areas of the PL spectra for pure CdS nanorods and ethanol, and oleylamine were all purchased from Aldrich and used as rethe Ag CdS heterostructures, respectively. As evidenced by ceived without further purification.
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Preparation of Organosulfur Precursor
[1] a) U. Banin, Y. Ben-Shahar, K. Vinokurov, Chem. Mater. 2014, 26, 97 – 110; b) K. Wu, H. Zhu, Z. Liu, W. Rodrguez-Cordoba, T. Lian, J. Am. Chem. Soc. 2012, 134, 10337 – 10340; c) A. H. Atwater, A. Polman, Nat. Mater. 2010, 9, 205 – 213; d) M. T. Sheldon, P. Trudeau, T. Mokari, L. W. Wang, A. P. Alivisatos, Nano Lett. 2009, 9, 3676 – 3682; e) T. Ming, L. Zhao, Z. Yang, H. Chen, L. Sun, J. Wang, C. H. Yan, Nano Lett. 2009, 9, 3896 – 3903. [2] a) K. Vinokurov, J. E. Macdonald, U. Banin, Chem. Mater. 2012, 24, 1822 – 1827; b) Z. Sun, Z. Yang, J. Zhou, M. Yeung, W. Ni, H. Wu, J. Wang, Angew. Chem. Int. Ed. 2009, 48, 2881 – 2885; Angew. Chem. 2009, 121, 2925 – 2929; c) C. OSullivan, R. D. Gunning, C. A. Barrett, A. Singh, K. M. Ryan, J. Mater. Chem. 2010, 20, 7875 – 7880; d) M. Berr, A. Vaneski, A. S. Susha, J. Rodriguez-Fernandez, M. Doeblinger, F. Jaeckel, A. L. Rogach, J. Feldmann, Appl. Phys. Lett. 2010, 97, 093108; e) H. Schlicke, D. Ghosh, L. K. Fong, H. L. Xin, H. Zheng, A. P. Alivisatos, Angew. Chem. Int. Ed. 2013, 52, 980 – 982; Angew. Chem. 2013, 125, 1014 – 1016; f) J. Lee, A. O. Govorov, J. Dulka, N. A. Kotov, Nano Lett. 2004, 4, 2323 – 2330. [3] P. Thakur, S. S. Joshi, S. Kapoor, T. Mukherjee, Langmuir 2009, 25, 6377 – 6384. [4] G. Manna, R. Bose, N. Pradhan, Angew. Chem. Int. Ed. 2014, 53, 6743 – 6746. [5] J. Qi, X. Dang, P. T. Hammond, A. M. Belcher, ACS Nano 2011, 5, 7108 – 7116. [6] a) S. Liang, X. Liu, Y. Yang, Y. Wang, J. Wang, Z. Yang, L. Wang, S. Jia, X. Yu, L. Zhou, J. Wang, J. Zeng, Q. Q. Wang, Z. Zhang, Nano Lett. 2012, 12, 5281 – 5286; b) J. Bang, S. Lee, J. Jang, W. Choi, H. Song, J. Phys. Chem. Lett. 2012, 3, 3781 – 3785; c) E. Shaviv, O. Schubert, M. A. Santos, G. Goldoni, R. D. Felice, F. Valle, N. D. Fatti, U. Banin, C. Sçnnichsen, ACS Nano 2011, 5, 4712 – 4719. [7] a) T. Mokari, C. G. Sztrum, A. Salant, E. Rabani, U. Banin, Nat. Mater. 2005, 4, 855 – 863; b) R. Costi, A. E. Saunders, U. Banin, Angew. Chem. Int. Ed. 2010, 49, 4878 – 4897; Angew. Chem. 2010, 122, 4996 – 5016; c) S. Chakrabortty, J. A. Yang, Y. M. Tan, N. Mishra, Y. Chan, Angew. Chem. Int. Ed. 2010, 49, 2888 – 2892; Angew. Chem. 2010, 122, 2950 – 2954; d) M. G. Alemseghed, T. P. A. Ruberu, J. Vela, Chem. Mater. 2011, 23, 3571 – 3579. [8] a) H. Shen, H. Shang, J. Niu, W. Xu, H. Wang, L. S. Li, Nanoscale 2012, 4, 6509 – 6514; b) X. Ye, D. R. Hickey, J. Fei, B. T. Diroll, T. Paik, J. Chen, C. B. Murray, J. Am. Chem. Soc. 2014, 136, 5106 – 5115. [9] H. Zhang, S. Delikanli, Y. Qin, S. He, M. Swihart, H. Zeng, Nano Res. 2008, 1, 314 – 320. [10] a) W. Han, L. Yi, N. Zhao, A. Tang, M. Y. Gao, Z. Y. Tang, J. Am. Chem. Soc. 2008, 130, 13152 – 13161; b) L. X. Yi, A. W. Tang, M. Niu, W. Han, Y. B. Hou, M. Y. Gao, CrystEngComm 2010, 12, 4124 – 4130; c) G. X. Zhu, Z. Xu, J. Am. Chem. Soc. 2011, 133, 148 – 157; d) J. Zhou, F. Huang, J. Xu, Y. Wang, Nanoscale 2013, 5, 9714 – 9719. [11] a) M. Pfeiffer, K. Lindfors, C. Wolpert, P. Atkinson, M. Benyoucef, A. Rastelli, O. G. Schmidt, H. Giessen, M. Lippitz, Nano Lett. 2010, 10, 4555 – 4558; b) S. Lin, M. M. Wong, P. K. Pat, C. Y. Wong, S. K. Chiu, E. Y. Pun, J. Phys. Chem. C 2011, 115, 21604 – 21609; c) S. Jin, E. D. Marco, M. J. Pellin, O. K. Farha, G. P. Wiederrecht, J. T. Hupp, J. Phys. Chem. Lett. 2013, 4, 3527 – 3533. [12] T. Shanmugapriya, P. Ramamurthy, J. Phys. Chem. C 2013, 117, 12272 – 12278. [13] A. Demortire, R. D. Schaller, T. Li, S. Chattopadhyay, G. Krylova, T. Shibata, P. C. Claro, C. E. Rowland, J. T. Miller, R. Cook, B. Lee, E. V. Shevchenko, J. Am. Chem. Soc. 2014, 136, 2342 – 2350.
The sulfur–oleylamine (S-OAm) precursor was prepared by dispersing sulfur powder (1 mmol) into oleylamine (10 mL) with the aid of ultrasound. Synthesis of Ag2S Nanocrystals Synthesis of spherical silver sulfide nanocrystals was accomplished through the reaction of AgNO3 and S-OAm precursor in oleylamine. Typically, oleylamine (8 mL) was heated to 120 8C in a flask with bubbling N2 and kept at this temperature for 20 min with stirring to remove water and other volatile impurities in oleylamine. Then AgNO3 (34 mg) and S-OAm precursor (2 mL) were added in turn. After the addition of S-OAm precursor, the color of the reaction mixture changed from straw yellow to jet black, which indicated the formation of Ag2S nanocrystals. After keeping the reaction mixture at 80 8C for 15 min, the Ag2S products were precipitated with ethanol and collected by centrifugation. Synthesis of Ag2S MS (M = Zn, Cd) Heterostructures In a typical synthesis of Ag2S CdS heterostructures, Ag2S nanocrystals (50 mg) were dissolved in oleylamine (8 mL) and heated in N2 at ambient temperature. When the temperature was increased to 180 8C, cadmium acetate (133 mg) and S-OAm precursor (5 mL) were added in turn and reacted for 20 min at 160 8C. The products were collected by centrifugation and dispersed in cyclohexane for further characterization. The synthesis of Ag2S ZnS heterostructures was similar to the procedure used for the Ag2S CdS heterostructures. Synthesis of Ag MS (M = Zn, Cd) Heterostructures In a typical synthesis of Ag CdS heterostructures, presynthesized Ag2S CdS heterostructures were dissolved in a mixed solvent containing oleylamine (8 mL) and TOP (2 mL). Then the temperature was increased to 160 8C in a flask with bubbling N2 and kept at this temperature for 20 min. The products were collected by centrifugation and dispersed in cyclohexane for further characterization. The synthesis of Ag ZnS heterostructures was similar to the procedure used for Ag CdS heterostructures. Characterizations XRD analyses were carried out with a powder diffractometer (DMAX 2500 RIGAKU) by using CuKa radiation (l = 0.154 nm). TEM observations were performed by means of a transmission electron microscope (JEM-2010) equipped with an energy-dispersive X-ray spectroscope (EDS). TEM specimens were prepared by directly drying a drop of the dilute dispersion of the products in cyclohexane on the surface of a carbon-coated copper grid. Absorption spectra were measured by means of a UV/Vis/IR spectrometer (Lambda 900). PL spectra were measured by using an Edinburgh Instruments FLS920 spectrofluorometer equipped with a xenon lamp. PL decay curves were measured under the excitation of a l = 370 nm nanosecond laser. All measurements were performed at room temperature.
Acknowledgements This work was supported by the National Natural Science Foundation of China(11204301, 21271170, 51202244, 51172231and 11304312), Natural Science Foundation of Fujian for Distinguished Young Scholars(2012J06014), Natural Science Foundation of Fujian(2014J05071), and Key Laboratory of Design and Assembly of Functional Nanostructures, Chinese Academy of Science(2013DP173231).
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Received: June 25, 2014 Published online: && &&, 0000
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FULL PAPER Semiconductors Jiangcong Zhou, Feng Huang,* Ju Xu, Yuansheng Wang* &&&&—&&&& Converting Ag2S CdS and Ag2S ZnS into Ag CdS and Ag ZnS Nanoheterostructures by Selective Extraction of Sulfur Pick and choose: A three-step strategy for synthesizing Ag MS (M = Zn, Cd) nanoheterostructures by following a solution–liquid–solid (SLS; see figure) mechanism with Ag2S nanoparticles as
catalysts, followed by conversion of Ag2S sections of the heterostructures into silver nanoparticles by selective extraction of sulfur, is reported.
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Converting Ag2S CdS and Ag2S ZnS into Ag CdS and Ag ZnS Nanoheterostructures by Selective Extraction of Sulfur Jiangcong Zhou, Feng Huang,* Ju Xu, and Yuansheng Wang*[a]
Abstract: A mild three-step solution strategy is developed to prepare Ag MS (M = Zn, Cd) nanoheterostructures composed of MS nanorods with silver tips. First, Ag2S MS heterostructures are synthesized by following a solution–liquid–solid mechanism with Ag2S nanoparticles as catalysts, then the Ag2S sections of the heterostructures are converted into silver nanoparticles
by selective extraction of sulfur. Notably, for the prepared Ag CdS heterostructures, the localized surface plasmon resonance of silver remarkably inKeywords: luminescence · nanostructures · photochemistry · semiconductors · surface plasmon resonance
tion of metal islands on semiconductor nanocrystals,[7] and metal nanoparticle seeded epitaxial growth of nano-semiconductors.[8] However, it is difficult to control the deposit locations of metal particles on a semiconductor surface through the former route, whereas accurate control of the exposed faces for metal seeds to minimize lattice mismatch between two nanophases is required for the latter route; these drawbacks limit their wide applications. Therefore, exploiting a new alternative route for preparing metal–semiconductor nanoheterostructures is still highly desired. Herein, a three-step strategy is developed to synthesize Ag MS (M = Zn, Cd) nanoheterostructures in solution. First, Ag2S MS heterostructures are synthesized through a solution–liquid–solid (SLS) mechanism[9, 10] with Ag2S as a catalyst, then selective extraction of sulfur to convert Ag2S catalysts into silver nanoparticles is carried out, yielding the desired Ag MS hybrid nanostructures. Importantly, for the synthesized Ag CdS heterostructures, in which the localized surface plasmon resonance (LSPR) absorption of silver overlaps well with the interband absorption of CdS, a remarkable enhancement in CdS photoluminescence (PL) is observed. The strategy reported herein would be useful for fabricating other metal–semiconductor hybrid nanostructures with desirable performances.
Introduction In the past decade, combining semiconductors and metals into a single nanostructure, for integrating their diverse functions and even generating novel properties, has attracted great attention, owing to their potential applications in fields such as photovoltaics, electronic devices, photocatalysis, and luminescence.[1, 2] For example, Joshi et al. succeeded in fabricating Ag CdS nanocomposites with enhanced luminescence;[3] Pradhan et al. constructed the Au Bi2S3 nanoheterostructure with advanced photocatalysis performance;[4] and Belcher et al. developed Ag@TiO2 core–shell nanoparticles for application in a high-performance photovoltage device.[5] Of the investigated metal–semiconductor hybrid nanostructures, metal-tipped semiconductor nanorods are of particular interest, in which the metal tips can provide anchor points for electrical connections, manifest light absorption, and generate charge separation at semiconductor– metal junctions.[6] To date, two synthetic routes have been used to prepare metal–semiconductor hybrid nanostructures: direct deposi-
[a] J. Zhou, Dr. F. Huang, J. Xu, Prof. Y. Wang State Key Laboratory of Structural Chemistry Fujian Institute of Research on the Structure of Matter Chinese Academy of Sciences University of Chinese Academy of Sciences Fuzhou, Fujian, 350002 (P.R. China) Fax: (+ 86) 591-83751402 . E-mail: [email protected] [email protected]
Results and Discussion The preparation of Ag CdS heterostructures is taken as an example to demonstrate the three-step solution strategy. First, monodisperse Ag2S nanospheres were synthesized by following the method reported by Xu and Zhu.[10c] As revealed by the XRD and TEM characterization results shown in Figure 1 a and b, the products are pure monoclinic
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402729.
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tensifies the photoluminescence of CdS by enhancing the excitation light absorption, which is beneficial for potential applications of CdS nanoparticles in the fields of biolabeling, light-emitting diodes, and so forth. The strategy reported herein would be useful for designing and fabricating other metal– semiconductor hybrid nanostructures with desirable performances.
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Figure 2. a) TEM micrograph of the Ag CdS heterostructures; inset: HRTEM image of an individual Ag CdS heterostructure. FFT patterns taken from Ag (b) and CdS regions (c) shown in the inset of a). d) XRD pattern of the Ag CdS nanocrystals; the bars on the bottom represent standard data of cubic Ag (JCPDS no. 65-2871) and hexagonal CdS (JCPDS no. 41-1049).
Figure 1. a) TEM micrograph of the Ag2S nanocrystals; inset: high-resolution (HR) TEM image of an individual Ag2S nanocrystal. b) XRD pattern of the Ag2S nanocrystals; the bars on the bottom represent standard data of monoclinic Ag2S (JCPDS no. 75-1061). c) TEM micrograph of the Ag2S CdS heterostructures; inset: HRTEM image of an Ag2S CdS heterostructure. Fast Fourier transform (FFT) patterns taken from Ag2S (d) and CdS (e) sections shown in the inset of c). f) XRD pattern of the Ag2S CdS heterostructures; the bars on the bottom represent standard data of hexagonal CdS (JCPDS no. 41-1049).
phase Ag2S (JCPDS no. 75-1061) with a mean grain size of 7.6 nm. Second, the Ag2S-tipped CdS nanorods were prepared by following the SLS mechanism with presynthesized Ag2S nanocrystals as catalysts.[10c] As revealed by the results in Figure 1 c–f, the prepared heterostructures comprise monoclinic Ag2S tips and hexagonally structured CdS rods. In the Ag2S CdS heterostructure, CdS nanorods grow along the [001] direction on the (121) surface of Ag2S particles to form a (002)CdS/ACHTUNGRE(121)Ag2S heterojunction. Finally, the Ag2S CdS heterostructures were reheated in a mixed solvent of tri-n-octylphosphine (TOP) and oleylamine at 160 8C for 15 min. Interestingly, the Ag2S tips convert into silver nanoparticles, whereas the CdS nanorods remain unchanged in shape, phase structure, and orientation, as revealed by XRD and TEM characterization results shown in Figure 2 a–d. Detailed HRTEM and FFT analyses demonstrate that, in each Ag CdS heterostructure, a (201)Ag/ACHTUNGRE(002)CdS heterojunction is generated between the silver tip and CdS rod, whereas the (111) lattices of silver consecutively connect with the (101) ones of CdS at the Ag CdS interface. It is supposed that, in this step, TOP acts as a selective reducing agent to extract sulfur from the Ag2S tips of Ag2S CdS heterostructures to leave Ag CdS heterostructures. In comparative experiments under the same conditions, after TOP treatment, the Ag2S nanospheres were completely reduced to silver nanocrystals, whereas the CdS nanorods, which were prepared without the assistance of Ag2S catalyst, exhibited no notable change, as demonstrated
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Figure 3. a) TEM micrograph of the Ag ZnS heterostructures; inset: HRTEM image of an individual Ag ZnS heterostructure. b) XRD pattern of the Ag ZnS heterostructures; the bars on the bottom represent standard data of hexagonal ZnS (JCPDS no. 36-1450). c) Schematic illustration of the three-step strategy to prepare Ag MS (M = Zn, Cd) heterostructures.
in Figure S1 in the Supporting Information. These results further confirm the selective reducing role of TOP. By using a similar strategy, Ag2S ZnS heterostructures were also prepared and then successfully converted into Ag ZnS heterostructures, as shown in Figure 3 a and b and Figure S2 in the Supporting Information. The whole preparation process for the Ag MS (M = Cd, Zn) heterostructures is illustrated in Figure 3 c. Notably, this strategy proceeds under mild conditions: all three steps were performed at relatively low temperatures ( 160 8C). To investigate the effect of silver formation on the luminescence of CdS, PL spectra of pure CdS nanorods, and Ag2S CdS and Ag CdS heterostructures, were measured, as presented in Figure 4 a; these results demonstrate that both
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the curve of Ih/Ip versus excitation wavelength shown in Figure 4 b, the luminescent enhancement factor varies with the excitation wavelength, and reaches a maximum of about 4 when the excitation wavelength matches the LSPR peak of the silver nanocrystals at l 420 nm. Figure 4 c and d display the luminescence decay curves of these samples. Notably, both emissions at l = 470 and 600 nm of the Ag CdS heterostructures exhibit a somewhat shorter lifetime than that of the pure CdS nanorods; this implies a slight increase in the radiative rate of CdS,[12] which indicates the existence of a weak emission enhancement effect in the Ag CdS heterostructures. As for the prepared Ag ZnS Figure 4. a) PL spectra of the pure CdS nanorods, and Ag2S CdS and Ag CdS heterostructures, excited at l = heterostructures, no lumines420 nm; inset: photographs of Ag CdS heterostructures and pure CdS nanorods under UV (l = 365 nm) irradicent enhancement was obation. b) Enhancement factor (Ih/Ip) versus the excitation wavelength accompanied by the absorption spectrum of silver nanoparticles. Luminescence decay curves of the pure CdS nanorods (c) and Ag CdS heterostrucserved; this can be explained by tures (d), monitored at l = 470 and 600 nm, respectively, are also shown. the Ag LSPR peak not matching with the interband absorption of ZnS. On the contrary, the luminescence intensity of Ag ZnS is weaker than that the interband transition emission at l 470 nm and the of pure ZnS, as presented in Figure S5 in the Supporting Indefect emission at l 600 nm of CdS in the Ag CdS heteroformation; this is attributed to the quenching effect of the structures exhibit a remarkable enhancement compared formed Ag ZnS heterojunctions.[13] with those in the pure CdS nanorods and Ag2S CdS heterostructures. It is well known that there are mainly two mechanisms that govern the luminescent enhancement effect in semiconConclusion ductor–metal heterostructures:[1a] one is the excitation enhancement related to the intensified excitations in the presA three-step solution strategy, which comprised the syntheence of the LSPR electric field, and the other is the emissis of Ag2S nanocatalysts, heterogrowth of MS (M = Zn, Cd) sion enhancement ascribed to the increased radiative rate nanorods on Ag2S surfaces through the SLS mechanism, and induced by coupling of the LSPR field with the transition conversion of Ag2S into silver through selective extraction dipole moment of the semiconductor. Herein, because the of sulfur, was developed to synthesize Ag MS nanoheterosinterband absorption of CdS overlaps well with the LSPR tructures. Importantly, the LSPR effect of silver in the Ag absorption of silver, as shown in Figure S3 in the Supporting CdS heterostructures remarkably intensified the CdS lumiInformation, the observed luminescent enhancement effect nescence by promoting excitation light harvesting. The stratis mainly attributed to the excitation enhancement mechaegy reported herein would be useful for fabricating other nism. metal–semiconductor hybrid nanostructures with desirable To reveal the relationship between the luminescent enperformances. hancement effect and excitation wavelength, PL spectra of the pure CdS nanorods and the Ag CdS heterostructures under various excitation wavelengths were measured, as preExperimental Section sented in Figure S4 in the Supporting Information. The enhancement factor of CdS luminescence in the Ag CdS hetMaterials erostructures is denoted by Ih/Ip,[11] in which Ip and Ih are the AgNO3, sulfur powder, cadmium acetate, zinc acetate, TOP, cyclohexane, integral areas of the PL spectra for pure CdS nanorods and ethanol, and oleylamine were all purchased from Aldrich and used as rethe Ag CdS heterostructures, respectively. As evidenced by ceived without further purification.
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Preparation of Organosulfur Precursor
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The sulfur–oleylamine (S-OAm) precursor was prepared by dispersing sulfur powder (1 mmol) into oleylamine (10 mL) with the aid of ultrasound. Synthesis of Ag2S Nanocrystals Synthesis of spherical silver sulfide nanocrystals was accomplished through the reaction of AgNO3 and S-OAm precursor in oleylamine. Typically, oleylamine (8 mL) was heated to 120 8C in a flask with bubbling N2 and kept at this temperature for 20 min with stirring to remove water and other volatile impurities in oleylamine. Then AgNO3 (34 mg) and S-OAm precursor (2 mL) were added in turn. After the addition of S-OAm precursor, the color of the reaction mixture changed from straw yellow to jet black, which indicated the formation of Ag2S nanocrystals. After keeping the reaction mixture at 80 8C for 15 min, the Ag2S products were precipitated with ethanol and collected by centrifugation. Synthesis of Ag2S MS (M = Zn, Cd) Heterostructures In a typical synthesis of Ag2S CdS heterostructures, Ag2S nanocrystals (50 mg) were dissolved in oleylamine (8 mL) and heated in N2 at ambient temperature. When the temperature was increased to 180 8C, cadmium acetate (133 mg) and S-OAm precursor (5 mL) were added in turn and reacted for 20 min at 160 8C. The products were collected by centrifugation and dispersed in cyclohexane for further characterization. The synthesis of Ag2S ZnS heterostructures was similar to the procedure used for the Ag2S CdS heterostructures. Synthesis of Ag MS (M = Zn, Cd) Heterostructures In a typical synthesis of Ag CdS heterostructures, presynthesized Ag2S CdS heterostructures were dissolved in a mixed solvent containing oleylamine (8 mL) and TOP (2 mL). Then the temperature was increased to 160 8C in a flask with bubbling N2 and kept at this temperature for 20 min. The products were collected by centrifugation and dispersed in cyclohexane for further characterization. The synthesis of Ag ZnS heterostructures was similar to the procedure used for Ag CdS heterostructures. Characterizations XRD analyses were carried out with a powder diffractometer (DMAX 2500 RIGAKU) by using CuKa radiation (l = 0.154 nm). TEM observations were performed by means of a transmission electron microscope (JEM-2010) equipped with an energy-dispersive X-ray spectroscope (EDS). TEM specimens were prepared by directly drying a drop of the dilute dispersion of the products in cyclohexane on the surface of a carbon-coated copper grid. Absorption spectra were measured by means of a UV/Vis/IR spectrometer (Lambda 900). PL spectra were measured by using an Edinburgh Instruments FLS920 spectrofluorometer equipped with a xenon lamp. PL decay curves were measured under the excitation of a l = 370 nm nanosecond laser. All measurements were performed at room temperature.
Acknowledgements This work was supported by the National Natural Science Foundation of China(11204301, 21271170, 51202244, 51172231and 11304312), Natural Science Foundation of Fujian for Distinguished Young Scholars(2012J06014), Natural Science Foundation of Fujian(2014J05071), and Key Laboratory of Design and Assembly of Functional Nanostructures, Chinese Academy of Science(2013DP173231).
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Received: June 25, 2014 Published online: && &&, 0000
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FULL PAPER Semiconductors Jiangcong Zhou, Feng Huang,* Ju Xu, Yuansheng Wang* &&&&—&&&& Converting Ag2S CdS and Ag2S ZnS into Ag CdS and Ag ZnS Nanoheterostructures by Selective Extraction of Sulfur Pick and choose: A three-step strategy for synthesizing Ag MS (M = Zn, Cd) nanoheterostructures by following a solution–liquid–solid (SLS; see figure) mechanism with Ag2S nanoparticles as
catalysts, followed by conversion of Ag2S sections of the heterostructures into silver nanoparticles by selective extraction of sulfur, is reported.
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