Stepwise synthesis of cubic Au-AgCdS core-shell nanostructures with tunable plasmon resonances and fluorescence Xiao-Li Liu, Shan Liang, Fan Nan, Yue-Yue Pan, Jun-Jun Shi, Li Zhou,* Shuang-Feng Jia, Jian-Bo Wang, Xue-Feng Yu, and Qu-Quan Wang Key Laboratory of Artificial Micro- and Nano-structures of the Ministry of Education, and School of Physics and Technology, Wuhan University, Wuhan, 430072, China * [email protected]
Abstract: Cubic Au-AgCdS core-shell nanostructures were synthesized through cation exchange method assisted by tributylphosphine (TBP) as a phase-transfer agent. Among intermediate products, Au-Ag core-shell nanocubes exhibited many high-order plasmon resonance modes related to the special cubic shape, and these plasmon bands red-shifted along with the increasing of particle size. The plasmon band of Au core first red-shifted and broadened at the step of Au-Ag2S and then blue-shifted and narrowed at the step of Au-AgCdS. Since TBP was very crucial for the efficient conversion from Ag2S to CdS, we found that both absorption and fluorescence of the final products could be controlled by TBP. ©2013 Optical Society of America OCIS codes: (160.3900) Metals; (160.6000) Semiconductor materials; (240.6680) Surface plasmons; (260.2510) Fluorescence.
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#194359 - $15.00 USD Received 22 Jul 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 9 Oct 2013 (C) 2013 OSA 21 October 2013 | Vol. 21, No. 21 | DOI:10.1364/OE.21.024793 | OPTICS EXPRESS 24793
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1. Introduction Metal-semiconductor hetero-nanostructures usually exhibit dramatically different properties than the individual ingredients [1,2], due to the intense interaction between metal and semiconductor. Surface plasmon resonances (SPR) in metal can be modified by the semiconductor ingredient in hetero-structures. Through near-field interactions, energy could flow from the excited state of semiconductor to the plasmon, leading to the loss compensation and amplification of plasmon [3–5]. On the other side, plasmon-exciton interaction can also
#194359 - $15.00 USD Received 22 Jul 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 9 Oct 2013 (C) 2013 OSA 21 October 2013 | Vol. 21, No. 21 | DOI:10.1364/OE.21.024793 | OPTICS EXPRESS 24794
bring a strong modification of the radiative and nonradiative properties for semiconductor, altering the emission behaviors of semiconductor (enhancement or quenching) [6,7]. With appropriate band structures, charge transfer process like photo-excited electrons in semiconductor transferring to metal could suppress the direct recombination of carriers and promote efficient charge separation, resulting in a significant enhancement in the photoelectric conversion efficiency in solar cell and photo-catalysis system [8,9]. Many studies have been devoted to preparing metal-semiconductor hybrids consisted of various metal nanostructures and semiconductor materials [10–15]. For instance, Wang’s group proposed a general approach to synthesize gold-metal sulfide core-shell and heterostructures [10], and Hsu’s group developed a facile approach for preparing Au-CdS core-shell nanocrystals with controllable shell thickness [11]. These methods prepared designated metal ion-molecule complex acting as precursor to bind on Au core for growth of semiconductor shell. Ouyang’s group established a nonepitaxial growth to synthesize spherical Au-CdS core-shell nanostructures with monocrystalline semiconductor shell through a cation exchange process, this method can overcome the constraint of lattice mismatches between two components and suit for growth of multiple type semiconductor shells (CdS, CdTe, ZnS, PbS, etc.) [14]. Metal-semiconductor hetero-structures have tunable optical properties since the SPR properties are strongly dependent on the composition, dimension, and morphology of metal nanoparticles (NPs) [16–18]. Compared with the spherical NPs, metallic nanorods support both longitudinal and transverse SPRs, and metallic nanocubes possess sharp corners with strong local field enhancements and multi-high-order SPRs induced by the local field coupling of pairs of parallel surface [18–20], which are very sensitive to the environment [21]. In this paper, we chose Au nanocubes as core to synthesize Au-AgCdS core-shell nanostructures through a stepwise method, including growth of cubic Ag on core, sulfuration of Ag shells, and cation exchange process (with tributylphosphine, TBP) from Ag2S to CdS. We observed five SPR bands and their unique evolution in the intermediate product of cubic Au-Ag NPs and found that both resonant absorption and fluorescence (FL) behavior of the final products were controlled by TBP. 2. Experimental section Au nanocubes were prepared using a seed-mediated growth as reported previously [19,22], and centrifuged and re-dispersed in water for future used. For growth of Au-Ag nanocubes, Au nanocubes (0.5 mL) and cetyltrimethylammoniumchloride (CTAC, 20 mM, 4.5 mL) were mixed and heated at 60 °C under magnetic stirring. After 20 mins, AgNO3 solution (2 mM, 2 mL) and 2 mL of mixture solution containing ascorbic acid (50 mM) and CTAC (40 mM) were simultaneously injected drop by drop, and the mixture was stirred for 4 hrs [23]. The Au-Ag nanocubes (1 mL) and NaHS solution (50 mM, 10 μL) were mixed and reacted for 20 mins at room temperature under magnetic stirring. The Au-Ag2S nanocubes were obtained by centrifugation (7600 rpm for 10 mins) and washed with water once. Au-AgCdS(II) core-shell NPs were synthesized as following procedure: Au-Ag2S nanocubes (1 mL) and CTAC (0.2 M, 1 mL) were mixed together and heated at 60 °C under magnetic stirring, then Cd(NO3)2 solution (50 mM, 10 μL) and TBP (10 μL) were added into the mixture. After 6 hrs, the final products were collected by centrifugation (7600 rpm for 10 mins). Au-AgCdS(I) were obtained through the same procedure but without adding TBP. The transmission electron microscope (TEM) images were performed with a JEOL 2010 HT transmission electron microscope operated at 200 kV. Energy-dispersive X-ray Spectrum (EDX) analysis was performed on an EDAX instrument incorporated in the HRTEM. The absorption spectra were measured using a TU-1810 UV-vis spectrophotometer. For fluorescence measurements, the excitation source was a mode-locked Ti:sapphire laser (Mira 900, Coherent) with a pulse width of around 3 ps and a repetition rate of 76 MHz. The FL spectra were recorded by a spectrometer (Spectrapro 2500i, Acton) with a liquid-nitrogen cooled CCD (SPEC-10: 100B, Princeton).
#194359 - $15.00 USD Received 22 Jul 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 9 Oct 2013 (C) 2013 OSA 21 October 2013 | Vol. 21, No. 21 | DOI:10.1364/OE.21.024793 | OPTICS EXPRESS 24795
3. Results and discussion The Au-AgCdS core-shell hetero-structures were synthesized by a stepwise method shown in Fig. 1(a). Firstly, Ag shells were grown on Au nanocubes to form Au-Ag core-shell nanocubes. Subsequently, the addition of S2- converted Ag to Ag2S. The final products were obtained through a cation exchange reaction between Ag+ and Cd2+ assisted by TBP or without TBP. The final shells were called AgCdS which were the complex of Ag2S and CdS. It can be seen from the TEM images that the samples at each step are all uniform and monodisperse. The average diameter of the Au nanocubes is about 43 ± 3 nm and that of the Au-Ag nanocubes is about 91 ± 11 nm, as shown in Figs. 1(b) and 1(c). The thickness of Ag shell could be controlled by adjusting the amount of AgNO3 in the reaction. Although Au and Au-Ag NPs are perfectly cubes, the shape of Au-Ag2S NPs in Fig. 1(d) becomes a little irregular after sulfidation reaction. At that process, accompanying the formation of polycrystalline Ag2S domains, the shell was enlarged in volume by incorporating S2- and expanded outward to form irregular shape [24,25]. The shape of Au-AgCdS in Figs. 1(e) and 1(f) is also irregular and that of the Au-AgCdS(II) changes more obviously. In the final cation exchange reaction, the TBP is very crucial for the efficient conversion from Ag2S to CdS, because the thermodynamic driving force for exchange between two cations is highly related to the reaction conditions [14,26]. In the Ag2S-CdS pair, valid conversion from Ag2S to CdS is favored by adding Cd2+ ions along with TBP. Here, the Ag2S is converted to CdS more effectively in the Au-AgCdS(II) than that in the Au-AgCdS(I).
Fig. 1. (a) Diagrammatic sketch for stepwise synthesis of Au-AgCdS core-shell nanocubes. (bf) TEM images of (b) Au, (c) Au-Ag, (d) Au-Ag2S, (e) Au-AgCdS(I) NPs synthesized without TBP, and (f) Au-AgCdS(II) NPs synthesized with TBP. All the bars represent 100 nm.
EDX analyses were carried out to confirm the conversion from Ag2S to CdS. Figure 2(a) shows the EDX elemental maps of Au, Ag, Cd, and S in a Au-AgCdS(II) nanocube. The data suggest that the shell is composed of Ag, Cd, and S. The distributions of Au, Ag, and Cd are also displayed by line-scan EDX spectra in Fig. 2(b). The intensity of Cd distribution in AuAgCdS(II) is stronger than that in Au-AgCdS(I), which manifests that the conversion from Ag2S to CdS is more effective with the assistance of TBP. The absorption spectral evolution reflects the distinctive SPR properties of metal NPs determined by composition, size, and shape. The spectral responses dependent on the thickness of Ag shells in Au-Ag core-shell nanocubes are shown in Fig. 3(a). The Au nanocubes with side length of 43 ± 3 nm have a plasmon band at ~545 nm due to the dipole resonance. As a thin Ag shell (~9 nm) is deposited on Au nanocubes, the dipole resonance of Au-Ag core-shell nanocubes blue-shifts to ~527 nm. Due to the dielectric function of Ag is different from Au, the blue-shift originates from the altering of effective dielectric function by
#194359 - $15.00 USD Received 22 Jul 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 9 Oct 2013 (C) 2013 OSA 21 October 2013 | Vol. 21, No. 21 | DOI:10.1364/OE.21.024793 | OPTICS EXPRESS 24796
Ag coating [27,28]. When the Ag thickness further increases, the dipole plasmon band (band B5) begins to red-shift because the size of whole particle increases. This band progressively red-shifts to ~617 nm when Au-Ag nanocubes grow to 111 ± 13 nm.
Fig. 2. (a) EDX elemental maps of Au, Ag, Cd, and S in a Au-AgCdS(II) nanocube. (b) Line profile of Au(II), Ag(II), and Cd(II) in the same Au-AgCdS(II) nanocube. All lines are normalized and the maximum intensity of Au is set to 1. Cd(I) line profile of a Au-AgCdS(I) nanocube (red) is also shown for comparison, which is normalized by it’s own Au distribution.
At the same time, multiple plasmon resonance absorption peaks gradually emerge in the range between 300 nm and 500 nm, and these new plasmon bands are very sensitive to the shape and size of Au-Ag nanocubes. The plasmon band near 340 nm (band B1) usually appeared when Ag NPs are cube shape, which may be derived from the octupole or dipole [21,29]. As the edge length of Au-Ag nanocubes increases to 75 ± 9 nm, a plasmon band attributed to the quadrupole resonance appears at ~461 nm (band B4). The band B3 near 410 nm is a mix of quadrupole and dipole plasmon resonance [29]. The band B2 arises and presents clearly as Au-Ag nanocubes grow large, which has been reported before when Ag nanocubes have a big size [30,31]. As shown in Fig. 3(b), these plasmon bands display a continuous red-shift with the increasing of size. An interesting phenomenon is that, although SPR in Au nanocubes is usually dominated by a single dipole resonance, a thin layer of Ag shell with cubic shape could cause the multiple plasmon resonances.
Fig. 3. (a) Absorption spectra of Au and Au-Ag nanocubes with different particle size. dAu and dAg represent the side length of Au nanocubes and the thickness of silver shell, respectively. (b) Multiple plasmon resonance modes vary with the size of the Au-Ag nanocubes. (c, d) Absorption spectra of Au-Ag2S NPs (0 hr), Au-AgCdS(II) and Au-AgCdS(I) NPs synthesized at different exchange reaction time T (T = 1, 2, 4 and 6 hrs).
Au-Ag core-shell nanocubes with edge length of 91 ± 11 nm were chosen to participate in the sulfidation reaction and cation exchange reaction. As shown in Fig. 3(c), the main absorption band of Au-Ag2S core-shell NPs is red-shifted to ~717 nm. Considering the dipole plasmon band at ~545 nm of Au nanocubes, the large shift results from the large medium dielectric constant of surrounding Ag2S (~2.9 at 700 nm) [32]. When the cation exchange reaction proceeds 6 hrs, the absorption peak of Au-AgCdS(II) NPs blue-shifts 82 nm from ~717 nm to ~635 nm, which is due to the relative small dielectric constant of CdS (~2.4 at 700 nm) [33] compared with Ag2S. It also can be seen that the full width half maximum (FWHM) of absorption becomes narrower from Au-Ag2S to Au-AgCdS, which is related to the plasmon damping effect interacted with semiconductor shell [34–36]. The FWHM of AuAg2S NPs is about 280 nm, while that of the Au-AgCdS(II) NPs narrows 55% to 126 nm.
#194359 - $15.00 USD Received 22 Jul 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 9 Oct 2013 (C) 2013 OSA 21 October 2013 | Vol. 21, No. 21 | DOI:10.1364/OE.21.024793 | OPTICS EXPRESS 24797
Comparing with CdS, Ag2S has larger imaginary part of the dielectric constant [32], which could cause faster decay of plasmon and broaden the plasmon band of Au core. The variation of absorption features also implies the conversion efficiency of cation exchange reaction between Ag+ and Cd2+. On the absorption spectrum of Au-Ag2S core-shell NPs, the existence of a valley at 320 nm (caused by the interband transition of Ag below 320 nm) and a weak band around 400 nm indicates the residual Ag domains in the shell. In Fig. 3(c), these features disappear in Au-AgCdS(II), which means the residual Ag has been reacted out. However, in Fig. 3(d), the retaining of these features in Au-AgCdS(I) implies the existence of residual Ag. Meanwhile, the band position blue-shifts only 17 nm and the band FWHM narrows only 25%, which indicates the extent of exchange between Ag+ and Cd2+ is small without adding TBP. The Au-AgCdS core-shell NPs show interesting FL behaviors in Fig. 4. The AuAgCdS(II) exhibit an emission peak at 700 nm, which is probably the defect emission of CdS nanocrystal. As the reaction time increases, the emission intensity increases gradually. Beside the emission peak at 700 nm, an emission band arises at 570 nm and reaches the maximum at 2 hrs. We attribute this emission to the Ag-Cd-S ternary complex located in some regions [37–39]. This emission decreases and disappears when the reaction proceeds forward, which means the emission is very sensitive to the ratio of Ag2S and CdS. By contrast, in Fig. 4(b), the Au-AgCdS(I) have no emission at 700 nm because the CdS component is small in AuAgCdS(I) shell, while the emission behavior at 570 nm is similar to that of the Au-AgCdS(II). This result accords with the conclusion deduced from the absorption evolution in Figs. 3(c) and 3(d) that the cation exchange reaction without TBP is inefficient.
Fig. 4. FL spectra of Au-Ag2S (0 hr), (a) Au-AgCdS(II) NPs and (b) Au-AgCdS(I) NPs synthesized at different reaction time T (T = 1, 2, 4 and 6 hrs) at 400 nm excitation wavelength.
4. Conclusions In summary, Au-AgCdS core-shell hetero-structures were synthesized by a stepwise method through the process as: Au nanocubes – Au-Ag nanocubes – Au-Ag2S nanocubes – AuAgCdS NPs. For Au-Ag nanocubes, the Ag shell growth on Au nanocubes causes the emergence of multiple high-order SPRs and these plasmon bands red-shift with the increasing of the particle size. The Au dipole plasmon band red-shifts and broadens at the step of AuAg2S and then blue-shifts and narrows at the step of Au-AgCdS, which is related to the variation of environment medium. TBP is very important for the conversion form Ag2S to CdS. Therefore, Au-AgCdS(II) and Au-AgCdS(I) exhibit different absorption and fluorescence properties due to the different component in their shells. The stepwise manner could be used to synthesize other metal-semiconductor hetero-structures and the Au-AgCdS metal-semicondutor core-shell NPs could find application in solar energy harvesting and photocatalysis. Acknowledgments This work was supported by the National Program on Key Science Research of China (2011CB922201), and the NSFC (61008043, 11174229 and 11204221).
#194359 - $15.00 USD Received 22 Jul 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 9 Oct 2013 (C) 2013 OSA 21 October 2013 | Vol. 21, No. 21 | DOI:10.1364/OE.21.024793 | OPTICS EXPRESS 24798
Abstract: Cubic Au-AgCdS core-shell nanostructures were synthesized through cation exchange method assisted by tributylphosphine (TBP) as a phase-transfer agent. Among intermediate products, Au-Ag core-shell nanocubes exhibited many high-order plasmon resonance modes related to the special cubic shape, and these plasmon bands red-shifted along with the increasing of particle size. The plasmon band of Au core first red-shifted and broadened at the step of Au-Ag2S and then blue-shifted and narrowed at the step of Au-AgCdS. Since TBP was very crucial for the efficient conversion from Ag2S to CdS, we found that both absorption and fluorescence of the final products could be controlled by TBP. ©2013 Optical Society of America OCIS codes: (160.3900) Metals; (160.6000) Semiconductor materials; (240.6680) Surface plasmons; (260.2510) Fluorescence.
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#194359 - $15.00 USD Received 22 Jul 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 9 Oct 2013 (C) 2013 OSA 21 October 2013 | Vol. 21, No. 21 | DOI:10.1364/OE.21.024793 | OPTICS EXPRESS 24793
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1. Introduction Metal-semiconductor hetero-nanostructures usually exhibit dramatically different properties than the individual ingredients [1,2], due to the intense interaction between metal and semiconductor. Surface plasmon resonances (SPR) in metal can be modified by the semiconductor ingredient in hetero-structures. Through near-field interactions, energy could flow from the excited state of semiconductor to the plasmon, leading to the loss compensation and amplification of plasmon [3–5]. On the other side, plasmon-exciton interaction can also
#194359 - $15.00 USD Received 22 Jul 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 9 Oct 2013 (C) 2013 OSA 21 October 2013 | Vol. 21, No. 21 | DOI:10.1364/OE.21.024793 | OPTICS EXPRESS 24794
bring a strong modification of the radiative and nonradiative properties for semiconductor, altering the emission behaviors of semiconductor (enhancement or quenching) [6,7]. With appropriate band structures, charge transfer process like photo-excited electrons in semiconductor transferring to metal could suppress the direct recombination of carriers and promote efficient charge separation, resulting in a significant enhancement in the photoelectric conversion efficiency in solar cell and photo-catalysis system [8,9]. Many studies have been devoted to preparing metal-semiconductor hybrids consisted of various metal nanostructures and semiconductor materials [10–15]. For instance, Wang’s group proposed a general approach to synthesize gold-metal sulfide core-shell and heterostructures [10], and Hsu’s group developed a facile approach for preparing Au-CdS core-shell nanocrystals with controllable shell thickness [11]. These methods prepared designated metal ion-molecule complex acting as precursor to bind on Au core for growth of semiconductor shell. Ouyang’s group established a nonepitaxial growth to synthesize spherical Au-CdS core-shell nanostructures with monocrystalline semiconductor shell through a cation exchange process, this method can overcome the constraint of lattice mismatches between two components and suit for growth of multiple type semiconductor shells (CdS, CdTe, ZnS, PbS, etc.) [14]. Metal-semiconductor hetero-structures have tunable optical properties since the SPR properties are strongly dependent on the composition, dimension, and morphology of metal nanoparticles (NPs) [16–18]. Compared with the spherical NPs, metallic nanorods support both longitudinal and transverse SPRs, and metallic nanocubes possess sharp corners with strong local field enhancements and multi-high-order SPRs induced by the local field coupling of pairs of parallel surface [18–20], which are very sensitive to the environment [21]. In this paper, we chose Au nanocubes as core to synthesize Au-AgCdS core-shell nanostructures through a stepwise method, including growth of cubic Ag on core, sulfuration of Ag shells, and cation exchange process (with tributylphosphine, TBP) from Ag2S to CdS. We observed five SPR bands and their unique evolution in the intermediate product of cubic Au-Ag NPs and found that both resonant absorption and fluorescence (FL) behavior of the final products were controlled by TBP. 2. Experimental section Au nanocubes were prepared using a seed-mediated growth as reported previously [19,22], and centrifuged and re-dispersed in water for future used. For growth of Au-Ag nanocubes, Au nanocubes (0.5 mL) and cetyltrimethylammoniumchloride (CTAC, 20 mM, 4.5 mL) were mixed and heated at 60 °C under magnetic stirring. After 20 mins, AgNO3 solution (2 mM, 2 mL) and 2 mL of mixture solution containing ascorbic acid (50 mM) and CTAC (40 mM) were simultaneously injected drop by drop, and the mixture was stirred for 4 hrs [23]. The Au-Ag nanocubes (1 mL) and NaHS solution (50 mM, 10 μL) were mixed and reacted for 20 mins at room temperature under magnetic stirring. The Au-Ag2S nanocubes were obtained by centrifugation (7600 rpm for 10 mins) and washed with water once. Au-AgCdS(II) core-shell NPs were synthesized as following procedure: Au-Ag2S nanocubes (1 mL) and CTAC (0.2 M, 1 mL) were mixed together and heated at 60 °C under magnetic stirring, then Cd(NO3)2 solution (50 mM, 10 μL) and TBP (10 μL) were added into the mixture. After 6 hrs, the final products were collected by centrifugation (7600 rpm for 10 mins). Au-AgCdS(I) were obtained through the same procedure but without adding TBP. The transmission electron microscope (TEM) images were performed with a JEOL 2010 HT transmission electron microscope operated at 200 kV. Energy-dispersive X-ray Spectrum (EDX) analysis was performed on an EDAX instrument incorporated in the HRTEM. The absorption spectra were measured using a TU-1810 UV-vis spectrophotometer. For fluorescence measurements, the excitation source was a mode-locked Ti:sapphire laser (Mira 900, Coherent) with a pulse width of around 3 ps and a repetition rate of 76 MHz. The FL spectra were recorded by a spectrometer (Spectrapro 2500i, Acton) with a liquid-nitrogen cooled CCD (SPEC-10: 100B, Princeton).
#194359 - $15.00 USD Received 22 Jul 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 9 Oct 2013 (C) 2013 OSA 21 October 2013 | Vol. 21, No. 21 | DOI:10.1364/OE.21.024793 | OPTICS EXPRESS 24795
3. Results and discussion The Au-AgCdS core-shell hetero-structures were synthesized by a stepwise method shown in Fig. 1(a). Firstly, Ag shells were grown on Au nanocubes to form Au-Ag core-shell nanocubes. Subsequently, the addition of S2- converted Ag to Ag2S. The final products were obtained through a cation exchange reaction between Ag+ and Cd2+ assisted by TBP or without TBP. The final shells were called AgCdS which were the complex of Ag2S and CdS. It can be seen from the TEM images that the samples at each step are all uniform and monodisperse. The average diameter of the Au nanocubes is about 43 ± 3 nm and that of the Au-Ag nanocubes is about 91 ± 11 nm, as shown in Figs. 1(b) and 1(c). The thickness of Ag shell could be controlled by adjusting the amount of AgNO3 in the reaction. Although Au and Au-Ag NPs are perfectly cubes, the shape of Au-Ag2S NPs in Fig. 1(d) becomes a little irregular after sulfidation reaction. At that process, accompanying the formation of polycrystalline Ag2S domains, the shell was enlarged in volume by incorporating S2- and expanded outward to form irregular shape [24,25]. The shape of Au-AgCdS in Figs. 1(e) and 1(f) is also irregular and that of the Au-AgCdS(II) changes more obviously. In the final cation exchange reaction, the TBP is very crucial for the efficient conversion from Ag2S to CdS, because the thermodynamic driving force for exchange between two cations is highly related to the reaction conditions [14,26]. In the Ag2S-CdS pair, valid conversion from Ag2S to CdS is favored by adding Cd2+ ions along with TBP. Here, the Ag2S is converted to CdS more effectively in the Au-AgCdS(II) than that in the Au-AgCdS(I).
Fig. 1. (a) Diagrammatic sketch for stepwise synthesis of Au-AgCdS core-shell nanocubes. (bf) TEM images of (b) Au, (c) Au-Ag, (d) Au-Ag2S, (e) Au-AgCdS(I) NPs synthesized without TBP, and (f) Au-AgCdS(II) NPs synthesized with TBP. All the bars represent 100 nm.
EDX analyses were carried out to confirm the conversion from Ag2S to CdS. Figure 2(a) shows the EDX elemental maps of Au, Ag, Cd, and S in a Au-AgCdS(II) nanocube. The data suggest that the shell is composed of Ag, Cd, and S. The distributions of Au, Ag, and Cd are also displayed by line-scan EDX spectra in Fig. 2(b). The intensity of Cd distribution in AuAgCdS(II) is stronger than that in Au-AgCdS(I), which manifests that the conversion from Ag2S to CdS is more effective with the assistance of TBP. The absorption spectral evolution reflects the distinctive SPR properties of metal NPs determined by composition, size, and shape. The spectral responses dependent on the thickness of Ag shells in Au-Ag core-shell nanocubes are shown in Fig. 3(a). The Au nanocubes with side length of 43 ± 3 nm have a plasmon band at ~545 nm due to the dipole resonance. As a thin Ag shell (~9 nm) is deposited on Au nanocubes, the dipole resonance of Au-Ag core-shell nanocubes blue-shifts to ~527 nm. Due to the dielectric function of Ag is different from Au, the blue-shift originates from the altering of effective dielectric function by
#194359 - $15.00 USD Received 22 Jul 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 9 Oct 2013 (C) 2013 OSA 21 October 2013 | Vol. 21, No. 21 | DOI:10.1364/OE.21.024793 | OPTICS EXPRESS 24796
Ag coating [27,28]. When the Ag thickness further increases, the dipole plasmon band (band B5) begins to red-shift because the size of whole particle increases. This band progressively red-shifts to ~617 nm when Au-Ag nanocubes grow to 111 ± 13 nm.
Fig. 2. (a) EDX elemental maps of Au, Ag, Cd, and S in a Au-AgCdS(II) nanocube. (b) Line profile of Au(II), Ag(II), and Cd(II) in the same Au-AgCdS(II) nanocube. All lines are normalized and the maximum intensity of Au is set to 1. Cd(I) line profile of a Au-AgCdS(I) nanocube (red) is also shown for comparison, which is normalized by it’s own Au distribution.
At the same time, multiple plasmon resonance absorption peaks gradually emerge in the range between 300 nm and 500 nm, and these new plasmon bands are very sensitive to the shape and size of Au-Ag nanocubes. The plasmon band near 340 nm (band B1) usually appeared when Ag NPs are cube shape, which may be derived from the octupole or dipole [21,29]. As the edge length of Au-Ag nanocubes increases to 75 ± 9 nm, a plasmon band attributed to the quadrupole resonance appears at ~461 nm (band B4). The band B3 near 410 nm is a mix of quadrupole and dipole plasmon resonance [29]. The band B2 arises and presents clearly as Au-Ag nanocubes grow large, which has been reported before when Ag nanocubes have a big size [30,31]. As shown in Fig. 3(b), these plasmon bands display a continuous red-shift with the increasing of size. An interesting phenomenon is that, although SPR in Au nanocubes is usually dominated by a single dipole resonance, a thin layer of Ag shell with cubic shape could cause the multiple plasmon resonances.
Fig. 3. (a) Absorption spectra of Au and Au-Ag nanocubes with different particle size. dAu and dAg represent the side length of Au nanocubes and the thickness of silver shell, respectively. (b) Multiple plasmon resonance modes vary with the size of the Au-Ag nanocubes. (c, d) Absorption spectra of Au-Ag2S NPs (0 hr), Au-AgCdS(II) and Au-AgCdS(I) NPs synthesized at different exchange reaction time T (T = 1, 2, 4 and 6 hrs).
Au-Ag core-shell nanocubes with edge length of 91 ± 11 nm were chosen to participate in the sulfidation reaction and cation exchange reaction. As shown in Fig. 3(c), the main absorption band of Au-Ag2S core-shell NPs is red-shifted to ~717 nm. Considering the dipole plasmon band at ~545 nm of Au nanocubes, the large shift results from the large medium dielectric constant of surrounding Ag2S (~2.9 at 700 nm) [32]. When the cation exchange reaction proceeds 6 hrs, the absorption peak of Au-AgCdS(II) NPs blue-shifts 82 nm from ~717 nm to ~635 nm, which is due to the relative small dielectric constant of CdS (~2.4 at 700 nm) [33] compared with Ag2S. It also can be seen that the full width half maximum (FWHM) of absorption becomes narrower from Au-Ag2S to Au-AgCdS, which is related to the plasmon damping effect interacted with semiconductor shell [34–36]. The FWHM of AuAg2S NPs is about 280 nm, while that of the Au-AgCdS(II) NPs narrows 55% to 126 nm.
#194359 - $15.00 USD Received 22 Jul 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 9 Oct 2013 (C) 2013 OSA 21 October 2013 | Vol. 21, No. 21 | DOI:10.1364/OE.21.024793 | OPTICS EXPRESS 24797
Comparing with CdS, Ag2S has larger imaginary part of the dielectric constant [32], which could cause faster decay of plasmon and broaden the plasmon band of Au core. The variation of absorption features also implies the conversion efficiency of cation exchange reaction between Ag+ and Cd2+. On the absorption spectrum of Au-Ag2S core-shell NPs, the existence of a valley at 320 nm (caused by the interband transition of Ag below 320 nm) and a weak band around 400 nm indicates the residual Ag domains in the shell. In Fig. 3(c), these features disappear in Au-AgCdS(II), which means the residual Ag has been reacted out. However, in Fig. 3(d), the retaining of these features in Au-AgCdS(I) implies the existence of residual Ag. Meanwhile, the band position blue-shifts only 17 nm and the band FWHM narrows only 25%, which indicates the extent of exchange between Ag+ and Cd2+ is small without adding TBP. The Au-AgCdS core-shell NPs show interesting FL behaviors in Fig. 4. The AuAgCdS(II) exhibit an emission peak at 700 nm, which is probably the defect emission of CdS nanocrystal. As the reaction time increases, the emission intensity increases gradually. Beside the emission peak at 700 nm, an emission band arises at 570 nm and reaches the maximum at 2 hrs. We attribute this emission to the Ag-Cd-S ternary complex located in some regions [37–39]. This emission decreases and disappears when the reaction proceeds forward, which means the emission is very sensitive to the ratio of Ag2S and CdS. By contrast, in Fig. 4(b), the Au-AgCdS(I) have no emission at 700 nm because the CdS component is small in AuAgCdS(I) shell, while the emission behavior at 570 nm is similar to that of the Au-AgCdS(II). This result accords with the conclusion deduced from the absorption evolution in Figs. 3(c) and 3(d) that the cation exchange reaction without TBP is inefficient.
Fig. 4. FL spectra of Au-Ag2S (0 hr), (a) Au-AgCdS(II) NPs and (b) Au-AgCdS(I) NPs synthesized at different reaction time T (T = 1, 2, 4 and 6 hrs) at 400 nm excitation wavelength.
4. Conclusions In summary, Au-AgCdS core-shell hetero-structures were synthesized by a stepwise method through the process as: Au nanocubes – Au-Ag nanocubes – Au-Ag2S nanocubes – AuAgCdS NPs. For Au-Ag nanocubes, the Ag shell growth on Au nanocubes causes the emergence of multiple high-order SPRs and these plasmon bands red-shift with the increasing of the particle size. The Au dipole plasmon band red-shifts and broadens at the step of AuAg2S and then blue-shifts and narrows at the step of Au-AgCdS, which is related to the variation of environment medium. TBP is very important for the conversion form Ag2S to CdS. Therefore, Au-AgCdS(II) and Au-AgCdS(I) exhibit different absorption and fluorescence properties due to the different component in their shells. The stepwise manner could be used to synthesize other metal-semiconductor hetero-structures and the Au-AgCdS metal-semicondutor core-shell NPs could find application in solar energy harvesting and photocatalysis. Acknowledgments This work was supported by the National Program on Key Science Research of China (2011CB922201), and the NSFC (61008043, 11174229 and 11204221).
#194359 - $15.00 USD Received 22 Jul 2013; revised 29 Sep 2013; accepted 30 Sep 2013; published 9 Oct 2013 (C) 2013 OSA 21 October 2013 | Vol. 21, No. 21 | DOI:10.1364/OE.21.024793 | OPTICS EXPRESS 24798
