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Formation of AgGaS2 nano-pyramids from Ag2S nanospheres through intermediate Ag2S–AgGaS2 heterostructures and AgGaS2 sensitized Mn2+ emission† Feng Huang,a Jiangcong Zhou,b Ju Xub and Yuansheng Wang*a A one-pot solution synthesis of monodisperse AgGaS2 nanocrystals with uniform pyramid-like shape is realized for the first time, in which an interesting phase and shape evolution from monodisperse Ag2S nanospheres to pure AgGaS2 nano-pyramids through an intermediate stage of Ag2S–AgGaS2 heterostructures, is revealed. Evidently, upon introducing Mn2+ ions into the reaction system, they are
Received 6th September 2013 Accepted 15th November 2013 DOI: 10.1039/c3nr04765b www.rsc.org/nanoscale
incorporated into AgGaS2 nano-pyramids which act as efficient sensitization matrixes for the red emission of Mn2+ d–d transition under blue excitation. Benefiting from their non-toxicity and facile fabrication, Mn:AgGaS2 nanocrystals may find potential applications in some fields such as blue chip excited LEDs and bio-labeling.
Introduction I–III–VI2 (I ¼ Cu, Ag; III ¼ Al, Ga, In; and VI ¼ S, Se, Te) ternary chalcogenide semiconductors have attracted great attention owing to their interesting chemical and physical properties.1–8 Among I–III–VI2 semiconductors, bulk AgGaS2 with a direct band gap of 2.5–2.7 eV exhibits unique characteristics such as strong optical nonlinearity and birefringence.9,10 Moreover, the exciton bohr radius of AgGaS2 is calculated to be 3.3 nm,11,12 close to those of the wildly studied binary chalcogenide semiconductors (such as CdSe, ZnSe, CdS, etc.), which means that the nano-sized AgGaS2 crystals would exhibit a remarkable dimensional effect. Therefore, it is highly desired to fabricate monodisperse AgGaS2 nanocrystals and investigate their structure related properties. However, because of the reactivity difference between the precursors of Ag and Ga which usually leads to the unexpected size-nonuniformity and/or phase complexity in the nal products,13,14 solution synthesis of such ternary suldes is much more difficult than that of the conventional binary suldes. Qian et al. developed a solvothermal route to obtain AgGaS2 nanocrystals,15 whereas the product exhibited remarkable shape irregularity and agglomeration; Yuan et al. synthesized
a
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002 P. R. China. E-mail: yswang@irsm.ac.cn
b
Key Laboratory of Design and Assembly of Functional Nanostructures, Chinese Academy of Sciences, Fuzhou, Fujian, 350002 P. R. China
† Electronic supplementary information (ESI) available: Fig. S1–S2. See DOI: 10.1039/c3nr04765b
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ower-like AgGaS2 nanocrystals,14 however the grain size was too large (>50 nm) to perform the dimensional effect. Herein, we report the successful preparation of monodisperse pyramid-like AgGaS2 nanocrystals from Ag2S nanospheres through an intermediate stage of Ag2S–AgGaS2 heterostructures. In addition, we nd that the prepared AgGaS2 nanocrystals may act as the efficient sensitization matrixes for the red emission of the doped Mn2+ ions.
Experimental Chemical reagents The used Gallium oxide (Ga2O3), sulfur (S), silver nitrate (AgNO3), potassium oleate (C17H32COOK, 95%), dodecanethiol (CH3(CH2)11–SH, 98%), oleylamine (CH3(CH2)7CH]CH(CH2)8– NH2, 90%), oleic acid (CH3(CH2)7CH]CH(CH2)7–COOH, 90%), 1-octadecene (CH3(CH2)7CH]CH(CH2)7CH3, 90%), acetylacetone (CH3COCH2COCH3, 90%) et al. were all purchased from Sinopharm Chemical Reagent Company. Synthesis of metal organic sources Ag–oleate was obtained by the reaction of AgNO3 with potassium oleate. In a typical reaction, 50 mmol AgNO3 and 50 mmol potassium oleate were dissolved in a solvent mixture composed of 30 mL distilled water, 50 mL ethanol and 60 mL hexane. The resulting solution was heated at 70 C for 4 h. When the reaction was completed, the upper organic layer containing the Ag– oleate complex was collected, washed three times with 30 mL distilled water in a separator funnel, then heated in a drying oven to evaporate the cyclohexane, yielding Ag–oleate complex in solid form.
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Ga-acetylacetone (Ga(acac)3) was obtained by the reaction of Ga(NO3)3 solution with acetylacetone. Typically, 10 mmol Ga2O3 was dissolved in HNO3 (63%). On the other hand, 8 mL acetylacetone was mixed with 20 mL distilled water accompanied with 6 mL ammonia (25%). Then the acetylacetone–water mixture was added to the Ga(NO3)3 solution drop wise. The pH of the mixture was adjusted to 6–7 by aqua ammonia. The yellow precipitate was generated, which was then collected, washed several times using distilled water and dried. Synthesis of AgGaS2 nano-pyramids In a typical synthesis, 0.05 g Ag–oleate and 0.06 g Ga(acac)3 were dissolved in a mixed solvent containing 3 mL oleylamine, 3 mL oleic acid, 2 mL dodecanethiol and 10 mL 1-octadecene. The resulting mixture was heated at 120 C for 30 min under argon ow to get rid of oxygen and water. 0.02 g sulfur was dissolved in 3 mL oleylamine and added into the reaction mixture, which was maintained at 120 C for another 15 min. Then, the temperature was risen to 280 C and kept for various durations (1 min, 3 min, 5 min, 10 min, 20 min, 30 min and 40 min). Aer the reaction system was cooled to room temperature, some amount of ethanol was added to precipitate the yellow product, which was then separated by centrifuging. Mn doping was achieved by adding Mn-oleate with a different Mole ratio (i.e. 0.5%, 2%, 3%, 5% and 7%) into the reaction system. Characterizations X-ray Diffraction (XRD) analyses were carried out with a powder diffractometer (DMAX2500) using CuKa radiation (l ¼ 0.154 nm). Transmission electron microscopy (TEM) observations were performed in a transmission electron microscope (JEM2010) equipped with an energy dispersive X-ray spectroscopy (EDS) system. TEM specimens were prepared by directly drying a drop of the dilute cyclohexane dispersion solution of the product on the carbon-coated copper grid. Absorbance spectra were measured by a UV-vis-IR spectrometer (Lambda 900). Photoluminescence (PL) spectra and quantum yields (QYs) were measured using an Edinburgh Instruments FLS920 spectrouorometer equipped with an integrating sphere. All the measurements were performed at room temperature.
Fig. 1 XRD pattern of product having reacted for 40 min; bars at bottom show the standard data of tetragonal phase AgGaS2 (PDF 651574).
bottom is identied to be the (002) plane, as schematically illustrated in Fig. 2d. To explore the formation process of these nano-pyramids, products at different reaction stages were monitored by XRD and HRTEM analyses. XRD results reveal that the product undergoes a phase evolution from the pure monoclinic Ag2S to a bi-phase intermediate of Ag2S–AgGaS2, and nally to the pure tetragonal AgGaS2, as demonstrated in Fig. 4. TEM observations reveal that the pre-formed Ag2S are monodisperse nanospheres (Fig. 5a). When the reaction proceeds to 3 min, the Ag2S nanosphere starts to convert to AgGaS2 partially, forming the dimeric Ag2S–AgGaS2 heterostructure (Fig. 5b). When the reaction proceeds to 5 min and 10 min, the Ag2S part reduces in size, while the AgGaS2 part grows bigger and bigger (Fig. 5c and d). When the reaction further proceeds to 20 min, the shape of AgGaS2 evolves to a pyramid, with the tiny Ag2S grain remaining on the “apex” (Fig. 5e). Aer a 30 min reaction, Ag2S disappears completely, resulting in the pure AgGaS2 nano-pyramids
Results and discussion A one-pot co-precipitation reaction was carried out at 280 C for 40 min in a mixed solvent of oleylamine, dodecanethiol, oleic and octadecylene, using Ag–oleate, Ga(acac)3 and sulfur as precursors. As demonstrated by X-ray diffraction (XRD) shown in Fig. 1, the product is pure tetragonal phase AgGaS2 with a mean size of 15 nm determined by Scherrer equation. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) observations reveal that AgGaS2 nanocrystals are monodisperse with uniform pyramid-like shape, and each AgGaS2 nano-pyramid is a monocrystal. Detailed HRTEM and FFT analyses, presented in Fig. 3, demonstrate that the four sides of the AgGaS2 nano-pyramid are h112i planes, while the Nanoscale
Fig. 2 (a) TEM micrograph, and (b) HRTEM image of AgGaS2 nanopyramids; (c) HRTEM image, and (d) 3D schematic illustration of an individual nano-pyramid.
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Fig. 3 (a) and (d) HRTEM images, (b) and (e) FFT patterns, (c) and (f) 3D schematic illustrations of AgGaS2 nano-pyramids with different zone axes.
(Fig. 5f). Further prolonging the reaction to 1 hour, no remarkable change in size and shape of the AgGaS2 nanopyramids is found (Fig. S1†). EDS measurements on the products at various growth stages were carried out to follow the
Fig. 5 HRTEM images of products having reacted for (a) 1 min, (b) 3 min, (c) 5 min, (d) 10 min, (e) 20 min, and (f) 30 min, respectively. (g) Schematic diagram showing the evolution from Ag2S to AgGaS2.
Fig. 4 XRD patterns of products having reacted for (a) 1 min, (b) 10 min and (c) 30 min, respectively; bars at bottom of (a) and (c) show the standard data of monoclinic Ag2S and tetragonal AgGaS2, respectively.
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composition variation, as shown in Fig. S2.† For the sample having reacted for 1 min, only Ag and S signals are detected. With further prolonging the reaction, i.e., with heterogrowth of AgGaS2, the signal of Ga element comes forth and intensies gradually. When the reaction proceeds to 30 min, the ratio of Ga to Ag reaches 1 : 1, owing to the formation of pure AgGaS2. Absorption spectra of the products at various reaction stages were measured, as presented in Fig. 6. For the product having reacted for 1 min, the absorption spectrum is dominated by the inter-band transition of Ag2S with absorption edge locating at 1000 nm. With heterogrowth of AgGaS2, the absorption of Ag2S reduces. When reacted for 30 min, absorption edge of the product shis to 450 nm (Eg ¼ 2.73 eV) which is ascribed to the inter-band transition of AgGaS2. Based on the results stated above, the mechanism of the phase evolution is proposed. At the beginning of the reaction, monodisperse Ag2S nanospheres are generated. Beneting from its unique feature as a fast ion conductor,16,17 the pre-formed Ag2S nanocrystal acts not only as a “reactant”, but also as a “catalyst”,18 allowing Ga cations in solution to diffuse into Ag2S and react with the Ag2S host there. Consequently, the dimeric Ag2S–AgGaS2 heterostructure is formed. With further
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Fig. 6 Absorption spectra of the products having reacted for 1 min, 3 min, 10 min, and 30 min, respectively; inset shows the (ahn)2 vs. hn curve fitted by the equation of (ahn)2 ¼ A(Eg hn) and the evaluated band gap Eg, for the products having reacted for 30 min.
Fig. 7 PLE and PL spectra of (a) pure AgGaS2 nanocrystals, and (b) 5% Mn doped AgGaS2 nanocrystals; (c) schematic diagram illustrating charge-transfer from AgGaS2 host to Mn2+ ions.
Fig. 8 PL spectra of Mn:AgGaS2 nanocrystals with various Mn2+
content, showing the growth and decline relation of Mn2+ 4T1 / 6A1 emission and exciton recombination radiation of AgGaS2; inset exhibits the curve of 650 nm emission intensity versus Mn content; images on the right side are photographs of Mn:AgGaS2 colloids with various Mn2+ content under ultraviolet (365 nm) irradiation.
proceeding of the reaction, the Ag2S part is gradually consumed and reduced in size, while the AgGaS2 part grows correspondingly and its shape evolves to a pyramid in the Ag2S–AgGaS2 heterostructure. Aer total exhaustion of Ag2S, a pure AgGaS2 nano-pyramid is eventually achieved. The monodispersity of Ag2S nanocrystals favors the size and shape uniformity of the nal AgGaS2 products. As conrmed by experiments, when
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introducing appropriate content of Mn2+ ions into the reaction system, the above studied phase and shape evolution is not affected, and the nal product is therefore the Mn:AgGaS2 nanopyramids. It is believed that, following the proposed mechanism, some multi-element suldes and selenides (such as AgInS2, AgInSe2, CuInS2, CuInSe2, etc.) of technological importance may also be synthesized, by pre-generating the monodisperse nanocrystals of fast ion conductors (such as Ag2S, Ag2Se, Cu2S, Cu2Se, etc.) as the precursor phases. The synthesized pure AgGaS2 nanocrystals exhibit a broad but somewhat dissymmetrical emission band at 500 nm, which is ascribed to the exciton recombination radiation in AgGaS2 accompanied with the weak defect luminescence at the relatively longer wavelength,14,19 as shown in Fig. 7a. Upon introducing Mn2+ ions into the system, they are expected to occupy the Ag+ and Ga3+ sites in AgGaS2 and act as both the trapping and the luminescent centers.20 As demonstrated in Fig. 7b, when doping 5% Mn2+ into AgGaS2, the exciton recombination radiation band nearly vanishes, while intense emission of Mn2+ 4T1 / 6A1 transition peaking at 650 nm comes forth. The photoluminescence excitation (PLE) spectrum monitoring the 650 nm emission of Mn2+ is basically the same as that monitoring the 500 nm emission of pure AgGaS2, demonstrating that it is dominated by the inter-band transitions of the AgGaS2 host. Therefore, it is concluded that, upon light excitation, the photogenerated charge carriers in the AgGaS2 host can be captured by the doped Mn2+ ions and transfer energy to the 3d-orbit of Mn2+ which then relaxes to emit photons,21–23 as schematically illustrated in Fig. 7c. In other words, the AgGaS2 nanocrystal is an efficient sensitization matrix for the red luminescence of Mn2+. It is worth noting that, due to the relatively narrow band gap of AgGaS2 (2.5–2.7 eV), red emission of Mn2+ in Mn:AgGaS2 can be excited by blue light, which favors LED and bio-labeling applications.20 PL spectra of Mn:AgGaS2 nano-pyramids with various Mn contents are measured, as presented in Fig. 8. With increase of Mn2+ content from 0.5% to 5%, the exciton recombination radiation band at 500 nm weakens gradually, while the Mn 4T1 / 6A1 emission at 650 nm intensies correspondingly. Further increasing Mn2+ content to 7%, the intensity of 650 nm emission turns to be reduced attributing to concentration quenching.
Conclusion In a one-pot solution reaction system, the pre-formed Ag2S nanospheres, as fast ion conductors, act not only as a “reactant”, but also as a “catalyst” to generate the monodisperse AgGaS2 nano-pyramids, through an intermediate stage of Ag2S– AgGaS2 heterostructures. Evidently, introducing Mn2+ ions into the system does not affect the phase and shape evolution, and the AgGaS2 host can act as the efficient sensitization matrix for red emission of the doped Mn2+ ions, under blue excitation. Beneting from their non-toxicity and facile fabrication, the red emitting Mn:AgGaS2 nano-pyramids may nd potential applications in some elds, such as blue chip excited LEDs and biolabeling.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (11204301,11304312, 21271170, 51202244 and 51172231), and the Fund of Key Laboratory of Optoelectronic Materials Chemistry and Physics, Chinese Academy of Sciences (2008DP173016).
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Notes and references 1 J. E. Jaffe and A. Zunger, Phys. Rev. B, 1984, 29, 1882– 1906. 2 C. Persson and A. Zunger, Phys. Rev. Lett., 2003, 91, 266401. 3 M. J. Hetzer, Y. M. Strzhemechny, M. Gao, M. A. Contreras, A. Zunger and L. J. Brillson, Appl. Phys. Lett., 2005, 86, 162105. 4 W. S. Song and H. S. Yang, Chem. Mater., 2012, 24, 1961– 1967. 5 E. P. Jang, W. S. Song, K. H. Lee and H. S Yang, Nanotechnology, 2013, 24, 045607. 6 M. G. Panthani, V. Akhavan, B. G. fellow, J. P. Schmidtke, L. Dunn, A. Dodabalapur, P. F. Barbara and B. A. Korgel, J. Am. Chem. Soc., 2008, 130, 16770–16777. 7 L. Tian, H. I. Elim, W. Ji and J. J. Vittal, Chem. Commun., 2006, 4276–4278. 8 S. T. Connor, C. M. Hsu, B. D. Weil, S. Aloni and Y. Cui, J. Am. Chem. Soc., 2009, 131, 4962–4966. 9 H. Matthes, R. Viehmann and N. Marschall, Appl. Phys. Lett., 1975, 26, 237–239.
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10 G. D. Boyd, H. M. Kasper and J. H. McFee, J. Appl. Phys., 1973, 44, 2809–2812. 11 T. Omata, K. Nose and S. O. Y. Matsuo, J. Appl. Phys., 2009, 105, 073106. 12 T. Uematsu, T. Doi, T. Torimoto and S. Kuwabata, J. Phys. Chem. Lett., 2010, 1, 3283–3287. 13 X. S. Tang, W. L. Cheng, E. S. G. Choo and J. M. Xue, Chem. Commun., 2011, 47, 5217–5219. 14 Y. P. Yuan, J. T. Zai, Y. Z. Su and X. F. Qian, J. Solid State Chem., 2011, 184, 1227–1235. 15 J. Q. Hu, Q. Y. Lu, K. B. Tang, Y. T. Qian, G. E. Zhou and X. M. Liu, Chem. Commun., 1999, 1093–1094. 16 G. X. Zhu and Z. Xu, J. Am. Chem. Soc., 2011, 133, 148–157. 17 W. P. Lim, Z. Zhang, H. Y. Low and W. S. Chin, Angew. Chem., Int. Ed., 2004, 43, 5685–5689. 18 W. Han, L. Yi, N. Zhao, A. Tang, M. Y. Gao and Z. Y. Tang, J. Am. Chem. Soc., 2008, 130, 13152–13161. 19 B. D. Mao, C. H. Chuang, J. W. Wang and C. Burda, J. Phys. Chem. C, 2011, 115, 8945–8954. 20 G. Manna, S. Jana, R. Bose and N. Pradhan, J. Phys. Chem. Lett., 2012, 3, 2528–2534. 21 K. Izumi, S. Miyazaki, S. Yoshida, T. Mizokawa and E. Hanamura, Phys. Rev. B: Condens. Matter Mater. Phys., 2007, 76, 075111. 22 S. L. Shen, Y. J. Zhang, Y. S. Liu, L. Peng, X. Y. Chen and Q. B. Wang, Chem. Mater., 2012, 24, 2407–2413. 23 R. Beaulac, S. T. Ochsenbein and D. R. Gamelin, in Nanocrystal Quantum Dots, ed. V. I. Klimov, 2nd edn, 2010, ch. 11, pp. 434–447.
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Formation of AgGaS2 nano-pyramids from Ag2S nanospheres through intermediate Ag2S–AgGaS2 heterostructures and AgGaS2 sensitized Mn2+ emission† Feng Huang,a Jiangcong Zhou,b Ju Xub and Yuansheng Wang*a A one-pot solution synthesis of monodisperse AgGaS2 nanocrystals with uniform pyramid-like shape is realized for the first time, in which an interesting phase and shape evolution from monodisperse Ag2S nanospheres to pure AgGaS2 nano-pyramids through an intermediate stage of Ag2S–AgGaS2 heterostructures, is revealed. Evidently, upon introducing Mn2+ ions into the reaction system, they are
Received 6th September 2013 Accepted 15th November 2013 DOI: 10.1039/c3nr04765b www.rsc.org/nanoscale
incorporated into AgGaS2 nano-pyramids which act as efficient sensitization matrixes for the red emission of Mn2+ d–d transition under blue excitation. Benefiting from their non-toxicity and facile fabrication, Mn:AgGaS2 nanocrystals may find potential applications in some fields such as blue chip excited LEDs and bio-labeling.
Introduction I–III–VI2 (I ¼ Cu, Ag; III ¼ Al, Ga, In; and VI ¼ S, Se, Te) ternary chalcogenide semiconductors have attracted great attention owing to their interesting chemical and physical properties.1–8 Among I–III–VI2 semiconductors, bulk AgGaS2 with a direct band gap of 2.5–2.7 eV exhibits unique characteristics such as strong optical nonlinearity and birefringence.9,10 Moreover, the exciton bohr radius of AgGaS2 is calculated to be 3.3 nm,11,12 close to those of the wildly studied binary chalcogenide semiconductors (such as CdSe, ZnSe, CdS, etc.), which means that the nano-sized AgGaS2 crystals would exhibit a remarkable dimensional effect. Therefore, it is highly desired to fabricate monodisperse AgGaS2 nanocrystals and investigate their structure related properties. However, because of the reactivity difference between the precursors of Ag and Ga which usually leads to the unexpected size-nonuniformity and/or phase complexity in the nal products,13,14 solution synthesis of such ternary suldes is much more difficult than that of the conventional binary suldes. Qian et al. developed a solvothermal route to obtain AgGaS2 nanocrystals,15 whereas the product exhibited remarkable shape irregularity and agglomeration; Yuan et al. synthesized
a
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002 P. R. China. E-mail: yswang@irsm.ac.cn
b
Key Laboratory of Design and Assembly of Functional Nanostructures, Chinese Academy of Sciences, Fuzhou, Fujian, 350002 P. R. China
† Electronic supplementary information (ESI) available: Fig. S1–S2. See DOI: 10.1039/c3nr04765b
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ower-like AgGaS2 nanocrystals,14 however the grain size was too large (>50 nm) to perform the dimensional effect. Herein, we report the successful preparation of monodisperse pyramid-like AgGaS2 nanocrystals from Ag2S nanospheres through an intermediate stage of Ag2S–AgGaS2 heterostructures. In addition, we nd that the prepared AgGaS2 nanocrystals may act as the efficient sensitization matrixes for the red emission of the doped Mn2+ ions.
Experimental Chemical reagents The used Gallium oxide (Ga2O3), sulfur (S), silver nitrate (AgNO3), potassium oleate (C17H32COOK, 95%), dodecanethiol (CH3(CH2)11–SH, 98%), oleylamine (CH3(CH2)7CH]CH(CH2)8– NH2, 90%), oleic acid (CH3(CH2)7CH]CH(CH2)7–COOH, 90%), 1-octadecene (CH3(CH2)7CH]CH(CH2)7CH3, 90%), acetylacetone (CH3COCH2COCH3, 90%) et al. were all purchased from Sinopharm Chemical Reagent Company. Synthesis of metal organic sources Ag–oleate was obtained by the reaction of AgNO3 with potassium oleate. In a typical reaction, 50 mmol AgNO3 and 50 mmol potassium oleate were dissolved in a solvent mixture composed of 30 mL distilled water, 50 mL ethanol and 60 mL hexane. The resulting solution was heated at 70 C for 4 h. When the reaction was completed, the upper organic layer containing the Ag– oleate complex was collected, washed three times with 30 mL distilled water in a separator funnel, then heated in a drying oven to evaporate the cyclohexane, yielding Ag–oleate complex in solid form.
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Ga-acetylacetone (Ga(acac)3) was obtained by the reaction of Ga(NO3)3 solution with acetylacetone. Typically, 10 mmol Ga2O3 was dissolved in HNO3 (63%). On the other hand, 8 mL acetylacetone was mixed with 20 mL distilled water accompanied with 6 mL ammonia (25%). Then the acetylacetone–water mixture was added to the Ga(NO3)3 solution drop wise. The pH of the mixture was adjusted to 6–7 by aqua ammonia. The yellow precipitate was generated, which was then collected, washed several times using distilled water and dried. Synthesis of AgGaS2 nano-pyramids In a typical synthesis, 0.05 g Ag–oleate and 0.06 g Ga(acac)3 were dissolved in a mixed solvent containing 3 mL oleylamine, 3 mL oleic acid, 2 mL dodecanethiol and 10 mL 1-octadecene. The resulting mixture was heated at 120 C for 30 min under argon ow to get rid of oxygen and water. 0.02 g sulfur was dissolved in 3 mL oleylamine and added into the reaction mixture, which was maintained at 120 C for another 15 min. Then, the temperature was risen to 280 C and kept for various durations (1 min, 3 min, 5 min, 10 min, 20 min, 30 min and 40 min). Aer the reaction system was cooled to room temperature, some amount of ethanol was added to precipitate the yellow product, which was then separated by centrifuging. Mn doping was achieved by adding Mn-oleate with a different Mole ratio (i.e. 0.5%, 2%, 3%, 5% and 7%) into the reaction system. Characterizations X-ray Diffraction (XRD) analyses were carried out with a powder diffractometer (DMAX2500) using CuKa radiation (l ¼ 0.154 nm). Transmission electron microscopy (TEM) observations were performed in a transmission electron microscope (JEM2010) equipped with an energy dispersive X-ray spectroscopy (EDS) system. TEM specimens were prepared by directly drying a drop of the dilute cyclohexane dispersion solution of the product on the carbon-coated copper grid. Absorbance spectra were measured by a UV-vis-IR spectrometer (Lambda 900). Photoluminescence (PL) spectra and quantum yields (QYs) were measured using an Edinburgh Instruments FLS920 spectrouorometer equipped with an integrating sphere. All the measurements were performed at room temperature.
Fig. 1 XRD pattern of product having reacted for 40 min; bars at bottom show the standard data of tetragonal phase AgGaS2 (PDF 651574).
bottom is identied to be the (002) plane, as schematically illustrated in Fig. 2d. To explore the formation process of these nano-pyramids, products at different reaction stages were monitored by XRD and HRTEM analyses. XRD results reveal that the product undergoes a phase evolution from the pure monoclinic Ag2S to a bi-phase intermediate of Ag2S–AgGaS2, and nally to the pure tetragonal AgGaS2, as demonstrated in Fig. 4. TEM observations reveal that the pre-formed Ag2S are monodisperse nanospheres (Fig. 5a). When the reaction proceeds to 3 min, the Ag2S nanosphere starts to convert to AgGaS2 partially, forming the dimeric Ag2S–AgGaS2 heterostructure (Fig. 5b). When the reaction proceeds to 5 min and 10 min, the Ag2S part reduces in size, while the AgGaS2 part grows bigger and bigger (Fig. 5c and d). When the reaction further proceeds to 20 min, the shape of AgGaS2 evolves to a pyramid, with the tiny Ag2S grain remaining on the “apex” (Fig. 5e). Aer a 30 min reaction, Ag2S disappears completely, resulting in the pure AgGaS2 nano-pyramids
Results and discussion A one-pot co-precipitation reaction was carried out at 280 C for 40 min in a mixed solvent of oleylamine, dodecanethiol, oleic and octadecylene, using Ag–oleate, Ga(acac)3 and sulfur as precursors. As demonstrated by X-ray diffraction (XRD) shown in Fig. 1, the product is pure tetragonal phase AgGaS2 with a mean size of 15 nm determined by Scherrer equation. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) observations reveal that AgGaS2 nanocrystals are monodisperse with uniform pyramid-like shape, and each AgGaS2 nano-pyramid is a monocrystal. Detailed HRTEM and FFT analyses, presented in Fig. 3, demonstrate that the four sides of the AgGaS2 nano-pyramid are h112i planes, while the Nanoscale
Fig. 2 (a) TEM micrograph, and (b) HRTEM image of AgGaS2 nanopyramids; (c) HRTEM image, and (d) 3D schematic illustration of an individual nano-pyramid.
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Fig. 3 (a) and (d) HRTEM images, (b) and (e) FFT patterns, (c) and (f) 3D schematic illustrations of AgGaS2 nano-pyramids with different zone axes.
(Fig. 5f). Further prolonging the reaction to 1 hour, no remarkable change in size and shape of the AgGaS2 nanopyramids is found (Fig. S1†). EDS measurements on the products at various growth stages were carried out to follow the
Fig. 5 HRTEM images of products having reacted for (a) 1 min, (b) 3 min, (c) 5 min, (d) 10 min, (e) 20 min, and (f) 30 min, respectively. (g) Schematic diagram showing the evolution from Ag2S to AgGaS2.
Fig. 4 XRD patterns of products having reacted for (a) 1 min, (b) 10 min and (c) 30 min, respectively; bars at bottom of (a) and (c) show the standard data of monoclinic Ag2S and tetragonal AgGaS2, respectively.
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composition variation, as shown in Fig. S2.† For the sample having reacted for 1 min, only Ag and S signals are detected. With further prolonging the reaction, i.e., with heterogrowth of AgGaS2, the signal of Ga element comes forth and intensies gradually. When the reaction proceeds to 30 min, the ratio of Ga to Ag reaches 1 : 1, owing to the formation of pure AgGaS2. Absorption spectra of the products at various reaction stages were measured, as presented in Fig. 6. For the product having reacted for 1 min, the absorption spectrum is dominated by the inter-band transition of Ag2S with absorption edge locating at 1000 nm. With heterogrowth of AgGaS2, the absorption of Ag2S reduces. When reacted for 30 min, absorption edge of the product shis to 450 nm (Eg ¼ 2.73 eV) which is ascribed to the inter-band transition of AgGaS2. Based on the results stated above, the mechanism of the phase evolution is proposed. At the beginning of the reaction, monodisperse Ag2S nanospheres are generated. Beneting from its unique feature as a fast ion conductor,16,17 the pre-formed Ag2S nanocrystal acts not only as a “reactant”, but also as a “catalyst”,18 allowing Ga cations in solution to diffuse into Ag2S and react with the Ag2S host there. Consequently, the dimeric Ag2S–AgGaS2 heterostructure is formed. With further
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Fig. 6 Absorption spectra of the products having reacted for 1 min, 3 min, 10 min, and 30 min, respectively; inset shows the (ahn)2 vs. hn curve fitted by the equation of (ahn)2 ¼ A(Eg hn) and the evaluated band gap Eg, for the products having reacted for 30 min.
Fig. 7 PLE and PL spectra of (a) pure AgGaS2 nanocrystals, and (b) 5% Mn doped AgGaS2 nanocrystals; (c) schematic diagram illustrating charge-transfer from AgGaS2 host to Mn2+ ions.
Fig. 8 PL spectra of Mn:AgGaS2 nanocrystals with various Mn2+
content, showing the growth and decline relation of Mn2+ 4T1 / 6A1 emission and exciton recombination radiation of AgGaS2; inset exhibits the curve of 650 nm emission intensity versus Mn content; images on the right side are photographs of Mn:AgGaS2 colloids with various Mn2+ content under ultraviolet (365 nm) irradiation.
proceeding of the reaction, the Ag2S part is gradually consumed and reduced in size, while the AgGaS2 part grows correspondingly and its shape evolves to a pyramid in the Ag2S–AgGaS2 heterostructure. Aer total exhaustion of Ag2S, a pure AgGaS2 nano-pyramid is eventually achieved. The monodispersity of Ag2S nanocrystals favors the size and shape uniformity of the nal AgGaS2 products. As conrmed by experiments, when
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introducing appropriate content of Mn2+ ions into the reaction system, the above studied phase and shape evolution is not affected, and the nal product is therefore the Mn:AgGaS2 nanopyramids. It is believed that, following the proposed mechanism, some multi-element suldes and selenides (such as AgInS2, AgInSe2, CuInS2, CuInSe2, etc.) of technological importance may also be synthesized, by pre-generating the monodisperse nanocrystals of fast ion conductors (such as Ag2S, Ag2Se, Cu2S, Cu2Se, etc.) as the precursor phases. The synthesized pure AgGaS2 nanocrystals exhibit a broad but somewhat dissymmetrical emission band at 500 nm, which is ascribed to the exciton recombination radiation in AgGaS2 accompanied with the weak defect luminescence at the relatively longer wavelength,14,19 as shown in Fig. 7a. Upon introducing Mn2+ ions into the system, they are expected to occupy the Ag+ and Ga3+ sites in AgGaS2 and act as both the trapping and the luminescent centers.20 As demonstrated in Fig. 7b, when doping 5% Mn2+ into AgGaS2, the exciton recombination radiation band nearly vanishes, while intense emission of Mn2+ 4T1 / 6A1 transition peaking at 650 nm comes forth. The photoluminescence excitation (PLE) spectrum monitoring the 650 nm emission of Mn2+ is basically the same as that monitoring the 500 nm emission of pure AgGaS2, demonstrating that it is dominated by the inter-band transitions of the AgGaS2 host. Therefore, it is concluded that, upon light excitation, the photogenerated charge carriers in the AgGaS2 host can be captured by the doped Mn2+ ions and transfer energy to the 3d-orbit of Mn2+ which then relaxes to emit photons,21–23 as schematically illustrated in Fig. 7c. In other words, the AgGaS2 nanocrystal is an efficient sensitization matrix for the red luminescence of Mn2+. It is worth noting that, due to the relatively narrow band gap of AgGaS2 (2.5–2.7 eV), red emission of Mn2+ in Mn:AgGaS2 can be excited by blue light, which favors LED and bio-labeling applications.20 PL spectra of Mn:AgGaS2 nano-pyramids with various Mn contents are measured, as presented in Fig. 8. With increase of Mn2+ content from 0.5% to 5%, the exciton recombination radiation band at 500 nm weakens gradually, while the Mn 4T1 / 6A1 emission at 650 nm intensies correspondingly. Further increasing Mn2+ content to 7%, the intensity of 650 nm emission turns to be reduced attributing to concentration quenching.
Conclusion In a one-pot solution reaction system, the pre-formed Ag2S nanospheres, as fast ion conductors, act not only as a “reactant”, but also as a “catalyst” to generate the monodisperse AgGaS2 nano-pyramids, through an intermediate stage of Ag2S– AgGaS2 heterostructures. Evidently, introducing Mn2+ ions into the system does not affect the phase and shape evolution, and the AgGaS2 host can act as the efficient sensitization matrix for red emission of the doped Mn2+ ions, under blue excitation. Beneting from their non-toxicity and facile fabrication, the red emitting Mn:AgGaS2 nano-pyramids may nd potential applications in some elds, such as blue chip excited LEDs and biolabeling.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (11204301,11304312, 21271170, 51202244 and 51172231), and the Fund of Key Laboratory of Optoelectronic Materials Chemistry and Physics, Chinese Academy of Sciences (2008DP173016).
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