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Cite this: Chem. Commun., 2015, 51, 6141 Received 5th December 2014, Accepted 22nd February 2015
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Aggregation-based abrupt crystallization from amorphous Ag2S to Ag2S nanocrystals† Ruding Zhang,ac Xiaogang Xue,ac Zanyong Zhuang,ac Jinsheng Zhengac and Zhang Lin*abc
DOI: 10.1039/c4cc09728a www.rsc.org/chemcomm
Abrupt crystallization from B2–5 nm (amorphous) to B12–15 nm (crystalline) was observed in hydrothermal coarsening of Ag2S. The desorption behavior of capping ligands could be associated with the aggregation and fusion of amorphous particles into crystals.
The evolution of particle size and morphology during the synthesis and growth of nanomaterials is closely related to the inherent crystal growth mechanisms. An in-depth understanding of crystal growth mechanisms can help to realize fine tailoring of crystal shapes and structures, which eventually determines functional properties of the materials.1 Currently, the descriptions of crystal growth mechanisms are mainly based on the classical Ostwald ripening crystallization mechanism2,3 and the newly proposed non-classical crystallization mechanisms4 such as mesocrystallization,5,6 oriented attachment,7,8 crystallization via amorphous intermediates9,10 and crystallization from the liquid precursor.11,12 In principle, the classical crystallization processes utilize ions, atoms, or molecules as primary assembly units; the morphology of crystals is codetermined by the crystal face energy and the external growth environment.13 While the non-classical crystallization pathway, which employs nanoparticles as building blocks (through fusing and crystallization),4 may form crystals of various kinds of morphologies, even the hierarchical structures.5,9,14 With the advancement of research in this area, studies on non-classical crystallization via an amorphous precursor have attracted broad attention.15 The amorphous phase is well-known as an isotropic body, which can be easily molded and overcome the directional a
Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China. E-mail: [email protected]; Fax: +86 591 83705474 b School of Environment and Energy, South China University of Technology, Guangzhou 510006, China c Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China † Electronic supplementary information (ESI) available: Details of the experimental procedure and characterization methods. See DOI: 10.1039/c4cc09728a
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restriction of crystallization during the establishment of crystal structures.4 Hence, the crystallization strategy via an amorphous precursor can open access to build complex structures and shapes,16,17 even create crystals with complex ordered hierarchical structures with superior excellent properties.18,19 A typical case is biomineralization, the organisms take advantage of the amorphous precursor based crystallization strategy to create biological mineral crystals.17 Revealing the underlying mechanism of phase transformation and crystal growth from the amorphous precursor to crystals is of significant importance for deciphering the overall non-classical crystallization mechanism.18,20 Recently, research studies have been focusing on the amorphous calcium carbonate (ACC) related system.21 However, less work reports on the amorphous phase related crystallization process about other material systems. Furthermore whether or not the crystallization transformation pathway from the amorphous phase of other materials is the same with that of ACC remains unclear? Therefore, it is necessary to clarify the courses of crystallization for other amorphous substances and disclose their phase change mechanisms. As a naturally occurring mineral and useful semiconductor material, Ag2S has a wide range of applications in many fields22,23 However, the initial phase state of the obtained Ag2S sample is not clearly defined in most existing studies, and the crystalline state is not obvious either.24,25 Based on these considerations, Ag2S may be an ideal candidate for studying the phase transformation from the amorphous type to crystalline. In this study, we first prepared the 3-mercaptopropionic acid-capped (3-MPA) amorphous Ag2S as a precursor. Followingly, hydrothermal treatment was performed to study the crystallization behavior of amorphous Ag2S. Our experiments showed a one-step abrupt phase transformation of amorphous Ag2S, which was accompanied by a steep size increasing from B2–5 nm (amorphous) to B12–15 nm (crystalline). Furthermore, detailed analysis of capping states of 3-MPA loading on Ag2S (adsorption–desorption behavior) revealed that the process could be associated with the aggregation of amorphous particles. The finding here may provide an in-depth mechanic understanding of the crystalline behavior and crystal growth of amorphous materials.
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Fig. 1 (a) XRD pattern and (b) TEM image of the as-synthesized Ag2S sample. The inset is the size distribution histogram of the Ag2S precursor obtained from the TEM statistics (the statistical number is over 100). (c) Energy disperse X-ray (EDX) spectrum, (d) typical HRTEM image and (e) ED pattern of the primary sample.
The XRD pattern of the as-synthesized sample shows a big broad peak between 301 and 401 (Fig. 1a). Combined with the HRTEM observations (Fig. 1d) and the electron diffraction (ED) pattern (Fig. 1c), it revealed that the as-synthesized Ag2S precursor is in the amorphous phase. EDX analysis further showed that the Ag and S elements in the sample were in similar stoichiometry of bulk Ag2S. According to absorption spectral data (ESI,† Fig. S1), the average particle size of the Ag2S precursor was B3.2 nm, well fitting with the particle size evaluated from TEM analysis (Fig. 1b). The hydrothermal crystallization of the Ag2S precursor was conducted at 125 1C in an autoclave. As shown in Fig. 2a, the samples coarsened for 2 h and 5 h have a broadening peak between 301 and 401 in the XRD pattern similar to the primary sample. This indicates that amorphous Ag2S is still the dominant component even after 5 hours of hydrothermal treatment. When time came to 6 h, the diffraction peaks of Ag2S suddenly appeared (Fig. 2a; ESI,† Fig. S2). This transition is also obvious in the particle size analysis shown in Fig. 2b. Accompanied by phase transition, the particle size increased from B2–5 nm (amorphous) to B12–15 nm (crystalline) (ESI,† Fig. S2). The above investigations indicate that unexpected and prompt non-classical crystallization and phase transition occurred. Typical TEM images and ED patterns for 0, 2, 5, 6, and 50 h further revealed the evolution of size and microstructure information of each sample (Fig. 3). The TEM images showed that all Ag2S
Fig. 2 (a) XRD patterns of MPA-capped Ag2S samples hydrothermally treated in water at 125 1C as a function of time. (b) Evaluated particle size of Ag2S in samples taken at different coarsening time at 125 1C.
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Fig. 3 Typical TEM images of low magnification (a–e) and high magnification (f–j) showing the primary sample hydrothermally treated in water at 125 1C for 0, 2, 5, 6, and 50 h, respectively. Scale bar in (a–e) represents 100 nm. Insets in (a–e) show electron diffraction (ED) patterns of corresponding samples, Scale bar represents 1 nm 1. Scale bar in (f j) represents 10 nm.
particles were about B2–5 nm within 5 h (Fig. 3f–h; ESI,† Fig. S3) and the corresponding ED patterns do not show obvious signals for crystals (Fig. 1e and insets in Fig. 3a–c). It indicates that the particle size and phase properties remained unchanged in the initial 5 h. This is consistent with the XRD and particle size analysis aforementioned. When the time is over 6 h, as shown in Fig. 3d, e, i and j, plenty of Ag2S nanocrystals (B12–15 nm) that emerged as amorphous particles (B2–5 nm) disappeared gradually. Meanwhile, the corresponding ED patterns showed typical polycrystalline diffraction rings (insets in Fig. 3i, j). It agreed well with the XRD analysis. Therefore, it is concluded that large nanocrystals abruptly formed taking the amorphous small particles as the precursors. Generally, the capping ligands could play an important role in controlling the growth of nanomaterials.26 Fig. 4 presents Fourier transform infrared (FTIR) spectroscopy of the collected samples, which reveals the change in the relative amount of 3-MPA as a function of time. It can be seen from Fig. 4b that the relative amount of 3-MPA decreased dramatically from 100% to B37% in the first 5 hours, which reveals a rapid removal and exclusion of 3-MPA from the Ag2S surface in this stage. The relative amount further decreased to B31% after 6 h and maintained at B20%–30% from 6 h to 120 h, indicating that it reached the adsorption–desorption equilibrium of the capping ligands. Based on the FTIR analysis and evidence from the above XRD and TEM, it is reasonable to infer that the irreversible desorption of 3-MPA in the first 5 h results in aggregation, which is
Fig. 4 (a) FTIR spectra of samples hydrothermally treated in water at 125 1C for different periods of crystallization time. (b) The percentage of adsorbed 3-MPA vs. crystallization time, setting the amount of 3-MPA in the primary sample as 100%.
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conducive to the subsequent abrupt crystallization. Results of Dynamic light scattering (DLS) also provide the detailed information regarding the change in the aggregation state of Ag2S from looseness to compactness (ESI,† Fig. S4). Although the aggregation was not enough to induce crystallization of amorphous Ag2S in this stage, it is the essential process that leads the whole system to the phase transition critical state. When the amount of 3-MPA on Ag2S decreased to a critical value (B31% at 6 h in this experiment), amorphous Ag2S fast crystallized and fused into nanocrystals. From 6 h to 120 h, the adsorption– desorption of capping ligands reached equilibrium, which leads to slow evolution of the particle size. Based on this, we suggest that the capping agents dominating the whole non-classical crystallization process naturally come to mind. Similar phenomena were observed when the coarsening temperature was set at 150 1C (ESI,† Fig. S5–S7). Only the difference lie in that the onset of the abrupt crystallization moved to B1.33 h, earlier than that of the samples coarsened at 125 1C (the onset time is 6 h). Fig. 5 illustrates the crystallization phase transition route proposed on the basis of the samples coarsened at 125 1C and 150 1C. In the first stage (r5 h in 125 1C series or r1 h in 150 1C series), 3-MPA is rapidly removed from the surface of amorphous Ag2S and it results in particle aggregation. The size and phase basically remain unchanged until the whole system reaches a critical state. When it enters the second stage (5–6 h in 125 1C series or 1–1.33 h in 150 1C series), Ag2S nanocrystals of B12–15 nm suddenly appear at the expense of B2–5 nm amorphous particles. Meanwhile, many defects existed inside the newly formed nanocrystals (ESI,† Fig. S8). As the hydrothermal treatment continues into the third stage (Z6 h in 125 1C series or Z1.33 h in 150 1C series), the adsorption– desorption of capping ligands reached an equilibrium, and the growth of nanocrystals forms a plateau. At this stage, it is speculated that the nanocrystals mainly modulate microstructures and defects. In theory, amorphous Ag2S is a thermodynamically
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unstable phase that tends to crystallize. However, herein, amorphous Ag2S remained stable for a long period of time even under hydrothermal conditions. This unusual behavior may be attributed to the 3-MPA ligand capping in process I (Fig. 5) which prevents the fusion and crystallization. As a matter of fact, this crystal growth pathway is ubiquitous in biomineralization. For example, a living body usually takes ACC as the metastable precursor by aggregation and crystallization processes to tailor-make mineral materials and superstructures compatible well with its own tissues. In this work, we clearly reveal a one-step abrupt phase transformation pathway of amorphous Ag2S. Apparently, this is quite different from the multistep crystallization pathway21b or the dissolution–recrystallization process27 of ACC. In the multistep crystallization process of ACC, nanocrystalline domains generate initially inside the large ACC particle, then crystalline domains gradually expand until the whole particle grows into a big monocrystal.21b In this study, the dissolution–recrystallization of amorphous Ag2S was not observed, and it is probably inhibited by the 3-MPA capping effect and low solubility of Ag2S. The crystallization of Ag2S nanocrystals completed in one-step. The phenomenon of one-step abrupt crystallization of amorphous Ag2S and multistep crystallization of ACC may indicate that transformation from the amorphous phase to the crystalline phase occurs once the particle size is dozens of nanometers. In a word, the finding here provide an in-depth mechanic understanding of the crystalline behavior and crystal growth of amorphous materials. In summary, we have successfully prepared 3-MPA coated amorphous Ag2S primary samples. During the hydrothermal coarsening of the Ag2S precusor, an abrupt one-step nonclassical crystallization from amorphous to crystalline Ag2S was observed. With the phase transition, the particle size increased from B2–5 nm (amorphous) to B12–15 nm (crystalline), indicating that the formation of nanocrystals could be related to aggregation and fusion of amorphous particles. The FTIR study demonstrated that the variation of capping states and adsorption–desorption behavior of 3-MPA were in line with the phase transformation and particle size evolution during the whole process. The findings in this paper may offer guidance for syntheses of other crystal materials and for further fundamental critical study. The financial support was provided by the National Basic Research Program of China (2010CB933501 and 2013CB934302), the Outstanding Youth Fund (21125730), and the National Science Foundation Grant (21273237, 21307130 and 51402295)
Notes and references
Fig. 5 Diagrams (1)–(4) illustrate the rapid crystallization and growth model. Stage I, II and III primarily show variation of ligand capping states, rapid crystallization of the amorphous phase, and microstructure adjustment of nanocrystals, respectively. Pictures (a)–(d) and the insets are typical TEM or HRTEM images of representative time points in the process, corresponding to (1)–(4) respectively. Scale bar in (a–d) represents 10 nm.
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1 (a) N. T. Thanh, N. Maclean and S. Mahiddine, Chem. Rev., 2014, 114, 7610–7630; (b) F. Wang, V. N. Richards, S. P. Shields and W. E. Buhro, Chem. Mater., 2013, 26, 5–21; (c) C. Zhu, J. Zeng, J. Tao, M. C. Johnson, I. Schmidt-Krey, L. Blubaugh, Y. Zhu, Z. Gu and Y. Xia, J. Am. Chem. Soc., 2012, 134, 15822–15831; (d) J. S. Zheng, F. Huang, S. G. Yin, Y. J. Wang, Z. Lin, X. L. Wu and Y. B. Zhao, ¨lfen and S. Mann, J. Am. Chem. Soc., 2010, 132, 9528–9530; (e) H. Co Angew. Chem., Int. Ed., 2003, 42, 2350–2365. 2 C. Wagner, Z. Elektrochem., 1961, 65, 581–591. 3 J. Huo, L. Wang, E. Irran, H. Yu, J. Gao, D. Fan, B. Li, J. Wang, W. Ding, A. M. Amin, C. Li and L. Ma, Angew. Chem., Int. Ed., 2010, 49, 9237–9241.
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Cite this: Chem. Commun., 2015, 51, 6141 Received 5th December 2014, Accepted 22nd February 2015
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Aggregation-based abrupt crystallization from amorphous Ag2S to Ag2S nanocrystals† Ruding Zhang,ac Xiaogang Xue,ac Zanyong Zhuang,ac Jinsheng Zhengac and Zhang Lin*abc
DOI: 10.1039/c4cc09728a www.rsc.org/chemcomm
Abrupt crystallization from B2–5 nm (amorphous) to B12–15 nm (crystalline) was observed in hydrothermal coarsening of Ag2S. The desorption behavior of capping ligands could be associated with the aggregation and fusion of amorphous particles into crystals.
The evolution of particle size and morphology during the synthesis and growth of nanomaterials is closely related to the inherent crystal growth mechanisms. An in-depth understanding of crystal growth mechanisms can help to realize fine tailoring of crystal shapes and structures, which eventually determines functional properties of the materials.1 Currently, the descriptions of crystal growth mechanisms are mainly based on the classical Ostwald ripening crystallization mechanism2,3 and the newly proposed non-classical crystallization mechanisms4 such as mesocrystallization,5,6 oriented attachment,7,8 crystallization via amorphous intermediates9,10 and crystallization from the liquid precursor.11,12 In principle, the classical crystallization processes utilize ions, atoms, or molecules as primary assembly units; the morphology of crystals is codetermined by the crystal face energy and the external growth environment.13 While the non-classical crystallization pathway, which employs nanoparticles as building blocks (through fusing and crystallization),4 may form crystals of various kinds of morphologies, even the hierarchical structures.5,9,14 With the advancement of research in this area, studies on non-classical crystallization via an amorphous precursor have attracted broad attention.15 The amorphous phase is well-known as an isotropic body, which can be easily molded and overcome the directional a
Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China. E-mail: [email protected]; Fax: +86 591 83705474 b School of Environment and Energy, South China University of Technology, Guangzhou 510006, China c Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China † Electronic supplementary information (ESI) available: Details of the experimental procedure and characterization methods. See DOI: 10.1039/c4cc09728a
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restriction of crystallization during the establishment of crystal structures.4 Hence, the crystallization strategy via an amorphous precursor can open access to build complex structures and shapes,16,17 even create crystals with complex ordered hierarchical structures with superior excellent properties.18,19 A typical case is biomineralization, the organisms take advantage of the amorphous precursor based crystallization strategy to create biological mineral crystals.17 Revealing the underlying mechanism of phase transformation and crystal growth from the amorphous precursor to crystals is of significant importance for deciphering the overall non-classical crystallization mechanism.18,20 Recently, research studies have been focusing on the amorphous calcium carbonate (ACC) related system.21 However, less work reports on the amorphous phase related crystallization process about other material systems. Furthermore whether or not the crystallization transformation pathway from the amorphous phase of other materials is the same with that of ACC remains unclear? Therefore, it is necessary to clarify the courses of crystallization for other amorphous substances and disclose their phase change mechanisms. As a naturally occurring mineral and useful semiconductor material, Ag2S has a wide range of applications in many fields22,23 However, the initial phase state of the obtained Ag2S sample is not clearly defined in most existing studies, and the crystalline state is not obvious either.24,25 Based on these considerations, Ag2S may be an ideal candidate for studying the phase transformation from the amorphous type to crystalline. In this study, we first prepared the 3-mercaptopropionic acid-capped (3-MPA) amorphous Ag2S as a precursor. Followingly, hydrothermal treatment was performed to study the crystallization behavior of amorphous Ag2S. Our experiments showed a one-step abrupt phase transformation of amorphous Ag2S, which was accompanied by a steep size increasing from B2–5 nm (amorphous) to B12–15 nm (crystalline). Furthermore, detailed analysis of capping states of 3-MPA loading on Ag2S (adsorption–desorption behavior) revealed that the process could be associated with the aggregation of amorphous particles. The finding here may provide an in-depth mechanic understanding of the crystalline behavior and crystal growth of amorphous materials.
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Fig. 1 (a) XRD pattern and (b) TEM image of the as-synthesized Ag2S sample. The inset is the size distribution histogram of the Ag2S precursor obtained from the TEM statistics (the statistical number is over 100). (c) Energy disperse X-ray (EDX) spectrum, (d) typical HRTEM image and (e) ED pattern of the primary sample.
The XRD pattern of the as-synthesized sample shows a big broad peak between 301 and 401 (Fig. 1a). Combined with the HRTEM observations (Fig. 1d) and the electron diffraction (ED) pattern (Fig. 1c), it revealed that the as-synthesized Ag2S precursor is in the amorphous phase. EDX analysis further showed that the Ag and S elements in the sample were in similar stoichiometry of bulk Ag2S. According to absorption spectral data (ESI,† Fig. S1), the average particle size of the Ag2S precursor was B3.2 nm, well fitting with the particle size evaluated from TEM analysis (Fig. 1b). The hydrothermal crystallization of the Ag2S precursor was conducted at 125 1C in an autoclave. As shown in Fig. 2a, the samples coarsened for 2 h and 5 h have a broadening peak between 301 and 401 in the XRD pattern similar to the primary sample. This indicates that amorphous Ag2S is still the dominant component even after 5 hours of hydrothermal treatment. When time came to 6 h, the diffraction peaks of Ag2S suddenly appeared (Fig. 2a; ESI,† Fig. S2). This transition is also obvious in the particle size analysis shown in Fig. 2b. Accompanied by phase transition, the particle size increased from B2–5 nm (amorphous) to B12–15 nm (crystalline) (ESI,† Fig. S2). The above investigations indicate that unexpected and prompt non-classical crystallization and phase transition occurred. Typical TEM images and ED patterns for 0, 2, 5, 6, and 50 h further revealed the evolution of size and microstructure information of each sample (Fig. 3). The TEM images showed that all Ag2S
Fig. 2 (a) XRD patterns of MPA-capped Ag2S samples hydrothermally treated in water at 125 1C as a function of time. (b) Evaluated particle size of Ag2S in samples taken at different coarsening time at 125 1C.
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Fig. 3 Typical TEM images of low magnification (a–e) and high magnification (f–j) showing the primary sample hydrothermally treated in water at 125 1C for 0, 2, 5, 6, and 50 h, respectively. Scale bar in (a–e) represents 100 nm. Insets in (a–e) show electron diffraction (ED) patterns of corresponding samples, Scale bar represents 1 nm 1. Scale bar in (f j) represents 10 nm.
particles were about B2–5 nm within 5 h (Fig. 3f–h; ESI,† Fig. S3) and the corresponding ED patterns do not show obvious signals for crystals (Fig. 1e and insets in Fig. 3a–c). It indicates that the particle size and phase properties remained unchanged in the initial 5 h. This is consistent with the XRD and particle size analysis aforementioned. When the time is over 6 h, as shown in Fig. 3d, e, i and j, plenty of Ag2S nanocrystals (B12–15 nm) that emerged as amorphous particles (B2–5 nm) disappeared gradually. Meanwhile, the corresponding ED patterns showed typical polycrystalline diffraction rings (insets in Fig. 3i, j). It agreed well with the XRD analysis. Therefore, it is concluded that large nanocrystals abruptly formed taking the amorphous small particles as the precursors. Generally, the capping ligands could play an important role in controlling the growth of nanomaterials.26 Fig. 4 presents Fourier transform infrared (FTIR) spectroscopy of the collected samples, which reveals the change in the relative amount of 3-MPA as a function of time. It can be seen from Fig. 4b that the relative amount of 3-MPA decreased dramatically from 100% to B37% in the first 5 hours, which reveals a rapid removal and exclusion of 3-MPA from the Ag2S surface in this stage. The relative amount further decreased to B31% after 6 h and maintained at B20%–30% from 6 h to 120 h, indicating that it reached the adsorption–desorption equilibrium of the capping ligands. Based on the FTIR analysis and evidence from the above XRD and TEM, it is reasonable to infer that the irreversible desorption of 3-MPA in the first 5 h results in aggregation, which is
Fig. 4 (a) FTIR spectra of samples hydrothermally treated in water at 125 1C for different periods of crystallization time. (b) The percentage of adsorbed 3-MPA vs. crystallization time, setting the amount of 3-MPA in the primary sample as 100%.
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conducive to the subsequent abrupt crystallization. Results of Dynamic light scattering (DLS) also provide the detailed information regarding the change in the aggregation state of Ag2S from looseness to compactness (ESI,† Fig. S4). Although the aggregation was not enough to induce crystallization of amorphous Ag2S in this stage, it is the essential process that leads the whole system to the phase transition critical state. When the amount of 3-MPA on Ag2S decreased to a critical value (B31% at 6 h in this experiment), amorphous Ag2S fast crystallized and fused into nanocrystals. From 6 h to 120 h, the adsorption– desorption of capping ligands reached equilibrium, which leads to slow evolution of the particle size. Based on this, we suggest that the capping agents dominating the whole non-classical crystallization process naturally come to mind. Similar phenomena were observed when the coarsening temperature was set at 150 1C (ESI,† Fig. S5–S7). Only the difference lie in that the onset of the abrupt crystallization moved to B1.33 h, earlier than that of the samples coarsened at 125 1C (the onset time is 6 h). Fig. 5 illustrates the crystallization phase transition route proposed on the basis of the samples coarsened at 125 1C and 150 1C. In the first stage (r5 h in 125 1C series or r1 h in 150 1C series), 3-MPA is rapidly removed from the surface of amorphous Ag2S and it results in particle aggregation. The size and phase basically remain unchanged until the whole system reaches a critical state. When it enters the second stage (5–6 h in 125 1C series or 1–1.33 h in 150 1C series), Ag2S nanocrystals of B12–15 nm suddenly appear at the expense of B2–5 nm amorphous particles. Meanwhile, many defects existed inside the newly formed nanocrystals (ESI,† Fig. S8). As the hydrothermal treatment continues into the third stage (Z6 h in 125 1C series or Z1.33 h in 150 1C series), the adsorption– desorption of capping ligands reached an equilibrium, and the growth of nanocrystals forms a plateau. At this stage, it is speculated that the nanocrystals mainly modulate microstructures and defects. In theory, amorphous Ag2S is a thermodynamically
Communication
unstable phase that tends to crystallize. However, herein, amorphous Ag2S remained stable for a long period of time even under hydrothermal conditions. This unusual behavior may be attributed to the 3-MPA ligand capping in process I (Fig. 5) which prevents the fusion and crystallization. As a matter of fact, this crystal growth pathway is ubiquitous in biomineralization. For example, a living body usually takes ACC as the metastable precursor by aggregation and crystallization processes to tailor-make mineral materials and superstructures compatible well with its own tissues. In this work, we clearly reveal a one-step abrupt phase transformation pathway of amorphous Ag2S. Apparently, this is quite different from the multistep crystallization pathway21b or the dissolution–recrystallization process27 of ACC. In the multistep crystallization process of ACC, nanocrystalline domains generate initially inside the large ACC particle, then crystalline domains gradually expand until the whole particle grows into a big monocrystal.21b In this study, the dissolution–recrystallization of amorphous Ag2S was not observed, and it is probably inhibited by the 3-MPA capping effect and low solubility of Ag2S. The crystallization of Ag2S nanocrystals completed in one-step. The phenomenon of one-step abrupt crystallization of amorphous Ag2S and multistep crystallization of ACC may indicate that transformation from the amorphous phase to the crystalline phase occurs once the particle size is dozens of nanometers. In a word, the finding here provide an in-depth mechanic understanding of the crystalline behavior and crystal growth of amorphous materials. In summary, we have successfully prepared 3-MPA coated amorphous Ag2S primary samples. During the hydrothermal coarsening of the Ag2S precusor, an abrupt one-step nonclassical crystallization from amorphous to crystalline Ag2S was observed. With the phase transition, the particle size increased from B2–5 nm (amorphous) to B12–15 nm (crystalline), indicating that the formation of nanocrystals could be related to aggregation and fusion of amorphous particles. The FTIR study demonstrated that the variation of capping states and adsorption–desorption behavior of 3-MPA were in line with the phase transformation and particle size evolution during the whole process. The findings in this paper may offer guidance for syntheses of other crystal materials and for further fundamental critical study. The financial support was provided by the National Basic Research Program of China (2010CB933501 and 2013CB934302), the Outstanding Youth Fund (21125730), and the National Science Foundation Grant (21273237, 21307130 and 51402295)
Notes and references
Fig. 5 Diagrams (1)–(4) illustrate the rapid crystallization and growth model. Stage I, II and III primarily show variation of ligand capping states, rapid crystallization of the amorphous phase, and microstructure adjustment of nanocrystals, respectively. Pictures (a)–(d) and the insets are typical TEM or HRTEM images of representative time points in the process, corresponding to (1)–(4) respectively. Scale bar in (a–d) represents 10 nm.
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