Insight into the Mechanism of Nanoparticle-Aggregated Crystallization for Mesocrystals

Rare Metal Materials and Engineering Volume 41, Issue 4, April 2012 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2012, 41(4): 0589-0593.

ARTICLE

Insight into the Mechanism of Nanoparticle-Aggregated Crystallization for Mesocrystals Shang Hongtao1, Yu Shuxiang2, He Wenxi1, Zhou Zeyuan1 1

Fourth Military Medical University, Xi’an 710032, China;

Lin Yuan1, 2

Zhang Ming1,

The 97 Hospital of Chinese PLA, Xuzhou 210003, China;

Zhu Chao3, 3

Northwest Insti-

tute for Nonferrous Metal Reseach, Xi’an 710016, China

Abstract: silver mesocrystals have been synthesized via oriented attachment of primary building nanoparticles in simple and fast replacement reactions between AgNO3 solution and Sn. The formation process of silver mesocrystals is discussed in detail and the growth mechanism is suggested to describe the formation of silver mesocrystals. In a 200 mmol/L solution, the primary nanoparticles orientationally aggregate along <211> directions to form a dendrite, the proposed attachment planes and directions for Ag nanoparticles are 3×{422} planes and <211> directions, respectively. When the concentration increases to 1 mol/L, the primary building nanoparticles orientotionally aggregate along <211> directions to form dendrites. The dendrites that can be proposed as the second building units orientationally attach along <110> directions to construct a porous monocrystalline plate, and finally transit to a mesocrystal with a thickness about 50 nm. When the AgNO 3 concentration is 2 mol/L, the building units are Ag triangle platelets. These platelets also orientationally attach along <110> directions to form a monocrystalline dense plate and finally the plate transforms to monocrystalline mesocrystal with a thickness over 100 nm. Key words: mesocrystal; crystal growth; nanoparticle; silver

Recently, mesocrystals have attracted great interests in many fields such as crystal growth theories, biomineralization, functional ceramics and so on[1-4]. A mesocrystal is defined as a superstructure of the crystalline nanoparticles with external crystal faces on the scale of some hundred nanometers to micrometers, all individual nanocrystals in the mesocrystal are aligned in a common crystallographic orientation[5-8]. As an exciting example of nonclassical crystallization, mesocrystals do not proceed through ion-by-ion attachment, but by modular building-block route[9-12]. Ding et al. reported that OA mechanism could lead to the formation of Ag dendrite mesocrystal and its transition to a single crystal or mesocrystals in a series of galvanic replacement reactions[13-16]. However, there still exist some unknown mechanisms. On what condition will the primary building nanoparticles attach or aggregate in an oriented fashion rather than in a random fashion? Which

direction is the preferred direction for the oriented attachment of Ag or Au nanoparticles to form a new single crystal or mesocrystal? So in the present work Ag mesocrystals have been synthesized via replacement reactions between Ag nitrate aqueous solution and Sn, and the influence of Ag nitrate concentration on the oriented attachment directions of building blocks has been investigated. The experimental results provide a unique insight into the mechanism of nanoparticle-aggregated crystallization for mesocrystals.

1 Experiment Sn plate (99.9%) was used as reducing metals in the replacement reactions. The plates were first treated with hydrochloric acid to remove surface contamination and rinsed with deionized water. The AgNO3 (aq) with a certain AgNO3 concentration was prepared using analytically pure AgNO3 reagent. The metal plates were immersed in the

Received date: March 25, 2011 Foundation item: National Science Foundation of China (50824001, 50771077, 50871080) Corresponding author: Shang Hongtao, Master, Associate Professor, School of Stomatology, Fourth Military Medical University, Xi’an 710032, P. R. China, E-mail: [email protected]; Zhou Zeyuan, Ph. D., E-mail: [email protected] Copyright © 2012, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

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nanoparticles distributed on the tip of the dendrite, the inset shows two contacted particles marked by dashed circle lines and their lattice fringes. The distance between two fringes of the two particles is 0.245 nm, corresponding to the 3×{422} supperlattice spacing in the [111] HRTEM image of the fcc Ag crystal[17]. The measured angle between fringes in these two particles is only 13º, which is in good agreement with 12º reported by Fang[14,15]. So these two particles nearly align in a common orientation and attach in an oriented fashion. For the fcc crystals the crystalline face indexes are perpendicular to the crystalline direction index. As a result, in a 200 mmol/L Ag nitrate solution, the oriented attachment planes of Ag nanoparticles are 3×{422} or {211} planes and the attached crystalline orientations are <211> directions. The small mismatched angle between attached particles will disappear via self-assembly when the reaction goes on and the attached nanoparticles will become a part of a single crystal. After 5 min reaction a dense dendrite will form as shown in Fig.1d. The inset is SAED pattern on the upper part of the dendrite and its indexes. Six spots attribute to 1/3{422} Bragg reflections and six spots attribute to {220} Bragg reflections arranged regularly, confirming that the dendrite is a single Ag crystal. Comparing the growth directions of the trunk and branches as marked by two arrows and <211> directions as noted in the inserted SAED patterns, one can find that the growth directions of the trunk and branches for the dendrite are all along <211> directions. In the solution the big particles can grow via primarily small nanoparticle aggregation or attachment. There are two pathways for the attachment of the primary building nanoparticles, one is random attachment and the other is oriented attachment. In the replacement reactions between Ag nitrate aqueous solution and Sn, when the AgNO3 concentration is low, for example 50 mmol/L, the primary reduced Ag nanoparticles aggregate randomly to form a secondary big particle, as can be confirmed by the SAED patterns consisting of diffraction rings[13-15]. The diffraction rings correspond to the randomly Bragg reflections of nanoparticles distributed without a same crystalline orientation, while the rings will disappear and regular spots will

AgNO3 solution in a container. The whole reaction was performed at room temperature and ambient pressure and lasted for a few min. After reactions, the plates coated with reactive products were washed with distilled water and ethanol in sequence. Finally, the products were collected for the observations of transmission electron microscope (TEM)，high transmission electron microscope (HRTEM) and field emission scanning electron microscope (FESEM). FESEM (JEOL JSM-7000F ) was used for morphology observation. TEM, selected area electron diffraction (SAED), energy dispersive X-ray (EDX) analysis were performed using a Hitachi Model H-800 TEM with an accelerating voltage of 200 kV. Samples for TEM observation were prepared by ultrasonic dispersion for 1 min of the as-prepared product with 10 mL of ethanol in a 30 mL conical flask. Then, the suspension was dropped onto a conventional carbon-coated copper grid and dried in air before observation.

2 Results and Discussion Fig.1a is a TEM image of a silver dendrite obtained via replacement reaction between 0.2 mol/L AgNO3 (aq) and Sn for 1 min, showing that the dendrite consists of numerous small Ag nanoparticles. The inserted EDX spectrum indicates that the composition of the particles was element Ag，the elemental Cu and C peaks come from the grid for supporting sample. Another inset is the corresponding SAED pattern of the circled area and its index. Six white spots marked by squares close to the center can be indexed as formally forbidden 1/3{422} Bragg reflections in [111] electron diffraction pattern[17]. However, these spots as shown in the inset are elongated, indicating a relatively imperfect single crystal characteristic. So the nanoparticles constructing the dendrite nearly share the same crystallographic orientation and they are an oriented dendrite aggregation. Fig.1b is a magnified image of the dendrite tip and the inset is the corresponding fast Fourier transform (FFT). Six white spots marked by squares contribute to the formally forbidden 1/3{422} Bragg reflections, indicating that all the particles share the same crystallographic orientation. Fig.1c is a HRTEM image of a

b

d

c 51 nm

20 nm Fig.1

10 nm

51 nm

5 nm

1 nm

500 nm

TEM image of a silver dendrite, the insets indicate the composition and SAED pattern of the dendrite, respectively(a); magnified TEM image of 1a, indicating that the dendrite consisted of numerous small nanoparticles, the inset is corresponding FFT image (b); HRTEM image of nanoparticles on the dendrite tip of 1b, the inset showing two oriented attached particles(c); TEM image of a dense Ag dendrite, the inset is SAED pattern of the top of the dendrite and its indexes (d)

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emerge when the nanoparticles distribute along a same crystalline direction. As shown in the inset in Fig.1b, the SAED patterns of aggregative nanoparticles present spotcharacteristic, suggesting that the nanoparticles reduced by Sn in 200 mmol/L Ag nitrate solution aggregate nearly along a same crystallographic orientation. As a result, 200 mmol/L may be the critical AgNO3 concentration above which the oriented aggregation will operate during the replacement reactions between Ag nitrate aqueous solution and Sn. High concentration of AgNO3 promotes the replacement reaction. When the concentration increases to 1 mol/L, the reaction product after 1 min reaction is Ag porous plate. Fig. 2a is a TEM image of a porous plate whose two dimensional size is several microns. The porous plate is an aggregation of dendrites. Fig.2b is a magnified TEM image of the left area of Fig.2a, showing the details of the dendrites. The length of the dendrites is over 1 μm, but the dendrites themselves are still comprised of primary nanoparticles. The inset is the SAED pattern of the dendrites and its indexes. Six weak spots marked by squares close to the center and six strong spots marked by circles can be indexed as formally forbidden 1/3{422} and {220} Bragg reflections in [111] electron diffraction pattern, respectively, presenting a single crystal characteristic. It can be found from Fig. 2b that the longitudinal direction of the dendritic trunk as noted by a dashed arrow is along <211> directions and the lateral direction is along <110> direction. This means that these dendrites grow longitudinally along <211> directions and attach laterally along <110> directions on {111} planes. So in this case the dendrites can be suggested as the second building units that attach along <110> directions and construct the porous plate as shown in Fig.2a. As indicated by white arrows, on the up-left area there are some dark stacked dendrites that construct the second dendrite layer on the former layer, suggesting that the porous plate will grow thicker in a layer-by-layer fashion in <111> directions. Fig.2c is a more magnified TEM image of Fig.2b, showing several particles on the tips of the dendrites. These particles a

already grow up and some of them are over 100 nm. On their surface distribute many much small particles whose size is about a few nanometers. During the particle growth period Ostwald ripening also plays an important role, where the very small nanoparticles that just attached on the surface of the larger particles become smaller and finally fuse. Here, it should be emphasized that the smaller particles finally fuse on the surface of bigger particles rather than dissolve in solution. When the reaction time prolongs to 5 min, the primary nanoparticles grew up continuously and the interface between them disappeared totally, resulting in the formation of dense plates. Fig.2d is a SEM image of overlapped Ag mesoscopic plates that still retained some dendrite characteristic. The inset shows a single mesocrystal with a thickness about 50 nm and the external crystal faces. Fig.3 displays a schematic representation of the formation process of a silver mesocrystal versus oriented attachment pathway. The formation starts from primary nanoparticles, forming a dendrite which grow along <211> directions on {111} planes. Then the dendrites as the second building units oriented attach along <110> directions forming a porous plate. Finally the dendrites oriented stack along <111> directions, forming a mesocrystal as shown in the inset of Fig.2d. When AgNO3 concentration increases to 2 mol/L, lots of Ag platelets are produced and they attach together to form a large triangle plate within 1 min. As shown in Fig.4a, the triangle plate is made of overlapped triangle platelets with a few pores. On the bottom part the platelets already grow up, their size is over 1 μm. On the top part a second or third layers (dark regions) are stacked on the first layer. Obviously, the plates grow thicker in a later-by-layer fashion, and the primary building units are small triangle platelets, as shown in Fig.4b. Fig. 4b is a magnified TEM image of the top part in Fig.4a, clear demonstrating that the overlapped triangle platelets on the second layer (the dark layer) and the third layer (the darkest layer). The sizes of the platelets on the second and third layers are over 0.5 μm.

b

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c 100 nm

500 nm

Fig.2

200 nm

51 nm

50 nm

1 µm

TEM image of a porous Ag nanoplate composed of dendrites (a), A magnified TEM image of Fig.2a, showing dendrites comprised of nanoparticles, the inset is SAED pattern and its index, indicating that the plate is a single crystal(b), A more magnified TEM image of Fig.2a, demonstrating many much small nanoparticles distribute on the big particle surface(c), SEM image of overlapped Ag mesocrystals, the inset shows a single mesocrystal with external faces (d).

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a

c

b

up and become mesoscopic plates. Fig.4d is another FESEM image of Ag mesoscopic plates with a thickness over 100 nm, clear demonstrating the external crystal faces.

3 Conclusions

Fig.3

A schematic representation of growth mechanism for the formation of a silver mesocrystal: (a) oriented attachment of nanoparticles along <211> directions, forming a dendrite; (b) oriented attachment of dendrite along <211> directions, forming a porous plate; (c) oriented attachment of dendrite along <111> directions, forming a mesocrystal

The inset in Fig.4b is the corresponding SAED pattern and its index. Six 1/3{422} spots, six {220} bright spots and other spots in [111] electron diffraction pattern can be clearly observed, demonstrating that the overlapped triangle platelets in Fig.4b construct a perfect Ag single crystal. Also it can be found in Fig. 4b that the longitudinal growth direction of the platelets noted by a blue arrow is along <211> directions because the blue line is parallel to another blue line noted in the inserted SAED pattern, and the lateral growth direction is along <110> directions. As a result, the primary building platelets will grow along <110> directions, and they stack together along <111> directions at the same time. Fig. 4c is a FESEM image of Ag plates obtained after 5 min reaction. It is clear that the plates have already grew

1) Silver mesocrystals can be synthesized via an oriented attachment of primary building nanoparticles in a series of simple and fast replacement reactions between AgNO3 solution and Sn metal. 2) In a 200 mmol/L solution, the primary nanoparticles orientationally aggregate or attach together along <211> directions to form a dendrite, the proposed attachment planes and directions for Ag nanoparticles are 3×{422} planes and <211> directions, respectively. 3) When the concentration increases to 1 mol/L, the primary building nanoparticles orientationally aggregate along <211> directions to form dendrites. The dendrites that can be proposed as the second building units orientationally attach along <110> directions to construct a porous monocrystalline plate, and finally transit to a mesocrystal with a thickness about 50 nm in 5 min. 4) When the AgNO3 concentration is 2 mol/L, the building units are Ag triangle platelets. These platelets also oriented attach along <110> directions to form a monocrystalline dense plate in 1 min and finally the plate transforms to monocrystalline mesocrystal with a thickness over 100 nm in 5 min.

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