Article pubs.acs.org/crystal
Charge Supported Growth and Superplasticity of Sodium Nanostructures Wan Neng,†,‡ Li-Tao Sun,*,† Xiao-Hui Hu,† Yi-Yu Zhu,† Zha Lin,† Xu Tao,† Heng-Chang Bi,† Sun Jun,† and Fang-Zhou Dong† †
SEU-FEI Nano-Pico Center, Key Laboratory of MEMS of Ministry of Education, School of Electrical Science and Engineering, Southeast University, 210096 Nanjing, China ‡ Laboratoire de Physico-Chimie des Matériaux Luminescents, Université Claude Bernard Lyon 1, UMR 5620 CNRS−UCBL, 69622 Villeurbanne Cedex, France S Supporting Information *
ABSTRACT: Single-crystal sodium nanostructures (SNs) grown from sodium chloride (NaCl) powder were observed under e-beam irradiation by in situ transmission electron microscopy technique. A novel charge-supported growth (CSG) mode was proposed to explain the rich growth dynamics observed in the in situ experiments, including diameterdependent growth velocity, self-avoiding growth mode, and seized growth. The electrostatic effect was found to be the main driving force of the CSG. Superplasticity was also found in the single-crystal SNs that showed plastic superplastic elongations of ∼300% upon tension. The small size effect was found to be the main reason for the superplasticity.
1. INTRODUCTION Nanotechnology has become one of the most interesting fields nowadays due to its importance in numerous applications.1−4 The preparation and properties of different nanostructured materials are two of the most important issues in nanotechnology.2−4 To date, different kinds of nanostructured materials (organic, inorganic, and organic−inorganic hybrid) have been prepared by various methods, and their properties have been extensively studied.2−4 However, there are still challenges encountered in studies on some nanostructure materials. As a common metal, sodium is characterized by a low melting point (371 K), softness (Young’s modulus = 10 GPa), and high activity. Due to these properties, sodium nanostructures (SNs) are very difficult to prepare in a well-controlled manner. Consequently, studies on their properties are also challenging. To date, reported methods for the preparation of SNs are limited to physical vapor deposition from sodium metal,5 plasma generation from sodium metal,6 and energetic particle/photon irradiation (EPI) of NaCl salt.7 Among these methods, EPI is the most widely used due to its advantages of easy operation and high yield. EPI is used to prepare various nanostructured materials.8−15 For example, P.-I. Wang et al.8 reported the growth of Cu nanorods by e-beam irradiation, and a model involving atom migration was proposed. S. S. Oh et al.9 fabricated indium nanowires by irradiating InGaN layers using a Ga+-focused ion beam, and a very large growth rate (∼102 nm/s) is obtained. Jay et al.10 observed the growth of lithium nanoparticles under © 2012 American Chemical Society
e-beam irradiation. Other reports indicate that although the growth of nanostructured materials under EPI is widely observed, the detailed growth mechanisms are much less studied. Recent studies found that charging,11 atom migration,8 and redeposition15 may work during the EPI nanostructure growth process, but more detailed studies are needed. There is even less information about the detailed growth mechanisms of SNs due to difficulty in realizing well-controllable sample preparation and preservation. The loss of lattice ions due to EPI collision and subsequent sodium accumulation are proposed to explain the formation of the sodium phase,7 but the pertinent mechanisms remain unknown. Thus, the growth mechanisms and properties of SNs must be thoroughly investigated. In the present study, e-beam irradiation and transmission electron microscopy (TEM) were used to fabricate and monitor the growth of SNs from sodium chloride (NaCl) powder. The detailed growth processes were studied in situ using selected area electron diffraction (SAED) and energy dispersive spectroscopy (EDS). Rich growth dynamics, including diameter-dependent growth velocity, self-avoiding growth mode, and seized growth, were observed during SN growth. A charge-sustaining growth mechanism based on the electrostatic effect is proposed based on the TEM observations. After studying the single-particle mechanical behaviors of the Received: February 23, 2012 Revised: June 1, 2012 Published: July 2, 2012 3899
dx.doi.org/10.1021/cg300264b | Cryst. Growth Des. 2012, 12, 3899−3905
Crystal Growth & Design
Article
SNs, superplasticity was observed, and the possible mechanisms are discussed.
2. EXPERIMENTAL SECTION NaCl powder (purity >99%) was used as the source material. It was ground into fine powder and then dried in a desiccator (humidity 5 A/cm2). This is totally different from the case when using conducting Cu meshes. Based on this, it is believed that the growth of SNs should be directly related to the charges in the NaCl particle. Moreover, most NaCl particles tend to detach easily from 3903
dx.doi.org/10.1021/cg300264b | Cryst. Growth Des. 2012, 12, 3899−3905
Crystal Growth & Design
Article
where Tb (371 K) is the bulk melting temperature, Tm is the melting temperature for a particle of radius R, L (113 kJ/kg) is the molar latent heat of fusion, and γ (γliq = 0.18 J/m2 and γsol = 0.2 J/m2)41 and ρ (ρliq = 0.93 g/cm3 and ρsol = 0.97 g/cm3) are the surface energy and density, respectively. The subscripts “sol” and “liq” refer to solid and liquid, respectively. A rough estimation using eq 2 gives a considerably low melting point in a 27 nm sized sodium nanoparticle even considering the several tens of times of increase in surface energy in small-sized nanoparticles.42,43 Previous reports also found decreased melting point especially in nanoparticles smaller than 100 nm.31 Consequently, a melting point decrease can cause solid− liquid transition during the in situ tension manipulations. Melt SNs may enhance plasticity by fast mass transport during tension, which also explains the uniformly thinned dynamics without necking.
4. CONCLUSIONS In conclusion, the growth of nanometer-sized single crystal SNs from NaCl powders was observed under e-beam irradiation by TEM. The growth of BCC single-crystal SNs was verified by different TEM techniques including electron EDS and SAED. The entire growth process was observed in situ. Rich growth dynamics such as diameter-related growth velocity, selfavoiding growth mode, and seized growth were observed. The largest growth velocity found was 38 nm/s. The growth mechanism was discussed based on the vapor−solid growth process. A CSG model was proposed to explain the experimental results. The combined effects of e-beam irradiation and electrostatic force were found to be the driving forces. Superplasticity was found in the SNs. An ∼300% superplastic elongation was found. The possible mechanisms including e-beam irradiation, SN melting, and small size effect were discussed based on the observations. The small size effect was considered to be the main effect.
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ASSOCIATED CONTENT
S Supporting Information *
Three movie files. This material is available free of charge via the Internet at http://pubs.acs.org.
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Figure 7. (a−f) Sequential TEM images show the pulling of an SN. The SN has an initial diameter of 60 nm (a). Before breaking, it is 27 nm in diameter and 180 nm long (e). After breaking, two droplet-like parts with a diameter of ∼30 nm are produced (f). Bar = 100 nm.
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[email protected]. Notes
The authors declare no competing financial interest.
be caused by two effects. First is the temperature increase caused by e-beam heating.39,40 However, as most SNs show facet crystalline structure under e-beam irradiation (as seen in the figures above), the temperature increase created by e-beam irradiation should be less than the melting point of the SNs (at least
