Solution–solid growth of α-monoclinic selenium nanowires at room temperature

Abstract

A wet chemical method for the preparation of α-monoclinic selenium nanowires at room temperature has been developed; in aqueous solution, selenium molecules produced from the decomposition of selenodiglutathiones continually stack on previously formed α-monoclinic selenium nanoparticles along the [001] direction, gradually producing α-monoclinic selenium nanowires.

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During the last few years, there has been growing interest in one-dimensional nanowires owing to their novel electrical and optical properties, and potential applications in nanometer-sized devices.¹ So far, many approaches have been developed for the synthesis of nanowires, and most of them are based on two growth mechanisms: (i) catalysis by metal nanoparticles in a fluid (vapor or liquid) phase² and (ii) the use of templates to confine the growth of materials.³ However, the problem of how to separate the metal catalysts or the templates from the nanowires produced by the above-mentioned methods is still a challenging one.

Here, we introduce a new and simple method for the fabrication of single crystal α-monoclinic selenium nanowires by using a solution–solid growth method. These pure, freestanding, single crystal selenium nanowires show novel physical properties and have potential applications in nanometer scale photoelectric devices, since selenium has valuable photoelectric properties, such as high photoconductivity (∼8 × 10⁴ s cm⁻¹)⁴ and low photomelting temperature (∼77 K),⁵ and has been widely used in the fields of solar cells, xerography, rectifiers, etc.⁴

Briefly, α-monoclinic selenium nanowires have been prepared by a wet chemical approach in aqueous solution at room temperature.⁶ This method is primarily concerned with the production of α-monoclinic selenium nanoparticles through reduction of sodium selenite with glutathione (GSH). Glutathione, a small peptide molecule with one thiol group, reacts vigorously with sodium selenite to form selenodiglutathione (GSSeSG), which slowly decomposes to produce selenium molecules and diglutathione (GSSG).⁷ These steps can be represented as follows:

Na₂SeO₃ + 4GSH → 2GSSeSG + 2NaOH + H₂O (1)

8GSSeSG → Se₈ + 8GSSG (2)

Reaction 1 proceeds very rapidly. Selenium molecules (Se₈) are slowly produced from GSSeSG through reaction 2 in weak alkaline solution, and then aggregate together to form selenium clusters or nanoparticles.^7,8 This process was performed by storing the solution for three days at room temperature. Transmission electron microscope (TEM) observations (JEOL 2010 high resolution transmission electron microscope, accelerating voltage 200 keV) revealed that spherical selenium nanoparticles were formed in the solution [Fig. 1(a)]. The diameters of 200 of these selenium nanoparticles were measured from the TEM images, and an average of 60 ± 5 nm was obtained. Selected-area electron diffraction (SAED) experiments showed that the nanoparticles have α-monoclinic crystalline structures [inset in Fig. 1(a)].⁹

Fig. 1 (a) TEM image of selenium nanoparticles. The inset shows the SAED pattern of a nanoparticle. (b) TEM image of selenium nanorods. The inset shows the SAED patterns of a nanorod. (c) TEM images of selenium nanowires. The inset shows the SAED pattern of a single nanowire.

When the solution containing selenium nanoparticles and GSSeSG was stored for a further four days at room temperature, some products in form of nanorods were obtained [Fig. 1(b)]. The SAED patterns of the nanorods [inset in Fig. 1(b)] revealed that the α-monoclinic crystalline nature of the nanoparticles was retained in the nanorods.⁹ TEM observations [Fig. 1(c)] also indicated that storage of the solution for an extended period leads to the formation of selenium nanowires. The nanowires were straight with smooth surfaces, and had almost uniform diameters along their lengths. Most of the selenium nanowires had diameters of about 60 nm, although some wires with smaller diameters were also observed. The SAED pattern [inset in Fig. 1(c)] indicates that the selenium nanowires are single crystalline with α-monoclinic structures.⁹

The growth direction of the α-monoclinic selenium nanowires was characterized directly by high resolution transmission electron microscopy (HRTEM). The HRTEM images in Fig. 2 clearly reveal (002) and (220) lattice fringes with spacings around 0.453 and 0.357 nm, respectively, which match those reported for α-monoclinic phase selenium (JCPDS File No. 24-1202). The (002) crystal planes are approximately vertical to the long axis of the selenium nanowires, which shows that the selenium nanowires predominantly grow along the [001] direction.

Fig. 2 HRTEM and TEM images of an α-monoclinic selenium nanowire.

The growth of single crystal α-monoclinic selenium nanowires through such a chemical route at room temperature is an interesting result. Previously, Abdelous et al.¹⁰ prepared selenium nanowires composed of nanoparticles, and Tang et al.¹¹ also found that CdTe nanoparticles could spontaneously reorganize into nanowires through dipole–dipole interactions in aqueous solution at room temperature. However, their products are different from our single crystal α-monoclinic selenium nanowires, as are the growth mechanisms. We suggest that the single crystal selenium nanowires result from the growth of α-monoclinic selenium nanoparticles along the [001] direction. As the nanoparticles are generated, the Se₈ molecules subsequently produced from GSSeSG stack on these particles, and the self-arrangement of Se₈ molecules along the [001] direction of the α-monoclinic selenium nanoparticles results from a tendency to decrease the free energy of the nanoparticles, since the {002} planes have higher surface energy,^8–10 considering that this process is thermodynamically favorable. As more and more Se₈ molecules stack on the α-monoclinic selenium nanoparticles along the [001] direction, selenium nanowires with α-monoclinic structures enclosed with lower surface energy {220} facets are produced. In this solution–solid growth process, the diameter of the nanowires is mainly influenced by the size of the selenium nanoparticles, which are the seeds for the final products. The growth mechanism of the selenium nanowires is shown in Fig. 3.

Fig. 3 Schematic illustration of the growth mechanism for the α-monoclinic selenium nanowires.

In summary, we have prepared pure, freestanding selenium nanowires with α-monoclinic phase structures through a solution–solid growth process. Since they were produced at room temperature without the assistance of templates or metal catalysts, the straight, long, defect-free selenium nanowires have the potential to be utilized in a wide variety of fields. In addition, the solution–solid growth mechanism could be adapted to fabricate other kinds of nanowires besides selenium.

Acknowledgements

This work was partially supported by the National Major Project of Fundamental Research: Nanomaterials and Nanostructures (Grant No. 19994506). The authors thank Prof. Shuyuan Zhang for his help with the HRTEM observations and Prof. Changhui Ye for his advice on the preparation of the manuscript.

Notes and references

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6. Preparation of α-monoclinic selenium nanowires and TEM specimens: sodium selenite and glutathione were purchased from the Shanghai Chemical Company and used without further purification. Sodium selenite (2.5 mmol) was dissolved in 500 ml double-distilled water in a glass beaker and glutathione (10 mmol) was subsequently added to react with the sodium selenite. During the above-mentioned process, the solution was stirred magnetically at 150 rpm. This solution was then kept still at room temperature for twelve days. During the storing process, the solution slowly became red in color and dark red products deposited from the solution. At various stages during the storing process, several droplets of solution containing products were deposited on Cu grids coated with C films and dried in air at room temperature for observation by TEM..
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Footnote

† Electronic supplementary information (ESI) available: EDS spectrum of α-monoclinic selenium nanowires. See http://www.rsc.org/suppdata/jm/b2/b209399e/

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