Synthesis Of Selenium Nanostructure In A Binary Soft-Template System Of SDS-PEG

Nano-selenium not only has the various physical and chemical properties, but also has low toxicity and high efficient biological activities. A number of methods are adapted to fabricate nano-selenium, among which using surfactants and polymers as soft-template in solution phase has attracted much attention. As a result, researches on nano-selenium have profound theoretical and practical value.In this paper, the necklace-like clusters composed of sodium dodecyl sulfate (SDS)-polyethylene glycol (PEG) were used as soft-template to control the synthesize of nano-selenium, from selenious acid (H₂SeO₃) reduced by hydrazine hydrate (N₂H₄·H₂O). The method has advantages of simple operation, low energy consumption and products with high purity.Surface tensions of SDS-PEG or SDS-polyvinylpyrrolidone (PVP) systems with or without selenious acid (H₂SeO₃) was measured by drop volume method, and steady-state fluorescence quenching technique with Py as a probe was used to detect the bound micelle aggregation number (N_b) and polarity of the micro-environment. The results show that, theγ-lgc curves of the system still have the characteristic of two critical concentrations. The value of c₁ keeps solid and c₂ increases with the increasing of PEG. The results of steady-state fluorescence quenching technique measured in SDS-PEG (PVP)-H₂SeO₃ systems show that, with the increasing concentration of SDS, the N_b increases and molecular microenvironment polarity of Py decreases. Fixing the concentration of polymers and SDS, with the increasing concentration of H₂SeO₃, the N_b of SDS-PEG (PVP) system increases and the molecular microenvironment polarity decreases. The increasing concentration of polymers causes a decrease of N_b and an increase of the molecular microenvironment polaritySelenium nanospheres were synthesized from H₂SeO₃ reduced by N₂H₄·H₂O in the soft template composed of SDS-PEG cluster in aqueous solution. XRD pattern suggests that the red product is amorphous selenium. DSC thermogram of the as-synthesized selenium nanospheres shows a melting peak at 221 oC. The concentrations of SDS and PEG have an influence on the size of selenium nanospheres. TEM images show that the average diameter of the products increases with increase in the concentration of SDS, while the increase in PEG concentration resultes in significant steric effect, the average diameter of the products reduces. UV-Vis spectrums show that the absorption of the products in solution becomes both stronger and red-shifted when the average diameter of selenium nanospheres increases. The TEM images of the products obtained in different reaction stages, clearly explained the growth process of the selenium nanospheres, and it shows that the binary soft-template has a two-level template effect. In the initial stage, selenious acid ions ( SeO₃^2-) are adsorbed to the SDS bound micelles by H⁺. With the reduction of SeO₃^2-, primary nano-selenium crystals with the diameter similar to SDS bound micelles are formed in the system; the SDS bound micelles assist the homogeneous nucleation, and SDS acts as the first level template effect. Along with the development of the reaction, more and more nano-selenium crystals attach on the chains of PEG, which makes nano-selenium crystals aggregate and grow into selenium nanospheres in the end, this is the second level template effect. Meanwhile, the soft clusters composed of SDS and PEG have a significant steric effect, which can avoid the selenium nanospheres getting assembled and improve the dispersion of the products.In ethanol/water system, with the regulation of SDS-PEG soft-template and spontaneously anisotropic crystal growth characteristics of the selenium nanocrystal, one-dimensional selenium nanostructure materials (selenium nanobelts and nanotubes) were successfully fabricated. XRD results show that the products are the standard trigonal selenium(t-Se); the results of HRTEM and SAED confirm that the products have the singal crystal structure, and grow along the longitudinal direction. TEM images of different reaction stages of the products inferent the growth process of one-dimensional selenium nanostructure, and confer the possible growth mechanism. In the preparation of selenium nanobelts, the selenium nanospheres prepared in the binary soft-template are dispersed in ethanol, with the reaction, amorphous selenium nanospheres gradually dissolved in ethanol, when getting saturated, the t-Se nanonucleations recrystallize and precipite in the solution. Selenium nanobelts are finally synthesized according to the crystal structure of t-Se with spontaneously anisotropic crystal growth characteristics, this process can be described as "solid-solution -solid" transformation. In the preparation of selenium nanotubulars, using ethanol as solvent, the initial reaction follows the "solid-liquid-solid" process as same as preparing nanobelts. Later in the nano-selenium crystal growth process, the soft clusters selectively adsorb on different crystalline grains of the nano-selenium surfaces and induce the nano-selenium crystals grow along a certain direction and eventually generate tubular structures. Low SDS concentrations generate nano-selenium thin and long selenium nanowires, with the SDS concentration increasing, the radial scale of selenium nanowires becomes wider, and shorter in length; in low PEG concentration system, irregular cross-linked nano-selenium structure are synthesized, and along with the increase in concentration of PEG, selenium nanostructures with tips appear. By changing the concentration of reducing agent and the content of ethanol, a variety of special morphologies of the nano-selenium structures are synthesized.