Nanosized Tellurium Preparation by Vibration Milling

Introduction Coarse powders of low-melting metals can be produced by spraying the molten metal, for example, commercial tellurium (m.p. 449 °C) is commonly prepared by this technique. Owing to the fast cooling step involved in this preparation method, the resulting tellurium solid drops have a polycrystalline structure. In order to obtain nanoscale powders a bottom-up approach (chemical methods) is usually carried out. In this paper, results on a top-down approach, based on dry vibration milling technology, to reduce the size of a brittle material such as tellurium and produce nanoscopic phases in a simple, effective, and inexpensive way, are reported. Indeed, nanostructures in the form of fine tellurium powder composed of grains with average size of a few nanometers were produced in air, without any temperature control and chemical reactions. Results and discussion To this aim, commercial pure tellurium powder (Aldrich, 99.8%, - 200 mesh) was grinded by the vibration milling technique using a Mixer Mill Retsch MM-200 apparatus, working for 7 hours at a frequency of 25 Hz. During the milling, in addition to the disaggregation in tellurium grains also a comminution process was involved (see Figure 1). In fact, the SEM micrograph in the left-side of Figure 1 shows that the starting tellurium powder was composed of solid drops having a size of ca. 30 mm and the XRD analysis showed that single crystal grains with an average size of 146 nm where included into the polycrystalline structure of these drops. On the other hand, the particles achieved after the milling treatment where much smaller with a diameter of ca. 4.8 nm, as displayed in the TEM micrograph on the right side of Figure 1. When the milling treatment is performed in air, also a slight oxidation process occurs at the tellurium grain surface. Raman scattering represents a very convenient approach for the study of the oxidation process involved in the nanoscopic tellurium preparation by vibration milling. In particular, the Raman measurements were carried out at room temperature with a Jasco Ventuno micro-Raman system in backscattering configuration, equipped with a Peltier-cooled charge-coupled device camera (operating temperature: - 50 °C) and a He-Ne laser (excitation wavelength: 632.8 nm). Proper laser power density was chosen to generate the best signal-to-noise ratio without broadening or shifting the Raman peaks due to local heating. In Figure 2 the Raman spectra of two tellurium samples prepared by coarse and milled powders are presented. Both spectra exhibit the main absorption peaks at 124, 144 and 645 cm-1. The last peak is originated from a vibrational band of TeO2, while the other peaks can be attributed to the vibrational modes of t-Te. As visible in the Figure 2, the sample of the starting pure tellurium powder contained only a very low amount of tellurium oxide (TeO2), while the amount of this solid phase increased significantly after the milling process. The amounts of elemental tellurium and TeO2 for both starting and milled samples were quantified by integrating the respective patterns in the XRD diffractogram It was found that the percentage of TeO2 in the starting material was ca. 19.8% by weight, while this percentage increased to 48.0% by weight after the milling process. Therefore, during the milling treatment the quantity of oxide phase significantly increases probably because of the much larger surface generated by the process. Nanoscopic tellurium can have many technological applications, for example it can be used in the fabrication of photoconductive polymeric films by binding the nanopowder with poly(methyl methacrylate) (PMMA).[2] The possibility to prepare tellurium nanostructures by a simple milling procedure makes this technique very promising for electronic device applications.