TiO2 Photocatalysis and Diamond Electrode

This paper presents a simple process for making tellurium (Te) nanoand microtubes of widely varying dimensions with the multiscale processing (MSP) technique. In this process, the Te metal was placed in a borosilicate glass reaction vessel and a fuse quartz substrate was added. Th vessel was evacuated and seal d unde vacuum with a torch. Then, the vessel was heated under a temperatur gradient where the portion of the tube with th substrate was under a decreasing temperatur gradient. Scanning and transmission electron microscopies hav sho n that multifaceted crystalline tubes have been formed extending from nanoup to micrometer scale with diameters ranging from 51.2 ± 5.9 to 1042 ± 134 nm between temperatures of 157 and 224 °C, respectively. One-dimensional tubular features are seen at lower temperatures and three-dimensional features at the higher temperatures. These features were characterized with X-ray diffraction and found to be trigonal Te with space group P3121. Our results show that the MSP can adequately be described using a simple Arrhenius equation. ■ INTRODUCTION Nanostructured materials have become a popular research topic since the early report of one-dimensional (1D) carbon nanotubes by Radushkevich and Lukyanovich. Since then, several 1D elemental microstructures have been fabricated that include (but are not limited to) Ag, Au, Bi, Co, Cu, Ge, Ni, Rh, and Si. Tellurium (Te), one of the more well-studied elements that often forms 1D structures, is an attractive material for many potential applications in energy and electronics,4,7−17 showing promise as a candidate material for photoelectrics, thermoelectrics, piezoelectrics, and gas sensors (e.g., for NH3, CO2, Cl2). 19−21 Tellurium nanomaterials have also been demonstrated to emit strong violet-blue and red radiation, making them useful for nano-optics. Fabrication of Te microand nanoscale structures has been demonstrated with a wide variety of techniques including biomolecule-assisted, chemical vapor deposition, physical vapor deposition, galvanic displacement, microwaveassisted, photothermally assisted, hydrothermal (or solvothermal),7,13−15 vapor transport, and electrochemical growth. The physical properties of the structures fabricated with these techniques vary widely in size and shape from several micrometers to a few nanometers in dimension. The most commonly observed features are 1D structures such as wires (or rods),4,7,8,11−15,17 tubes, and belts. Nanofeathers and peapods have also been reported, although these structures are not as prevalent in the literature. Here, we present our simple technique to fabricate multiscale Te structures that we have termed multiscale processing (MSP). This synthesis method, described in detail previously for AsxS100−x, 23−25 is a sublimation−condensation process where a small quantity of starting precursor is heated in an evacuated and sealed glass tube (ampule) under a measured temperature gradient, resulting in the production of various macro-, micro-, and nanostructures of a specific chemical stoichiometry in a single short heat treatment (2−4 h). Also, this technique does not involve any decomposition byproducts or generation of wastes, making it more environmentally friendly than some of the other aforementioned techniques. With the MSP technique, the distribution of macro-, micro-, and nanostructures can be tailored with careful control of (1) the initial pressure inside the reaction vessel, (2) the initial mass of material that will affect the amount of vapor available for deposition, and (3) the temperature gradient inside the reaction vessel. This technique allows for predictable and reproducible fabrication of a wide range of material structures in a single experiment and also provides a cheaper alternative to techniques that require high-pressure reactions (hydrothermal) or solvent-based reactions (solvothermal), because the only materials required for MSP are a reaction vessel, a substrate, and pure tellurium. ■ EXPERIMENTAL METHODS Specimen Preparation. The experimental design for the current work is presented in Figure 1. The reaction vessel used in this experiment was a 22 × 25 mm Pyrex tube that was necked down slightly around 200 mm up from the base (sealed end) with a torch. The substrates were two 20 × 50 mm (1 mm thick) fused quartz slides (substrate, GM Associates, Inc., Oakland, CA) tied together with Pt wire (Alfa Aesar, Ward Hill, Received: January 11, 2013 Revised: March 5, 2013 Published: March 11, 2013 Article