Ag2Te Nanowires Research Articles

Controlled Synthesis of Tellurium Nanostructures from Nanotubes to Nanorods and Nanowires and Their Template Applications

In the present study, we report a facile hydrothermal route developed to synthesize various 1D tellurium nanostructures including nanotubes, nanowires, and nanorods on a large scale. Te nanotubes with a tunable diameter from 200 nm to 2 μm are fabricated in NaOH solution. With poly(vinyl pyrrolidone) as the surfactant, ultrathin Te nanowires with a diameter of 5−8 nm are synthesized in NaOH solution. However, in both cases, only Te nanorods are obtained if NaOH is not added. The formation of Te nanotubes is attributed to the depletion of Te atoms at the surface of seeds. The influence of the reaction conditions including pH value, reaction temperature, reducing agent, and reactant concentration on the size and morphology of the Te nanostructures is investigated and the structural evolution with different growth rate is illustrated. A reduction reaction rate controlled growth mechanism has been proposed for the variable nanostructures. In addition, the obtained Te nanostructures are ideal templates to synthesize other Te-related nanocompounds, which is demonstrated by the synthesis of ultrathin Ag2Te nanowires through the direct reaction of ultrathin Te nanowires with AgNO3 at room temperature.

Hybrid A–B–A type nanowires through cation exchange

Hybrid A–B–A type nanowires (NWs) with Ag5Te3–HgTe–Ag5Te3 composition have been created by the reaction of Hg2+ with Ag2Te NWs. The NW morphology of Ag2Te is preserved upon reaction with minor changes and the two separate phases formed are spatially separated within the same NW. The reaction of Hg2+ with Ag2Te NWs was monitored at different concentrations and the reactivity was attributed to cationic exchange depending on solubility products. Hybrid NWs were formed by partial cation exchange only at low concentrations (below 50 ppm) resulting in Ag5Te3 and HgTe within the same NW. However, at high concentrations (above 100 ppm), the HgTe phase alone was formed. These studies have been extended to other metal ions such as Pb2+, Cd2+, and Zn2+ whose reactivity towards Ag2Te NWs is different from that of Hg2+. These ions form a passivating Te oxide layer upon reaction with other metal ions. The mechanism of reactivity of Hg2+ is explained on the basis of free energy of formation of the ionic solid. Phase transition of Hg2+-reacted NWs occurs at a lower temperature than the parent (Ag2Te NWs) and other metal ions-reacted Ag2Te NWs. Details of the process were elucidated using microscopic and spectroscopic investigations. The physical and chemical properties of the individual components within a NW are expected to provide a novel functionality to the metal chalcogenide systems.

In Situ TEM Observation of Heterogeneous Phase Transition of a Constrained Single-Crystalline Ag2Te Nanowire

Laterally epitaxial single crystalline Ag2Te nanowires (NWs) are synthesized on sapphire substrates by the vapor transport method. We observed the phase transitions of these Ag2Te NWs via in situ transmission electron microscopy (TEM) after covering them with Pt layers. The constrained NW shows phase transition from monoclinic to a body-centered cubic (bcc) structure near the interfaces, which is ascribed to the thermal stress caused by differences in the thermal expansion coefficients. Furthermore, we observed the nucleation and growth of bcc phase penetrating into the face-centered cubic matrix at 200 °C by high-resolution TEM in real time. Our results would provide valuable insight into how compressive stresses imposed by overlayers affect behaviors of nanodevices.

Chemical Transformations in Ultrathin Chalcogenide Nanowires

We have studied the chemical transformations in ultrathin chalcogenide nanowires with an aim to understand the parameters that control the morphology and crystal structure of the product. Ultrathin Te nanowires were transformed into Ag2Te nanowires with preservation of the single crystallinity. The Ag2Te nanowires were then converted into CdTe, ZnTe, and PbTe using cation-exchange reactions, and the CdTe nanowires were further transformed into PtTe2 nanotubes. On the basis of the solubility products of the ionic solids, the crystal structures of the involved solids, the reaction kinetics, and the reaction conditions for transformations, we were able to reach the following conclusions: (i) The solubility products of ionic solids can be used as a rough criterion to predict if the transformation is thermodynamically favorable or not. (ii) The morphological preservation of reactant nanowires is more sensitive to the change in length rather than the total volume in addition to the lattice matching between the reactant and product nanowires. (iii) The crystal structure resulting from a transformation should be determined by the free energy of formation and the stability of the products. (iv) The transformation involving small volume change or topotactic lattice matching is considered homogeneous along the entire length of the nanowires, preserving both the single crystallinity and the morphology of the reactant nanowires.

Optical limiting properties of Te and Ag 2Te nanowires

We have prepared tellurium (Te) and silver telluride (Ag 2Te) nanowires in solution and investigated their nonlinear optical transmission by the open-aperture z-scan technique, using nanosecond laser pulses, at the excitation wavelengths of 532 and 1064 nm. The nonlinearity has contributions from absorption saturation, excited state absorption and nonlinear scattering processes. The saturation intensity and nonlinear absorption coefficient are calculated numerically. Nonlinear absorption in Ag 2Te nanowires is found to be one order of magnitude higher than that in Te nanowires. Observation of optical limiting at both excitation wavelengths indicates the potential of these materials to be used as broadband optical limiters.

Phase-Transition-Dependent Conductivity and Thermoelectric Property of Silver Telluride Nanowires

Silver telluride nanowires have been synthesized by the composite-hydroxide-mediated (CHM) method at temperatures of 180−225 °C. The products were single-crystalline nanowires and monoclinic phase Ag2Te with a high purity. The reversible structural phase transition from monoclinic structure (β-Ag2Te) to face-centered cubic structure (α-Ag2Te) around 148/133 °C was observed during the heating/cooling process. The latent heat of the phase transition was calculated. The electrical resistance and thermal emf versus temperature were for the first time investigated on the Ag2Te nanowires. Abrupt changes of conductivity and Seebeck coefficient around the phase transition temperature were observed. The maximum Seebeck coefficient is ∼170 and ∼160 μV K−1 during the heating and cooling process, respectively, indicating a promising one-dimensional thermoelectric material as a building block to construct nanodevices.

The Effect of Stabilizer Density on Transformation of CdTe Nanoparticles Induced by Ag Cations

In this study the strong effect of surface density of stabilizers on the outcome of the typical cation replacement reaction in nanoparticles is demonstrated. The density of 2-aminoethanethiol coating determines the growth mode and morphology of the transformation products of CdTe nanoparticles induced by Ag cations. Using quantitative measurements of the stabilizer's surface density, it is demonstrated that CdTe nanoparticles produce Ag2Te nanowire networks upon partial removal of the stabilizer. This process follows the kinetically controlled growth mode but the mechanism changes drastically once stabilizer density is increased. The formation of spherical Ag2Te nanoparticles was observed when CdTe retained the original density of 2-aminoethanethiol. This study provides better understanding of (1) non-classical growth mechanisms of crystals at the nanoscale, (2) effect of stabilizer density on chemical transformations of nanoparticles, and (3) new routes for preparation of nanomaterials.

Silver telluride nanowires prepared by dc electrodeposition in porous anodic alumina templates

Silver telluride nanowire arrays were synthesized by cathodic electrolysis into porous anodic alumina membranes from dimethyl sulfoxide (DMSO) solutions containing 0.1 M NaNO3, 5.0 mM AgNO3 and 6.0 mM TeCl4. XRD and SAED patterns of the nanowires were indexed to be a monoclinic structure of Ag2Te. TEM images show that the nanowires have smooth surfaces with uniform diameters. Near stoichiometric Ag2Te nanowires were obtained for potentials between −0.55 and −0.65 V vs. Ag/AgCl. Furthermore, the composition of the nanowires can be controlled continuously from Ag-rich to Te-rich mainly by changing the concentration of TeCl4 in the solutions.