A study published in the journal Materials Chemistry and Physics reported the nanorod synthesis of single crystalline silver telluride (Ag2Te) in two stages: tellurium (Te) nanorod electroplating inside an ionized liquid, and then chemical cementation in an aqueous solution of silver nitrate.
Study: Insights in the two-step synthesis of single crystalline Ag2Te nanorods. Image Credit: Pomorzev/Shutterstock.com
The Importance of Thermoelectric Materials
Silver tellurides (such as Ag2Te, AgTe, Ag7Te4, and Ag5Te3) are semiconductor compounds utilized in data storing systems, optical components, and magnetic studies. Ag2Te, like many other metallic tellurides (Bi2Te3, Cu1.75Te, PbTe), is of great importance as a thermoelectric substance, capable of transforming heat into electrical energy and vice versa.
With the looming energy crisis, the thermoelectric effect looks to be a viable renewable power avenue for collecting wasted heat and limiting energy loss. The optimum thermoelectric substances have low heat conduction in combination with a high electric conduction and Seebeck coefficient.
Properties and Synthesis Methods of Silver Telluride
There are three distinct forms of Ag2Te. Ag2Te has the finest thermoelectric characteristics in the monoclinic form compared to the others. This material shows the behavior of a semiconducting material and has a tiny band gap, making it suitable for thermoelectric purposes in low-temperature settings.
The makeup may be changed to influence the characteristics of the charge carriers and produce tellurium-rich (p-type) or silver-rich (n-type) formations.
Ag2Te possesses a high power factor because of its excellent charge carrier mobility, weak heat conduction, and good electrical conduction.
The various approaches that may be used for Ag2Te nanorod synthesis include hydrothermal or solvothermal reduction, electroplating, and the vapor transportation method. The most popular fabrication techniques are solvothermal and hydrothermal processes, which may be split into two categories.
Single-Step Synthesis of Silver Telluride
In a one-step fabrication process, metal precursors Ag(I) and Te(IV) are mixed under the influence of a reduction agent (such as ascorbic acid, ethylene glycol (EG), hydrazine).
The process kinetics are sluggish and take one to two days to employ one-step techniques. Furthermore, contaminants like Te-rich nanostructures and Ag particles are harder to avoid.
The process is extremely responsive to pH, which must be set to the basic levels to prevent the creation of silver particles and Ag7Te4 on the surface of silver telluride nanorods.
To regulate reaction kinetics and prevent the appearance of Ag contaminants, complex substances such as Na2(EDTA) or MPA are often used. Another drawback of single-step fabrication is the struggle to control the crystallinity and dimensions of the resultant nanorods.
The Two-Step Synthesis Approach
A two-step fabrication may be performed in which silver or tellurium (nanoparticles or sheets) are first created and then transformed into Ag2Te through an oxidation-reduction reaction. However, the mechanism for this reaction is hitherto unknown.
This two-step approach preserves the purity and crystalline nature of tellurium nanorods. Furthermore, it inhibits the development of silver particles and derivatives (such as Ag5Te3, Ag4.5Te3, Ag7Te4) on the silver telluride nanorod surface.
Reaction duration, bath temperature, molar ratio of Ag(I) to Te, the introduction of a reduction agent (RA), and molar ratio of RA to Ag(I) all have an effect on the makeup, crystallinity, and structure of silver telluride. Ethylene glycol is usually utilized as a solvent and an Ag(I) RA as well, avoiding the development of problematic derivatives.
Findings of the Study
The two-step approach employed in this study enables silver tellurium nanorod synthesis with extremely pure monoclinic crystalline nature via chemical cementation, with tellurium nanorods acting as a reduction agent and a template as well.
The hexagon-shaped single-crystalline self-sustained Te nanorods with diameters of 60 ± 13 nm and lengths of not more than 200 nm were first electroplated in an ionized liquid medium.
The subsequent cementation process involved Te reducing Ag(I) to Ag(0) on the nanorod surface. Afterward, the integration of the silver atoms in a tellurium hexagon-shaped lattice occurred. A phase change occurred when the stoichiometry approached the desired silver telluride concentration.
The finding of an intermediary stützite stage prompted the experimental focus on the dynamics of this process for the first time. Single crystalline hessite Ag2Te nanorod synthesis was ultimately achieved with consistent chemical makeup and size.
This two-step fabrication technique seems appealing for the targeted production of high-quality Ag2Te nanorods as it is accomplished in ambient settings, without including harmful solvents, reduction agents, or surfactants.
Al-Hokayem, K., Ghanbaja, J., Michel, S., Legeai, S., & Stein, N. (2022). Insights in the two-step synthesis of single crystalline Ag2Te nanorods. Materials Chemistry and Physics. Available at: https://doi.org/10.1016/j.matchemphys.2022.126487