PbTe Nanorods by Sonoelectrochemistry

Abstract

Size, shape, and compositional control are at the heart of nanochemistry. Herein, we present a novel method that allows control over all three variables in a simple one-step, wet-chemical procedure. One-dimensional (1D) nanomaterials are of great interest for the construction of highperformance thermoelectric (TE) devices. Theoretical calculations indicate that improvement in TE efficiency can be achieved as the diameter of the 1D structures approaches a few nanometers. To date, the most successful synthesis of 1D TE materials has been achieved by electrodeposition within alumina templates. A series of Bi2Te3, [4,5] Bi2 xSbxTe3, [6] Bi2Te3 ySey, [7] and Bi1 xSbx [8, 9] nanowires were prepared by using the template-based method. The advantages of the electrodeposition method include high efficiency, ease of control over composition, highly crystalline products, and room-temperature reaction conditions. However, the diameters of the nanowires synthesized by the template method are well above 10 nm. To get into the sub-10-nm regime, one needs to obtain templates with very narrow channel diameters, which is currently the limiting factor of this technique. However, advances in combining sonochemistry and electrochemistry have provided a new strategy for the synthesis of nanomaterials. The synthesis of quite sophisticated 1D nanomaterials has recently been demonstrated to be possible through careful control of the electrochemistry, sonochemistry, and initial composition of the precursor solutions. As observed, the advantage of the sonoelectrochemical method is that it achieves 1D control without any template, thereby practically overcoming the limitation of generating nanorods with diameters below 10 nm. This diameter is the size regime in which TE properties become enhanced and a controlled synthesis can produce technologically relevant nanomaterials. Herein, we report the first synthesis of monodispersed PbTe nanorods that are sub10 nm in diameter through a sonoelectrochemical technique. Furthermore, we present the effect of changing the concentration of the coordinating ligand on the resulting composition of the synthesized nanomaterials. Changing the metal/ ligand ratio enabled us to tune the composition of the product from pure Te to pure PbTe nanorods. Lead telluride is the material of choice because of its great potential in high-performance TE devices. Furthermore, it allows the mechanisms that lead to control over the resulting nanorod size and composition to be studied. Basically, the synthesis of PbTe nanorods consists of two steps: First, the electrodeposition of PbTe on the surface of the Ti sonication horn, and second, the dispersion of the PbTe nuclei into solution by pulsed sonication. Control over the electrodeposition process is crucial in obtaining pure and highly crystalline PbTe nanoparticles. Interestingly, we found that the Pb/nitrilotriacetic acid (NTA) ratio plays a key role in the synthesis of PbTe nanorods. In contrast to previous studies on the electrodeposition of bulk PbTe, we used a stoichiometric reactant ratio in the synthesis of the nanorods. The introduction of NTA helps to prevent the precipitation of Pb ions as Pb(OH)2. Furthermore, electrochemical studies show that NTA acts as more than a coordinating capping agent: it actually determines the final products of the process. The pH value of the solution was monitored during the electrodeposition, and TeO2 was shown to be sufficiently soluble at pH 7. However, the solubility of NTA was too poor at low pH values to keep the Pb ions coordinated and in solution. Therefore, the solution was kept at approximately pH 8 under our experimental conditions, as TeO2 is dissolved as TeO3 2 ions at this pH value. The overall PbTe reaction equation [Eq. (1)] is (the NTA ligand is omitted for clarity):