# One-Dimensional Nanostructure Engineering of Conducting Polymers for Thermoelectric Applications
## 2 D-Conducting Polymers and Their Thermoelectric Properties
### Ppy  
PPy is amongst the family of conducting polymers that is widely used in broad applications such as anti-electrostatic coatings, biosensors, solid electrolytic capacitors, polymer batteries, etc. [64; 65; 66]. It possesses a wide variety of advantages such as simple synthetic methods, commendable electrical conductivity, low thermal conductivity, good environmental stability and low toxicity, and proved to be a potential thermoelectric material to yield great thermoelectric properties [67; 68; 69; 70]. In 2014, [PERSON] et al. first reported the fabrication of free-standing PPy nanotube films for thermoelectric applications [71]. Two kinds of PPy nanotubes (Ppy-1 and Ppy-2) were prepared via similar self-degraded template methods which involves the oxidation of pyrrole monomers in the presence of methyl orange as the soft templates. The as-prepared PPy nanotube suspensions were dried in petri dishes to afford free-standing flexible films through self-assembly (Figure 6a,b). As shown in the SEM and TEM images (Figure 6c-f), PPy-2 nanotubes exhibit longer length and smaller diameters (-40 nm), leading to higher electrical conductivity and Seebeck coefficient at different temperatures (Figure 6g,h). At 310 K, the PPy-2 films exhibit a power factor of 0.31 \(\upmu\)W m\({}^{-1}\) K\({}^{-2}\) and a ZT value of 5.71 \(\times\) 10\({}^{-4}\). The ZT\({}_{\text{max}}\) of PPy-2 film can reach 7.84 \(\times\) 10\({}^{-4}\) at 370 K, which is triple that of PPy-1 film. The developed free-standing PPy nanotube films can be applied to fabricate flexible thermoelectric devices.  
In 2018, [PERSON] et al. successfully prepared PPy of different texture and topography (nanoparticles, nanotubes and nanowires) with or without various surfactants (methyl orange (MO), cetyltrimethyl ammonium bromide (CTAB)) and using different oxidants (ammonium peroxdyisulfate (APS), FeCl\({}_{3}\)) through a chemical oxidative polymerization approach (Figure 7) [72]. The effects of different  
Figure 5: TEM images of (**a**) neutral and (**b**) Ag doped P3 HT nanonet (the inset shows a SEM image of the silver particles. (**c**) Comparison of the power factors measured for the bulk and nanonet structured P3 HT, doped with 1 mM AgClO\({}_{4}\) solution. (**d**) Maximum power factor of the P3 HT nanonet at different oxidant concentrations. (**e**) Scanning force microscopy (SFM) image of a P3 HT nanofiber mat (10 \(\times\) 10 m) and (**f**) corresponding binarized image. Readapted with permission from The Royal Society of Chemistry [62] and MDPI [63].  
morphologies on their thermoelectric properties have been studied, and PPy nanowires with a diameter of 30-60 nm were found to exhibit the highest electrical conductivity and Seebeck coefficient, leading to an optimal power factor of 0.3 \(\upmu\)W m\({}^{-1}\) K\({}^{-2}\) at 380 K. Using the similar approach, [PERSON] et al. also studied the controlled synthesis of 1D nanostructured PPy with various morphologies (straight wires, curly wires, necklace-like structure, etc.) by adjusting the experimental conditions, including type of oxidizing agent, concentration of oxidizing agent, duration of polymerization and the solvent used for the reaction mixture [73]. It was found that the electrical conductivity and power factor were more easily affected than the Seebeck coefficient, by the morphology of PPy.