Influence of Polyoxyethylene Stearyl Ether and Subsequent Annealing on n-Type Doping of CNT Yarns for Flexible Thermoelectric Generators

Carbon nanotube (CNT) yarn is a promising element for flexible/wearable thermoelectric (TE) generators due to its high electrical conductivity and structural flexibility. However, one of the challenges is controlling the n-type doping and its air stability, which are essential for fabricating p-n-combined π-type cells and optimizing their performance. An obstacle to doping control is the unintentional p-type doping caused by oxygen in the atmosphere. This paper demonstrates a simple and effective way to fabricate weakly doped n-type CNT yarns using a nonionic oligomer surfactant, polyoxyethylene (50) stearyl ether, and postannealing. Although the as-prepared CNT yarn showed p-type characteristics in air as frequently reported, surfactant-adsorbed yarn became relatively stable n-type after annealing at 200-300 °C. However, annealing at 400-500 °C turned the CNT yarn into p-type again. Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy revealed that the surfactant physisorbed on CNT is positively charged to be multivalent cations and acts as an n-type dopant after annealing at 200-300 °C. However, the surfactant is removed after annealing at 400-500 °C, and negatively charged oxygen in the carboxy group, chemisorbed on CNT, acts as a p-type dopant. These results imply that the polyoxyethylene (50) stearyl ether plays multiple roles in obtaining n-type CNT yarn: a surfactant to disperse CNTs in water, an electron donor to make CNTs n-type, and a protective cap against oxygen attack on the CNT.