New energy technologies have emerged that aim at harvesting environmental energy, which more often than not goes to waste, in a fashion that is clean and sustainable. The nanogenerator based on the pyroelectric effect, and more recently the thermoelectric effect, has been demonstrated to be an efficient and effective approach to harvesting environmental energy though both strategies exhibit shortcomings such as either low energy output or low efficiency. Thermoelectric nanocomposites have been developed by combining several nanomaterials to overcome the low efficiency, with special attention to the two-dimensional nanomaterial molybdenum disulfide, and graphene, which integrate the advantages of the two materials and avoids their weaknesses.
In a recent research published in Semiconductor Science and Technology, Yannan Xie and Zong-Hong Lin developed a flexible thermoelectric nanogenerator that is based on the molybdenum disulfide-graphene nanocomposite, which exhibits enhanced energy output and which can be applied as a temperature measurement sensor that is self-powered.
The authors prepared the molybdenum disulfide nanomaterials using a hydrothermal process, and mixed this with graphene paste in order to form the nanocomposite. The mixture was applied onto an indium tin oxide-polyethylene terephthalate substrate and dried to form the thermoelectric layer. The indium tin oxide layer and a silver paste were employed as the bottom and top conductive electrodes respectively, and the as-prepared nanocomposite as the thermoelectric layer in-between.
The research team observed that the molybdenum disulfide nanomaterial exhibits a nanoflower-like morphology of 1µm average size, and is made up of nanosheets that are well-dispersed. The nanosheets are composed of layered morphologies with 0.62 nm lattice spacing. Further, the nanomaterials exhibited 2 characteristic Raman bands at 360 cm-1 and 406-1. The thicknesses of the substrate and the molybdenum disulfide-graphene nanocomposite layers were noted to be 780 µm and 580 µm respectively.
From the analysis of the current-voltage characteristics between the as-developed device and pristine graphene and pristine molybdenum disulfide, it was noted that an ohmic contact was formed between the thermoelectric materials and the electrodes. Also, the as-developed nanocomposite displayed improved electrical conductivity as compared with pristine molybdenum disulfide, largely attributed to the presence of graphene which facilitates additional electron transport channels.
Further tests showed that with an increasing trend of temperature difference in the as-prepared device, there was a corresponding increase in voltage output, and the reverse was also true. At -35K, the as-developed device generates a -0.73mV output voltage, which is about 2 and 8 times larger as compared with devices having pristine molybdenum disulfide and pristine graphene respectively. The as-developed device has a power density of about 8.8nW/cm2, and a Seebeck coefficient of about 21µV/K as compared with 11µV/K for pristine molybdenum disulfide.
The polyethylene terephthalate substrate imparted the device with shape-adaptive and ultra-flexible properties, which grants it a wide range of applications for thermoelectric nanogenerators such as self-powered sensing. The authors tested this with a glass bottle having the thermoelectric nanogenerator to detect solution temperature, and noted the variation tendency between the output voltage and temperature differences fit a good linear relationship, which indicates its likely application in temperature sensors that are self-powered.
Yannan Xie, Ting-Mao Chou, Weifeng Yang, Minghui He, Yingru Zhao, Ning Li, Zong-Hong Lin. Flexible thermoelectric nanogenerator based on the MoS2/graphene nanocomposite and its application for a self-powered temperature sensor. Semiconductor Science and Technology, 32 (2017) 044003.Go To Semiconductor Science and Technology