Huang Huang, Li Junxiao, Ke Haibo, Du Yangjian, Peng Wenlian, Dai Mengyan, Zhang Yufeng, Zhang Xue-Ao
College of Physical Science and Technology, Xiamen University, Xiamen, Fujian 361000, China.
Institute of Chemical Defense, Academy of Military Science, Beijing 102205, China.
ACS Appl Mater Interfaces. 2021 Jun 9;13(22):26256-26263. doi: 10.1021/acsami.0c22167. Epub 2021 May 24.
Multilayer graphene has been employed as a functional material for tuning the emissivity in mid- and long-infrared range, which shows great potential for various applications, such as radiative cooling and thermal camouflage. However, the stability of the multilayer graphene is not sufficient for practical applications yet. Even though it is reported that the integrity of the multilayer graphene is compromised by ion intercalation, the detailed mechanism is rather unclear. Here, a set of ionic liquids is deployed as sources of electronic charges for tuning the emissivity of multilayer graphene. It is found that the emissivity modulator using 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([EMIm]NTf) as the ionic liquid provides a modulation depth of about 0.52 (i.e., about 21% larger than the best-reported value) while maintaining a reasonable device lifetime. The microscopic structures of the multilayer graphene in an operational and failure modulator are investigated by scanning electron microscopy, Raman spectroscopy, X-ray diffraction. The results indicate that the modulation depth of emissivity is negatively correlated with the initial voltage, which represents the reaction potential between the ionic liquid and graphene. Furthermore, not only the chemical reactivity but also the size of both anion and cation in the ionic liquids play important roles in maintaining stability of the modulator. Therefore, a set of criteria (e.g., low initial voltage and small size of anion and cation) is proposed to select proper ionic liquids for emissivity modulation. This not only sheds light on the underlying physics of the modulator but also promotes its practical applications.
多层石墨烯已被用作一种功能材料，用于调节中红外和长红外范围内的发射率，这在诸如辐射冷却和热伪装等各种应用中显示出巨大潜力。然而，多层石墨烯的稳定性对于实际应用来说仍不够。尽管有报道称多层石墨烯的完整性会因离子插层而受到损害，但其详细机制尚不清楚。在此，一组离子液体被用作电荷源来调节多层石墨烯的发射率。研究发现，以1-乙基-3-甲基咪唑双（三氟甲基磺酰）亚胺（[EMIm]NTf）作为离子液体的发射率调制器在保持合理器件寿命的同时，提供了约0.52的调制深度（即比报道的最佳值大约21%）。通过扫描电子显微镜、拉曼光谱和X射线衍射研究了工作和失效调制器中多层石墨烯的微观结构。结果表明，发射率的调制深度与初始电压呈负相关，初始电压代表离子液体与石墨烯之间的反应电位。此外，离子液体中不仅化学反应性，而且阴离子和阳离子的大小在维持调制器的稳定性方面都起着重要作用。因此，提出了一组标准（例如，低初始电压以及小尺寸的阴离子和阳离子）来选择用于发射率调制的合适离子液体。这不仅揭示了调制器的潜在物理原理，也促进了其实际应用。
