The deleterious effect of electromagnetic radiation on human health and sensitive electronic devices is matter of concern. The vast growth in use of portable and wearable smart electronics warrant development of thin, lightweight and flexible electromagnetic-interference (EMI) shielding materials.
Researchers at State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology, China and School of Materials Science and Engineering, Henan Polytechnic University, China have demonstrated the fabrication of freestanding, hydrophobic, lightweight, and flexible 2D transition-metal carbide (MXene) foams by assembling MXene sheets into films followed by a hydrazine-induced foaming process.
A stack of Ti3AlC2 sheets were used as the precursor. The Al in Ti3AlC2 sheets was selectively etched using LiF/HCl. Delamination of the sheets induced during etching has resulted in the formation of loosely stacked structure of Ti3C2Tx (MXene) with weakened interlayer interactions. The MXene film was prepared by vacuum-assisted filtration of an aqueous suspension of MXene using a polypropylene membrane. MXene films with desired thickness were obtained by suitably adjusting the concentration and volume of the MXene suspension. The freestanding MXene film exhibits excellent mechanical flexibility and withstand repeated folding and stretching. The MXene film sandwiched between two ceramic wafers was treated with hydrazine at 90 °C. Infiltration of hydrazine molecules into the interior of the MXene film through the numerous tiny channels created during vacuum filtration process has enabled the formation of a lightweight MXene foam with a cellular structure. The various stages involved in the fabrication of MXene foam is schematically illustrated in Fig. 1 along with the photographs of MXene suspension, film and foam.
Fig. 1 Schematic illustration of the various stages involved in the fabrication of MXene foam along with photographs of MXene suspension, film and foam
The morphological features acquired at the cross-section indicate that the MXene film possesses a compact structure with its layers arranged parallel to each other (Figs. 2(a) and 2(b)). This structural arrangement enables the MXene film a good flexibility and excellent mechanical properties. During hydrazine treatment, introduction of numerous small pores between the parallel layers which is accompanied by volume expansion has enabled the formation of MXene foam with a cellular structure (Figs. 2(c) and 2(d)). The reaction of hydrazine with the oxygen-containing groups of MXene accompanied by the rapid release large amounts of gaseous species overcome the van der Waals forces that hold the sheets together, resulting in a lightweight and flexible MXene foam with a cellular structure containing numerous pores.
Fig. 2 Cross-sectional SEM of: (a, b) MXene film; and (c, d) MXene foam
The MXene film and foam exhibit distinct wetting behaviors due to their difference in chemical composition. The MXene film is hydrophilic (water contact angle: 59.5°), an attribute which is originated from the MXene sheets containing oxygen and fluorine terminal groups. In contrast, the MXene foam is hydrophobic (water contact angle: 94.0°), resulting from the reaction of hydrazine with the oxygen-containing groups in the MXene film. The hydrophobic nature and porous structure of the MXene foam will be useful for selective absorption of organic solvents and oils.
The MXene film possesses a very high electrical conductivity of 400000 S/m and offers an excellent EMI-shielding performance at different thicknesses; ≈29 dB (1 μm), ≈47 dB (3 μm), and ≈53 dB (6 μm). During the preparation of MXene foams, the sample thickness is increased from 1 to 6 μm, 3 to 18 μm, and 6 to 60 μm and the introduction of insulating pores has lead to a decrease in their electrical conductivity to 58820, 62500, and 58000 S/m, respectively. It is difficult to retain the high electrical conductivity while increasing the thickness of MXene films by foaming. Nevertheless, the increment in thickness of the MXene foam outweighs the decrease in conductivity and improves its EMI-shielding performance. A 6 μm thick MXene foam offers an EMI-shielding effect of 70 dB as opposed to 53 dB for MXene film of similar thickness (Fig. 3).
Fig. 3 EMI-shielding efficiency: (a) MXene films; and (b) MXene foams
The lightweight, flexible, hydrophobic MXene foam with high strength, reasonable electrical conductivity and excellent EMI-shielding performance will be suitable for applications in defense, aerospace, and wearable electronics.
T.S.N. Sankara Narayanan
For more information, the reader may kindly refer: Ji Liu et al., Hydrophobic, Flexible, and Lightweight MXene Foams for High-Performance Electromagnetic-Interference Shielding, Adv. Mater. 2017, 1702367, DOI: 10.1002/adma.201702367
Recent Advancements in Science 