Freestanding, Hydrophobic, Flexible, Lightweight 2D Transition-Metal Carbide Foams for Electromagnetic-Interference Shielding

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 Ti₃C₂T_x (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

Formation of Luminescent Carbon Nitride Nanosheets by Spontaneous Liquid Phase Dissolution

2D materials due to their unique physical and chemical properties assume significance in a wide variety of applications. Among the various strategies employed for the synthesis of 2D materials, liquid phase exfoliation from layered crystalline precursors (bottom-up route) is considered to be beneficial. Nevertheless, use of aggressive chemicals and formation of fragmented or chemically modified nanosheets limit the applicability of this methodology. Researchers at University College London, Imperial College London, University of Bristol, United Kingdom and École Polytechnique Fédérale de Lausanne, Switzerland have demonstrated a liquid phase dissolution route for the synthesis of 2D carbon nitride (CN) nanosheets using poly(triazine imide)-lithium bromide (PTI-LiBr) as the crystalline precursor and aprotic polar solvents as the liquid phase. The spontaneous dissolution of PTI-LiBr in organic solvents yield solutions containing defect-free, crystalline, 2D CN nanosheets.

Dicyandiamide (DCDA), lithium bromide (LiBr) and potassium bromide (KBr) were used as the starting materials. 2 g of DCDA was mixed with 10 g of the LiBr/KBr (52%:48%) and thoroughly ground. 7 g of the ground homogeneous powder was heated to 400 °C under flowing N₂ and soaked at 400 °C for 6 h. 4 g of this pretreated mixture was placed inside a quartz tube sealed at one end. The quartz tube was evacuated to < 10^-6 mbar and sealed. The quartz ampoule was heated to 600 °C for 12 h. The resultant brown coloured  material was removed from the ampoule, repeatedly washed with hot deionized water, centrifuged at 4000 rpm and the retrieved PTI-LiBr was washed with methanol. The structural and morphological properties of PTI-LiBr are shown in Fig. 1

Fig. 1 (a) XRD pattern of crystalline PTI·LiBr (Inset: one unit cell of a PTI·LiBr); (b) SEM image of an aggregate of hexagonal prismatic PTI·LiBr crystallites (Inset: TEM image of hexagonal PTI·LiBr crystallites).

Dissolution of as-synthesized PTI-LiBr crystals in N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) indicates a change in the color of the liquid over time (Fig. 2(a)). The extent of dissolution of PTI-LiBr crystals is enhanced under UV-light illumination (Fig. 2(b)).

Fig. 2 Time-lapse photographs depicting spontaneous dissolution of PTI-LiBr in DMSO up to 48 h under visible and UV illuminations.

The high-resolution TEM images of CN nanosheets deposited from solutions containing PTI-LiBr dissolved in NMP (Figs. 3(a)-3(c)) indicate that the CN nanosheets are atomically intact with well-defined edges and maintained the hexagonal shape with its lateral dimensions close to that of the precursor bulk crystals. No evidence of any dislocations or point defects could be observed.

Fig. 3 (a-c) HR-TEM images of CN nanosheets deposited from solutions containing PTI-LiBr dissolved in NMP

Both bulk and exfoliated CN exhibit luminescence in the UV/visible range. The normalized photoluminescence (PL) emission spectra of CN nanosheets dissolved in DMF exhibit a broad peak ∼380 nm, which slightly shift toward blue-green range with an increase in wavelength excitation from 260 to 330 nm (Fig. 4(a)). The PL spectra of stacked or aggregated films of CN nanosheets deposited from dissolved solution also exhibit a broad peak centered ∼480 nm (red-shift when compared to PL spectra of dissolved CN nanosheet) (Fig. 4(b)). The broadening of the PL spectra of CN nanosheets dissolved in DMF as well as the stacked or aggregated CN film deposited from dissolved solution indicates that they could be composed of 9 to 40 layers in thickness. These inferences indicate that depending on the thickness of CN nanosheets, it would be possible to tune the PL wavelength from narrow UV to broad-band white.

Fig. 4 PL spectra of CN nanosheets at varying excitation wavelength: (a) CN nanosheets dissolved in DMF; (b) stacked or aggregated CN film deposited from dissolved nanosheets

The methodology employed for the synthesis of 2D CN nanosheets is simple  and easily scalable. The spontaneous dissolution of PTI-LiBr crystals in NMP, DMF, and DMSO results in the formation of stable solutions of pristine, defect-free CN nanosheets with well-defined functional properties. The luminescence property of dissolved as well as stacked film of CN nanosheets indicate that they can be explored as potential next-generation materials for photocatalysis. The tunability of PL spectra depending on the stack thickness of CN nanosheets makes them as suitable candidate materials for UV-blue and white LED emitters. The CN nanosheets prepared by this method can be used for a wide range of optoelectronic devices.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Thomas S. Miller et al., Single Crystal, Luminescent Carbon Nitride Nanosheets Formed by Spontaneous Dissolution, Nano Lett. 2017, 17, 5891−5896.

Synthesis of atomically thin metal oxides at room temperature using liquid metals – A novel approach to expand the realm of 2D materials

Metals when exposed to air under ambient conditions leads to the formation of self-limiting atomically thin oxide layer at the metal-air interface, which is considered to be a naturally occurring two-dimensional (2D) material. However, isolation of 2D metal oxides from the metal surface poses considerable challenges.

Researchers at RMIT University Australia, Queensland University of Technology,  Australia and California NanoSystems Institute, University of California, USA have shown that it would be possible to synthesis atomically thin metal oxides (2D metal oxide) at room-temperature using liquid metals as reaction environment (Reference: Ali Zavabeti et al., A liquid metal reaction environment for the room-temperature synthesis of atomically thin metal oxides. Science, 2017; 358 (6361): 332 DOI: 10.1126/science.aao4249)

In this study galinstan (liquid metal alloy containing gallium, indium and tin) was used as a reaction environment. Galinstan alloyed ~1 wt % of elemental hafnium, aluminum, or gadolinium served as the precursors for the formation of their respective oxides (HfO₂, Al₂O₃ and Gd₂O₃). The choice of these alloying elements were made on the basis of thermodynamic considerations (Gibbs free energy (ΔG_f) value).

Two different methods were proposed for isolating the surface oxides; (i) van der Waals (vdW) exfoliation technique; and (ii) gas injection method.

The van der Waals (vdW) exfoliation technique is quite similar to the method for obtaining monolayer of graphene which involves touching the liquid metal droplet with a solid substrate. The liquid nature of the parent metal allows a clean delamination of the oxide layer (Fig. 1). This technique is suitable for the production of high-quality thin oxide sheets on substrates.

The second technique relies on the injection of pressurized air into the liquid metal, in which the metal oxide forms rapidly on the inside of air bubbles and rose through the liquid metal. When the released air bubbles pass through deionized water placed above the liquid metal, allows dispersion of the oxide sheets in the aqueous suspension. Subsequently, the suspension can be subjected to drop casting to prepare 2D metal oxide films on suitable substrates (Fig. 2). This technique is highly scalable and hence suitable for the synthesis of the target oxide nanosheets with high yield.

Fig. 1 Schematic representation of the van der Waals exfoliation technique. The pristine liquid metal droplet is first exposed to an oxygen-containing environment. Touching the liquid metal with a suitable substrate allows transfer of the interfacial oxide layer.

Fig. 2 Schematic representation of the gas injection method (left), photographs of the bubble bursting through the liquid metal (center), and an optical image of the resulting sheets drop-cast onto a SiO₂/Si wafer (right)

The findings of the study indicate that oxide layers formed on liquid metals can be manipulated by an appropriate choice of alloying elements based on Gibbs free energy. The two method proposed to isolate the 2D nanosheets require simple experimental set-up and allows either a direct deposition on solid surfaces or formation of an aqueous suspension that can be drop cast over a variety of substrates. The methodology outlined in this study provides a novel pathway for the synthesis and easy isolation of 2D materials that was previously inaccessible.

The 2D materials, viz., HfO₂, Al₂O₃ and Gd₂O₃, synthesized in this study hold promise for applications in energy storage, such as supercapacitors and batteries. HfO₂ can be used as an ultrathin insulator dielectric material for the fabrication of field-effect transistors.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer Ali Zavabeti et al., Science, 2017; 358 (6361): 332 DOI: 10.1126/science.aao4249