Sequential Surface-Modification of Graphene Oxide

The formation and growth of ice crystals is considered to be a critical issue in aerospace and automotive industries as well as in cryopreservation of cells. Pure water undergoes homogenous nucleation of ice at ~ −40 °C. However, the presence of dusts, minerals, birch and conifer pollen and some species of fungus could serve as a nucleator and promotes nucleation of ice much above this temperature. Carbon nanotubes, graphene nano-flakes and carbon soots (from burning of fuels) are promising candidates to promote nucleation of ice crystals.

Base-washing has been shown to be effective in removing oxidative debris from graphene oxide (GO) and enables effective functionalization of the surface of GO with thiols, Au nanoparticles and polymers. Base-washed graphene oxide (bwGO) is a distinct graphene-like material with better qualities than the normal GO. Researchers at Department of Chemistry, Warwick Medical School and Department of Physics, University of Warwick, UK have suggested that surface modification of bwGO would offer a versatile template to evaluate the potential of 2D carbon nanomaterials as ice-nucleating agents as well as to serve as a versatile scaffold to probe the role of surface chemistry.

GO was synthesized by Hummer’s method. About 140 mg of GO was re-dispersed in 250 ml of deionized H₂O by mild sonication followed by addition of 0.140 g of NaOH and heating of the solution to 70 °C for 1 h. The resultant dark brown solution was centrifuged (@12,500 rpm for 30 min). The dark brown solid was washed with water and re-centrifuged. The solid was re-protonated using 0.014 M HCl at 70 °C for 1 h, filtered, thoroughly washed with deionized H₂O and dried under vacuum to yield bwGO (a black solid), which was dispersed in a H₂O/CH₃CN mixture via sonication. Poly(N-isopropylacrylamide), (pNIPAM) with degree of polymerization of 55 and 140 were prepared by polymerization of N-isopropylacrylamide (Fig. 1). pNIPAM hexanethiol, dodecanethiol and octadecanethiol were grafted on the surface of bwGO under Schlenk conditions in N₂ atmosphere (Fig. 2).

Fig. 1 Scheme depicting polymerization of N-isopropylacrylamide

Fig. 2 Scheme depicting polymerization of N-isopropylacrylamide and grafting of polymers and thiols on the surface of base-washed graphene oxide

The ice nucleation activity of unmodified and surface modified GO was quantified by determining the average nucleation temperature to freeze a droplet (1 μL) of water. The droplets were cooled under an atmosphere of dry nitrogen, and the freezing point of each droplet was recorded by visual observation using a microscope. When tested for the nucleation activity, ultra-pure Milli-Q water nucleated at -26 °C, suggesting a heterogeneous nucleation (Fig. 3); Both bwGO and bwGO-Cyst increased the nucleation temperature by over 5 °C, to -20 and -18 °C (Fig. 4).

Fig. 3 Ice nucleation assay: No water droplet is frozen at -20 °C; At -23 °C, two water droplets (marked by red circles) are frozen while all water droplets are frozen at -30 °C.

Fig. 4 Comparison of ice nucleation activity of Milli-Q water, GO and cysteine-functionalized GO

A remarkable nucleation promotion activity is observed for bwGO surface modified with alkane thiols; octadecanethiol modified bwGO increased the nucleation temperature by > 15 °C to –12 °C (Fig. 5(a)). All the alkyl modified GOs are more active than bwGO and the cysteine modified bwGO, which suggests that the increased hydrophobicity plays a dominant role in determining the ice nucleation. The similar activity of pNIPAM-bwGO with that of bwGO (Fig. 5(b)) suggests that modification of the surface of bwGO with polymer molecules exert a very little influence on the ice nucleation temperature.

Fig. 5 Comparison of ice nucleation activity of (a) Milli Q water, GO and GO functionalized with hexanethiol, octadecanethiol and dodecanethiol; and (b) Milli Q water, GO, pNIPAM55 and pNIPAM140.

The surface modified bwGO may find application in cryopreservation and cloud seeding.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Caroline I. Biggs et al., Impact of sequential surface-modification of graphene oxide on ice nucleation, Phys. Chem. Chem. Phys., 2017,19, 21929-21932

Achieving Enhanced Antibacterial Activity by Suitably Aligning Graphene Oxide Nanosheets

Graphene-based nanomaterials (GBNs) due to their exceptional mechanical, electronic, and thermal properties assumed significance in a variety of applications. The cytotoxic properties of GBNs are also important for their biomedical applications. GBNs have been shown to be cytotoxic toward a variety of cell types. However, the impact of alignment of nanosheets on the antibacterial activity has not been established. Researchers at Department of Chemical and Environmental Engineering, Yale University, USA, have investigated orientation-dependent interaction of graphene oxide (GO) nanosheets aligned in different orientations using a magnetic filed with Escherichia coli (E. coli). The GO nanosheets with vertical orientation exhibit an enhanced antibacterial activity when compared to those with random and horizontal orientations and the mechanism responsible is also suggested.

The schematic illustration of alignment of GO nanosheets with different orientations using magnetic field and alignment quality of GO nanosheets suspended in the monomer solution at different field strengths evaluated by 2D small-angle X-ray scattering (SAXS) are shown in Fig. 1.

Fig. 1 (a) Schematic illustration of alignment of GO nanosheets with different orientations using magnetic field; and (b) alignment quality of GO nanosheets

The various stages involved in the fabrication of GO composite films is shown in Fig. 2. Suspensions of GO nanosheets (with a thickness of ∼0.8 nm) in 2-hydroxyethyl methacrylate (HEMA), doped with cross-linker and photo initiator, were sealed between two glass substrates with a 300-μm spacer and aligned in a magnetic field of 6 T. Samples were subsequently cross-linked under UV irradiation to form polymer films, which preserved the orientation of the aligned GO nanosheets. The composite films were then detached from the glass substrates and irradiated using UV/O₃ to etch away the outer polymer and expose GO nanosheets on the surface. The resultant films are tough, mechanically coherent and resistant to water swelling, which are critical in preserving the GO orientation in aqueous environments.

Fig. 2 Various stages involved in the fabrication of GO composite film

The GO composite films were contacted with E. coli in suspension for 3 h. The bacteria attached on the surface were stained using SYTO 9 dye and propidium iodide and evaluated for live and dead cells. The vertical-GO film showed a lower cell viability (56.0 ± 8.7%) when compared to those with random (75.3 ± 3.5%) and planar (81.8 ± 5.1%) orientation. Morphological features indicate that E. coli on No-GO film showed an intact cell morphology, indicating no cytotoxicity of the pure polymer. E. coli on planar- and random-GO films largely retained their morphological integrity whereas cells on vertical-GO films became flattened and wrinkled, suggesting loss of viability and possible damage to the cell membrane (Fig. 3).Fig. 3 SEM micrographs of E. coli cells on etched GO composite films. The scale bar is 1 μm.

The mechanism for the enhanced antibacterial activity of vertically aligned GO nanosheets is explained based on (i) physical disruption; and (ii) chemical oxidation using lipid vesicles and oxidation of glutathione, respectively. GO nanosheets with a vertical orientation induced physical disruption of the lipid bilayer structure, resulting in loss of membrane integrity of the GO/lipid vesicle system. GO nanosheets with a vertical orientation also increased the extent of oxidation of glutathione (27.6%) with limited generation of reactive oxygen species, suggesting that the oxidation occurs through a direct electron transfer mechanism. Thus, both mechanisms contribute to the enhanced antibacterial activity of the vertical-GO film. Nevertheless, both of them require direct, edge-mediated contact with cells. The exposed edges of GO nanosheets with a vertical orientation could induce enhanced physical penetration and promote greater levels of electron transfer. Hence, the enhanced antibacterial activity of the film with vertically aligned GO nanosheets can be attributed to the increased density of edges with a preferential orientation for membrane disruption. The orientation-dependent cytotoxicity of GO nanosheets has direct implications on the design of engineering surfaces using graphene based nanomaterials.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: X. Lu et al., Enhanced antibacterial activity through the controlled alignment of graphene oxide nanosheets, PNAS 2017 114: E9793-E9801

 

Synthesis of Reduced Graphene Oxide by Microwave Exfoliation Using Graphite as a Catalyst

Achieving large scale synthesis of high quality graphene is a critical step to exploit the practical application of graphene in a variety of fields. Researchers at State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, China have reported a environmental friendly and ultrafast catalytic microwave method in which a small amount of graphite flake served as the catalyst to promote microwave exfoliation and reduction of graphene oxide (GO).

Microwave irradiation of GO powder leads to the formation of microwave exfoliated graphene oxide (MEGO), which usually occurs in ~15 min. This reaction is triggered in presence of a small amount of graphite powder (< 1 mg) and the exfoliation process is completed within 5 s with the formation of a large volume of  catalytic microwave exfoliated graphite oxide (CMEGO) (Fig. 1). The graphite flakes with highly extended π-system efficiently absorb the microwave as a susceptor and convert the energy to activate the nearby gas molecules while the microwave plasma generates a local ultrahigh energy environment.

Fig. 1 Schematic of the synthesis of CMEGO

The exfoliation and reduction of GO in the ultrahigh energy environment created by catalytic microwave plasma leads to a more complete removal of oxygen functional groups and yields CMEGO with a lower lattice defects, higher specific surface area (886 m²/g), large C/O ratio (19.4), good electrical conductivity (53180 S/m) as well as excellent solvent dispersability and processability. The morphology of CMEGO indicates that it is thoroughly exfoliated as evidenced by the formation of smooth, thinner and transparent graphene sheets with a weak lattice distortion (Figs. 2(a) and 2(b)). HR-TEM images show that the CMEGO has a more regular lattice and fewer graphene layers (Figs. 2(c) and 2(d)).

Fig. 2 (a, b) SEM; and (c, d) HR-TEM images of CMEGO

Use of CMEGO as an anode material in lithium-ion batteries has enabled very high reversible capacities of 2260 mAh/g and 469 mAh/g at a charge/discharge rate of 0.1 A/g and 30 A/g, respectively, and an outstanding capacity retention of 91.4% after 1000 cycles at 5.0 A/g. Similarly, use of CMEGO as an anode material in sodium-ion batteries offered very high reversible capacities of 424 mAh/g and 218 mAh/g at 0.1 A/g and 30 A/g, respectively, and a stable capacity retention of 85.7% after 1000 cycles at 5.0 A/g.

The catalytic microwave irradiation strategy employed for the large-scale synthesis of high quality graphene is promising for applications in energy storage and conversion.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Runze Liu, A Catalytic Microwave Process for Superfast Preparation of High-Quality Reduced Graphene Oxide, Angew. Chem. Int. Ed. 10.1002/anie.201708714

Converting Wood to Graphene by Laser Scribing

Graphene has proved its supremacy in a wide variety of applications due to its high electrical conductivity, and excellent chemical and mechanical properties. The conventional method of synthesis of graphene by chemical vapor deposition (CVD) has certain limitations such as need for high temperature, type of substrates, requirement for post-treatments such as etching and critical point drying, or aerogel formation. Laser-induced graphene (LIG) can be formed on commercial polyimide (PI) films, which is considered as an alternate method for CVD to prepare graphene. Researchers at Rice University, USA and Beihang University, China have demonstrated that LIG can be formed on the surface of wood with a high electrical conductivity. Moreover, the graphene layer can be further modified by electrodeposition of polyaniline (PANI) to make it as a supercapacitor or plate it with Co-P and Ni-Fe hydroxides to catalyze hydrogen evolution and oxygen evolution reactions (OERs).

The 3D porous LIG graphene was formed on the surface of pine wood by irradiating it with a 10.6 μm CO₂ laser under Ar or H₂ atmosphere (Fig. 1(a)). The LIG graphene is formed only on the area of the pine wood, which is scribed by the laser whereas other areas remain unchanged (inset of Fig. 1(a)). This attribute helps patterning of LIG with different shapes (Fig. 1(b)). The inert atmosphere helps in the development of graphene with a stable structure while ablation of wood in air has lead to decomposition of the lignocellulose structure. The LIGs were prepared from pine wood using 10% (1.6 W), 30% (4.1 W), 50% (6.3 W), 70% (7.8 W), and 90% (8.6 W). The LIGs prepared using pine, oak and birch woods are designated as P-LIG-x, O-LIG-x and B-LIG-x, respectively, where x signifies the power percentage of the laser.

Fig. 1 (a) Schematic illustration of the formation of LIG using CO₂ laser under Ar or H₂ atmosphere; (b) Photograph of LIG patterned with letter ‘R’ on wood.

The morphology, chemical composition, structure, porosity, crystallite size, electrical properties and I_D/I_G ratio of P-LIG show a strong dependence on the laser power. Morphological features of the pine wood, laser scribed at varying laser powers indicate the evolution of a porous structure due to the liberation of gas during laser irradiation. The pore size of the P-LIGs is decreased with an increase in laser power from 10% to 70% (Figs. 2). The thermal stability and electrical conductivity of P-LIGs are increased with an increase in laser power.

Fig. 2 Morphological features of the laser-scribed pine wood at varying powers: (a) 30%; (b) 50%; and (f) 70%.

The chemical composition of the P-LIG is changed during laser irradiation; an increase in laser power leads to a decrease in C-O content and an increase an in C-C and carboxyl contents (Fig. 3(a)). The structural changes of P-LIG with laser power are confirmed by Raman spectra and TEM. At 10% laser power, no apparent Raman signal is detected while at 30% formation of amorphous carbon is evident by the broad D peak at ≈1350 cm⁻¹ and the weak 2D peak at ≈2700 cm⁻¹. As the laser power is increased from 50% to 90%, sharpening of the D and G peaks along with an enhancement in  the intensity of 2D peak indicate the formation of the graphene structure (Fig. 3(b)).

Fig. 3 (a) Change in chemical composition derived from XPS; and (b) Raman spectra of P-LIG as a function of laser power

The predominance of amorphous carbon without a clear graphene lattice at 30% laser power and graphene carbon over 50% laser power is confirmed by TEM (Fig. 4). The crystalline size of P-LIG reach a maximum at 70% power. The large I_2D/I_G ratio indicate that P-LIG-70 has a stacking of fewest-layered graphene. An increase in laser power, in general, has resulted in the formation of graphene with desired characteristics. Nevertheless, at 90% laser power, overheating of the pine wood has lead to an inferior LIG structure.

Fig. 4 TEM images of (a) P-LIG-30; (b) P-LIG-50; and (c) P-LIG-70

Similar to pine wood, 3D porous LIG is also formed using oak and birch woods. A comparison of the characteristics of LIGs formed using pine, oak and birch woods at a laser power of 70% indicates a lower I_D/I_G ratio of 0.48 for O-LIG-70, followed by at 0.73 and 0.85 for B-LIG-70 and P-LIG-70, respectively. Since aliphatic carbon moieties are more reactive than aromatic carbon, the hemicellulose and cellulose of oak and birch woods are easily decomposed during laser irradiation, thus resulting in O-LIG-70 and B-LIG-70 with more defects. In contrast, a higher aromatic lignin content favours the  generation of P-LIG-70 with a lower degree of defects. In spite of the lignin component, the formation of LIG also depends on the inherent composite structure of wood consisting of cellulose, hemicelluloses, and lignin.

The P-LIG is converted to a supercapacitor by electrodepositing polyaniline (P-LIG-PANI). The cyclic voltammetry curves of P-LIG-PANI in the potential window of −0.2 to 0.8 V indicate two characteristic pairs of redox peaks that correspond to leucoemeraldine/emeraldine and emeraldine/ pernigraniline transition of PANI, thus confirming the pseudocapacitive characteristics of PANI upon P-LIG (Fig. 5(a)). The galvanostatic charge-discharge curves of P-LIG-PANI indicate a specific areal capacitance of ≈780 and ≈320 mF/cm² at 1 and 10 mA/cm², respectively (Fig. 5(b)).

Fig. 5 (a) CV of P-LIG-PANI and P-LIG in 1 M H₂SO₄ at a scan rate of 20 mV/s; and (b) Galvanostatic charge–discharge curves of P-LIG-PANI at varying current densities.

Electrodeposition of Co-P on P-LIG-70 (P-LIG-Co-P) shows that if can be tuned to catalyze the hydrogen and oxygen evolution reaction (HER and OER). The polarization curves of P-LIG-Co-P in 1 M KOH delivers a HER current density of ≈62 mA/cm² at 200 mV overpotential and an OER current density of ≈20 mA/cm² at 400 mV overpotential over an area of ≈0.5 cm² (Fig. 6(a)). The Tafel slopes extrapolated from the polarization curves are ≈35 and 280 mV/decade for HER and OER, respectively (Fig. 6(b)). The OER performance can be improved by electrodepositing NiFe hydroxides on P-LIG (P-LIG-NiFe). Photographic image of P-LIG-Co-P as cathode and P-LIG-NiFe as anode, powered by two 1.5 V batteries in series are shown in Fig. 6(c)).

Fig. 6 (a) HER and OER windows of P-LIG-Co-P and P-LIG-NiFe in 1 M KOH; (b) HER and OER Tafel slopes of P-LIG-Co-P and P-LIG-NiFe; and (c) Photograph of P-LIG-Co-P and P-LIG-NiFe are powered by two 1.5 V batteries in series.

By a suitable choice of materials that can be deposited on LIG, it can be tailored to suit diverse applications such as supercapacitors and water splitting systems. The methodology opens up new avenues to engineer woods surfaces for diverse electronic applications.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Ruquan Ye et al., Laser-Induced Graphene Formation on Wood, Adv. Mater. 2017, 1702211, DOI: 10.1002/adma.201702211

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