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

Rapid charging of your smart phones – Are we getting closer to reality?

Supercapacitors are used as an alternative power source for rechargeable batteries due to their efficient operation at high power density, long cycle life and improved safety. Nevertheless, the limited energy density, typically of the order of 5-8 Wh/L, limits their widespread use for many practical applications. Boosting capacitance and extending window of cell voltage are the available options to impart further improvement in their energy density. Researchers at University of Waterloo, Canada and Jain University, Bangalore, India have proposed a novel approach towards the development of high voltage super capacitors with high energy density (ACS Nano, 2017, 11 (10), pp 10077–10087).

Ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI) and Tween 20 (nonionic surfactant) were mixed together to obtain a stable microemulsion with nanometer sized particles. Upon mixing it with graphene oxide (GO), the surfactant stabilized microemulsion spontaneously adsorbs on the surface of GO. This dispersion was directly casted onto copper with the formation of a dense nanocomposite film of GO/IL/Tween 20. Subsequent thermal treatment leads to the removal of IL by evaporation and reduction of GO to reduced graphene oxide (rGO). The resultant electrode is referred as IL-mediated reduced graphene oxide (IM-rGO).

Fig. 1 Schematic of fabrication of IM-rGO electrode assembly: (a) spontaneous adsorption of surfactant stabilized microemulsion particles on the surface of GO; (b) enlarged view of EMImTFSI/Tween 20/H₂O microemulsion particle; (c) film structure after drop-casting and water evaporation; and (d) film structure after evaporation of Tween 20 following thermal reduction

The surface morphology of the nanocomposite film reveals the presence of macropores (Fig. 2(a)) due to evaporation of water and Tween 20 during thermal treatment. Morphology at the cross-section indicates a layered structure (Fig. 2(b)), in which the sheets lay parallel to the current collector, thus providing a relatively high bulk density.

Fig. 2 Morphology of the IM-rGO film fabricated using 60% IL: (a) at the surface; and (b) at the cross-section

The electrochemical performance of the IM-rGO electrode fabricated using 60 wt% of IL at RT is depicted in Fig. 3. The formation of a dense film enabled a CV of 218 F/cm³. This electrode offered a maximum energy density of 45 Wh/L at a power density of 571.4 W/L and maintained a high energy density of 21.7 Wh/L at a power density as high as 6.04 kW/L at RT.

Fig. 3 Electrochemical performance of 60% IL electrodes at RT: (a) CVs and (b) GCDs for IM-rGO at RT; (c) specific capacitance at varying current density

Eliminating the macropores of the film still remains a challenge and elimination of macropores would help to achieve even higher bulk density. The easy adoptability of the proposed methodology provides new avenues for the manufacturing of large-scale supercapacitors.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Zimin She et al., ACS Nano, 2017, 11 (10), pp 10077–10087