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Shen, Tian-Zi,Hong, Seung-Ho,Guo, Jin-Kun,Song, Jang-Kun Elsevier 2016 Carbon Vol.105 No.-
<P>Electro-optical switching of graphene oxide (GO) liquid crystals is of great interest owing to its potential applications. However, a GO dispersion undergoes vinylogous carboxylic acidic reaction with time, accompanied by a decrease in pH, which deteriorates the electro-optical performance of a GO cell. We find that the vinylogous carboxylic reaction is hindered in high concentration GO dispersions, and also demonstrate that the deteriorating electro-optical performance can be recovered either by re-cleaning or by ionic neutralization treatment. The electro-optical performance is systematically analyzed in terms of the acidity of the neutralizing solvent, GO dispersion, and supernatant liquid. The best performance is obtained in a GO dispersion with a pH-7 supernatant liquid after neutralization by weakly alkaline water. The neutralization process decreases the bulk conductivity, and also improves the surface conductivity on GO. Approximately 56% of newly generated H+ ions reside near the GO surface, and the rest diffuse away into the bulk solvent. These findings may provide better understanding for the electro-optical switching phenomenon and its applicability. (C) 2016 Elsevier Ltd. All rights reserved.</P>
Electro-optical switching of graphene oxide liquid crystals with an extremely large Kerr coefficient
Shen, Tian-Zi,Hong, Seung-Ho,Song, Jang-Kun Nature Publishing Group, a division of Macmillan P 2014 NATURE MATERIALS Vol.13 No.4
The sensitive response of the nematic graphene oxide (GO) phase to external stimuli makes this phase attractive for extending the applicability of GO and reduced GO to solution processes and electro-optic devices. However, contrary to expectations, the alignment of nematic GO has been difficult to control through the application of electric fields or surface treatments. Here, we show that when interflake interactions are sufficiently weak, both the degree of microscopic ordering and the direction of macroscopic alignment of GO liquid crystals (LCs) can be readily controlled by applying low electric fields. We also show that the large polarizability anisotropy of GO and Onsager excluded-volume effect cooperatively give rise to Kerr coefficients that are about three orders of magnitude larger than the maximum value obtained so far in molecular LCs. The extremely large Kerr coefficient allowed us to fabricate electro-optic devices with macroscopic electrodes, as well as well-aligned, defect-free GO over wide areas.
Shen, Tian-Zi,Hong, Seung-Ho,Lee, Bomi,Song, Jang-Kun Nature Publishing Group 2016 NPG Asia Materials Vol.8 No.-
<P>The recent discovery of structural coloration in aqueous graphene-oxide (GO) dispersions has increased the applicability of carbon-based two-dimensional materials. However, the origin of the photonic band-gap is still poorly understood, and its practical manipulation is still in an early developmental stage. Here, we demonstrate full-color reflection with first-and second-order Bragg reflections in a GO dispersion, and we use two fundamental approaches to manipulate GO photonic crystals, namely, bottom-up and top-down manipulation by controlling the Debye length and using shear or surface fields, respectively. Nanoscopic tailoring of the electrostatic effective thickness and macroscopic smoothing of the curvature of the GO sheet result in similar modifications of the quality and pitch of the photonic crystallinity. Direct observation of the GO particle alignments reveals excellent electrostatic layer-to-layer packing assembly and rather poor in-layer assembly. These results elucidate the mechanism that governs the nematic nature of GO (rather than its lamellar mesophase) and the origin of its photonic crystalline periodicity and provide new methodologies for exploiting these attractive features in actual applications.</P>
Electro-optical Characteristics of Aqueous Graphene Oxide Dispersion Depending on Ion Concentration
Hong, Seung-Ho,Shen, Tian-Zi,Song, Jang-Kun American Chemical Society 2014 The Journal of Physical Chemistry Part C Vol.118 No.45
<P>Facile alignment control of graphene oxide (GO) particles in aqueous dispersions is a highly useful technique for various potential applications. Aqueous GO dispersions were recently reported to have an extremely large Kerr coefficient, which may provide a valuable pathway to the alignment control of GO particles. We investigated the electro-optic responses of GO dispersions in various ionic solutes and with varying ionic concentrations. We found that the addition of NaOH actually improved the electro-optic sensitivity of the GO dispersion, while other ionic additives resulted in desensitization. We experimentally and theoretically elucidated the underlying mechanism of the phenomena. The mechanism is closely related to the acidic nature of the GO dispersion, which is neutralized by the addition of NaOH. The addition of ionic solutes caused only a mild change in the surface conductivity of GO particles, but it brought about a large variation in the bulk solvent conductivity. The electro-optical sensitivity agreed well with the variation in solvent conductivity. Thus, the electro-optic response of GO dispersion was influenced more by the electric properties of solvent rather than by those of the GO particle itself. We also found that the cation-exchange capacity for H<SUP>+</SUP> ions in the electrical double layer is quite high; i.e., H<SUP>+</SUP> ions are not likely to be replaced by other ions.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2014/jpccck.2014.118.issue-45/jp504892s/production/images/medium/jp-2014-04892s_0012.gif'></P>
Wijewardhana, K. Rohana,Shen, Tian-Zi,Jayaweera, E.N.,Shahzad, Amir,Song, Jang-Kun Elsevier 2018 Nano energy Vol.52 No.-
<P><B>Abstract</B></P> <P>Artificially embedded surface charges can dramatically improve the output energy of water–solid contact electrification devices, but the embedded charges easily escape or are cancelled out by adsorption of opponent ions in ambient conditions. Here, we demonstrate a hybrid energy harvesting nanogenerator by combing water–solid and solid–solid contact electrification schemes. It consists of a water–solid contact electrification device and wind-fluttering ribbons that regularly supplies triboelectric surface charges on the solid surface and hence, the high density of surface charges can remain on the surface. This scheme can enhance the harvesting energy by up to a factor of 30 during rain. Moreover, the fluttering ribbons independently collect electric energy from mild winds without rain. This scheme provides a facile, cost-effective, and robust approach to enhance the energy-harvesting efficiency of a water–solid contact electrification device and to combine the energy harvesting from simultaneous rain and wind.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Hybrid nanogenerator combining an EDL device and triboelectric ribbons was demonstrated. </LI> <LI> The hybrid nanogenerator produces electricity in both rainy and windy conditions. </LI> <LI> Wind-driven ribbons provides triboelectric charges, producing 30-times more electricity. </LI> <LI> It provides a solution to enhance the electrification efficiency in water–solid nanogenerators. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>