The preparation and exfoliation of graphite oxide at low temperatures (near room temperature) to produce exfoliated graphite (EG) instead of rapid heating to a high temperature (conventional process) are reviewed. The exfoliation by microwave irradiation, electrochemical exfoliation and surfactant-assisted exfoliation of graphite are also included because these techniques can be applied under ambient atmosphere, although last two techniques were mainly applied for thinning the graphite flakes to obtain “graphene” flakes. The applications of the resultant exfoliated graphite (EG) for oil/water separation, adsorptive removal of the environment pollutants and microwave shielding are shortly reviewed.

The synthesis of graphene and graphene quantum dots (GQDs) employing various approaches with a range of precursors, chemicals, and parameters has been reported. Most of the top-down and bottom-up techniques employ strong and hazardous chemical environments, complicated and tedious procedures, are time-consuming, and often require special equipment. Another drawback of the techniques reported is the production of agglomerated, inhomogeneous, and non-dispersible graphene in aqueous solvents or organic solvents, thus limiting its application. This work specifically and comprehensively describes the electrochemical exfoliation of graphene and GQDs, which is often considered as a simple one-step, facile, non-hazardous, and highly efficient technique yet favourable for mass production. A brief discussion on the advantageous and challenges of the electrochemical technique and applications of the electrochemically exfoliated graphene and GQDs is also presented.

Graphene is an unconventional material with a two-dimensional hexagonal crystalline array of elemental carbon atoms and outstanding properties; accordingly, a desirable objective in the line of research of graphene is the development of novel and more productive methods of synthesis, validating its properties and applications. In our exploratory research, we have effectively exfoliated graphene from graphite using supercritical fluids (water, ethanol and carbon dioxide). The exfoliated graphene was properly characterized; via scanning electron microscopy, the morphology of graphene was observed; Raman spectra confirmed the exfoliation of graphene depicting the characteristic shift towards smaller Raman number in the 2D band (2676 cm−1) compared to that of graphite (≈ 2700 cm−1); transmission electron microscopy analysis exhibited the crystalline structure of graphene attesting also the expected transparency of exfoliated layers. Graphene exfoliation from graphite by supercritical fluids promises to be a simple large-scale method for graphene production.

Large-size graphene samples are successfully prepared by combining ultrosonic assisted liquid phase exfoliation process with oxidation-deoxidation method. Different from previous works, we used an ultrasound-treated expanded graphite as the raw material and prepared the graphene via a facile oxidation-reduction reaction. Results of X-ray diffraction and Raman spectroscopy confirm the crystal structure of the as-prepared graphene. Scanning electron microscopy images show that this kind of graphene has a large size (with a diameter over 100 μm), larger than the graphene from graphite powder and flake graphite prepared through single oxidation-deoxidation method. Transmission electron microscopy results also reveal the thin layers of the prepared graphene (number of layers ≤3). Furthermore, the importance of preprocessing the raw materials is also proven. Therefore, this method is an attractive way for preparing graphene with large size.

결정성 탄소물질은 결합의 형태에 따라 carbyne (sp1, 1D 구조), graphite (sp2+π, 2D), diamond (sp3, 3D) 구조 로 나뉜다. 특히 sp2 결합에 기반한 나노물질은 fullerene (0D), 탄소나노튜브 (1D or quasi-2D), 그래핀 (2D) 으로 나뉜 다. 탄소나노튜브와 그래핀은 물리적으로 여러 가지 뛰어난 특성이 있어 구조재나 광전자 재료, 멤브레인 등 다양한 분 야에 응용가치가 높다. 하지만 이들 나노재료는 강하게 응집되는 성질이 있어 용액에 분산할 필요가 있다. 특히 이는 용 액 상에서 박리, 안정화의 과정을 거쳐야 안정적으로 분산된 상태를 유지할 수 있다. 본 고에서는 탄노나노튜브나 그래 핀이 용매에서 박리되어

Graphene oxide has been synthesized by microwave-assisted exfoliation of graphite oxide prepared by modified Hummers method. Graphite was oxidized in a solution of H2O2 and KMnO4 at 65~80˚C, followed by 10 % H2O2 solution treatment at 80~90˚C. The graphite oxide was exfoliated under microwave irradiation of 1 kW and was reduced to graphene effectively by hydrazine hydrate (H4N2·H2O) treatment. The exfoliation of graphene oxide was significantly affected by the microwave irradiation on (heating)/off (cooling) period. An on/off period of 10 s/20 s resulted in much more effective exfoliation than that of 5 s/10 s with the same total treatment time of 10 min. This can be explained by the higher exfoliation temperature of 10 s/20 s. Repetition of the graphite oxidation and exfoliation processes also enhanced the exfoliation of graphene oxide. The thickness of the final graphene products was estimated to be several layers. The D band peaks of the Raman spectra of the final graphene products were quite low, suggesting a high crystal quality.

Graphene oxide powders prepared by two different drying processes, freeze drying and spray drying, were studied to compare the effect of the drying method on the physical properties of graphene oxide powder. The graphene oxide dispersion was prepared from graphite by chemical delamination with the aid of sulfuric acid and permanganic acid, and the dispersion was further washed and re-dispersed in a mixed solvent of water and isopropyl alcohol. A freeze drying method can feasibly minimize damage to the sample, but it requires a long process time. In contrast, spray drying is able to remove a solvent in a relatively short time, though this process requires exposure to a high temperature for a rapid evaporation of the solvent. The powders prepared by freeze drying and spray drying were characterized and compared by Raman spectroscopy, X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, and by an elemental analysis. The graphene oxide powders showed similar chemical compositions; however, the morphologies of the powders differed in that the graphene oxide prepared by spray drying had a winkled morphology and a higher apparent density compared to the powder prepared by freeze drying. The graphene oxide powders were reduced at 900˚C in an atmosphere of N2. The effect of the drying process on the properties of the reduced graphene oxide was examined by SEM, TEM and Raman spectroscopy.

하이드록시 산(hydroxy acid, HA)은 피부 각질층에 대한 박리효과로 노화방지와 피부 보습을 높이는데 효과를 보여, 피부 외용제와 화장품으로서 많은 활용을 하고 있다. 그 중에서 가장 효과적으로 빠른 시간 내에 나타나는 각질 박리는 화장품 제형 pH에 의해 효과를 보이는 것으로 많은 보고가 있다. 그러나 pH에 의한 자극, 부작용에 의한 염려로 인해 사용에 많은 어려움이 있다. 본 연구의 목적은 피부 각질층에 (1) 하이드록시 산의 농도와 (2) 종류, (3) pH를 변화시킨 화장품을 인체 피부 에 도포하여 각질 박리 효과에 대한 영향을 측정하는 데에 있다. 건강한 성인 22명을 대상으로 하박 내측에 DHA (dihydroxyacetone), DC (dansyl chloride)로 피부 표면 각질을 염색하여, 시험제품을 도포한 뒤 각질 박리 효과를 측정하였다. (1) GA (glycolic acid)의 농도에 따라 각질 박리 효과는 농도 의존적으로 증가하는 것으로 나타났다. (2 )하이드록시 산의 종류에 따라 pH를 산성과 중성으로 제조한 제품을 대상으로 각질 박리 효과를 측정한 결과, 중성 pH의 GA는 각질 박리 효과가 나타나지 않았다. 이에 반해 SA (salicylic acid)는 산성 pH와 중성 pH에서 모두 통계적으로 유의한 각질 박리 효과가 나타났다. (3) 중성 pH의 SA는 DHA와 DC로 염색한 피부 표면에서 모두 우수한 각질 박리 효과를 나타내었다. 이러한 결과는 pH에 민감한 사람들에게 각질 박리 효과를 기대하는 화장품을 사용할 수 있는 기회를 제공하며, 피부 장벽의 손상 없이 안전한 화장품을 제조할 수 있을 것이라 사료된다.