Graphene hollow capsules prepared by the layer-by-layer deposition

Abstract

본 연구는 Layer-by-Layer 적층법을 이용하여 중공형 그래핀 캡슐을 형성하고 이를 구성하는 산화 그래핀이 캡슐의 기계적 특성에 미치는 영향에 대한 연구로서, 산화 그래핀의 적층 횟수에 따른 두께, 크기, 그리고 열적 환원에 의한 캡슐의 구조적 변화를 다각적으로 관찰하였다. 중공형 캡슐 구조를 구현하기 위해 표면을 카르복실기로 치환시킨 실리카 구형 입자를 템플레이트로 사용하여 불화수소산(hydrofluoric acid)으로 선택적으로 제거하였으며, 산화 그래핀의 크기를 조절하기 위해 용매의 pH 값과 초음파처리 시간을 변경하여 다양한 크기 (59 ~ 485 nm)의 산화 그래핀 용액을 형성하였다. 이와 같은 방법으로 준비된 캡슐은 적층 횟수가 많거나 산화 그래핀의 크기가 클수록 본래 템플레이트의 구형 구조를 유지하는 경향을 보였으며, 특히 열적 환원을 가할 경우 그 변화가 가장 분명하게 관찰되었다. 이러한 경향은 힘 분광기 (force spectroscopy)를 통한 각 캡슐의 상대적 경도분석에서도 동일하게 확인되었다.

Since the first successful isolation of graphene in 2004, the field of graphene research has blossomed over the last several years owing to their superior properties such as high electrical and thermal conductivity, strong mechanical strength, flexibility, and so forth. Among many production methods of graphene, the exfoliation of graphite oxide (i.e. graphene oxide (GO)) enables the layer- by-layer (LbL) deposition, which takes advantages of electrostatic attractions between GO sheets. In the present study, we report graphene hollow capsules, recently reported 3-dimensional graphene structure, made of differently sized GO sheets with aim of accurate control of their mechanical properties. A fabrication of graphene hollow capsule was carried out by electrostatic LbL deposition of oppositely charged GO sheets on silica colloidal templates followed by template removal with hydrofluoric acid treatment. A dispersion of random-sized GO sheets was separated by modulating their dispersion stability and controlling destruction in grain structure. Dispersion stability of GO sheets was controlled by pH value of solution, directly related to the degree of ionization of their functional groups, and centrifugation. Destruction in grains of GO sheet was simply controlled by ultrasonication. Those size-fractionized GO sheets provide controllability of mechanical properties of capsules, which were confirmed by scanning electron microscopy (SEM) and force spectroscopy by scanning probe microscopy (SPM). Furthermore, the effect of reduction was also investigated via thermal reduction in inert atmosphere. The present study offers hollow capsules with narrowly size-distributed GO sheets which allow the clear pathway for overcoming intrinsic problem of GO composite system

a heterogeneity in size of GO sheets due to random destruction during exfoliation. This novel graphene hollow capsule system may be appropriate for replacing organic/inorganic capsules widely studied in drug delivery, bio-imaging, and framework for catalysts as graphene provides unique characteristics of biocompatibility, mechanical strength, electrical and thermal conductivity.