Hydrogen storage properties in carbon nanotubes(CNTs) were investigated from the view point of physical hydrogen adsorption in nano-hole but also defects on nanotube wall. Carbon nanotubes were prepared by thermal CVD using floating catalyst method. Ferrocene and xylene were used as a catalyst and carbon source. A large amount of vertically aligned carbon nanotubes was obtained by controlling ferrocene/xylene ratio. As usually observed in CNTs grown by thermal CVD, CNTs synthesized at this condition showed intrinsic closed structures, which mean the blocked hole and entry induced by highly defective layers and tube cap. In order to change CNTs with open structure that could accomodate more hydrogen, etching method using oxygen added atmospheric plasma(O2/He) has been introduced. The structure of CNTs gradually changed with increasing oxygen addition amount. Carbon nanotubes with open cap were obtained by atmospheric plasma etching with 2% $O_2$ addition. The pore size distribution measured by liquid nitrogen adsorption displayed that when carbon nanotubes were etched by oxygen plasma, nano-pore of 2-3 nm, identical size with nano-hole, account for the largest volume. These results implied that a large amount of CNTs with closed structure was changed to open structure by oxygen plasma etching available for physical hydrogen gas adsorption. Raman spectroscopy was used to analyze a crystallinity of closed and open capped carbon nanotubes. carbon nanotubes with open cap showed low crystallinity, because many defective graphite layers were produced at nanotube tip and on wall by oxygen plasma etching. The hydrogen storage properties were studied through hydrogen thermal desorption and compared between CNTs with closed structure and open CNTs. The precise analysis on thermal desorption spectra on CNTs with closed structure showed that hydrogen gas was released at a major temperature range as 100-150K where the evolved hydrogen amount were ～4.9wt%. However, in case of aligned and open CNTs, hydrogen gas was released at two major temperature ranges such as sub-ambient temperature (100-150K) and ambient temperature(300-330K). The amount of released hydrogen was 5.1 wt% and 0.6 wt% respectively. According to the past researches, the sub-ambient temperature peaks was related with hydrogen adsorption on surface. And ambient temperature peak was related with hydrogen adsortion in nano-hole and defects on nanotube wall. These hydrogen storage sites were reversible sites st ambient temperature. In physical hydrogen adsorption in nano-hole and defects on nanotube wall, the higher hydrogen storage capacity was obtained at open CNTs by oxygen added atmospheric plasma etching. The hydrogen physi-sorption process treated by oxygen plasma could be suggested as the effective hydrogen storage method at an ambient temperature.
탄소 나노 튜브의 수소 저장 특성이 내부 hole과 표면의 결함에 물리 흡착되는 관점에서 연구되었다. 탄소 나노 튜브는 촉매인 ferrocene을 탄소 계열의 xylene에 녹여 대량 생산이 가능한 기상 화학 증착법으로 제조되었다. 제조된 탄소 나노 튜브는 수직으로 정렬되었고 내부 hole이 튜브의 끝에 의해 막힌 구조를 가진다. 탄소 나노 튜브의 구조를 좀더 수소 저장 특성에 알맞게 변화시키기 위하여 연속적으로 대량의 시편처리가 가능한 상압 플라즈마에 의해 튜브의 끝 부분이 etching되었다. etching gas로써 산소가 사용되었으며 그 비율을 달리하여 구조를 관찰한 결과 2%의 산소를 첨가하여 etching하였을 경우 탄소 나노 튜브의 끝 부분이 제거되어 내부 hole이 열린 상태임을 관찰할 수 있었다. pore의 크기를 분석한 결과 플라즈마 처리후 2-3nm 크기의 pore가 크게 증가한 것을 확인하였으며 Raman spectroscopy로 플라즈마 처리후 나노 튜브의 결정성이 크게 감소한 것을 볼 수 있었다. 이상에서 제조된 시편의 수소 저장 특성이 연속적인 온도변화와 함께 분석되었다. 기상 화학 증착법에 의해 제조된 cap이 막힌 나노 튜브의 경우 실제 사용할 수 없는 온도인 100-150K에서 약 4.9wt%의 수소가 방출되었으나 상압 플라즈마 처리 후 100-150K에서 5.1wt%의 수소방출 외에도 상온 부근인 300-330K에서도 0.6wt%의 수소가 방출되는 것을 확인할 수 있었다. 기존의 연구 결과에 따르면 저온에서의 수소방출은 탄소 나노 튜브의 외부벽에 물리 흡착된 수소에 의한 것으로 보여지며 상온에서의 수소 방출은 플라즈마 처리에 의해 생성된 defect와 open된 내부 hole에 기인하는 것으로 보여진다. 이상에서 실제 공정에 적용가능하고 scalable한 방법인 기상 화학 증착법과 상압 플라즈마로 탄소 나노 튜브의 수소 저장 특성을 향상시킬 수 있었으며 좀더 세밀한 조건 변화를 통해 그 저장량을 증가시킬 수 있을 것으로 생각된다.
