Ever since the first report by Fujishima and Honda in 1972 on the splitting of water through photo-electro catalytic reactions, the photocatalytic generation of hydrogen over semiconductors has received considerable attention from the viewpoint of potentially solving current environmental and energy problems. Nowadays, a metal-free polymeric photocatalyst, graphitic carbon nitride (g- $C_3N_4$ ), has been introduced in the field of solar driven applications for the splitting of water. However, its photocatalytic activity for hydrogen generation is poor due to its limited utilization of the solar spectrum ( $\lambda$ <460 nm), low surface area, and high recombination rate of the carriers.
The ball milling process was adopted to improve the photocatalytic activity of g- $C_3N_4$ . Sulfur doped g- $C_3N_4$ , which shows better photoconversion efficiency than pristine g- $C_3N_4$ , was used as starting material for the modification. The photocatalyst was prepared by calcinating thiourea and the replacement of sulfur with nitrogen was observed. An enhanced dispersion property and enlarged surface area, which can be ascribed to the reduced particle size, were observed after the modification. Furthermore, a reduced recombination rate was measured as a result of the eliminating the amino spices on the surface. The sample modified by for 48 hours showed an excellent hydrogen production rate under visible light irradiation with the photocatalytic hydrogen production activity 5.3 times higher than that of the unmodified sample. The prolonged lifetime of the carriers was also identified and the improvement of the photocatalytic activity can be attributed to the synergic effect of the reduced particle size and decreased surface defect concentration.
Furthermore, the study on the relationship between the surface defect concentration and the photocatalytic activity of the catalyst was carried out. The structural defects in g- $C_3N_4$ play an important role for photocatalytic activity of g- $C_3N_4$ for hydrogen production. The ball milling process for the elimination of the defects on the surface was adopted. The amino group created by incomplete polymerization of g- $C_3N_4$ may be considered as the surface defects and the lowered concentration of amino species was observed as the modification time became longer. The enhanced photocatalytic activity was measured after the modification and the linear plot for photocatalytic activity versus the molar ratio between bridging nitrogen and amino species was obtained. It was identified that the molar ratio is a major factor for enhancement of the photocatalytic hydrogen production rate.In order to oxidize water effectively, two ruthenium complexes having benzene and anthracene moieties on the axial ligands are synthesized and characterized. The aromatic moieties introduced in the axial ligand in-duce aggregation of catalyst. Dimeric aggregation properties are studied using UV-visible absorption, emission and 1H NMR spectral techniques. Water oxidation reaction is carried out with two ruthenium complexes using cerium (IV) as sacrificial oxidant. The water oxidation activity of the reaction is enhanced due to the aggregation property of the axial ligands. The effect of aggregation on catalytic activity is studied using emission spectral technique.
21세기 인류의 생존을 담보하기 위해서 친환경적 특성을 갖는 수소에너지가 새로운 에너지원으로 각광 받고 있다. 하지만 재생 가능한 수소 생산은 비싼 가격문제로 인해 아직까지 널리 사용되지 못하고 있다. 태양광 에너지는 주요한 재생 가능한 에너지원으로, 재생가능한 수소생산을 가능케 해 줄 수 있다. 수소를 에너지원으로 활용할 경우, 연소 시 발생하는 물질이 물로 이산화탄소 배출이 없는 장점을 갖는다.
g- $C_3N_4$ 물질의 촉매 성능을 증대시키기 위해 ball milling process가 도입되었다. 기존의 g-$C_3N_4$ 보다 높은 photo conversion efficiency를 갖는다고 알려진sulfur doped g- $C_3N_4$ 물질을 starting material로 사용하였으며, thiourea를 전구체로 촉매를 합성하였을 때, nitrogen 자리에 도핑된 sulfur를 확인할 수 있었다. Modification 후 작아진 particle size에 따른 증가된 비표면적, 향상된 dispersion property가 확인 되었으며 촉매 표면의 amino spices 제거에 따라 줄어든 recombination rate를 확인 할 수 있었다. 48시간동안 ball milling 처리된 촉매의 경우, ball milling 처리하지 않은 촉매에 비해 5.3배 증가된 촉매 성능을 보였으며 prolonged life time 및 증가된 광반응을 통한 수소 생산 비율이 측정되었다. 이는 작아진 particle size 및 줄어든 defect concentration의 synergic effect라 볼 수 있다.
뿐만 아니라 surface defect concentration과 촉매 성능에 관한 연구를 진행하였다. g- $C_3N_4$ 촉매에서 defect는 수소생산에 있어 중요한 역할을 한다고 알려져 있으며 본 연구에서는 촉매의 defect concentration을 조절하기 위하여 ball milling process를 도입하였다. Incomplete polymerization에 의해 형성된 amino species가 structure defect로 밝혀졌으며, modification 시간이 길어짐에 따라 줄어든 농도가 확인되었다. Modification 도입에 따라 증가된 촉매 성능이 확인되었고, bridging nitrogen과 amino species의 molar ratio에 따른 촉매 성능을 plot하였을 때 molar ratio가 촉매 성능의 증가에 있어 major factor라는 것이 규명되었다.
효율적인 물산화 반응을 위해 두개의 각각 benzene과 anthracene을 moiety로 하는 ruthenium complex를 합성하였다. Aromatic moiety의 도입을 통해 촉매의 aggregation을 유도하였으며, dimeric aggregation의 property에 관한 연구를 UV-vis 분석 및 NMR 분석을 통해 진행하였다. Cerium(IV)을 sacrificial agent로 하는 물산화 반응을 진행하였으며 axial ligand의 aggregation 특성에 따라 증가된 물산화 효율을 확인하였고, 다양한 emission spectral technique를 통해 촉매 aggregation에 따른 그 효과를 연구하였다.
