Photocatalytic applications of graphitic carbon nitride; Designing the photo-electronic properties

Abstract

Developing an efficient photocatalyst and find out the mechanisms of it give us the new opportunities to direct utilization of the sun light as an alternative energy source. The sun light which arrived earth ground through the atmosphere is mostly consisted with long wave length ranged light such as visible light. As known for the most efficient photocatalyst, titanium oxide $(TiO_2)$ reacts only under above UV light. Additionally, the quantum efficiency which is indicated photon conversion to products is pretty low to commercially use. Here, the strategies for developing efficient photocatalyst and its various applications which based on organic carbon material is studied. The organic materials have lots of advantages when we use it as catalyst such as, mild synthesis conditions, easy modulation of composition and structure, and cheap fabrication cost. However, the organic based photocatalyst is known for weak physical and optical stability than metal based semiconductors. The carbon nitride $(C_xN_y)$, which is consisted with alternatively covalent bonded nitrogen and carbon atom, can be divided into 3 types, i.e. atomic, crystal, and graphitic. The graphitic carbon nitride (g-CN) is the most physically and chemically stable isotope at STP condition. The g-CN can be divided into two types, triazine and tri-s-triazine unit structure. Even though the good physical and chemical properties, the g-CN has limit to apply as photocatalyst because of the low photon efficiency, fast charge recombination rate, and pretty large band gap. Within this work, we synthesized mesoporous graphitic carbon nitrides with 3D-cubic structure via nano casting method using mesoporous silica hard template and template free micro sized hollow sphere using melamine and cyanuric acid network. The optical and electrical properties of semiconductor varies depending on its chemical compositions, and structures. The synthesized mesoporous graphitic carbon nitride was modulated by defect engineering and controlling condensation reaction for suitable utilization of environmental, energy, and biomedical applications. For environmental photocatalytic application, we designed the defect rich 3D-cubic mesoporous g-CN. In previous work, we already demonstrated the RhB degradation using 3D-cubic mesoporous g-CN. Almost 6 hr is required to remove 10 $\mu$ mol/ml of RhB under visible light irradiation. The result comparing with bulk and commercially use $TiO_2$ nanoparticle, mesoporous graphitic carbon nitride has the best activity. When we use the carbon nitride as an organic molecular reaction, the surface primary amine functionalities are favorable because the electron pathway might possible only defects and termination sites of the graphitic sheet which promoting the electron re-localization. Above all the structure of RhB is resembled with heterocyclic benzene rings which could be possible to form a charge transfer complex. For the further enhancement of photocatalytic efficiency of mesoporous graphitic carbon nitride, the malonic acid incorporated with cyanamide precursors. The malonic acid is one of the dicarboxylic acid used as stabilizer of high concentrated aqueous cyanamide solution by inherent the rapid dimerization, resulted in dicyandiamide. 0.01 ~ 0.05 weight portion of malonic acid shows the best stabilizing action. Here we used the malonic acid as a defect site inducing agent in mesoporous graphitic carbon nitride structure. The interfered thermal poly-condensation would be resulted in less condensed mesoporous graphitic carbon nitride which have lots of primary amine termination inside the structure. Moreover, we utilized defect rich mesoporous graphitic carbon nitride as for photocatalytic hydrogen gas evolution. Interestingly when we used 0.05g of malonic acid used (0.0065 wt fraction) has the longest life time of separated charge carriers. Because the defect structure of photocatalyst could act as charge separation center or charge recombination center. With this respect view, the introducing defect structure into photocatalyst should be controlled carefully. Opto-electro property of $CNM_{0.05}$ can be directly explained the enhancement of hydrogen evolution. The optimum amounts of malonic acid incorporated g-CN can produce the hydrogen gas in aqueous solution almost 6 time better than pristine g-CN under visible light irradiation (≥420nm). The hydrogen gas evolution rate and efficiency is 25.31 ？mol/hr and 1.8 % respectively without performance decreasing during several re-use. Template free g-CN can be fabricated using melamine and cyanuric acid network by its hydrogen bonding. Recently reported sulfur mediated synthesis of g-CN indicated us extra synthetic ways of condensation. The presence of sulfur motifs in framework or circumstances evoke enhanced condensation/polymerization of precursors via implementing of easy leaving group of ？SH. Synthesis of highly condensed but large surface area material is usually contradicted to fulfill both requirements. However, the sulfur mediated strategies can offer much simple, but efficient synthetic pathway to fabricate the large surface area and condensed graphitic carbon nitride. In this research, the condensed graphitic carbon nitride was prepared by $H_2S$ gas stream during thermal poly-condensation of as synthesized melamine and cyanuric acid supramolecular adducts (MCA). The melamine and cyanuric acid were utilized as morphological and constitutional precursors for graphitic carbon nitride. 1:1 hydrogen bonding networks of precursors formed precipitated in dimethylsulforxide (DMSO) media. The condensed structure of MCA was utilized for photocatalytic conversion of aqueous carbon dioxide into valuable hydrocarbons under visible light irradiation. The rod structure of g-CN was transformed into nano particle using inorganic salt melts to utilize it as a bio-medicine. In therapeutic view, developing efficient photodynamic nanomaterials is required to replace the typically used homogeneous photosensitizers in photodynamic therapy (PDT) of cancer. Previously g-CN is already known as a biocompatible material rather than toxic metal base materials which indicated the potential of bio-medical application of g-CN. For bio-medical applications, the small size of material is required to transport into cell. We fabricated the nano particle g-CN using rod type of melamine-cyanuric acid network and inorganic salt melts. The small size of g-CN was utilized both as therapeutic agent of PDT and cell imaging without further modifications. The low toxicity and biocompatibility of nano particle g-CN can transported inside the cells and kill the cancer cells by visible light irradiation with red fluorescence emission. Moreover, low concentration of nanoparticle g-CN can destroy cancer cells rather than normal cells which indicated the possibility of selective therapy to minimize the damage of normal cells. This study suggests the promising utilization of g-CN for bio-medical applications, especially cancer diagnosis and therapy.