Tuning Photocatalytic Redox Ability Of G-C₃N₄ For Its Environmental Catalytic Activity Enhancement

Since Fujishima and Honda discovered the evolution of oxygen and hydrogen at a TiO2 electrode under the irradiation of light in 1972, semiconductor photocatalysis have. attracted worldwide interests because of its potential applications in the fields of environment and energy. Unfortunately, the visible light photocatalytic activity of TiO2 is very poor because it can be only activated under UV light irradiation. Although many methods have been developed to increase the efficiency of TiO2, including anionic or cationic doping, coupling with other semiconductors, porousification and surface area enhancement, as well as quantum size confinement, its visible light photocatalytic activity is still too low for practical application. Many researchers thus turn to develop new non-titania based photocatalysts active under visible light.Graphitic carbon nitride (g-C3N4) has attracted increasing attention because of its high thermal and chemical stability, semiconductivity, and special optical features. Recently it was proven to be a promising metal-free visible light photocatalyst to generate hydrogen from water, oxidize benzyl alcohol and decompose pollutants. However, pure g-C3N4 can only absorb the blue light up to 450 nn and suffers from the fast recombination of photogenerated electrons and holes. Moreover, it also suffers the self decomposition by photogenerated holes during the photocatalysis because of its organic nature. All these disadvantages limit the utilization of solar energy to some degree. Therefore, the purpose of this dissertation is to improve the photoreactivity and stability of g-C3N4, and clarify the reasons for the enhanced activity and stability. The detailed results were summarized as follows.1. We demonstrated that g-C3N4 could photoreduce CO2 to CO in the presence of water vapor and exhibited interesting porous structure dependent photoreactivity under visible light (λ> 420 nm) irradiation. g-C3N4 was synthesized by directly heating the inexpensive melamine and the replacement of melamine with melamine hydrochloride could result in porousification in the final g-C3N4 with much higher surface area (39 times) and more abundant pores, accompanying with band gap increase of 0.13 eV. The porousification could significantly enhance the photoreactivity of g-C3N4 on rhodamine B photooxidation by 9.4 times, but lower its activity on CO2 photoreduction by 4.6 times, while porousification induced band gap enlargement did not favour either photooxidation or photoreduction. The reasons for the porous structure dependent photoreactivity were investigated in detail.2. We demonstrated that carbon self doping could induce intrinsic electronic and band structure change of g-C3N4 via a homogeneous substitution of lattice nitrogen with carbon, accompanying with a surface area increase without the introduction of impurity. The homogeneous substitution of carbon could increase the visible light absorbance and electrical conductivity resulted from the formation of big delocalized π bonds among the substituted carbons and the hexatomic rings. The resulting carbon self doped g-C3N4 exhibited enhanced activity on the RhB photooxdiation and the photoreduction of Cr(VI) as well as the photocatalytic H2 evolution under visible light irradiation. We assigned the photoredox activity enhancement to easier photoexcitation due to smaller band gap, better separation and transfer of photogenerated carriers arisen from smaller particle sizes and higher electrical conductivity, and higher surface area induced by carbon self doping. Interestingly, we found that photogenerated carries and surface area contributed differently to RhB photooxidation and Cr(Ⅵ) photoreduction because of their different adsorption behaviors over carbon self doped g-C3N4.3. We reported on the synthesis of formate anion containing g-C3N4 and its dramatically enhanced activity and stability on Cr(Ⅵ) photoreduction under visible light irradiation. We found that the incorporated formate anions could not only trap the photogenerated holes to produce more photogenerated electrons, but also change two-step superoxide ions mediated indirect reduction to one-step direct photogenerated electrons reduction of Cr(Ⅵ) over g-C3N4 under visible light through inhibiting surface dioxygen adsorption, and thus enhance Cr(Ⅵ) photoreduction. This study could not only develop a novel strategy to improve the Cr(Ⅵ) photoreduction activity and stability of semiconductors, but also shed light on the deep understanding of the relationship between intrinsic structure and Cr(Ⅵ) photoreduction activity of semiconductor photocatalysts.4. We demonstrated that oxygen functionalization could endow g-C3N4 with anoxic photocatalytic organic pollutant oxidation ability. The oxygen functionalization could increase the anoxic photocatalytic pollutant degradation and mineralization constants by about 18 and 7 times under visible light, respectively. After systematically investigated the relationship between oxygen functional groups and anoxic photo-oxidation property of g-C3N4, we attributed the anoxic photocatalytic oxidation ability of g-C3N4 to the holes remained after photogenerated electron trapping by oxygen functional groups for hydrogen evolution. The anoxic photo-oxidation activity of oxygen functionalized g-C3N4 did not significantly decline after four cycles, suggesting its high stability. This study could provide some new insight into the correlation between oxygen functionalization and semiconductor photocatalysis as well as the design and fabrication of anoxic photocatalysts.5. We demonstrated that p-type g-C3N4 could reduce CO2 to CO more efficiently and selectively under visible light than the n-type counterpart. We found that p-type g-C3N4 of ultrathin nanostructure with abundant surface defect sites could harvest more visible light, and favor the separation and transfer of photogenerated carriers, as well as a strong chemisorption of CO2, all contributing to its higher photoreactivity. Meanwhile, the surface defects of p-type g-C3N4 shifted the adsorption mode of CO2 from N-CO2- for the n-type counterpart to N-O-C=O, eventually resulting in its higher selective reduction of CO2 to CO.