Inorganic Ions Promoted Photocatalysis Based On G-C₃N₄

Semiconductor photocatalysis has fascinated tremendous researchers because it is the most promising technology to settle intractable energy and environment issues using abundant solar energy for the useful of degradation of environmental pollutants, splitting of water into hydrogen fuel and reducing of CO2 into organic fuel. To use this technique in large scale application, a photocatalyst material which would be cheap, abundant and efficient is needed. Recently, as a free-metal semiconductor with a wealth of attractive properties such as excellent chemical and thermal endurance and unique energy band structure, polymeric graphitic carbon nitride (g-C3N4) are becoming interestingly significant as a visible light active photocatalyst in the sustainable utilization of solar energy. However, the g-C3N4 exhibits unsatisfactory photocatalytic activity. A possible reason for the poor photocatalytic activity of g-C3N4 photocatalyst is the inefficiency in separating and transporting charges for the chemical processes due to unavoidable disordered structure or defects produced during the synthesis of the crystal. The electron and hole are probably recombined at these specific surface termination sites, thus decreasing the photocatalytic activities.Optimizing the transmission path and reducing the recombination of photo-generated carriers are effective approach for improving the quantum efficiency of g-C3N4. In this thesis, based on the modification of inorganic ions for the polymer, the inorganic ions were used to overcome the shortcoming of the g-C3N4. The samples and reaction systems were characterized by all sorts of measurements method. We have demonstrated the mechism of high photocatalytic activity of g-C3N4 on present inorganic ions.The main contents and innovations of this investigation are as follows:Promoting the photodegradation of organic dyes over g-C3N4 by SO32- anions significantly. Sulfite is a common anion in the printing and dyeing industrial wastewater. We found that the photocatalytic degradation of organic dye over g-C3N4 was significantly improved by addition of SO32-. This may owe to two reasons:firstly, the photogenerated holes on g-C3N4 oxidize the SO32- into ·SO3-, and then induced a chain reaction for production of active species such as ·OH. Another, the consumption of h+ by SO32- caused the increase in the concentration of photogenerated electron which accelerated the formation of ·OH via multistep reduction of O2. This investigation offers the new opportunitie for using the g-C3N4 with low cost and facile adjustable function to treat industrial wastewater containing organic pollutants. Moreover, the consumption of sulfite during the g-C3N4 photocatalysis also provides a useful method for capture and removal of SO2.Improving the photoactivity of g-C3N4 by facile surface metal ion coordin-ation. The graphene-like C-N sheet of g-C3N4 is compound by tri-s-triazine (C6N7). The large nitrogen pots in the plane of g-C3N4 are composed of a 3-fold N-bridge linking the triazine units, which are filled with six nitrogen lone-pair electrons. The nitrogen atoms of the nitrogen pots of g-C3N4 also are potentially ideal sites for metal ion inclusion with the stronger coordination ability due to more lone-pair electrons. It is found that the metal ions (such as K+, Na+, Mg2+ et al.) coordinating into the plane of g-C3N4 significantly reduced fluorescence intensity, lifetime and non-radiative rate, indicating the formation of new, nonradiative deactivation pathways, which is an efficient surface modification method to improve the separation and transport of charge carriers. Enhanced charge separation and transport contribute to a drastic increase of the photocatalytic activity more than 5 times in solar hydrogen production as well as in the photodegradation of organic pollutants.Improving the photocatalytic splitting water H2 evolution over carbon nitride by ion intercalation. For layered materials, intercalation provides to the host material a means for controlled variation of many physical properties over a wide range. Carbon nitride intercalation compound (CNIC) synthesized by a simple molten salt route is an efficient polymer photocatalyst exhibits a quantum yield as high as 21.2% under 420 nm light irradiation for solar hydrogen production. We found that coordinating the alkali metals into the C-N plane of carbon nitride will induce the un-uniform spatial charge distribution. The electrons are confined in the intercalated region while the holes are in the far intercalated region, which promoted efficient separation of photogenerated carriers. The donor-type alkali metal ions coordinating into the N6 pots of carbon nitrides increase the free carrier concentration and lead to the formation of novel nonradiative paths. This should favor improved transport of the photogenerated electron and hole and decrease the electron-hole recombination rate.Effect of inserted metal ions in g-C3N4 on the structure and properties. For the different ionic radical and electronegativity, the alka metal ions may play different effect for the photocatalytic activity of g-C3N4 under visible light irradiation. It was found that Li+ ion embedded in the carbon nitride tend to form the PTI/Li+Cl-structure. This structure promotes the binding energies of the exciton for the materials and induces a serious band edge recombination, thus leads to the inhibition of photocatalytic activity of the material. In differently, intercalated structure was formed by coordinated K+ ions with the big N6 rings between the plane of g-C3N4, the inserted of K+ ions give rise to deform of the C-N plane which in favor of the separation and transport of carriers, thus improving the photocatalytic activity. Moreover, Combined with improving condensation of g-C3N4, pre-protonated g-C3N4 and surface alkal metal ion coordination, the carbon nitride intercalation compound shown the photocatalytic H2 evolution rate higher than 1 mmol/h under the visible light irradiation.