Carbon Nitride Based New Type Photocatalyst With High Photocatalytic Activity And Wide Spectral Response

Semiconductor photocatalysis has attracted a great deal of attentions due to their wide application in the environmental remediation, especially for organic pollutants removal. The practical application of the traditional photocatalytic materials with wide band gap, especially TiO2, is subject to severe restrictions, due to their limited ultraviolet light utilization and low quantum yield contributing to the easy recombination of photogenerated electrons/holes. We must broaden the photocatalytic materials’range of light response and discover a new generation of photocatalytic materials capable of absorbing visible light, for the purpose of improving the photocatalytic material’s utilization of sunlight and thereby breaking the bottlenecks of photocatalytic application. The two-dimensional (2D) structure nanomaterials are of unique structure and performance characteristics that are different from the bulk material owing to the unique structure associated with their ultra-thin thickness and two-dimensional planar morphology.2D nanomaterials have unique advantages and particularity as photocatalysts. As 2D layered material that has advantageous properties of suitable band-gap (-2.7eV), appropriate band edges, metal-free, non-toxicity, stability and low-cost, novel photocatalyst graphitic carbon nitride (g-C3N4) has great potential of practical utilization. This dissertation is devoted to develop g-C3N4-based photocatalysts with high photocatalytic activity and/or wide spectrum response by modulating the compositions and heterostructures according to the basic procedures of photocatalytic degradation of organic pollutants. The modulation is based on the 2D structure and the surface properties, and the possible influencing factors of photocatalytic activity and spectral response of photocatalyst were discussed.(1) Water-soluble magnetic-functionalized graphitic carbon nitride (g-C3N4) composites were synthesized successfully by in situ decorating spinel nanoparticles on g-C3N4 sheets (CN-ZnFe) through a one-step solvothermal method. The magnetic properties of CN-ZnFe can be effectively controlled via tuning the coverage density and the size of ZnFe2O4 nanoparticles. The results indicate that the CN-ZnFe exhibits excellent photocatalytic efficiency for methyl orange (MO) and fast separation from aqueous solution by magnet. Interestingly, the catalytic performance of the CN-ZnFe is strongly dependent on the loading of ZnFe2O4. The optimum activity of 160CN-ZnFe photocatalyst is almost 6.4 and 5.6 times higher than those of individual g-C3N4 and ZnFe2O4 toward MO degradation, respectively. By carefully investigating the influence factors, a possible mechanism is proposed and it is believed that the synergistic effect of g-C3N4 and ZnFe2O4, the smaller particle size, and the high solubility in water contribute to the effective electron-hole pairs separation and excellent photocatalytic efficiency. This work could provide new insights that g-C3N4 sheets function as good support to develop highly efficient g-C3N4-based magnetic photocatalysts in environmental pollution cleanup.(2) A novel efficient Ag@AgCl/g-C3N4 plasmonic photocatalyst was synthesized by a rational in situ ion exchange approach between exfoliated g-C3N4 nanosheets with porous 2D morphology and AgNO3. The as-prepared Ag@AgCl-9/g-C3N4 plasmonic photocatalyst exhibited excellent photocatalytic performance under visible light irradiation for rhodamine B degradation with a rate constant of 0.1954 min-1, which is-41.6 and-16.8 times higher than those of the g-C3N4 (-0.0047 min-1) and Ag/AgCl (～0.0116 min-1), respectively. The degradation of methylene blue, methyl orange and colorless phenol further confirmed the broad spectrum photocatalytic degradation abilities of Ag@AgCl-9/g-C3N4. These results suggested that an integration of the synergetic effect of suitable size plasmonic Ag@AgCl and strong coupling effect between the Ag@AgCl nanoparticles and the exfoliated porous g-C3N4 nanosheets was superior for visible-light-responsive and fast separation of photogenerated electron-hole pairs, thus significantly improving the photocatalytic efficiency. This work may provide a novel concept for the rational design of stable and high performance g-C3N4-based plasmonic photocatalysts for unique photochemical reaction.(3) 1D Ag@AgVO3 nanowire/graphene/protonated g-C3N4 nanosheet (Ag@AgVO3/rGO/PCN) heterojunctions are fabricated via a simple electrostatic self-assembly process followed by a photochemical reduction method. In this hybrid structure, 1D Ag@AgVO3 nanowires penetrate through 2D nanosheets (graphene and PCN), forming a 3D hybrid photocatalyst, which is applied as an efficient visible light driven photocatalyst for organic pollutant degradation. Its enhanced photocatalytic activity is ascribed to the well-known electronic conductivity of 2D graphene, the intense visible light absorption of 1D Ag@AgVO3 nanowires, large surface areas and rapid photogenerated charge interface transfer and separation. Our results provide a facile way to fabricate hierarchical g-C3N4-based photocatalysts in a controlled manner and highlight promising prospects by adopting an integrative 1D and 2D nanomaterial strategy to design more efficient semiconductor-based composite photocatalysts with high photocatalytic activities and a wide spectral response toward environmental and energy applications.(4) Bandgap narrowing and a more positive valence band (VB) potential are generally considered to be effective methods for improving visible-light-driven photocatalysts because of the significant enhancement of visible-light absorption and oxidation ability. Herein, an approach is reported for the synthesis of a novel visible-light-driven high performance polymer photocatalyst based on band structure control and nonmetal and metal ion codoping, that is, C and Fe-codoped as a model, by a simple thermal conversion method.The results indicate that compared to pristine graphitic carbon nitride (g-C3N4), C+Fe-codoped g-C3N4 show a narrower bandgap and remarkable positively shifted VB; as a result the light-absorption range was expanded and the oxidation capability was increased. Experimental results show that the catalytic efficiency of C+Fe-codoped g-C3N4 for photodegradation of rhodamine B (RhB) increased 14 times, compared with pristine g-C3N4 under visible-light absorption at λ＞ 420 nm. The synergistic enhancement in C+Fe-codoped g-C3N4 photocatalyst could be attributed to the following features:(1) C+Fe-codoping of g-C3N4 tuned the bandgap and improved visible-light absorption; (2) the porous lamellar structure and decreased particle size could provide a high surface area and greatly improve photogenerated charge separation and electron transfer; and (3) both increased electrical conductivity and a more positive VB ensured the superior electron-transport property and high oxidation capability. The results imply that a high-performance photocatalyst can be obtained by combining bandgap control and doping modification; this may provide a basic concept for the rational design of high performance polymer photocatalysts with reasonable electronic structures for unique photochemical reaction.