| Graphitic carbon nitride(g-C3N4) is a kind of novel semiconductive polymeric photocatalysts, characterized by its lamellar structure, which is similar to graphite, has adaptive band structure and high chemical stability. It has been widely used with great performance in pollutant degradation, water splitting to produce H2 and carbon dioxide reduction. However, the rapid recombination of photogenerated charges restrict its photocatalytic activities. In this work, different modification methods were explored to modify g-C3N4 with inorganic acid and introduce functional bridge in the fabricated heterojunction nanocomposites in order to improve the separation of photogenerated charge, and consequently lead to enhanced photocatalytic activities. The influences of inorganic acid and introduce functional bridge on photogenerated carrier properties and photocatalytic activities were investigated in detail.O2 is a common and important oxidant, which could be activated and trigger subsequent reactions by capturing photoelectrons in photocatalysis processes during organic pollutants degradation. Notably, it is a feasible route for the promotion of O2 adsorption on photocatalysts to improve its photocatalytic activities. In this paper, g-C3N4 was modified with phosphoric acid by the simple method of dip-molding. The results showed that the photocatalytic activity for degrading pollutants could be greatly improved after modification with a proper amount of phosphoric acid, which is attributed to the obvious increase in the adsorbed O2 and its capturing of photoelectrons on the surface of g-C3N4 so as to prolong the lifetime and enhance the separation of photogenerated carriers, mainly based on the results of O2 temperature-programmed desorption curves(O2-TPD), atmosphere-controlled(AC-SPS) and time-resolved(TPV) surface photovoltage measurement. Furthermore, it is confirmed that modification with sulfuric acid or hydrofluoric acid could increase the adsorbed O2 on g-C3N4, leading to enhanced photocatalytic activity. It is further proved that the method of modification with inorganic acid is feasible for promoting O2 adsorption on the surface of g-C3N4 and improving its photocatalytic activities.In addition, one of the most general methods to enhance the separation of photogenerated carriers for g-C3N4 is to construct a suitable heterojunctional composite, according to the principle of matching energy levels. Moreover, the interface contact greatly influence the charge transfer and separation so as to greatly influence its photocatalytic activities, in fabricated heterojunction nanocomposites. So, in this paper, the Zn O/g-C3N4 nanocomposites were successfully fabricated by a simple wet chemical process and silicate-oxygen bridge(-Si-O-bridge) was successfully introduced in the fabricated Zn O/g-C3N4 nanocomposite. Our results of AC-SPS, TPV and EIS show that the photogenerated electrons transfer from g-C3N4 to Zn O. More importantly, the introduction of ‘-Si-O-bridge’ improved the connection between Zn O and g-C3N4, which is much favourable for charge transfer between g-C3N4 and Zn O so as to further prolong the lifetime and enhance the separation of photogenerated carriers. Hence, the formed silicate-oxygen bridged Zn O/g-C3N4 nanocomposites showed higher photocatalytic activity for degrading pollutants and splitting water to H2.In order to further verify and expand our so called ‘ bridging ’ idea, the phosphate-oxygen bridged Ti O2/g-C3N4 nanocomposites were also fabricated by the same wet chemical process. The results showed that, the introduction of Ti O2 enhance the separation of photogenerated carriers of the Ti O2/g-C3N4 nanocomposites, especially after the introduction of ‘-P-O-bridge’ because, the formed bridge could improve connection between Ti O2 and g-C3N4, which is much favourable for charge transfer between Ti O2 and g-C3N4 for the higher photocatalytic activity of pollutants degradation and carbon dioxide reduction. It is further proved that the method of fabricating bridged heterojunction nanocomposites is feasible to enhance the separation of photogenerated charges for improved photocatalytic activities of g-C3N4.Based on the above strategies, the problems of the recombination of the photogenerated carriers in g-C3N4 are solved well, leading to improved photocatalytic activities for degrading pollutants, splitting water to H2 and carbon dioxide reduction under light irradiation. The corresponding mechanism was studied in order to compensate for the lack of existing research to some extent. It provides references for designing and synthesizing g-C3N4 based photocatalysts with high activities. |
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