Surface Engineerings of Graphitic Carbon Nitride Films for Photoelectrochemical Applications


Student thesis: Doctoral Thesis

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Award date19 Aug 2021


Graphitic carbon nitride (g-CN) materials are promising photocatalysts for photoelectrochemcial (PEC) applications such as water splitting into H2 and O2 which are ideal clean chemical energy carriers producing only water after combustion. While depositions of uniform, large area, and pinhole free g-CN thin films required for PEC applications have been successfully achieved recently but the film’s charge carrier conductivity and stability are still low for PEC applications. The PEC performance of g-CN can be improved by modifying its photocatalytic properties because it has been widely considered as a promising agent for water splitting, having a reasonable band gap, and being metal free. Although doping and forming heterojunction are straightforward approaches for improving the conductivity and electron-hole charge separation, interstingly, we have recently observed that certain variation in surface composition of g-CN thin film could not only improve the conductivity of charge carriers but also significantly improve the chemical stability of the g-CN films.

Here, in this thesis we employed metal free elements like boron for doping by surface modification of g-CN films and discovered its influence on the PEC performance experimentally. The as-prepared films modified by surface doping boost the photocurrent density by a factor of 3 vs reversible hydrogen electrodes without sacrificial reagents as compared to pristine g-CN films. The B atoms readily substitutes the nitrogen atoms when film is coated 5 times and reduces the bandgap by raising the valence band edge. We demonstrated that such improvement is owing to the enhanced charge carrier separation, improved charge migration, reduced space charge region and lowed recombination rate.

The electronic structures and physicochemical properties of g-CN photoanodes can also be varied by coupling it with tin disulphide (SnS2) nanosheets to form 2D/2D heterojunction photocatalysts, leading to high photocatalytic activity. In this work, we obtained the 2D/2D type of heterojunction photocatalysts fabricated by horizontal loading ultrathin hexagonal SnS2 nanosheets on g-C3N4 thin film through a facile spin coating. The sheet-like structures of these two nanomaterials induce a large contact region in the heterojunction interface, as evidenced by electron microscopic analyses. By taking advantage of this feature, the as-fabricated SnS2/g-C3N4 heterojunction exhibits considerable improvement on the photocatalytic activities for the efficient PEC performance as compared to pure g-C3N4 and SnS2 nanosheets. After spin coating, the SnS2 nanosheets on g-CN film were annealed at three different temperatures. The optimal heterojunction with 400ºC annealing temperature shows 125 µA/cm2 photo electric current density which is apparently higher than that of pure g-CN thin films and SnS2 nanosheets by factors of 5 and 4, respectively. Further findings by transient photoluminescence spectroscopy indicate that the photo synergistic effect of SnS2/g-C3N4 heterojunction can remarkably enrich the photoinduced interfacial charge transfer, thereby expanding the charge separation during the photocatalytic reaction.

Furthermore, studies about the surface exfoliation of g-CN using ethanol and methanol as solvents were also carried out which shows significant changes in the photocatalytic behaviour of g-CN film. The ultrasonication of g-CN film in various solvents changed the film’s morphology and enhanced the electron-hole charge separation which in turn ease the charge migration and boosted the photocurrent density.

In summary, the g-CN films were rationally designed, modified, and studied in-depth in the current work, showing the correlation between the component and their PEC performance. It is expected that the tactics and conclusions in this thesis can shed new glow on the compositional model of semiconductor materials for cost-effective PEC energy storage/conversion applications.