Insight on Shallow Trap States-Introduced Photocathodic Performance in n-Type Polymer Photocatalysts

Graphitic carbon nitride (g-C₃N₄) is a robust organic semiconductor photocatalyst with proven H₂ evolution ability. However, its application in a photoelectrochemical system as a photocathode for H₂ production is extremely challenging with the majority of reports representing it as a photoanode. Despite research into constructing g-C₃N₄ photocathodes in recent years, factors affecting an n-type semiconductor's properties as a photocathode are still not well-understood. The current work demonstrates an effective strategy to transform an n-type g-C₃N₄ photoanode material into an efficient photocathode through introducing electron trap states associated with both N-defects and C-OH terminal groups. As compared to the g-C₃N₄ photoelectrode, this strategy develops 2 orders of magnitude higher conductivity and 3 orders of magnitude longer-lived shallow-trapped charges. Furthermore, the average OCVD lifetime observed for def-g-C₃N₄ is 5 times longer than that observed for g-C₃N₄. Thus, clear photocathode behavior has been observed with negative photocurrent densities of around -10 μA/cm² at 0 V vs RHE. Open circuit photovoltage decay (OCVD), Mott-Schottky (MS) plot, and transient absorption spectroscopy (TAS) provide consistent evidence that long-lived shallow-trapped electrons that exist at about the microsecond time scale after photoexcitation are key to the photocathode behavior observed for defect-rich g-C₃N₄, thus further demonstrating g-C₃N₄ can be both a photoanode and a photocathode candidate.