Boosting photoanodic activity for water splitting in carbon dots aqueous solution without any traditional supporting electrolyte

• PEC water oxidation was executed directly in a Carbon Dots (CDs) aqueous solution. • Optimized photocurrent achieves dozens of times for that in traditional electrolyte. • CDs solution results into accelerated charge transfer during PEC proceeding. • CDs solution acts as electrolyte and built dynamic heterojunction with photoanode. • Robustness roots from static and dynamic interaction between photoanode and CDs. We present a PEC water oxidation system with boosting activity in Carbon Dots (CDs) aqueous solution directly without any traditional supporting electrolyte. For α-Fe 2 O 3 photoanode, photocurrent was significantly enhanced by dozens of folds compared to that in Na 2 SO 4 solution under similar conditions. Based on electrochemical impedance spectroscopy, energy alignment measurements, photovoltage decay and Mott-Schottky plot, mechanism for charge transfer and function of CDs was well characterized. As semiconductors, CDs can combine with α-Fe 2 O 3 into heterojunction for charge separation. As electrolyte, it can induce downward shift in conduction band of α-Fe 2 O 3 , allowing for stronger driving force for charge migration. Since CDs solution possesses a negative Zeta potential, it is proposed that during PEC proceeding, static interaction between CDs and positively charged photoanodic substrate can induce a dynamic balance of CDs absorbing or desorbing on/off α-Fe 2 O 3 surface, which accounts for fast charge transfer, boosted photocurrent and high stability in PEC water splitting.