Developing positively surface-charged carbon dots as "semiconductor electrolyte" for photo-electrochemical H2O2 production based on oxygen reduction.

[Display omitted] • Positively surface-charged carbon dots (CDs) function as electrolytes in PEC system. • CDs solution regulates PEC O 2 reduction into H 2 O 2 with high yield and selectivity. • CDs and photocathode unite into a dynamic p-n heterojunction for charge separation. • CDs adjust electronic structure and hydrophilic property of catalytic interface. • CDs incorporate active sites that reduce kinetic barrier for H 2 O 2 generation. Electrolyte modulation for tailoring photoelectrochemical (PEC) activity and selectivity is highly effective but has proved challenging. The major obstacle is lack of a universal strategy that aims for improving the system charge separation based on electrolyte optimization. Herein, we presented an idea as "semiconductor electrolyte", which was developed from the imidazole chloride delivered carbon dot (CDs), and featured for the positive surface charge. The CDs aqueous solution can accumulate H 2 O 2 on a CuBi 2 O 4 photocathode via PEC O 2 reduction reaction (ORR) at neutral pH without any traditional electrolyte. Combined with experimental measurements and theoretical calculation, the mechanism of system and function of CDs was characterized. As electrolytes, CDs solution enabled to tune the energy band location of CuBi 2 O 4. The electrostatic interaction between positively-charged CDs and negatively-charged photocathode triggered the fast charge and mass migration during PEC proceeding. As semiconductors, CDs can fabricate a dynamic p-n heterojunction with CuBi 2 O 4. These advantages facilitated the photo-induced charge separation, which contributed to the boosted photocurrent and improved stability in PEC system. As ORR occurred interface that incorporated active sites, CDs provided a hydrophilic and adjusted-electronic surface, which fascinated to confine O 2 and reduced the kinetic barrier for H 2 O 2 generation. As a result, H 2 O 2 generation was regulated with 0.44 mM and faraday efficiency over 84 % within 10 min, nearly 3 times to the commonly-used Na 2 SO 4 solution. Our work therefore provides a striking and facile strategy to tailor PEC activity and selectivity in a durable and robust pathway. Importantly, it will evoke to develop a universal protocol for regulating PEC performance by electrolyte modulation. [ABSTRACT FROM AUTHOR]