Photocatalysis over NH2-UiO-66/CoFe2O4/CdIn2S4 double p-n junction: Significantly promoting photocatalytic performance by double internal electric fields

Providing sufficient driving force for charge separation is a critical requirement in photocatalytic H2 evolution (PHE), but it remains challenging to precisely adjust the charge separation. Herein, we address this challenge by constructing a NH2-UiO-66/CoFe2O4/CdIn2S4 (NU6/CFO/CIS) double p-n junction with double internal electric fields. Compared with p-n junction with single internal electric field, double p-n junction enables photocatalysts with dual carrier transfer channels, faster carrier separation rate and stronger redox capacity. Additionally, PHE rate is related to the mass of photocatalyst used and a different photocatalyst weight may lead to a different PHE rate. When an excessive amount of photocatalyst is used, most of the photocatalyst cannot adsorb sufficient light and the photocatalytic rate is lower when normalized by the unit photocatalyst mass. Therefore, the effect of photocatalyst mass on PHE rate and apparent quantum yield are investigated in depth. The 25% NU6/2% CFO/CIS double p-n junction exhibits comparable PHE activity, its activity is about 13.5 times and 2.5 times higher than that of CIS and 2% CFO/CIS single p-n junction. In-situ XPS, density functional theory calculations, and Pt ion probe method are used to confirm where the photogenerated charges go and where they react, which is very in accordance with the "double p-n junction with double internal electric fields" mechanism. This work not only provides some ideas for the design of high-efficiency photocatalysts, but also offers a reference for the photocatalytic performance evaluation system.