Real roles of perylenetetracarboxylic diimide for enhancing photocatalytic H₂-production

Highlights: • H₂-evolution is markedly promoted with doping minute quantity of π-conjugate PDI molecules to the Zn_0.5Cd_0.5S semiconductor. • Mechanism is proposed that PDI functions as an electron collector and transporter to inhibit the charge recombination. • The charge separation and the H₂-evolution efficiency depend heavily on the modulated molecular structure of PDIs. On account of the steps of water splitting reaction, suppression of the electron–hole recombination is one key factor to improve the photocatalytic activity. Composites with π-conjugated molecules are promising to inhibit the recombination process by delocalization of the photo-generated electron. In this study, we synthesized perylenetetracarboxylic diimide (PDI) decorated Zn_0.5Cd_0.5S hybrid photocatalysts to reveal the function of the PDI in promoting the charge separation by electron transfer process. To understand the mechanisms that govern the carrier separation, transport, extraction and their recombination within this inorganic/organic nanocomposite, three PDIs, namely PDI-1, PDI-2 and PDI-3 with different molecular structure were loaded in the Zn_0.5Cd_0.5S, respectively, and were investigated comparatively. It is demonstrated that PDIs play great roles in increasing the specific surface area and stabilizing the photogenerated electron–hole pairs, resulting in enhancement of the overall hydrogen production rate. The highest rate of 1.32 mmol h⁻¹ g⁻¹ was achieved on the Zn_0.5Cd_0.5S-PDI-1 composite, which is 6 times higher than the pristine Zn_0.5Cd_0.5S, owning to the effective photo-driven electron transport between the Zn_0.5Cd_0.5S and the PDI-1. The results of the current work are relevant in understanding the nature of charge-transfer pathways in photoinduced catalysis.