Synergistic Effect of Charge Generation and Separation in Epitaxially Grown BiOCl/Bi₂S₃ Nano-Heterostructure

Nano-heterostructures are widely used in the field of optoelectronic devices, and an optimal proportion usually exists between the constituents that make up the structures. Investigation on the mechanism underlying the optimal ratio is instructive for fabricating nano-heterostructures with high efficiency. In this work, BiOCl/Bi₂S₃ type-II nano-heterostructures with different Bi₂S₃/BiOCl ratios have been prepared via epitaxial growth of Bi₂S₃ nanorods on BiOCl nanosheets with solvothermal treatment at different sulfuration temperatures (110-180 °C) and their photoelectrochemical (PEC) performances as photoanodes have been studied. Results indicate that the Bi₂S₃ content increases with the sulfuration temperature. BiOCl/Bi₂S₃-170 (i.e., sulfurized@170 °C) exhibits the highest PEC performance under visible-light illumination, whereas BiOCl/Bi₂S₃-180 with the maximum Bi₂S₃ content shows the highest visible-light absorption, i.e., possessing the best potential for charge generation. Further analysis indicates that the BiOCl/Bi₂S₃ heterojunction interface is also crucial in determining the PEC performance of the obtained heterostructures by influencing the charge separation process. With increasing Bi₂S₃ content, the interface area in the BiOCl/Bi₂S₃ nano-heterostructures increases first and then decreases due to the mechanical fragility of the nanosheet-nanorod structure and the structural instability in the [010] direction of Bi₂S₃ with higher Bi₂S₃ content. Therefore, the increasing content of the Bi₂S₃ does not necessarily correspond to higher heterojunction area. The optimal performance of BiOCl/Bi₂S₃-170 results from the maximum of the synthetic coordination of the charge generation and separation. This is the first time ever to figure out the detailed explanation of the optimal property in the nano-heterostructures. The result is inspiring in designing high-performance nano-heterostructures from the point of synthesizing morphological mechanically robust heterostructure and structurally stable constituents to reach a high interfacial area, as well as high light-absorption ability.