Heterostructures based on covalent organic frameworks (COFs) and other two-dimensional (2D) materials attract considerable attention due to their extraordinary properties and tremendous application potential. Substrate effects play a crucial role in the integration of ultrathin COF films onto 2D materials through direct polymerization. In this study, highly ordered monolayer COFs were successfully constructed on the surfaces of highly oriented pyrolytic graphite (HOPG), hexagonal boron nitride (hBN), and molybdenum disulfide (MoS₂). High-resolution atomic force microscopy (HR-AFM) imaging clearly reveals the substrate orientation effect in COFs/2D materials heterostructure. Honeycomb networks formed via Schiff-base reaction and boronic acid condensation reaction can epitaxially grow in specific orientations relative to the underlying substrate lattices. This work provides direct evidence for substrate effects in the on-surface synthesis of COFs and paves the way for further investigation into the intrinsic electronic properties of monolayer COFs and the development of multifunctional hybrid devices.

As an electrochemical energy conversion system, fuel cell has the advantages of high energy conversion efficiency and high cleanliness. Oxygen reduction reaction (ORR), as an important cathode reaction in fuel cells, has received extensive attention. At present, the electrocatalysts are still one of the key materials restricting the further commercialization of fuel cells. The fundamental understanding on the catalytic mechanism of ORR is conducive to the development of electrocatalysts with the enhanced activity and high selectivity. This review aims to summarize the in situ characterization techniques used to study ORR. From this perspective, we first briefly introduce the advantages of various in situ techniques in ORR research, including electrochemical scanning tunneling microscopy, infrared spectroscopy, Raman spectroscopy, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. Then, the applications of various in situ characterization techniques in characterizing of the catalyst morphological evolution and electronic structure as well as the identification of reactants and intermediates in the catalytic process are summarized. Finally, the future development of in situ technology is outlooked.