Molybdenum disulfide (MoS ) is extensively studied because of its potential applications in catalysis, electronic and optoelectronic devices, and composite nanostructures. However, a recent experimental study indicated that, contrary to current beliefs, MoS monolayers lack long-term stability in air. Here, a study is presented on the oxidation of MoS monolayers based on density functional theory (DFT) calculations. The results suggest that single-layer MoS samples with exposed edge sites are indeed unstable to oxidation, which occurs because of the low energetic barrier to dissociation of oxygen molecules at the Mo-edges of MoS . After an oxygen molecule dissociates, oxygen atoms replace sulfur atoms, and further oxidation causes the formation of a one-dimensional chain-like structure resembling that of bulk MoO . This MoO structure facilitates the spread of oxidation onto the surface, and the stress associated with the misfit between the MoS and MoO lattices may cause the experimentally observed cracking of MoS flakes.

Succeeding graphene, monoelemental two-dimensional (2D) materials such as germanene and silicene, coined as "Xenes", have attracted vast scientific and technological interests. Adding covalently bonded hydrogen on both sides of germanene leads to germanane (i.e., hydrogen-terminated germanene, GeH). Further, the covalent functionalization of germanane allows the tuning of its physical and chemical properties. Diverse variants of germananes have been synthesized, but current research is primarily focused on their fundamental properties. As a case in point, their applications as photo-and electrocatalysts in the field of modern energy conversion have not been explored. Here, we prepare 2D germanene-based materials, specifically germanane and germananes functionalized by various alkyl chains with different terminal groupsgermanane with methyl, propyl, hydroxypropyl, and 2-(methoxycarbonyl)ethyland investigate their structural, morphological, optical, electronic, and electrochemical properties. The bond geometries of the functionalized structures, their formation energies, and band gap values are investigated by density functional theory calculations. The functionalized germananes are tested as photoelectrocatalysts in the hydrogen evolution reaction (HER) and photo-oxidation of water. The performance of the germananes is influenced by the functionalized groups, where the germanane with −CH 2 CH 2 CH 2 OH termination records the lowest HER overpotentials and with −H termination reaches the highest photocurrent densities for water oxidation over the entire visible spectral region. These positive findings serve as an overview of organic functionalization of 2D germananes that can be expanded to other "Xanes" for targeted tuning of the optical and electronic properties for photo-and electrochemical energy conversion applications.

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