Growth of Layered Copper–Alkanethiolate Frameworks\nfrom Thin Anodic Copper Oxide Films

Abstract

We\ninvestigated the conversion of electrochemically grown CuO into\ncopper–alkanethiolate lamellar structures supported on a Cu\nsubstrate upon immersion in solutions of <i>n</i>-alkanethiols.\nAfter considering different preparation conditions (alkyl chain length,\nthiol concentration, and type of solvent), we found that the oxide\ncan be rapidly converted into compact and highly passivating lamellar layers\nin 1-octanethiolate/0.1 M NaOH forming\nsolutions. Alkaline forming solutions play a key role in favoring\nthe conversion of CuO into the lamellar compound vs the competing\nmechanism in which the oxide is reduced and the thiolates are oxidized\nto disulfides. The oxide conversion into the layered structure is\nincomplete for both short and long alkanethiols, and it is almost\ncomplete in the case of 1-octanethiol. The use of less polar solvents\nsuch as ethanol or <i>n</i>-hexane is much less effective\nin producing layered Cu–thiolates. Spectroscopic ellipsometry\nmeasurements showed that 10-nm-thick CuO is converted into a 100-nm-thick\nlamellar compound, which corresponds to around 40 lamellae. After\nthe breakage of the layers during O₂ evolution at high\nanodic potentials, a healing behavior was observed: the copper surface\nbecame passivated again after exposure of the broken layer in the\nair for a few minutes. The layered compound was extensively characterized\nby X-ray photoelectron spectroscopy and Raman spectroscopy. The decrease\nof O 1s/Cu 3p ratio and the increase of S 2p/Cu 3p and C 1s/Cu 3p\nratios clearly showed the conversion of CuO into a copper–thiolate\ncomplex. The Raman spectrum of the Cu–octanethiolate compound\nshows a considerable enhancement of all bands as compared to a self-assembled\nmonolayer. The sharpness of the most prominent peaks indicates the\nhigh crystallinity of the alkyl chains in the layered compound. Density\nfunctional theory calculations showed that the alkyl chains lie nearly\nperpendicular to the layer of Cu atoms. The calculation of Raman vibrational\nfrequencies using density functional perturbation theory allowed an\nunambiguous assignment of experimental Raman peaks.