Cobalt(II) Metal-Organic Framework Coatings for CO2 Photo-Reduction

Abstract

This thesis describes synthesis, analysis, photo- and electro-catalytic experiments of cobalt(II) based metal organic framework (MOF) thin films. Organic molecules 4,4’-bipyridine (44BP), 2,4-dihydroxy terephthalic acid (DHTA) and amino terephthalic acid (ATA) were used as linkers. MOFs thin films were prepared on an acid or amine functionalized gold substrate or FTO glass. Homogeneous films of Co44BP were prepared using a layer-by-layer synthesis approach. The coordination of 44BP to the cobalt was observed with IR and Raman measurements. Identical crystalline thin films of Co44BP anchored on acid, hydroxide and pyridine terminated surfaces were revealed by XRD measurements. Homogeneous distributed Co44BP crystals over the surfaces were confirmed with Time of Flight Secondary Ion Mass Spectrometry (ToFSIMS) measurements. Thin films of CoDHTA and CoATA were prepared by spin coating. Vibrational analysis with IR and Raman, and DFT calculations showed that not only the acid groups, but the substituents, -OH and -NH2, were also involved in the coordination. Combined with XRD analysis, the synthesis of the thin films resulted in poly crystalline CoDHTA and amorphous CoATA coatings. XRD measurements and Photo induced Force Microscopy (PiFM) on the CoDHTA thin films indicated that the crystalline orientation of CoDHTA di↵ered on acid terminated surfaces compared to hydroxide terminated surfaces. For CoATA thin films, these measurements revealed very homogeneous amorphous films in both spatial and chemical perspectives. Ligand to metal charge transfer (LMCT) energies of CoDHTA and CoATA were observed at 382 nm and 375 nm respectively. Transients were observed with transient absorption spectroscopy (TAS) in CoDHTA and Co44BP with half-lifes up to respectively 139 and 141 ns. Activity in the catalytic testing of CoDHTA@FTO on the photoreduction of CO2 and electrochemical reduction of methylene blue were barely observed. During the electrocatalysis, the coating was destroyed.