Structure‐Dependent Anchoring of Organic Molecules to Atomically Defined Oxide Surfaces: Phthalic Acid on Co₃O₄(111), CoO(100), and CoO(111)

Abstract

We have performed a model study to explore the influence of surface structure on the anchoring of organic molecules on oxide materials. Specifically, we have investigated the adsorption of phthalic acid (PA) on three different, well-ordered, and atomically defined cobalt oxide surfaces, namely 1) Co3O4(111), 2) CoO(111), and 3) CoO(100) on Ir(100). PA was deposited by physical vapor deposition (PVD). The formation of the PA films and interfacial reactions were monitored in situ during growth by isothermal time-resolved IR reflection absorption spectroscopy (TR-IRAS) under ultrahigh vacuum (UHV) conditions. We observed a pronounced structure dependence on the three surfaces with three distinctively different binding geometries and characteristic differences depending on the temperature and coverage. 1) PA initially binds to Co3O4(111) through the formation of a chelating bis-carboxylate with the molecular plane oriented perpendicularly to the surface. Similar species were observed both at low temperature (130 K) and at room temperature (300 K). With increasing exposure, chelating mono-carboxylates became more abundant and partially replaced the bis-carboxylate. 2) PA binds to CoO(100) in the form of a bridging bis-carboxylate for low coverage. Upon prolonged deposition of PA at low temperature, the bis-carboxylates were converted into mono-carboxylate species. In contrast, the bis-carboxylate layer was very stable at 300 K. 3) For CoO(111) we observed a temperature-dependent change in the adsorption mechanism. Although PA binds as a mono-carboxylate in a bridging bidentate fashion at low temperature (130 K), a strongly distorted bis-carboxylate was formed at 300 K, possibly as a result of temperature-dependent restructuring of the surface. The results show that the adsorption geometry of PA depends on the atomic structure of the oxide surface. The structure dependence can be rationalized by the different arrangements of cobalt ions at the three surfaces.

The surface structure matters: thermal stability of phthalic acid anchored to atomically-defined cobalt oxide films

“…In a previous study we showed that PA forms a bridging biscarboxylate at 300 K, with some tendency to form additional co-adsorbed mono-carboxylate species at higher exposure. 18 In the TP-IRAS spectra we again observe changes in intensity and peak width, but no indication for the appearance of new bands. Similar as for the experiments on the other two surfaces, we analyze the peak intensity and peak width quantitatively.…”

“…The deposition rate is estimated to be around 0.03 ML per min on Co 3 O 4 (111) and CoO(111) and 0.08 ML per min on CoO(100), respectively, as determined from low temperature adsorption experiments where we can distinguish monolayer from multilayer. 18…”

“…The different desorption temperatures T des -or different stabilities of the bis-carboxylate species -can be rationalized on the basis of the distribution and accessibility of the Co 2+ ions on the surface. On CoO(100), a nearly perfectly bridging biscarboxylate is formed at 300 K. 18 The bridging adsorption geometry is typically the most stable one, as reflected by the preferential formation of bridging species for other carboxylic acids and oxides (e.g. formic acid on ZnO(10% 10), 37 rutile TiO 2 (110), 38 anatase TiO 2 (110) 39 and on CeO 2 (111) 40 or benzoic acid on rutile TiO 2 (110) 41 and on MgO(100) 27 ).…”

“…Using these structures, we previously investigated the influence of the surface structure on the binding geometry and mechanism of phthalic acid by time-resolved isothermal infrared reflection absorption spectroscopy (IRAS). 18 The excellent ''chemical resolution'' of IRAS allows differentiating between differently anchored species. Taking advantage of the metal surface selection rule (MSSR), 19 which is also valid for thin oxide films on metal surfaces, 20 we simultaneously obtain information on the molecular orientation.…”

“…We have found that the binding mechanisms are very different on the three cobalt oxide surfaces. 18 This finding would suggest that the different binding geometries could also lead to different bonding strength and stability. In this work, we explore the thermal stability of the different surface bound carboxylates on Co 3 O 4 (111), CoO(111) and CoO(100) by temperature-programmed in situ spectroscopy.…”