Abstract
In light of increasing interest in the development of organic–organic multicomponent heterostructures on metals, this molecular-scale study investigates prototypical composite systems of ultrathin porphyrin and ionic liquid (IL) films on metallic supports under well-defined ultrahigh vacuum conditions. By means of angle-resolved X-ray photoelectron spectroscopy, we investigated the adsorption, stability, and thermal exchange of the resulting films after sequential physical vapor deposition of the free-base porphyrin 5,10,15,20-tetraphenylporphyrin, 2H-TPP, and the IL 1-methyl-3-octylimidazolium hexafluorophosphate, [C8C1Im][PF6], on Ag(111) and Au(111). 2H-TPP shows two-dimensional growth of up to two closed molecular layers on Ag(111) and Au(111) and three-dimensional island growth for thicker films. IL films on top of a monolayer of 2H-TPP exhibit Stranski–Krastanov-like growth and are stable up to 385 K. The 2H-TPP layer leads to destabilization of the IL films, compared to the IL in direct contact with the bare metals, by inhibiting the specific adsorption of the ions on the metal surfaces. When the porphyrin is deposited on top of [C8C1Im][PF6] at low temperature, the 2H-TPP molecules adsorb on top of the IL film at first but replace the IL at the IL/metal interfaces upon heating above 240 K. This exchange process is most likely driven by the higher adsorption energy of 2H-TPP on Ag(111) and Au(111) surfaces, as compared to the IL. The behavior observed on Ag(111) and Au(111) is identical. The results are highly relevant for the stability of porphyrin/IL-based thin film catalyst systems and molecular devices, and more generally, stacked organic multilayer architectures.
Highlights
One key ingredient for the development of high-performance, multicomponent, molecular thin-film devices is a molecularlevel understanding of the adsorption and assembly of organic constituents on the solid substrate
With reference to the rapidly growing interest in the development of organic−organic multicomponent heterostructures on metals,[10,20−28] recent studies on porphyrins and related aromatic molecules highlight the importance of the strength of the organic−metal interaction and how it relates to the stability and arrangement of stacked multilayer architectures within organic bilayer structures on metals.[10,27,28]
Self-assembly of the porphyrin molecules into extended, ordered 2D networks on Ag(111) and Au(111) has been reported at room temperature (RT) and below 100 K.33,93,95−97,132,133 Isolated 2H-TPP molecules are considered highly mobile on both metal surfaces at 300 K,96 and the formation of ordered 2D networks is driven by the relatively strong molecule− molecule interaction.[5,33,93,96,97,122,123,132−139]
Summary
One key ingredient for the development of high-performance, multicomponent, molecular thin-film devices is a molecularlevel understanding of the adsorption and assembly of organic constituents on the solid substrate. Deposited on high surface area solid supports, they are the key ingredient to effective SCILL36 (solid catalyst with IL layer) and SILP37−39 (supported IL phase) catalysts.[36−38,40−53] they show an increasing potential for widespread use, for example, in sensors,[48,54−57] lubrication,[58−60] separation,[39,48,57,61−63] electrochemistry,[48,57,64−75] and dye-sensitized solar cell[76,77] technologies For these thin IL film applications, the function, performance, and stability of the system are strongly determined by the interface properties.[53,78−83] Understanding in detail the interaction of the IL with the support is of high importance. XPS analysis did not give any indication of decomposition upon heating up to 500 K
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