Deconstructing the Heterogeneity of Surface-Bound Catalysts: Rutile Surface Structure Affects Molecular Properties

Abstract

Catalytic interfaces involving surface-bound molecular catalysts often exhibit a large structural heterogeneity from uncontrolled variation in surface morphology. Conventional spectroscopic techniques typically average over these different structural motifs within the sample, making it difficult to link the underlying surface morphology to the properties of the immobilized catalyst. Here we present the first direct comparison of the vibrational dynamics of a CO2 reduction catalyst bound to two different single-crystalline TiO2 surfaces, rutile (001) and (110), probed with transient surface-specific sum-frequency generation spectroscopy. We find that the change in surface structure between crystallographic faces alters both the vibrational frequency and relaxation time of the symmetric carbonyl stretching mode of the catalyst, with (001) displaying a lower frequency and longer relaxation time. This results from a change in the catalyst electronic structure and indicates that the molecular properties of the catalyst, likely including the catalytic properties, depend on the specific TiO2 surface to which it is bound. The comparison of the molecular properties on these two single crystal surfaces is an essential step toward understanding how semiconductor surface structure influences catalyst behavior and identifying optimal surface structures for improved catalytic performance.

Plasmonic Substrates Do Not Promote Vibrational Energy Transfer at Solid–Liquid Interfaces

“…49,[67][68][69][70] In this context, particularly purposely tailored nanostructures such as nano-antennae or nano-slits with plasmon resonances located directly in the mid-IR spectral range have been shown to dramatically enhance IR absorbances. 67,[69][70][71][72][73][74] In that case, the quasi-static approximation, which we used in the derivation of Among the previous 2D IR investigations of Re-carbonyl complexes on various types of surfaces, 18,20,24,25,[38][39][40][41][42][43][44][45] the studies by Xiong et al 42,43 are the most closely related to our present report. That works has specifically looked at substrate-adsorbate interactions between an immobilized Re-complex and an underlying Au surface, suggesting that dipole/image-dipole interactions cause severe differences in lineshapes on the same molecule.…”

Ultrafast Vibrational Energy Transfer in Catalytic Monolayers at Solid–Liquid Interfaces

“…These observations indicate that first, the coupling between molecules is weak, and that second, each molecule exists as a single species on the surface, in contrast to previous related studies, which differ in the way how the samples have been prepared. [37][38][39] But even when coupling is too weak to result in direct cross-peaks at early population times, vibrational energy transfer may occur as a result of fluctuations in the coupling Hamiltonian (in analogy to NOESY in NMR), that reveals cross-peaks growing in as a function of population time. 20,[40][41][42] To explore that effect, Figure 4 shows a time-series of 2D ATR IR spectra, where indeed distinct cross-peaks show up above and below the diagonal after a few picoseconds ((b) -(d)).…”

“…Particularly for potentially catalytically-active systems, the ultrafast intramolecular vibrational dynamics of Re-carbonyl complexes in electronic ground and excited states have been investigated in detail before, mainly for bulk solution samples, but recently also for immobilized systems under a variety of experimental conditions. [37][38][39]45,47,48,50,52,[61][62][63][64][65] However, intermolecular dynamics are largely unexplored to date, since femtosecond multi-dimensional vibrational spectroscopy on surfaces became available only recently. 17,47,48,66 Nonetheless, it has been proposed that if the vibrational states of two molecules are coupled, the electronic states will be as well, which in turn may have a decisive impact on the performance of working devices.…”

Surface-Sensitive and Surface-Specific Ultrafast Two-Dimensional Vibrational Spectroscopy

“…It is known that these interfacial structures can influence the conformations of molecules near interfaces and thus also determine their properties. [19][20][21][22][23] In this regard, many examples exist ranging from fundamental science to technical applications. Possibly the bestknown process is related to molecular self-assembly of covalently bound adsorbates at surfaces 19,20,[24][25][26][27] and nanostructures [28][29][30][31][32] .…”

“…216,217 . The vibrational dynamics can again be followed over multiple integers of the vibrational lifetime (here: about 20 ps) 22 . An important effect in 2D ATR IR spectroscopy is related to distortions of the vibrational lineshapes for AOIs close to θc.…”