arXiv subjects

Faith J. Lewis

Publications and source records attributed to Faith J. Lewis.

4 recordsLinked to original sources

arXiv2026

The absence of a central metal ion destabilizes phthalocyanine on In$_2$O$_3$(111)

Metal phthalocyanines (MPc) are a versatile molecular platform for applications ranging from organic optoelectronic devices to single-atom catalysis (SAC). Their adsorption and layer formation on the prototypical transparent electrode substrates of organic optoelectronic devices is directly relevant for charge injection and transport across the organic-oxide interface. Moreover, the well-defined M-N$_4$ coordination of the metal cation defines their activity as SACs for (electro-)catalysis. Here, the adsorption of the metal-free phthalocyanine (H$_2$Pc) is characterized on In$_2$O$_3$(111) using low-temperature STM, nc-AFM, and STS. In$_2$O$_3$ is not only a model system of indium tin oxide (ITO) but also an active catalytic material for CO$_2$ reduction. H$_2$Pc adsorbs in the same site and configuration reported for copper phthalocyanine [J. Mater. Chem. C 13, 17650-17661 (2025)] and for the majority of cobalt phthalocyanine [Surf. Sci. 722, 122065 (2022)]. Despite this shared preference in adsorption site, H$_2$Pc cannot be organized into extended and ordered monolayer structures by gentle annealing: the molecule starts to decompose at $\approx$50$^\circ$C, well below the temperature used to grow monolayers of CoPc and CuPc. Self-metalation is not observed on stoichiometric In$_2$O$_3$(111). On the reduced surface where In$^0$ adatoms are present, new H$_2$Pc-related features appear but cannot be identified by imaging only. The comparison of H$_2$Pc with CoPc and CuPc identifies distinct roles of the central metal ion in the metal-Pc/In$_2$O$_3$(111) systems: it acts as a structural anchor that stabilizes the macrocycle against decomposition on the surface, and it modifies the frontier-orbital character in ways that determine whether a second adsorption configuration is populated.

cond-mat.mtrl-sci↗

arXiv2026

CO on a Rh/Fe3O4 single-atom catalyst: high-resolution infrared spectroscopy and near-ambient-pressure scanning tunnelling microscopy

Infrared reflection absorption spectroscopy (IRAS) offers a powerful route to bridging the materials and pressure gaps between surface science and powder catalysis. Using a newly developed IRAS setup optimised for dielectric single crystals, we investigate CO adsorption on the model single-atom catalyst Rh/Fe3O4(001). IRAS resolves three species: monocarbonyls at isolated, twofold-coordinated Rh adatoms, monocarbonyls at fivefold-coordinated Rh atoms embedded in the surface, and gem-dicarbonyls at isolated, twofold-coordinated Rh adatoms. Under ultra-high vacuum (UHV) conditions, RhCO monocarbonyl species at adatom sites dominate. Rh(CO)2 gem-dicarbonyl formation is kinetically hindered and occurs predominantly through CO-induced dissociation of Rh dimers rather than sequential adsorption of two CO molecules at an isolated, twofold Rh adatom. The sequential-adsorption pathway to Rh(CO)2 becomes accessible at millibar CO pressures as evidenced by near-ambient-pressure scanning tunnelling microscopy (NAP-STM). These findings link the UHV behaviour to that expected under realistic reaction conditions. Assignments of the vibrational frequencies to individual species rely on isotopic labelling, thermal treatments, and a review of previous SPM, XPS, and TPD data, supported by density functional theory (DFT)-based calculations. While theory provides qualitative insight, such as the instability of dicarbonyls on fivefold-coordinated Rh atoms, it does not yet reproduce experimental frequencies quantitatively and is sensitive to the computational parameters, highlighting the need for robust experimental benchmarks. The spectroscopic fingerprints established here provide a reliable foundation for identifying Rh coordination environments in oxide-supported single-atom catalysts.

cond-mat.mtrl-sci↗

arXiv2025

Multi-Technique Characterization of Rhodium Gem-Dicarbonyls on TiO$_2$(110)

Gem-dicarbonyls of transition metals supported on metal (oxide) surfaces are common intermediates in heterogeneous catalysis. While infrared (IR) spectroscopy is a standard tool for detecting these species on applied catalysts, the ill-defined crystallographic environment of species observed on powder catalysts renders data interpretation challenging. In this work, we apply a multi-technique surface science approach to investigate rhodium gem-dicarbonyls on a single-crystalline rutile TiO$_2$(110) surface. We combine spectroscopy, scanning probe microscopy, and Density Functional Theory (DFT) to determine their location and coordination on the surface. IR spectroscopy shows the successful creation of gem-dicarbonyls on a titania single crystal by exposing deposited Rh atoms to CO gas, followed by annealing to 200-250 K. Low-temperature scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) data reveal that these complexes are mostly aligned along the [001] crystallographic direction, corroborating theoretical predictions. Notably, x-ray photoelectron spectroscopy (XPS) data reveal multiple rhodium species on the surface, even when the IR spectra show only the signature of rhodium gem-dicarbonyls. As such, our results highlight the complex behavior of carbonyls on metal oxide surfaces, and demonstrate the necessity of multi-technique approaches for the adequate characterization of single-atom catalysts.

physics.chem-ph↗

arXiv2024

Digging its own Site: Linear Coordination Stabilizes a Pt1/Fe2O3 Single-Atom Catalyst

Determining the local coordination of the active site is a pre-requisite for the reliable modeling of single-atom catalysts (SACs). Obtaining such information is difficult on powder-based systems, so much emphasis is placed on density functional theory-based computations based on idealized low-index surfaces of the support. In this work, we investigate how Pt atoms bind to the (1-102) facet of Fe2O3, a common support material in SAC. Using a combination of scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), and an extensive computational evolutionary search, we find that Pt atoms significantly reconfigure the support lattice to facilitate a pseudo-linear coordination to surface oxygen atoms. Despite breaking three surface Fe-O bonds, this geometry is favored by 0.84 eV over the best configuration involving an unperturbed support. We suggest that the linear O-Pt-O configuration is common in reactive Pt-based SAC systems because it balances thermal stability with the ability to adsorb reactants from the gas phase, and that extensive structural searches are likely necessary to determine realistic active site geometry in single-atom catalysis.

cond-mat.mtrl-sci↗