Pith. sign in

REVIEW 3 major objections 5 minor 6 references

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

T0 review · 3 major / 5 minor · reviewed 2026-08-06 · deepseek-v4-flash

Challenge this review Re-run · record.json Download PDF Read on arXiv ↗ Review JSON

Pith's one-line read Rh gem-dicarbonyls on TiO2(110) are square-planar, aligned along [001], and infrared-invisible Rh clusters coexist with them.

desk verdict Direct STM/nc-AFM images settle the 25-year orientation debate for Rh gem-dicarbonyls on TiO2(110), with a modest but addressable gap between the imaged and infrared-detected species. read the letter →

arxiv 2506.21068 v1 pith:MPJFTEZZ submitted 2025-06-26 physics.chem-ph cond-mat.mtrl-sci

Moritz Eder , Faith J. Lewis , Johanna I. Hütner , Panukorn Sombut , Maosheng Hao , David Rath , Jan Balajka , Margareta Wagner
show 7 more authors
Matthias Meier Cesare Franchini Ulrike Diebold Michael Schmid Florian Libisch Jiří Pavelec Gareth S. Parkinson
This is my paper · ORCID
classification physics.chem-phcond-mat.mtrl-sci
keywords rhodiumgem-dicarbonylTiO2(110)single-atomcatalysisscanningtunnelingmicroscopynon-contactatomicforceinfraredreflectionabsorptionspectroscopyX-rayphotoelectrondensityfunctionaltheory
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper sets out to establish what rhodium gem-dicarbonyls—individual Rh atoms each holding two CO molecules—look like and where they sit on the model surface rutile TiO2(110), and to show that infrared spectroscopy alone can misrepresent such systems. Combining IRAS, STM, nc-AFM, XPS, and DFT, it argues that the dominant Rh(CO)2 species is square-planar with both CO molecules aligned along the [001] direction on top of the bridging oxygen rows, a geometry that matches theory and contradicts an earlier claim of perpendicular alignment. It further reports that XPS detects more Rh species than the IR spectra reveal, specifically small Rh clusters that carry little or no CO and are therefore invisible to infrared. If correct, the work provides the first direct visualization of this catalytic intermediate on a single-crystal oxide and a concrete warning that IR-only studies of single-atom catalysts can overstate sample uniformity. A sympathetic reader would take the paper's central contribution to be the convergence of four independent techniques on one structural assignment, plus the demonstration that missing species can hide in plain sight.

What carries the argument

The central object is the gem-dicarbonyl complex Rh+(CO)2 on TiO2(110): a single Rh adatom coordinated by two surface bridging-oxygen anions and two CO molecules in a square-planar arrangement. The argument is carried by the coincidence of four independent measurements on this object: the symmetric/asymmetric IRAS pair at 2104/2043 cm−1; the azimuthal dependence of p-polarized IRAS on a dielectric single crystal, which lets the orientation of the CO dipoles be read from the spectra; the double-lobed STM and nc-AFM contrast with [001] orientation and a position centered between bridging O atoms; and DFT+U with HSE06 frequency calculations that reproduce the 59 cm−1 mode splitting. A supporting mechanism is the initial-state approximation for Rh 3d core-level shifts combined with Bader charge analysis, which the paper uses to argue that the high-binding-energy XPS component is a Rh+ carbonyl rather than a Rh3+ species.

What would settle it

A direct test would be to run IRAS and STM on one sample at the same Rh coverage and annealing temperature, so the number of [001]-oriented double-lobes per area can be compared quantitatively with the 2104/2043 cm−1 band intensities as CO is dosed and annealed; if the two do not track together, the image-to-spectrum assignment is wrong. A second test is to heat an SPM-prepared surface past the dicarbonyl decomposition temperature and then re-dose CO: if the double-lobe features reappear without the dicarbonyl IR bands, or persist after the bands are gone, the imaged species is not the gem-dicarbonyl. Measuring XPS and IR on the same preparation would also settle whether the 310.1 eV component and the two CO stretch bands grow and decay together on the same sample.

Watch

Extended reading notes

Core claim

On rutile TiO2(110), rhodium gem-dicarbonyls prepared by depositing Rh at low temperature, dosing CO, and annealing to 200–250 K are square-planar Rh+(CO)2 units whose two CO ligands lie along the [001] crystallographic direction, parallel to the Ti and bridging-O rows. The paper reports direct STM and nc-AFM images of these complexes as double-lobed features centered between two bridging oxygen anions, with the two lobes oriented along [001], and it assigns the IRAS bands at 2104 and 2043 cm−1 to their symmetric and asymmetric CO stretches on the basis of HSE06 frequencies of 2098.9 and 2042.6 cm−1. The XPS Rh 3d5/2 feature at 310.1 eV that appears and disappears with the dicarbonyl bands is assigned to this complex; despite the high binding energy, Bader analysis (+0.68 e) and the absence of multiplet splitting place the Rh in the +1 state, not +3. The paper also establishes that other Rh species coexist with the dicarbonyls: a substantial cluster population is visible in STM, and the C 1s signal after oxide CO desorption is only about half of what it would be if every Rh atom carried two CO molecules.

Load-bearing premise

The load-bearing premise is that the double-lobed features imaged by STM and nc-AFM are the same chemical species that produces the 2104 and 2043 cm−1 infrared bands, even though the microscopy and spectroscopy samples were prepared at different Rh coverages and slightly different annealing temperatures and the imaged complexes were not spectroscopically tagged.

Editorial extensions

If this is right

  • On TiO2(110), the square-planar, [001]-aligned Rh(CO)2 structure, not the perpendicular one proposed by earlier work, is the starting geometry for interpreting Rh carbonyl chemistry on this surface.
  • Infrared-only characterization of supported Rh can miss a substantial population of clustered species, so conclusions about single-atom uniformity drawn from IR band intensities alone should be treated with caution.
  • Rh gem-dicarbonyls on TiO2(110) are stable only up to roughly 250 K; above that they decompose and CO-covered Rh clusters form, which delimits the temperature window in which this intermediate can participate in catalysis.
  • Surface hydroxyl groups can break the symmetry of the two CO ligands in the imaged complexes, a distortion the paper links to a plausible mechanism for tuning the geometry and reactivity of the species.
  • The low-temperature synthesis route—Rh deposition at 80 K followed by CO exposure and mild annealing—avoids the high CO pressures and chloride contamination of earlier preparations and is proposed to transfer to other carbonyl-forming metals.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If the image-to-spectrum assignment holds, the double-lobe contrast gives a site-by-site way to count gem-dicarbonyls and watch them form, distort, and decompose on TiO2(110), something ensemble IR measurements cannot do; a natural next step would be a temperature-programmed STM series on one and the same sample.
  • The IR-invisible cluster population suggests an explicit test for operando single-atom catalysis studies: whenever IR shows only gem-dicarbonyl bands, an independent counting technique such as XPS, STEM, or TPD should be used to check whether undetected clusters could be carrying the catalytic turnover.
  • The OH-induced asymmetry seen here implies that hydroxyl coverage could be used deliberately to tune CO stretching frequencies and binding strengths of surface carbonyls; DFT could map which OH arrangements produce which frequency shifts and predict whether the distorted species is more or less reactive.
  • The same dielectric-IRAS azimuthal analysis could be ported to other metal-oxide-supported gem-dicarbonyls such as Ir, Pd, Pt, and Ni to determine their orientation even on surfaces where scanning probe imaging is difficult.
Share X Bluesky LinkedIn Reddit HN

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 5 minor

Summary. The manuscript reports a multi-technique surface-science study of rhodium gem-dicarbonyls, Rh(CO)2, on rutile TiO2(110). The authors synthesize the species by depositing Rh atoms at low temperature, exposing them to CO, and annealing to 200–250 K. Infrared reflection absorption spectroscopy (IRAS) shows peaks at 2104 and 2043 cm−1, which they assign to the symmetric and asymmetric C–O stretches of Rh+(CO)2. DFT+U and HSE06 calculations reproduce these frequencies and predict a square-planar geometry in which the two CO molecules are aligned along the [001] direction, parallel to the bridging O rows. Low-temperature STM and nc-AFM images show bright double-lobed features oriented along [001] on top of bridging O rows, which the authors identify as the gem-dicarbonyl complexes. XPS shows multiple Rh species, including a component at 310.1 eV that correlates in thermal behavior with the IRAS gem-dicarbonyl bands and a lower-binding-energy component assigned to Rh clusters that are not detected by IR. The central claims are that this is the first direct visualization of the gem-dicarbonyl on this surface, that the geometry is square-planar with CO along [001], and that IR-invisible Rh clusters coexist with the gem-dicarbonyl species.

Significance. If the identification of the scanning-probe features with the IRAS-detected gem-dicarbonyl is correct, the paper provides the first direct structural visualization of a Rh gem-dicarbonyl on a well-defined oxide surface, resolving a previous conflict between IR-based alignment assignments (Hayden et al. versus Tang et al.). The multi-technique approach is well suited to the single-atom catalysis problem, where IR spectroscopy alone can miss or misassign species. Strengths include the use of two independent DFT functionals with a clearly stated gas-phase CO scaling procedure, the combination of XPS and IRAS thermal-evolution data, and the explicit discussion of IR-invisible species. The paper is generally careful and does not overstate the theoretical agreement; the main gap is the lack of a direct spectroscopic tag connecting the imaged double-lobed features to the 2104/2043 cm−1 IRAS species.

major comments (3)
  1. [Section 3.2, Figs. 2 and 3] The central claim that the STM/nc-AFM double-lobed features are Rh gem-dicarbonyls is not spectroscopically verified. The SPM sample was prepared at 0.005 ML Rh with 1 L CO and annealed to about 270 K, whereas the IRAS sample used 0.05 ML Rh with 0.5 ML CO and annealing at 200–250 K. The manuscript offers no measurement that tags the imaged lobes as the species producing the 2104 and 2043 cm−1 IRAS bands; the assignment rests on the geometric match to the DFT minimum and on the thermal behavior of separate samples. Because the paper also leaves the 2113 cm−1 carbonyl species unassigned and explicitly notes the presence of clusters in the SPM images, the possibility remains that the double lobes belong to a different Rh carbonyl or a CO-decorated cluster. To support the "first direct visualization" claim, the authors should either acquire IRAS under the SPM preparation conditions, perform a CO-removal/re-imaging experiment, or clearly temper the claim to reflect the indirect nature of the assignment.
  2. [Section 4 vs. Conclusions] There is an internal inconsistency about the CO coverage on the IR-invisible Rh clusters. In the Discussion, the authors state that the IR-invisible species likely bind "to very few or no CO molecules, or the dipole moment of those CO species being too low," which leaves open the possibility that the clusters are bare or nearly bare. The Conclusions, however, assert that the clusters are "almost certainly covered by CO at these temperatures." These statements should be reconciled. If the clusters are indeed CO-covered, the absence of their IR signal requires a quantitative explanation, for example a strong orientation/selection-rule effect or very low intrinsic intensity; if they are not CO-covered, the concluding sentence should be revised to avoid an unsupported assertion.
  3. [Section 3.1.1, Fig. 1 and Section 4] The 2113 cm−1 peak is observed alongside the gem-dicarbonyl doublet and disappears upon annealing to 250 K, yet the manuscript explicitly leaves it unassigned and states that it is unclear whether it transforms into the "normal" gem-dicarbonyl or into Rh clusters. This is a load-bearing ambiguity because the SPM images are assigned partly on the basis of the IRAS sample preparation, and the 2113 cm−1 species may represent a second Rh carbonyl that could appear in the SPM images. The authors should either provide a candidate assignment (with supporting DFT calculations or literature comparison) or state more explicitly how the presence of this species affects the interpretation of the SPM and XPS data.
minor comments (5)
  1. [Section 4] Typo: "located hat higher binding energies" should read "located at higher binding energies."
  2. [Section 2] The phrase "The p signal is more pronounced in our design compared to other setups" would benefit from a brief explanation of what "p signal" refers to (presumably the p-polarized reflectivity difference or the signal-to-noise of the p-polarized measurement).
  3. [Section 3.2, Fig. 3] The sentence "The opposite orientations of these inequivalent CO molecules" is ambiguous; the authors appear to mean opposite asymmetries or opposite tilts of the two lobes, not opposite orientations in a crystallographic sense. Please rephrase.
  4. [References] Reference 10 is formatted inconsistently ("Yang C., G. C. W."); please correct the author list and title formatting.
  5. [Section 3.1.1] The sentence "We have observed these vibrational frequencies of the carbonyl species most frequently in our experiments" is awkward; consider rephrasing to "In most experiments, the gem-dicarbonyl bands appeared at 2104 and 2043 cm−1, with occasional 1–4 cm−1 shifts..."

Circularity Check

0 steps flagged · score 2.0 of 10

No circular derivation: the central square-planar [001] gem-dicarbonyl claim rests on direct SPM imaging, independent IRAS/DFT frequencies, and XPS assignments, with only non-load-bearing self-citations.

full rationale

The paper's central claims — that Rh gem-dicarbonyls on TiO2(110) adopt a square-planar geometry with both CO ligands aligned along [001], and that IR-invisible Rh clusters coexist — are supported by direct measurements rather than by construction. The IRAS peaks at 2104 and 2043 cm−1 are assigned to the symmetric and asymmetric CO stretches of Rh+(CO)2 using their characteristic positions and the 61 cm−1 experimental splitting, which the HSE06 calculation reproduces as 59 cm−1; the DFT frequencies are calibrated against gas-phase CO (scaling by 2143 cm−1 / computed gas-phase value), an external reference that is not fitted to the target surface spectra. The SPM assignment is based on direct imaging: a bright double-lobe feature centered on a bridging-O row in STM and two dark lobes in nc-AFM, matching the DFT-optimized structure and the earlier independent calculation by Tang et al. The [001] orientation conclusion follows from the observed geometry in the images, not from a parameter fitted to those images. The IRAS azimuthal inference is supported by Fresnel simulations from the group's separate instrument paper (ref 24), but that is a technical, parameter-free optical calculation, not a fit to the present data. XPS assignments are tied to the temperature-dependent disappearance of the IRAS bands and to DFT initial-state core-level shifts, both independent of the conclusion. The only genuine weakness is experimental, not circular: the SPM and IRAS samples were prepared under somewhat different conditions, and the imaged double-lobes are not spectroscopically tagged as the same species that produces the 2104/2043 cm−1 doublet; the 2113 cm−1 species is also left unassigned. That is an addressable species-identification gap rather than a reduction of a prediction to its input by construction. Minor self-citations appear (refs 24 and 46), but neither is load-bearing: ref 24 validates the IRAS setup and ref 46 provides prior Rh immobilization methodology. I therefore find no circular step that can be exhibited as an equation-level or fit-level equivalence.

Assumptions & free parameters 1 free parameters · 5 assumptions · 0 invented entities

The central claim rests on DFT approximations (optPBE-vdW, HSE06, Ueff = 3.9 eV), the harmonic approximation, initial-state core-level shifts, and the transferability of the SPM/IR sample preparations. No new entities are introduced. One method-dependent scaling factor for CO frequencies is used, calibrated on gas-phase CO. No parameters are fitted to the target data.

free parameters (1)
  • CO stretch vibrational frequency scaling factor = 2143/2114 = 1.0137 (optPBE-vdW); 2143/2232 = 0.9601 (HSE06)
    The computed gas-phase CO frequency is scaled to the experimental value of 2143 cm-1, and the same factor is applied to the adsorbed CO frequencies. This is a method calibration against gas-phase CO, not a fit to the target dicarbonyl bands, but it is a parameter that determines the reported numbers.
assumptions (5)
  • domain assumption optPBE-vdW and HSE06 functionals, with Ueff = 3.9 eV on Ti, adequately describe the Rh/TiO2(110) electronic structure.
    Used throughout the DFT calculations (Computational methods); this approximation determines the computed geometry, frequencies, and core-level shifts.
  • domain assumption The harmonic approximation and finite-difference method yield accurate CO stretch frequencies for adsorbed CO.
    Used to compute frequencies; anharmonicity is neither calculated nor corrected beyond the gas-phase scaling factor.
  • domain assumption The initial-state approximation is sufficient for Rh 3d core-level shifts.
    Used for the XPS assignment of the 310.1 eV peak to the gem-dicarbonyl; final-state effects are only discussed qualitatively.
  • domain assumption Pseudo-hydrogen saturation of the bottom slab makes the bottom surface bulk-like without affecting the top surface.
    Used to allow a thinner slab; the assertion is not separately validated in this paper.
  • domain assumption The IRAS intensity and orientation interpretation on dielectric substrates via Fresnel simulations is valid.
    The azimuthal orientation is inferred from the appearance of the 2043 cm-1 peak in p-polarization, relying on the setup of Rath et al. (2024).

how reviews work

0 comments
Cite this review

Pith. "Pith review of Multi-Technique Characterization of Rhodium Gem-Dicarbonyls on TiO$_2$(110)." pith.science (2026). https://pith.science/paper/MPJFTEZZ

@misc{pith2026250621068,
  author       = {Pith},
  title        = {Pith review of: Multi-Technique Characterization of Rhodium Gem-Dicarbonyls on TiO$_2$(110)},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/MPJFTEZZ}},
  note         = {Machine review of arXiv:2506.21068}
}
abstract

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.

Figures

Figures reproduced from arXiv: 2506.21068 by the authors.

Figure 2
Figure 2. Low-temperature scanning probe images of a Rh gem-dicarbonyl on TiO2(110) taken at 14 K with a Cu-terminated tip. a) Empty-state STM image showing the two CO molecules of the Rh [PITH_FULL_IMAGE:figures/full_fig_p014_2.png] view at source ↗

Discussion (0). Continue with ORCID to comment.

Reference graph

Works this paper leans on

6 extracted references · 6 canonical work pages

  1. [2]

    https://doi.org/10.1039/D0CY00146E

    Catalysis Science & Technology 2020, 10 (6), 1597–1601. https://doi.org/10.1039/D0CY00146E. (18) Gu, F.; Qin, X.; Li, M.; Xu, Y.; Hong, S.; Ouyang, M.; Giannakakis, G.; Cao, S.; Peng, M.; Xie, J.; Wang, M.; Han, D.; Xiao, D.; Wang, X.; Wang, Z.; Ma, D. Selective Catalytic Oxidation of Methane to Methanol in Aqueous Medium over Copper Cations Promoted by A...

    work page doi:10.1039/d0cy00146e 2020

  2. [3]

    https://doi.org/10.1002/anie.202201540

    Angewandte Chemie International Edition 2022, 61 (18). https://doi.org/10.1002/anie.202201540. (19) Marino, S.; Wei, L.; Cortes-Reyes, M.; Cheng, Y.; Laing, P.; Cavataio, G.; Paolucci, C.; Epling, W. Rhodium Catalyst Structural Changes during, and Their Impacts on the Kinetics of, CO Oxidation. JACS Au 2023, 3 (2), 459–467. https://doi.org/10.1021/jacsau....

    work page doi:10.1002/anie.202201540 2022

  3. [488]

    (51) Wang, Y.; Wöll, C

    https://doi.org/10.1016/S0009-2614(97)00305-9. (51) Wang, Y.; Wöll, C. IR Spectroscopic Investigations of Chemical and Photochemical Reactions on Metal Oxides: Bridging the Materials Gap. Chemical Society Reviews 2017, 46 (7), 1875–

    work page doi:10.1016/s0009-2614(97)00305-9 2017

  4. [971]

    27 (23) Xu, R.; Jiang, Z.; Yang, Q.; Bloino, J.; Biczysko, M

    https://doi.org/10.1021/ic50051a026. 27 (23) Xu, R.; Jiang, Z.; Yang, Q.; Bloino, J.; Biczysko, M. Harmonic and Anharmonic Vibrational Computations for Biomolecular Building Blocks: Benchmarking DFT and Basis Sets by Theoretical and Experimental IR Spectrum of Glycine Conformers. Journal of Computational Chemistry 2024, 45 (21), 1846–1869. https://doi.org...

    work page arXiv

  5. [1932]

    inline"> <mrow> <msub> <mrow> <mi Mathvariant=

    https://doi.org/10.1039/C6CS00914J. (52) Krenn, G.; Bako, I.; Schennach, R. CO Adsorption and CO and O Coadsorption on Rh(111) Studied by Reflection Absorption Infrared Spectroscopy and Density Functional Theory. The Journal of Chemical Physics 2006, 124 (14). https://doi.org/10.1063/1.2184308. 29 (53) Parkinson, G. S.; Dohnálek, Z.; Smith, R. S.; Kay, B....

    work page doi:10.1039/c6cs00914j

  6. [3786]

    (50) Hayden, B

    https://doi.org/10.1021/ja011688y. (50) Hayden, B. E.; King, A.; Newton, M. A. The Alignment of a Surface Species Determined by FT- RAIRS: Rhodium Gem-Dicarbonyl on TiO2(110). Chemical Physics Letters 1997, 269 (5), 485–

    work page doi:10.1021/ja011688y 1997

Pith tools

Reviewed August 6, 2026 · model on record in the stance chip above.