AVS 69 Session SS+2D+AS+HC-WeM: Surface Science of 2D Materials

We investigated the (photo)chemistry of alcohols on TiO₂(110) and Fe₂O₃(012) in ultra-high vacuum. Our studies focused on the role of the metal co-catalyst in the photocatalytic reaction by comparing the reactivity of bare and metal-loaded surfaces. We show that photocatalytic reactions are not merely a couple of redox reactions, but an interplay of thermal and photon-driven surface reactions.

Our results demonstrated that the co-catalyst plays a crucial role in the outcome of the reaction. On TiO₂(110), alcohols are oxidized to the aldehyde/ketone and hydrogen surface species upon illumination. The hydrogen surface species were thermally converted to H₂ by the co-catalyst, allowing for a steady-state photocatalytic conversion of alcohols and the continuous production of molecular hydrogen. Using mass spectrometry, we determined turnover frequencies and rate constants. The identification of surface mechanisms on Fe₂O₃ is less advanced, but there seem to be strong parallels in the photochemistry.

Our studies shed light on the fundamental processes involved in photocatalytic reactions on metal-loaded surfaces and contribute to the development of sustainable energy technologies.

Transition metal carbides are of broad interest for both heterogeneous and electro-catalysis. However, fundamental understanding of chemical factors governing reactivities and selectivities at the vapor/solid and liquid/solid interfaces remain sparse. Herein, in situ XPS results, electrochemical measurements, and DFT-based calculations are presented regarding the reactivities of NbC and TaC in the presence of O₂ vapor, and reactivity in solution towards the reduction of N₂ to NH₃. NbC and TaC films were prepared by DC magnetron sputtering deposition, then exposed to O₂ vapor at room temperature, and analyzed by in situ XPS without exposure to ambient. Similarly prepared samples were also analyzed by ex situ XRD. These data show that, although Nb and Ta have similar oxophilicities, (a) deposited NbC films contain significant amounts of Nb oxide phases throughout the film, whereas TaC films deposited under similar conditions do not, and (b) the exposure of NbC films to O₂ at 300 K results in significant Nb oxide formation, but that TaC films remain inert towards O₂ under these conditions. DFT calculations indicate that this significant reactivity difference towards O₂ is due in large part to the greater Ta-C bond strength compared to Nb-C, and in part due to the relative energetic stabilities of the corresponding oxides. Electrochemical studies show that ambient-exposed NbC, with a Nb₂O₅ surface layer, becomes reactive towards N₂ reduction to NH₃ under acidic conditions, but only after etching in NaOH to remove the surface oxide layer. Additionally, chronoamperometric data indicate that this reactive NbC surface is eventually modified under electrochemical conditions and becomes relatively inert towards N₂ reduction with time. Experiments involving in situ sample transfer between UHV and electrochemistry environments demonstrate that electrochemically active NbC surfaces in solution comprise Nb sub-oxide surface layers, in line with previous studies showing that effective NRR catalysts contain surface transition metal ions in intermediate oxidation states, supporting both N₂ lone pair attraction and pi-backbonding to bind and activate the NN triple bond.

Acknowledgement This work was supported in part by the UNT College of Science through COS grants 1600089 and RSG-2023-002 and and in part by the NSF under grant no. DMR 2112864.

CO₂ activation is a fundamental process in heterogeneous catalysis. ZnO-based catalyst has been extensively used in commercial methanol synthesis from CO₂ gas and the reverse water gas shift reaction. The adsorption behavior of CO₂ on the catalyst surface is pivotal to the reactivity. Whereas ZnO(0001)-Zn physisorbed or weakly chemisorbed CO₂,¹ strong chemisorption of the molecule happens on non-polar surfaces, such as ZnO(10-10), resulting in a tridentate carbonate.² In Operando TEM investigation during methanol synthesis shows that ZnO single atomic layer stacks distortedly around Cu nanoparticles via strong metal-support interaction. The lake of interlayer ordering between the layers suggests a weak interlayer interaction; therefore, each layer resembles a free-standing sheet.³ DFT modeling concluded that free-standing ZnO(0001) layer adopts an graphitic-like co-planner structure. The co-planner feature was verified experimentally for the bi-layer ZnO(0001) supported on Ag(111) and Au(111).⁴ On Au(111) substrate, TPD shows that CO₂ adsorbs on the low coordinate sites at the layer edges.⁵ In the present study, we investigate the CO₂ adsorption on bi-layer ZnO/Ag(111) film using NAP-XPS to extend the pressure condition towards reality. We found a more considerable CO₂ chemisorption at elevated pressure. The presentation will also address how the surface hydroxyl group influences CO₂ adsorption.

1. Wang, J.;Hokkanen, B.; Burghaus, U., Adsorption of CO2 on pristine Zn–ZnO (0 0 0 1) and defected Zn–ZnO (0 0 0 1): A thermal desorption spectroscopy study. Surf. Sci. 2005,577 (2-3), 158-166.
2. Schott, V.;Oberhofer, H.; Birkner, A.;Xu, M.;Wang, Y.;Muhler, M.;Reuter, K.; Wöll, C., Chemical activity of thin oxide layers: strong interactions with the support yield a new thin‐film phase of ZnO. Angewandte Chemie International Edition 2013,52 (45), 11925-11929.
3. Lunkenbein, T.;Schumann, J.;Behrens, M.;Schlögl, R.; Willinger, M. G., Formation of a ZnO overlayer in industrial Cu/ZnO/Al2O3 catalysts induced by strong metal–support interactions. Angewandte Chemie 2015,127 (15), 4627-4631.
4. Tusche, C.;Meyerheim, H.; Kirschner, J., Observation of depolarized ZnO (0001) monolayers: formation of unreconstructed planar sheets. Phys. Rev. Lett. 2007,99 (2), 026102.
5. Deng, X.;Sorescu, D. C.; Lee, J., Enhanced adsorption of CO 2 at steps of ultrathin ZnO: the importance of Zn–O geometry and coordination. Phys. Chem. Chem. Phys. 2017,19 (7), 5296-5303.

This work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-06ER46317.

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Recently, Chen et al. reported the observation of tunneling magnetoresistance in an all-antiferromagnetic tunnel junction consisting of Mn₃Sn/MgO/Mn₃Sn.¹ Furthermore, Bangar et al. demonstrated a technique for engineering the spin Hall conductivity of Mn₃Sn films by changing the Mn: Sn composition.² These works show the potential of studying this Kagome antiferromagnetic material and the importance of being able to grow smooth films. This work uses molecular beam epitaxy to investigate the growth of Mn₃Sn (1120) on Al₂O₃ (0001). The growth is monitored in-situ using reflection high energy electron diffraction and measured ex-situ using X-ray diffraction, Rutherford backscattering, and atomic force microscopy.In our previous work, we carried out a single-step growth at 450°C, which resulted in a crystalline but discontiguous a-plane-oriented (~43% 1120) Mn₃Sn film with a mix of other orientations including 0002.³ Leading from this work, changes were made to the growth recipe, which involved carrying out a two-step growth procedure at room temperature, resulting in a contiguous, epitaxial Mn₃Sn film with up to ~82% 1120-orientation. We are also exploring the effect of varying the Mn: Sn flux ratio and the film thicknesses (in the range of 5 – 200 nm) on the film crystallinity and orientation. We observe that varying the Mn: Sn flux ratio leads to a change in the RHEED patterns from pointy to streaky, and the XRD shows that the 1120 peak can be varied between~ 82 % to ~38 % of all the peaks’ total intensity. We also plan to present the first results on ultra-high vacuum scanning tunneling microscopy imaging of the (11­20) Mn₃Sn surface.

Acknowledgments:

The authors acknowledge support from the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-06ER46317. The authors would like to thank Dr. Eric Stinaff and his students for back-coating the sapphire (0001) substrates.

¹ X. Chen et al., "Octupole–driven magnetoresistance in an antiferromagnetic tunnel junction.” Nature 613, 490 (2023).

² H. Bangar et al., “Large Spin Hall Conductivity in Epitaxial thin films of Kagome Antiferromagnet Mn₃Sn at room temperature”, Adv.Quantum Technol. 6, 2200115 (2023).

³ S. Upadhyay et al., “Molecular beam epitaxy and crystal structure of majority a-plane oriented and substrate strained Mn₃Sn thin films grown directly on sapphire (0001)”, Journal of Vacuum Science and Technology A, to be published (2023).