AVS 67 Session LI-TuA1: Tuesday Afternoon Live Session I: Harnessing and Understanding Interfacial Chemistry

Computation-driven search for thermoelectrics (TEs) has recently shown several successes, but a principal bottleneck appears to be that predicted materials often prove to be difficult to dope in the lab. Here we will present our recent computational and experimental efforts to tailor the defect chemistry and dopability of a set of ordered vacancy compounds that are of interest for their TE potential. Ordered vacancy compounds (OVCs) are a family of materials whose structure is closely related to diamond like semiconductors, but contain an ordered array of vacancies. Diamond like semiconductors and OVCs are typically observed to be p-type, but our TE materials descriptor suggests that they would be more effective as thermoelectrics if they could be doped n-type. Therefore, we screen through a chemically-diverse set of 54 OVCs to assess their TE potential and dopability. For the most promising candidates, using calculations of phase stability defect formation energies, we comprehensively establish the achievable range of carrier concentrations achievable. Using phase boundary mapping, experimental carrier concentrations are measured and compared to the predicted values, typically showing correspondence within a few orders of magnitude. For all compounds, we find that a delicate competition between a set of dominant, competing defects governs the achievable range of carrier concentrations. For instance in the II₂-IV-VI₄ OVC structural prototype, enhancing n-type dopability requires suppressing the IV_II antisite and promoting the formation of the group II vacancy. Using this observation as a design strategy, we identify candidates that may be more amenable to n-type doping. The results of this search are used to generate a chemically intuitive framework for predicting dopabilities in this family of materials without the need to carry out full-scale first-principles analysis.

Niobium (Nb) is the current standard material for superconducting radio frequency (SRF) accelerator cavities due to its ultra-low surface resistance and high cavity quality factor (Q) at operating temperatures of ~ 2 K. However, prohibitively expensive operating and infrastructure costs prevent the implementation of accelerator-based technologies across a variety of sectors. Nb₃Sn is a promising next-generation material for SRF cavities and can operate at ~ 4.2 K, significantly reducing operating costs of cryogenic infrastructure. To increase the efficiency and accessibility of accelerator-based technologies, SRF materials must be developed that can reliably produce intense particle beams above 4 K.

Nb surface composition and contaminant incorporation directly affect Q. Hydrogen incorporated during Nb cavity fabrication results in the formation of Nb hydrides, which lower Q at high fields and reduce cavity performance. Nitrogen infusion of Nb cavities mitigates hydride formation, but the growth and suppression mechanisms of Nb hydrides are not thoroughly understood. Utilizing low-temperature scanning tunneling microscopy (LT-STM), scanning tunneling spectroscopy (STS), and density functional theory (DFT) calculations, we have investigated the growth and suppression mechanisms of Nb nano-hydrides on (3×1)-O Nb(100). STM and STS results identified the structural and electronic evolution of undoped, hydrogen doped, and co-doped Nb(100) resulting from Nb nano-hydride growth and suppression. DFT calculations corroborate experimental data and identified unique near-surface phases stabilized by dopant incorporation. These results provide the first in situ, real-time nanoscale characterization of the effects of dopant incorporation on Nb nano-hydride growth and suppression, elucidating the interplay between nitrogen and hydrogen regarding Nb SRF materials performance.

High-entropy alloys (HEAs) and high-entropy metal-sublattice ceramics (HESCs) have recently gained particular attraction in the field of materials research due to their promising properties, such as high hardness, high strength, and thermal stability. Within this work, we report on the phase formation and thermal stability of high entropy metal-sublattice carbides to provide a further insight to a more extensive understanding of the high-entropy effect, according to which, based on the Gibbs-free energy, such materials should be stabilised in the high-temperature regime. Therefore, (Hf,Ta,Ti,V,Zr)C coatings were reactively and non-reactively sputtered from a single powder-metallurgically produced composite target (either metallic or consisting of the respective binary carbides). Reactively sputtered coatings were synthesised using an C₂H₂ – Ar mixture with different C₂H₂/(C₂H₂+Ar) ratios (f_C2H2). After deposition, the coatings were investigated in as-deposited state and after vacuum annealing between 800 and 1200°C. The structure and morphology, the chemical composition, the mechanical properties, and the thermal stability of the coatings were investigated by scanning electron microscopy, X-ray diffraction, and nanoindentation.

The non-reactively sputtered as well as reactively sputtered coatings with f_C2H2= 20 % show a single-phased face-centred cubic (fcc) structure. The hardness for the non-reactively sputtered HESCs is with ~41 GPa higher than that of the reactively sputtered one which exhibits a hardness of 35 GPa. This indicates that due to the use of C₂H₂ also regions of amorphous carbon form, which slightly weaken the coating already in the as-deposited state. After vacuum annealing up to 1200 °C the non-reactively sputtered coatings maintain a hardness of ~40 GPa indicating retarded softening mechanisms due to sluggish diffusion. This behaviour was also observed in previous studies on different material classes such as nitrides, borides, and oxides indicating a stabilisation due to the high-entropy metal sublattice.

Single atoms have shown different structural and catalytic properties with deep coupling between the oxide and metallic structures. This phenomenum, unraveled by theory might have a high impact in the understanding of the chemical properties of the materials and might affect our current understading for materials included those that are active in energy-conversion proceses.

Methane remains a valuable yet underutilized resource, with great interest for the direct upgrade to valued products. Our recent studies indicate that, in spite of the high stability of methane, systems such as Ni-CeO2(111) and CeO2-Cu(111) can break C-H bonds even at room temperature, through careful manipulation of interfaces and metal-support interactions, using light oxidants (CO2, O2/H2O). the selective oxidation on the surface sites can lead to conversion steps that produce CH3OH and CO+H2. AP-XPS with the help of DFT has been essential to elucidate C-H activation and oxidation steps with high resolution C 1s and O 1s spectroscopies.

We have established a method for the direct conversion of methane to methanol building on these surface results yielding low methanol selectivity (~30%). Subsequently, and in remarkable contrast we have found that a CeO2/Cu2O/Cu(111) inverse system is able to activate methane at room temperature and then, with the help of water, performs a highly selective (70%) catalytic cycle, for the production of methanol. The interfacial interaction between CeOx-CuOx is crucial while the concentration of water has a strong effect on the selectivity towards the production of methanol.

We have used AP-XPS to track the sequence of pathways that initially activate C-H bonds into -OCH3 surface species that leads to the production of methanol. DFT and KMC were then used to evaluate the most prevalent steps that are likely essential for such selective chemistry.

This work is funded by the U.S. Department of Energy under contract No. DE-SC0012704. XPS measurements were performed at beamline 9.3.2 at the Advanced Light Source of LBNL under contract no. DE-AC02-05CH11231.

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From the earliest days of UHV single-crystal and thin-film research related to fundamental aspects of heterogeneous catalysis, researchers have recognized the need to address various technical gaps relating fundamental observations under pristine conditions to factors affecting the working catalyst.Some of those observations scale, and others do not, but in both cases the atomic/molecular scale understanding obtained from research bridging those divides has been invaluable to the advancement of heterogeneous catalysis as practiced.Nevertheless, significant technical gaps still exist in advancing catalyst technology through the concept, design, and development stages to commercial application.Catalyst technology often gets stuck at a particular level, characterized by a combination of factors best described as “knowledge gaps.”At that point, industry tends to focus on optimization around a core (often proprietary) technology, and the academic community heads off to new pastures.Federal funding agencies and our national laboratories sit in a unique position to forge research partnerships between academic and industry researchers that bridge knowledge gaps.I will provide specific examples of key knowledge gaps in heterogeneous catalysis, and how NSF is funding partnerships between academic institutions and industry to bridge those gaps.