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1. Surface Segregation Studies in Ternary Noble Metal Alloys: Comparing DFT and Machine Learning with Experimental Data

   https://doi.org/10.1002/cphc.202400073  

   Broderick, Kirby; Burnley, Robert A; Gellman, Andrew J; Kitchin, John R (July 2024, ChemPhysChem)

   Abstract Surface segregation, whereby the surface composition of an alloy differs systematically from the bulk, has historically been hard to study, because it requires experimental and modeling methods that span alloy composition space. In this work, we study surface segregation in catalytically relevant noble and platinum‐group metal alloys with a focus on three ternary systems: AgAuCu, AuCuPd, and CuPdPt. We develop a data set of 2478 fcc slabs with those compositions including all three low‐index crystallographic orientations relaxed with Density Functional Theory using the PBEsol functional with D3 dispersion corrections. We fine‐tune a machine learning model on this data and use the model in a series of 1800 Monte Carlo simulations spanning ternary composition space for each surface orientation and ternary chemical system. The results of these simulations are validated against prior experimental surface segregation data collected using composition spread alloy films for AgAuCu and AuCuPd. Our findings reveal that simulations conducted using the (110) orientation most closely match experimentally observed surface segregation trends, and while predicted trends qualitatively match observation, biases in the PBEsol functional limit numeric accuracy. This study advances understanding of surface segregation and the utility of computational studies and highlights the need for further improvements in simulation accuracy. 

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2. Colloidal Synthesis of Homogeneous Ge1-x-ySiySnx Nanoalloys with Composition-Tunable Visible to Near IR Optical Properties

   https://doi.org/10.1021/acs.chemmater.3c01644  

   Spera, Drew; Pate, David; Spence, Griffin C; Villot, Corentin; Onukwughara, Chineme Jeanfrances; White, Daulton; Lao, Ka Un; Özgür, Ümit; Arachchige, Indika U (November 2023, Chemistry of Materials)

   Group IV alloy nanocrystals (NCs) are a class of direct energy gap semiconductors that show high elemental abundance, low to non-toxicity, and composition-tunable absorption and emission properties. These properties have distinguished Ge1-xSnx NCs as an intriguing material for near-infrared (IR) optical studies. Achieving a material with efficient visible emission requires a modified class of Group IV alloys and the computational studies suggest that this can be achieved with Ge1-x-ySiySnx NCs. Herein, we report a colloidal strategy for the synthesis of bulk-like (10.3 ± 2.5 – 25.5 ± 5.3 nm) and quantum-confined (3.2 ± 0.6 – 4.2 ± 1.1 nm) Ge1-x-ySiySnx alloys that show strong size confinement effects and composition-tunable visible to near IR absorption and emission properties. This synthesis produces a homogeneous alloy with diamond cubic Ge structure and tunable Si (0.9 – 16.1%) and Sn (1.8 – 14.9%) compositions, exceeding the equilibrium solubility of Sn (<1%) in crystalline Si and Ge. Raman spectra of Ge1-x-ySiySnx alloys show a prominent redshift of the Ge-Ge peak and the emergence of a Ge-Si peak with increasing Si/Sn, suggesting the growth of homogeneous alloys. The smaller Ge1-x-ySiySnx NCs exhibit absorption onsets from 1.21 to 1.94 eV for x = 1.8 – 6.8% and y = 0.9 – 16.1% compositions, which are blueshifted from those reported for Ge1-x-ySiySnx bulk alloy films and Ge1-xSnx alloy NCs, indicating the influence of Si incorporation and strong size confinement effects. Solid-state photoluminescence (PL) spectra reveal core-related PL maxima from 1.77 – 1.97 eV in agreement with absorption onsets, consistent with the energy gaps calculated for ~3–4 nm alloy NCs. With facile low-temperature solution synthesis and direct control over physical properties, this methodology presents a noteworthy advancement in the synthesis of bulk-like and quantum-confined Ge1-x-ySiySnx alloys as versatile materials for future optical and electronic studies. 

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3. Study of growth twins and phase formation in CuNiAl alloys via a combinatorial approach

   https://doi.org/10.1557/s43578-024-01491-6  

   Alwen, A.; Peter, Nicolas_J; Schwaiger, Ruth; Hodge, A_M (November 2024, Journal of Materials Research)

   Abstract In this study, a combinatorial and high-throughput approach was leveraged to investigate nanotwin behavior in the ternary CuNiAl alloy system. Combinatorial co-sputtering was used to synthesize 169 unique CuNiAl alloy compositions, which were characterized in both the as-sputtered and annealed conditions to elucidate relationships between composition, nanotwin formation, and phase evolution. Compositional effects on phase formation were investigated using high-throughput X-ray diffraction, while scanning transmission electron microscopy was used to identify nanotwin compositional boundaries and isolate the roles of varied composition and nanotwin formation on microstructural evolution. It was determined that Al content was the primary variable influencing thermal evolution in the nanotwinned CuNiAl alloys, as it altered the thermodynamic driving forces by changing composition and reducing the as-sputtered twin boundary spacing. Overall, this work demonstrates a novel approach to globally study unexplored nanotwin synthesis domains beyond binary alloys. Graphical Abstract 

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4. H2−D2 Exchange Activity and Electronic Structure of AgxPd1−x Alloy Catalysts Spanning Composition Space

   Golio, Nicholas; Sen, Irem; Yu, Xiaoxiao; Kondratyuk, Petro; Gellman, Andrew J (July 2024, ACS catalysis)

   Many computational studies of catalytic surface reaction kinetics have demonstrated the existence of linear scaling relationships between physical descriptors of catalysts and reaction barriers on their surfaces. In this work, the relationship between catalyst activity, electronic structure, and alloy composition was investigated experimentally using a AgxPd1−x Composition Spread Alloy Film (CSAF) and a multichannel reactor array that allows measurement of steady-state reaction kinetics at 100 alloy compositions simultaneously. Steady-state H2 −D2 exchange kinetics were measured at atmospheric pressure on AgxPd1−x catalysts over a temperature range of 333−593 K and a range of inlet H2 and D2 partial pressures. X-ray photoelectron spectroscopy (XPS) was used to characterize the CSAF by determining the local surface compositions and the valence band electronic structure at each composition. The valence band photoemission spectra showed that the average energy of the valence band, ε̅v, shifts linearly with composition from −6.2 eV for pure Ag to −3.4 eV for pure Pd. At all reaction conditions, the H2 −D2 exchange activity was found to be highest on pure Pd and gradually decreased as the alloy was diluted with Ag until no activity was observed for compositions with xPd < 0.58. Measured H2 −D2 exchange rates across the CSAF were fit using the Dual Subsurface Hydrogen (2H′) mechanism to extract estimates for the activation energy barriers to dissociative adsorption, ΔEads ‡ , associative desorption, ΔEdes ‡ , and the surface-to-subsurface diffusion energy, ΔEss, as a function of alloy composition, xPd. The 2H′ mechanism predicts ΔEads ‡ = 0−10 kJ/mol, ΔEdes ‡ = 30−65 kJ/mol, and ΔEss = 20−30 kJ/mol for all alloy compositions with xPd ≥ 0.64, including for the pure Pd catalyst (i.e., xPd = 1). For these Pd-rich catalysts, ΔEdes ‡ and ΔEss appeared to increase by ∼5 kJ/mol with decreasing xPd. However, due to the coupling of kinetic parameters in the 2H′ mechanism, we are unable to exclude the possibility that the kinetic parameters predicted when xPd ≥ 0.64 are identical to those predicted for pure Pd. This suggests that H2 −D2 exchange occurs only on bulk-like Pd domains, presumably due to the strong interactions between H2 and Pd. In this case, the decrease in catalytic activity with decreasing xPd can be explained by a reduction in the availability of surface Pd at high Ag compositions. 

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5. Adaptive kinetic Monte Carlo simulations of surface segregation in PdAu nanoparticles

   https://doi.org/10.1039/C9NR01858A  

   Li, Lei; Li, Xinyu; Duan, Zhiyao; Meyer, Randall J.; Carr, Rob; Raman, Sumathy; Koziol, Lucas; Henkelman, Graeme (May 2019, Nanoscale)

   Surface segregation in bimetallic nanoparticles (NPs) is critically important for their catalytic activity because the activity is largely determined by the surface composition. Little, however, is known about the atomic scale mechanisms and kinetics of surface segregation. One reason is that it is hard to resolve atomic rearrangements experimentally. It is also difficult to model surface segregation at the atomic scale because the atomic rearrangements can take place on time scales of seconds or minutes – much longer than can be modeled with molecular dynamics. Here we use the adaptive kinetic Monte Carlo (AKMC) method to model the segregation dynamics in PdAu NPs over experimentally relevant time scales, and reveal the origin of kinetic stability of the core@shell and random alloy NPs at the atomic level. Our focus on PdAu NPs is motivated by experimental work showing that both core@shell and random alloy PdAu NPs with diameters of less than 2 nm are stable, indicating that one of these structures must be metastable and kinetically trapped. Our simulations show that both the Au@Pd and the PdAu random alloy NPs are metastable and kinetically trapped below 400 K over time scales of hours. These AKMC simulations provide insight into the energy landscape of the two NP structures, and the diffusion mechanisms that lead to segregation. In the core–shell NP, surface segregation occurs primarily on the (100) facet through both a vacancy-mediated and a concerted mechanism. The system becomes kinetically trapped when all corner sites in the core of the NP are occupied by Pd atoms. Higher energy barriers are required for further segregation, so that the metastable NP has a partially alloyed shell. In contrast, surface segregation in the random alloy PdAu NP is suppressed because the random alloy NP has reduced strain as compared to the Au@Pd NP, and the segregation mechanisms in the alloy require more elastic energy for exchange of Pd and Au and between the surface and subsurface. 

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