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At least 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Solid breeder material crystal structure evolution due to Li burn up–Loss of crystal stability

Solid breeder materials for fusion power plants are subjected to extreme conditions of radiation damage, high temperature and designed component burn-up. Here, the loss of lattice stability of Li 2 TiO 3 due to Li burn-up was investigated using first principles-based approaches to aid evaluation of upper limit of component lifetime. The lattice stability is analyzed using calculated phonon dispersion in the Li 2 TiO 3 supercell, while Li burn-up is modeled by the introduction of Li-vacancies. It has been determined that the studied ceramic can be structurally stable up to ~40% of Li atoms burn-up. At 50% burn-up, the negative frequency branch in the phonon dispersion spectrum appears indicating the loss of lattice dynamical stability. Moreover, the structure obtained by introduction of unstable frozen phonons with minima energy amplitude also results in a phonon dispersion with negative branches in the structure obtained. Therefore, the system completely loses stability when approximately half of Li atoms are consumed.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Rapid Screening of Single-Atom Catalyst Synthesis Conditions Using ToF-SIMS and Facet-Dependent Single-Crystal Substrates

Single-atom catalysts (SACs) offer superior catalytic performance compared to traditional nanoparticle catalysts but are challenging to develop because of the need for extensive optimization and specialized characterization techniques. Here, this study presents a rapid and versatile method for detecting synthesis conditions and elucidating deposition mechanisms of SACs on various substrates. By depositing active elements (Au, Cu, Ni and Rh) on facet-specific single-crystalline substrates (CeO 2 , TiO 2 , MgO and Al 2 O 3 ) and employing time-of-flight secondary ion mass spectroscopy (ToF-SIMS), we assessed facet-dependent deposition behaviors and identified optimal conditions for solution-based SAC synthesis. On CeO 2 and TiO 2 , we confirmed facet-dependent deposition, primarily influenced by oxygen vacancy density and photocatalytic activity, respectively. MgO exhibited the formation of metal oxide/hydroxide clusters for all active elements, and the degree of clustering for Cu and Ni was correlated with the facet hydrolysis susceptibility. Notably, Au and Rh deposition on MgO was facet-independent, attributed to the formation of hydroxide species in solution. Al 2 O 3 , due to its chemical stability and lack of surface defects, did not show active element deposition. This study not only provides a time and cost-efficient method for prescreening SAC synthesis conditions, but it also provides valuable insights into the various deposition mechanisms governing SAC formation on different substrates, paving the way for the rational design of tailored SACs for various catalytic applications.

ToF-SIMS↗
DOE OSTI2022

Understanding the Fluorination of Disordered Rocksalt Cathodes through Rational Exploration of Synthesis Pathways

Here, we have designed and tested several synthesis routes targeting a highly fluorinated disordered rocksalt (DRX) cathode, Li 1.2 Mn 0.4 Ti 0.4 O 1.6 F 0.4 , with each route rationalized by thermochemical analysis. Precursor combinations were screened to raise the F chemical potential and avoid the formation of LiF, which inhibits fluorination of the targeted DRX phase. MnF 2 was used as a reactive source of F, and Li 6 MnO 4 , LiMnO 2 , and Li 2 Mn 0.33 Ti 0.66 O 3 were tested as alternative Li sources. Each synthesis procedure was monitored using a multi-modal suite of characterization techniques including X-ray diffraction, nuclear magnetic resonance, thermogravimetric analysis, and differential scanning calorimetry. From the resulting data, we advance the understanding of oxyfluoride synthesis by outlining the key factors limiting F solubility. At low temperatures, MnF 2 consistently reacts with the Li source to form LiF as an intermediate phase, thereby trapping F in strong Li-F bonds. LiF can react with Li 2 TiO 3 to form a highly lithiated and fluorinated DRX (Li 3 TiO 3 F); however, MnO is not easily incorporated into this DRX phase. Although higher temperatures typically increase solubility, the volatility of LiF above its melting point (848 °C) inhibits fluorination of the DRX phase. Based on these findings, metastable synthesis techniques are suggested for future work on DRX fluorination.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Nb 2 O 5 Protection Layers for Hydrogen Evolution by p-InP Photocathodes in Acidic Electrolytes

Coatings of HfO 2 , Ta 2 O 5 , TiO 2 , and Nb 2 O 5 were evaluated as protection layers for p-InP photocathodes in aqueous acidic electrolytes. Each candidate protection layer was characterized based on interfacial conduction of photogenerated electrons, the thermodynamic and kinetic stability of the overlayer over the relevant potential and pH range for cathodic fuel-forming half-reactions, and inhibition of the primary anodic and/or chemical dissolution processes that limit electrode stability. The conduction of photogenerated electrons and inhibition of anodic oxidation was evaluated using V 3+ / 2+ in 5.0 M HCl(aq) as a one-electron redox couple with a potential close to that of hydrogen evolution in acidic media. Failure modes due to cathodic plating of metal, as well as anodic dissolution and chemical dissolution processes, were evaluated for photocathodes made from etched p-InP, platinized p-InP, and p-InP photoelectrodes that had been coated with the various oxide films and then platinized for use in photoelectrochemical hydrogen evolution in acidic aqueous electrolytes. Nb 2 O 5 best met all of the criteria, contrasting with TiO 2 , which is thermodynamically and kinetically unstable in acid over the potential range relevant for the hydrogen evolution and/or CO 2 reduction electrochemical half-reactions. Furthermore, these protocols should be useful for other materials and semiconductors to determine the effectiveness of protection layers for photocathodes.

Electrodes↗
DOE OSTI2021

Modulation of Photoinduced Iodine Expulsion in Mixed Halide Perovskites with Electrochemical Bias

Hole trapping at iodine (I) sites in MAPbBr 1.5 I 1.5 mixed halide perovskites (MHP) is responsible for iodine migration and its eventual expulsion into solution. We have now modulated the photoinduced iodine expulsion in MHP through an externally applied electrochemical bias. At positive potentials, electron extraction at TiO 2 /MHP interfaces becomes efficient, leading to hole buildup within MHP films. This improved charge separation, in turn, favors iodine migration as evident from the increased apparent rate constant of iodine expulsion (k expulsion = 0.0030 s –1 ). Conversely, at negative potentials (-0.3 V vs Ag/AgCl) electron–hole recombination is facilitated within MHP, slowing down iodine expulsion by an order of magnitude (k expulsion = 0.00018 s –1 ). The tuning of the EFermi level through external bias modulates electron extraction at the TiO 2 /MHP interface and indirectly controls the buildup of holes, ultimately inducing iodine migration/expulsion. Suppressing iodine migration in perovskite solar cells is important for attaining greater stability since they operate under internal electrical bias.

14 SOLAR ENERGY↗
DOE OSTI2021

Proton-Coupled Defects Impact O–H Bond Dissociation Free Energies on Metal Oxide Surfaces

Proton-coupled electron transfer (PCET) reactions on metal oxides require coupling between proton transfer at the solid-liquid interface and electron transfer involving defects at or near the band edge. Herein, hybrid functional periodic density functional theory is used to elucidate the impact of proton-coupled defects on the bond dissociation free energies (BDFEs) of O-H bonds on anatase TiO 2 surfaces. These O-H BDFEs are directly related to interfacial PCET thermochemistry. Comparison between geometrically similar O-H bonds associated with different defect types, namely conduction d-band electrons or valence p-band holes, reveals that the BDFEs differ by ~81 kcal/mol (3.50 eV), comparable to the wide TiO 2 band gap. These differences are shown to be determined primarily by differences in electron transfer driving forces, which are analyzed by using band energies and inner-sphere reorganization energies within a Marcus theory framework. Furthermore, these fundamental insights about the impact of proton-coupled defects on PCET thermochemistry at semiconductor surfaces have broad implications for electrocatalysis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Real-Space Constrained Density Functional Theory Investigation of Site-Specific, Interfacial Charge Recombination Dynamics Across the Au Nanoparticle/TiO 2 Heterojunction

Au nanoparticle (NP)/TiO 2 heterojunction is a representative system to study interfacial charge transfer in photocatalysis and photovoltaics, where suppressing recombination from TiO 2 to Au can enhance hot carrier extraction. We apply real-space constrained density functional theory (CDFT) with Marcus theory to quantify charge recombination time scales across Au/TiO 2 . This approach enables direct control and visualization of charge-separated states, aligning with site-specific probes like time-resolved X-ray photoelectron spectroscopy (trXPS). We find that the charge-separated state features a bipolaron, with recombination dominated by TiO 2 LUMO to Au HOMO transitions, primarily at interfacial Au sites. Marcus rate predictions are benchmarked with surface hopping methods, quantifying differences in time scales and computational efficiency. Lastly, we examine how the Au cluster size affects the free energy change (ΔG) and reorganization energy (λ), explaining trends in closed-shell systems and highlighting challenges for open-shell extrapolations. Overall, CDFT + Marcus theory provides efficient, mechanistically transparent interfacial charge transfer modeling, and we clearly defined its applicability and limitation.

Glenna, Drew M. [Univ. of Idaho, Idaho Falls, ID (↗
DOE OSTI2021

Modulating Interactions between Molten Polystyrene and Porous Solids Using Atomic Layer Deposition

Understanding and modulating the interactions between molten polymers and porous solids is important for numerous processes and phenomena including catalytic conversion of polymers and fabrication of nanocomposites and nanostructured materials. Although changing the surface composition of pores would enable modulation of interactions between polymer and nanoporous solids, it is challenging to achieve such a control without inducing significant changes to the size and structure of nanopores. In this work, we demonstrate that the interactions between molten polystyrene (PS) and disordered packings of SiO 2 nanoparticles (NPs) can be modulated by changing the surface composition of the NPs using atomic layer deposition (ALD). A disordered packing of silica NPs is modified with varying surface coverages of TiO 2 , WO 3 and CaCO 3 , with coverages estimated by the mass gain and the refractive index change of NP packings. Based on the time required to fully infiltrate these ALD-modified NP packings via capillarity, the contact angles for PS on different surfaces prepared via ALD are determined. The contact angle gradually changes from that of pure SiO 2 to that of the fully covered surfaces. The contact angles for PS on SiO 2 , TiO 2 , WO 3 and CaCO 3 are found to be 20°, 62°, 70° and 10°, respectively. Interestingly, the contact angles and interfacial energies between PS and the ALD-modified surfaces do not correlate strongly with the water contact angle of these surfaces; thus, caution must be exercised in predicting how a polymer would wet or interact with porous solids solely based on their hydrophilicity. Furthermore, the method presented in this work can be extended to study the interactions between a wide range of polymers and surfaces in porous media, which will have important implications for designing new catalytic materials for polymer upcycling reactions as well as novel NP-polymer composite films and membranes with enhanced mechanical and transport properties.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Spontaneous Lithiation of Binary Oxides during Epitaxial Growth on LiCoO 2

Epitaxial growth is a powerful tool for synthesizing heterostructures and integrating multiple functionalities. Interfacial mixing can readily occur at temperatures required for complete film crystallization and can significantly modify the properties of layered structures, particularly for those containing energy storage materials with smaller cations. Here we show a two-step sequence involving the growth of an epitaxial LiCoO 2 cathode layer followed by the deposition of a binary transition metal oxide (WO 3 , TiO 2 , and others) in which controlled lithiation of the binary oxide occurs. Orientation-controlled epitaxial synthesis of the model solid-state-electrolyte Li 2 WO 4 and model anode material Li 4 Ti 5 O 12 occurs as WO 3 and TiO 2 nucleate their respective host lattices and attract Li ions from the underlying cathode. The cathode layer readily provides a tunable amount of Li ions for the controlled lithiation of the subsequent layer. We demonstrate that this approach can be used for energy materials discovery and exploring different combinations of epitaxial interfaces that can serve as well-defined model systems for mechanistic studies of energy storage and conversion processes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Transport Mediating Core–Shell Photocatalyst Architecture for Selective Alkane Oxidation

The high activation barrier of the C-H bond in methane, combined with the high propensity of methanol and other liquid oxygenates toward overoxidation to CO 2 , have historically posed significant scientific and industrial challenges to the selective and direct conversion of methane to energy-dense fuels and chemical feedstocks. Here, we report a unique core-shell nanostructured photocatalyst, silica encapsulated TiO 2 decorated with AuPd nanoparticles (TiO 2 @SiO 2 -AuPd), that prevents methanol overoxidation on its surface and possesses high selectivity and yield of oxygenates even at high UV intensity. This room-temperature approach achieves high selectivity for oxygenates (94.5%) with a total oxygenate yield of 15.4 mmol/g cat ·h at 9.65 bar total pressure of CH 4 and O 2 . The working principles of this core-shell photocatalyst were also systematically investigated. Finally, this design concept was further demonstrated to be generalizable for the selective oxidation of other alkanes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Microwave-Absorbing Foams with Adjustable Absorption Frequency and Structural Coloration

Microwave-absorbing materials with regulatable absorption frequency and optical camouflage hold great significance in intelligent electronic devices and advanced stealth technology. Herein, we present an innovative microwave-absorbing foam that can dynamically tune microwave absorption frequencies via a simple mechanical compression while in parallel enabling optical camouflage over broad spectral ranges by adjusting the structural colors. In this study, the vivid colors spanning different color categories generated from thin-film interference can be precisely regulated by adjusting the thickness of the conformal TiO 2 coatings on Ni/melamine foam. Enhanced interfacial and defect-induced polarizations resulting from the introduction of TiO 2 coating synergistically contribute to the dielectric attenuation performance. Consequently, such a foam exhibits exceptional microwave absorption capabilities, and the absorption frequency can be dynamically tuned from the S band to the Ku band by manipulating its compression ratio. Additionally, simulation calculations validate the adjustable electromagnetic wave loss behavior, offering valuable insights for the development of next-generation intelligent electromagnetic devices across diverse fields.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Role of Elemental Substitution on Hydrogen Incorporation in Characteristic Ti/Fe-Based Oxides

Elemental substitution is well-known to benefit hydrogen storage in the intermetallic compound TiFe; however, the impact of these substituents on the native and passivating surface oxide has not been well-established. Here, we perform density functional theory calculations on several characteristic oxides that are likely to form on exposed TiFe surfaces, specifically, TiO 2 , FeO, TiFeO 3 , and Ti 4 Fe 2 O. In TiO 2 and TiFeO 3 , additional acceptor species substituting for Ti can increase the solubility of protonic hydrogen (H i + ), likely enhancing hydrogen permeation of the oxide layer. In FeO, few dopants will act as acceptors, and hydrogen will incorporate as substitutional H O + , which is likely to be less mobile and will hinder hydrogen permeation. Finally, we find that Ti 4 Fe 2 O, which is thermodynamically metastable, may be stabilized by defects under O-poor conditions, although the presence of certain substitutional elements can destabilize it. In general, Mn, V, and Cr are among the best substituents for enhancing hydrogenation through the oxide layer. In conclusion, our results help to explain what features make additional elements particularly effective for enabling reversible hydrogen storage in TiFe, and they can guide further technological development of high-performing TiFe-based alloys for future energy applications.

organic↗
DOE OSTI2020

Tuning Irreversible Magnetoresistance in Pr 0.67 Sr 0.33 MnO 3 Film via Octahedral Rotation

Herein, the oxygen octahedral rotation around the out-of-plane axis is explored to study its effect on metastable status, magnetic cluster glass in manganite. The antiphase rotation around the out-of-plane axis (TiO 6 α Ο α Ο c – ) of SrTiO 3 enhances the Mn–O bond anisotropy along in-plane and out-of-plane directions and weakens the ferromagnetic interactions in a 12 nm Pr 0.67 Sr 0.33 MnO 3 film on the (001) SrTiO 3 substrate, which together promote the formation of magnetic cluster-glassiness and enlarges the irreversible magnetoresistance (MR) effect with enhanced relaxation time of charge carriers. The effect of TiO 6 α Ο α Ο c – in the SrTiO 3 substrate on material properties is obvious with a large irreversible MR effect for thin films, which fades away with the increase in film thickness. At 10 K, the irreversible MR is 0.91 for the 12 nm film and 0.22 for the 30 nm film. This work extends current understanding on interfacial coupling to metastable status and could help explore other systems in the perovskite structure with octahedral rotation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Structure-Based Design of Dual Bactericidal and Bacteria-Releasing Nanosurfaces

Here, we report synergistic nanostructured surfaces combining bactericidal and bacteria-releasing properties. A polystyrene-block-poly(methyl methacrylate) (PS-block-PMMA) diblock copolymer is used to fabricate vertically oriented cylindrical PS structures (“PS nanopillars”) on silicon substrates. The results demonstrate that the PS nanopillars (with a height of about 10 nm, size of about 50 nm, and spacing of about 70 nm) exhibit highly effective bactericidal and bacteria-releasing properties (“dual properties”) against Escherichia coli for at least 36 h of immersion in an E. coli solution. Interestingly, the PS nanopillars coated with a thin layer (≈3 nm thick) of titanium oxide (TiO 2 ) (“TiO 2 nanopillars”) show much improved dual properties against E. coli (a Gram-negative bacterium) compared to the PS nanopillars. Moreover, the dual properties emerge against Listeria monocytogenes (a Gram-positive bacterium). To understand the mechanisms underlying the multifaceted property of the nanopillars, coarse-grained molecular dynamics (MD) simulations of a lipid bilayer (as a simplified model for E. coli) in contact with a substrate containing hexagonally packed hydrophilic nanopillars were performed. The MD results demonstrate that when the bacterium–substrate interaction is strong, the lipid heads adsorb onto the nanopillar surfaces, conforming the shape of a lipid bilayer to the structure/curvature of nanopillars and generating high stress concentrations within the membrane (i.e., the driving force for rupture) at the edge of the nanopillars. Membrane rupture begins with the formation of pores between nanopillars (i.e., bactericidal activity) and ultimately leads to the membrane withdrawal from the nanopillar surface (i.e., bacteria-releasing activity). In the case of Gram-positive bacteria, the adhesion area to the pillar surface is limited due to the inherent stiffness of the bacteria, creating higher stress concentrations within a bacterial cell wall. In conclusion, the present study provides insight into the mechanism underlying the “adhesion-mediated” multifaceted property of nanosurfaces, which is crucial for the development of next-generation antibacterial surface coatings for relevant medical applications.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Study of CO 2 Adsorption Properties on the SrTiO 3 (001) Surface with Ambient Pressure XPS

The adsorption properties of CO 2 on the SrTiO 3 (001) surface were investigated using ambient pressure X-ray photoelectron spectroscopy under elevated pressure and temperature conditions. On the Nb-doped TiO 2 -enriched (1 × 1) SrTiO 3 surface, CO 2 adsorption, i.e., the formation of CO 3 surface species, occurs first at the oxygen lattice site under 10 –6 mbar CO 2 at room temperature. The interaction of CO 2 molecules with oxygen vacancies begins when the CO 2 pressure increases to 0.25 mbar. The adsorbed CO 3 species on the Nb-doped SrTiO 3 surface increases continuously as the pressure increases but starts to leave the surface as the surface temperature increases, which occurs at approximately 373 K on the defect-free surface. On the undoped TiO 2 -enriched (1 × 1) SrTiO 3 surface, CO 2 adsorption also occurs first at the lattice oxygen sites. Both the doped and undoped SrTiO 3 surfaces exhibit an enhancement of the CO 3 species with the presence of oxygen vacancies, thus indicating the important role of oxygen vacancies in CO 2 dissociation. When OH species are removed from the undoped SrTiO 3 surface, the CO 3 species begin to form under 10 –6 mbar at 573 K, thus indicating the critical role of OH in preventing CO 2 adsorption. The observed CO 2 adsorption properties of the various SrTiO 3 surfaces provide valuable information for designing SrTiO 3 -based CO 2 catalysts.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Probing the Effects of Dimethyl Aluminum Isopropoxide Surface Reaction Byproducts on Atomic Layer Deposition Nucleation

Atomic layer deposition (ALD) processes that leverage a myriad of metal–organic and complementary reactant combinations have been identified to realize precise and conformal thin film growth. However, the effects of the ALD reaction byproducts on nucleation and growth mechanisms are rarely considered. Site-selective ALD processes provide an opportunity for the detailed investigation of uniform surface sites with atomistic accuracy. Intentional pretreatment with a known ALD reaction byproduct – isopropanol – enables a significant improvement in the nucleation rate reproducibility of dimethylaluminum isopropoxide and water ALD on rutile TiO 2 (110). In situ spectroscopic ellipsometry reveals a partially reversible byproduct binding that is site-selective for TiO 2 (110) surface oxygen vacancies. First-principles calculations reveal surface site-specific thermodynamics for adsorption of isopropanol and water that may influence ALD nucleation. In conclusion, the sensitivity of site-selective ALD motivates consideration of secondary surface reactions when designing precision deposition processes, including area- or site-selective ALD reactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Tandem Catalytic Antioxidant Nanoparticles Comprising Cerium Carbonate and Photoactive Metal Oxides

While photoactive metal oxides such as TiO 2 find widespread use in paints and coatings as well as cosmetics and suncare products, they also generate reactive oxygen species (ROS), which degrade materials and are associated with human-health pathologies. Here, we demonstrate a robust and potent catalytic antioxidant consisting of earth-abundant cerium carbonate nanoparticles and micron-size Ce 2 (CO 3 ) 3 . 8H 2 O, which are characterized by powder X-ray diffraction and scanning nanobeam electron diffraction. When dispersed with photoactive metal oxides, these cerium carbonate catalysts decrease the photodecomposition rate of organic dyes and commercial pigment colorants in aqueous media by up to 820-fold, as well as in acrylic coatings. X-ray photoelectron spectroscopy and kinetic experiments support the same tandem catalysis mechanism of photoprotection when using both micron-size Ce 2 (CO 3 ) 3 . 8H 2 O and cerium carbonate nanoparticles. This mechanism involves ROS disproportionation (catalyzed by cerium carbonate) and H 2 O 2 decomposition (partially catalyzed by TiO 2 ) pathways, both of which cerium carbonate also catalyzes on its own, crudely mimicking the function of the cascade system of superoxide dismutase and catalase enzymes. When cerium carbonate nanoparticles were dispersed at 2 wt % in polymethylmethacrylate, the transparency of the polymer film was preserved and the photo-oxidative degradation of the polymer was prevented following UV irradiation at 254 nm, which otherwise resulted in the loss of optical properties and hydroxylation as characterized by ATR-FTIR spectroscopy, in the control polymer lacking cerium carbonate. Similar observations were made regarding color preservation in paint films comprising dye and insoluble commercial colorant pigments. The material chemistry associated with this photoprotection catalysis is subtle and emphasizes the importance of both Ce(III) and carbonate together, as both CePO 4 and Na 2 CO 3 are inactive. In conclusion, this emphasis is also apparent in comparisons of photoprotection catalysis with previously reported cerium oxide nanoparticles, which are significantly less active compared with Ce 2 (CO 3 ) 3 . 8H 2 O under the same conditions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗