Search

We report giant anisotropy in thermoelectric power factor ($S^2\sigma$) of marcasite structure-type PtSb$_{1.4}$Sn$_{0.6}$. PtSb$_{1.4}$Sn$_{0.6}$, synthesized using ambient pressure flux growth method upon mixing Sb and Sn on the same atomic site, is a new phase different from both PtSb$_2$ and PtSn$_2$ that crystallize in the cubic $Pa\bar{3}$ pyrite and $Fm\bar{3}m$ fluorite unit cell symmetry, respectively. Giant difference in $S^2\sigma$ for heat flow applied along different principal directions of the orthorhombic unit cell mostly stems from anisotropic Seebeck coefficients.

Connections between crystal chemistry and critical temperature Tc have been in the focus of superconductivity, one of the most widely studied phenomena in physics, chemistry, and materials science alike. In most Fe-based superconductors, materials chemistry and physics conspire so that Tc correlates with the average anion height above the Fe plane, i.e., with the geometry of the FeAs4 or FeCh4 (Ch = Te, Se, or S) tetrahedron. By synthesizing Fe1–ySe1–xSx (0 ≤ x ≤ 1; y ≤ 0.1), we find that in alloyed crystals Tc is not correlated with the anion height like it is for most other Fe superconductors. Instead, changes in Tc(x) and tetragonal-to-orthorhombic (nematic) transition Ts(x) upon cooling are correlated with disorder in Fe vibrations in the direction orthogonal to Fe planes, along the crystallographic c-axis. The disorder stems from the random nature of S substitution, causing deformed Fe(Se,S)4 tetrahedra with different Fe–Se and Fe–S bond distances. Our results provide evidence of Tc and Ts suppression by disorder in anion height. The connection to local crystal chemistry may be exploited in computational prediction of new superconducting materials with FeSe/S building blocks.

Atomic dispersion of metal catalysts on a substrate accounts for the increased atomic efficiency of single-atom catalysts (SACs) in various catalytic schemes compared to the nanoparticle counterparts. However, lacking neighboring metal sites has been shown to deteriorate the catalytic performance of SACs in a few industrially important reactions, such as dehalogenation, CO oxidation, and hydrogenation. Metal ensemble catalysts (M n ), an extended concept to SACs, have emerged as a promising alternative to overcome such limitation. Inspired by the fact that the performance of fully isolated SACs can be enhanced by tailoring their coordination environment (CE), we here evaluate whether the CE of M n can also be manipulated in order to enhance their catalytic activity. We synthesized a set of Pd ensembles (Pd n ) on doped graphene supports (Pd n /X-graphene where X = O, S, B, and N). We found that introducing S and N onto oxidized graphene modifies the first shell of Pd n converting Pd–O to Pd–S and Pd–N, respectively. We further found that the B dopant significantly affected the electronic structure of Pd n by serving as an electron donor in the second shell. We examined the performance of Pd n /X-graphene toward selective reductive catalysis, such as bromate reduction, brominated organic hydrogenation, and aqueous-phase CO 2 reduction. We observed that Pd n /N-graphene exhibited superior performance by lowering the activation energy of the rate-limiting step, i.e., H 2 dissociation into atomic hydrogen. The results collectively suggest controlling the CE of SACs in an ensemble configuration is a viable strategy to optimize and enhance their catalytic performance.

Aqueous Zn/MnO2 batteries with their environmental sustainability and competitive cost, are becoming a promising, safe alternative for grid-scale electrochemical energy storage.

We introduce a new graphene oxide (GO)-based membrane architecture that hosts cobalt catalysts within its nanoscale pore walls. Such an architecture would not be possible with catalysts in nanoscale, the current benchmark, since they would block the pores or alter the pore structure. Therefore, we developed a new synthesis procedure to load cobalt in an atomically dispersed fashion, the theoretical limit in material downsizing. The use of vitamin C as a mild reducing agent was critical to load Co as dispersed atoms (Co

Charge density waves (CDWs) with superconductivity, competing Fermi surface instabilities, and collective orders have captured much interest in two-dimensional van der Waals (vdW) materials. Understanding the CDW suppression mechanism, its connection to the emerging superconducting state, and electronic correlations provides opportunities for engineering the electronic properties of vdW heterostructures and thin-film devices. Using a combination of the thermal transport, x-ray photoemission spectroscopy, Raman measurements, and first-principles calculations, we observe an increase in electronic correlations of the conducting states as the CDW is suppressed in ZrTe3 with 5% Cu and Ni intercalation in the vdW gap. As superconductivity emerges, intercalation brings not only decoupling of quasi-one-dimensional conduction electrons with phonons as a consequence of intercalation-induced lattice expansion but also a drastic increase in Zr2+ at the expense of Zr4+ metal atoms. These observations not only demonstrate the potential of atomic intercalates in the vdW gap for ground-state tuning but also illustrate the crucial role of the Zr metal valence in the formation of collective electronic orders.

Understanding and controlling the physical and chemical processes at molten salt‐alloy interfaces is vital for molten‐salt nuclear reactors. Corrosion processes in molten salts are highly dependent on the redox potential of the solution that changes with the addition of fission and corrosion products. Therefore, reactor designers develop online electrochemical methods of salt monitoring. But electrochemical spectroscopy relies on the deconvolution of broad peaks, a process that may be imprecise in the presence of multiple species in the solution. Here, we describe our developments towards monitoring the concentration and the chemical state of corrosion products in the melt by a combination of electrochemistry and X-ray absorption spectroscopy. We placed NiCr foil in molten FLiNaK and found the presence of both Ni2+ ions and metallic Ni in the melt, which we attribute to the disintegration of the corroding foil due to Cr dealloying. Although extremely challenging, spectroelectrochemical measurements add a promising rich new data stream for online salt monitoring.

A new magnetic insulator Mn2MnSbO6 with a polar crystal structure and an ordered Mn2+ and Mn3+ arrangement was synthesized under a high pressure of 7.5 GPa and 1300 °C. The crystal structure of Mn2...

Two-dimensional magnetic materials (2DMMs) are significant not only for studies on the nature of 2D long-range magnetic order but also for future spintronic devices. Of particular interest are 2DMMs where spins can be manipulated by electrical conduction. Whereas Cr

Tailoring the coordination number (CN) of metal atoms has been increasingly recognized as one of the strategies to enhance the catalytic performance of single-atom catalysts (SACs). We here present...

In situ synchrotron X-ray absorption near-edge structure characterization of thin-film titania growth by atomic layer deposition (ALD) over ZnO nanowires reveals persistent low-coordinated Ti motif...

Separating the redox sites of photocatalysts is one of most promising strategies to promote an efficient photoinduced charge transfer of semiconductor photocatalysis. Herein, we present a site-sele...

Ni-rich NMC materials are a particularly promising class of Li-ion cathodes for various applications. LiNi0.6Mn0.2Co0.2O2 (NMC622) offers a unique balance of thermal stability and energy density, t...

Environmentally friendly and energy-efficient ways to produce ammonia are essential to meet global food demands. Here, a new approach for ammonia production at nominally ambient conditions is intro...

Ceria–zirconia-supported Ni catalysts (Ni/Ce0.83Zr0.17O2 or Ni/CZ) are prepared by dry impregnation, strong electrostatic adsorption, coprecipitation (CP), and combustion synthesis (CS). The nature...

CO2 hydrogenation to methanol has attracted increasing attention with the development of renewable hydrogen. A big challenge is to identify catalysts able to achieve high conversion and selectivity...

We here present a novel Ti4O7-based electrode loaded with amorphous Pd clusters that achieve efficient anodic destruction of perfluorooctanoic acid (PFOA), a persistent water pollutant with significant environmental and human health concerns. These amorphous Pd clusters were characterized by the disordered, noncrystalline arrangement of Pd single atoms in close proximity, in contrast to crystalline Pd nanoparticles that have been often employed to tailor the electronic properties of an electrode. We found that the Ti4O7 electrode loaded with amorphous Pd clusters significantly outperformed the Ti4O7 electrode loaded with crystalline Pd particles due to enhanced electron transfer through dominant Pd-O bonds. Combined with the efficient binding of PFOA and its degradation intermediates to the fluorinated electrode surface, this electrode was capable of mineralizing PFOA and releasing fluoride as F-. The reaction pathway was found to proceed without involving reactive oxygen species and therefore was not quenched by common anions in complex natural water systems such as chloride ions.

Understanding surface reactions of biomass-derived oxygenates on metal oxides is important for designing catalysts for valorization of biomass. This work elucidated the effect of different pretreat...

Titanium disulfide (TiS2) with high electric conductivity, fast rate capability, and good cycling performance is a promising candidate for electrode material in sodium (Na)-ion batteries. Despite the well-studied electrochemical behaviors of TiS2 in Li-ion batteries, the detailed reaction mechanism of TiS2 in Na-ion batteries is not yet fully understood due to a more complex multi-phase conversion process. In this work, reactions of TiS2 in Na-ion batteries are investigated via a multi-modal synchrotron approach: operando X-ray Absorption Spectroscopy (XAS) – including X-ray Absorption Near-Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) – and ex situ X-ray Powder Diffraction (XPD), coupled with computational modeling. Operando XANES spectra indicate that the redox reactions occur in both Ti and S during the electrochemically driven phase transformation. Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) analysis of XAS suggests that different numbers of components are involved in the lithiation and sodiation of TiS2, with the sodiation including at least one intermediate phase in addition to the starting material and the final sodiation product. Ex situ XPD and Rietveld refinement further determined and quantified the unknown phases, showing that three phases, TiS2, Na0.55TiS2, and NaTiS2, participate in the sodiation of TiS2. Operando EXAFS results show the changes in the Ti–Ti coordination number and interatomic distance. This explains the coulombic efficiency decay due to the incomplete recovery of the coordination number of Ti after cycling. Overall, this work reveals the reaction mechanism occurring in Na–TiS2 batteries with a greater quantitative understanding of the structural evolution. By combining the multi-modal synchrotron approach and computational work, this study provides a framework for studying a broader range of electrochemically driven phase-transformation systems towards advanced energy storage and conversion applications.

Precise control of lattice mismatch accommodation and interfacial cation diffusion is critical to modulate correlated functionalities in epitaxial heterostructures, particularly when the interface composition is positioned near a compositional phase transition boundary. Here we select La1−xSrxMnO3 (LSMO) as a prototype phase transition oxide and establish vertical epitaxial interfaces with NiO for exploring the strong interplay between strain accommodation, stoichiometry modification, and localized electron transport across the interface. It is found that localized stoichiometry modification overcomes the dead layer problem plaguing LSMO and leads to strongly directional conductivity, as manifested by a more than three orders of magnitude difference between out-of-plane and in-plane conductivity. Comprehensive atomic scale structural characterization and transport measurements reveal that this emerging behavior is created by a compositional change produced by preferential cation diffusion that pushes the LSMO phase transition from insulating into metallic within an ultrathin interface region. This study explores the nature of unusual electrical conductivity at vertical epitaxial interfaces and highlights the beneficial role of controllable cation diffusion that enables emerging functionalities for a broad range of potential applications such as memristors, spintronic devices, and novel nanoelectronic devices using strongly correlated materials.

Oxidation of inorganic molecules (e.g., arsenite (As(III)) to arsenate (As(V))) by Cu(II)–chitosan in the presence of dissolved oxygen is examined to elucidate the ability and mechanism of Cu(II)–chitosan and Cu(II)–n-TiO2–chitosan participation in Fenton-like reactions. To form the Cu(II)–chitosan complex, Cu(II) binds with the amine groups of the chitosan backbone resulting in a Cu(II)-complex with cationic behavior. Arsenic is then adsorbed to the copper binding site through a combination of Lewis acid–base coordinate bonding and electrostatics. As K-edge XANES indicate that arsenite is fully oxidized to arsenate when adsorbed to a Cu(II)–chitosan complex in the dark without introduction of a photo- or chemical-oxidant. The oxidation of arsenite by this complex is strongly controlled by the presence of dissolved oxygen as suggested by linear combination fitting where the % As(V) bound decreases from 100% ± 0.0% to 60.2 ± 0.1% upon removal of dissolved oxygen via the freeze pump thaw method. Cu K-edge XANES indicate that Cu(II) acts as a catalyst rather than a reactant, as it remains present as Cu(II) after As oxidation in each system condition examined. For Cu(II)–n-TiO2–chitosan, the amount of As(III) oxidized to As(V) is strongly controlled by the loading of Cu(II), with a higher loading of Cu(II) leading to more As(III) oxidized and bound on the surface of the adsorbent as As(V). As(III) removal by both Cu(II)–chitosan and Cu(II)–nTiO2–chitosan is significantly improved due to the Fenton-like oxidation of As(III) to As(V). For Cu(II)–n-TiO2–chitosan, higher loadings of Cu(II) relative to n-TiO2 lead to greater improvement of As(III) removal performance under oxic vs. anoxic conditions.

Two-dimensional transition metal trichalcogenides (TMTC's) feature covalently bonded metal-chalcogen layers separated by the van der Waals (vdW) gap. Similar to transition metal dichalcogenides (TMDCs), TMTCs often host charge density waves (CDWs) and superconductivity but unlike TMDCs atomic chains in the crystal structure give rise to quasi one-dimensional (quasi 1D) conduction. ZrTe$_3$ features CDW below $T_{\textrm{CDW}}$ = 63 K and filamentary superconductivity below 2 K that can be enhanced by pressure or chemical substitution. Here we report the presence of mixed valent Zr$^{2+}$ and Zr$^{4+}$ atoms in ZrTe$_3$ crystals that is reduced by doping in ZrTe$_{3-x}$Se$_x$ and Zr$_{1-y}$Hf$_y$Te$_3$. Superconductivity is enhanced via disorder in Te2-Te3 atomic chains that are associated with CDW formation. Hf substitution on Zr atomic site enhances $T_{\textrm{CDW}}$ due to unperturbed Te2-Te3 chain periodicity and enhanced electron-phonon coupling. Weak electronic correlations in ZrTe$_{3-x}$Se$_x$ are likely governed by the lattice contraction effects.

Two-dimensional magnetic materials (2DMM) are significant for studies on the nature of 2D long range magnetic order but also for future spintronic devices. Of particular interest are 2DMM where spins can be manipulated by electrical conduction. Whereas Cr$_2$Si$_2$Te$_6$ exhibits magnetic order in few-layer crystals, its large band gap inhibits electronic conduction. Here we show that the defect-induced short-range crystal order in Cr$_2$Si$_2$Te$_6$ on the length scale below 0.6 nm induces substantially reduced band gap and robust semiconducting behavior down to 2 K that turns to metallic above 10 GPa. Our results will be helpful to design conducting state in 2DMM and call for spin-resolved measurement of the electronic structure in exfoliated ultrathin crystals.

In this study, we loaded Pd catalysts onto a reduced graphene oxide (rGO) support in an atomically dispersed fashion [i.e., Pd single-atom catalysts (SACs) on rGO or Pd1/rGO] via a facile and scalable synthesis based on anchor-site and photoreduction techniques. The as-synthesized Pd1/rGO significantly outperformed the Pd nanoparticle (Pdnano) counterparts in the electrocatalytic hydrodechlorination of chlorinated phenols. Downsizing Pdnano to Pd1 leads to a substantially higher Pd atomic efficiency (14 times that of Pdnano), remarkably reducing the cost for practical applications. The unique single-atom architecture of Pd1 additionally affects the desorption energy of the intermediate, suppressing the catalyst poisoning by Cl-, which is a prevalent challenge with Pdnano. Characterization and experimental results demonstrate that the superior performance of Pd1/rGO originates from (1) enhanced interfacial electron transfer through Pd-O bonds due to the electronic metal-support interaction and (2) increased atomic H (H*) utilization efficiency by inhibiting H2 evolution on Pd1. This work presents an important example of how the unique geometric and electronic structure of SACs can tune their catalytic performance toward beneficial use in environmental remediation applications.

Effects of corrosion products on molten salt properties are a focus of many studies motivated by the needs of molten-salt nuclear reactors. We demonstrate in-situ measurements of corrosion products from NiCr (80%-20%) foil in molten FLiNaK. We used X-ray absorption spectroscopy (XAS) and a combination of XAS with electrochemical spectroscopy. Using XAS, we measured both the space and time distributions of Ni and Cr ions leaching from the foil and characterized their valence state and the local structure. We also correlated the XAS results with simultaneous electrochemical measurements. We also demonstrated a novel sample environment that we developed that enables simultaneous XAS and electrochemical measurements of molten fluoride salts. The next step of this program is to measure the corrosion on structural alloys such as stainless steel in molten FLiBe.

The use of molten salts for large-scale solar concentrated power plants and molten salt reactors has been driving the research to better understand how metals and alloys interact with the molten salt. As the metals may undergo morphological, chemical, and structural change in molten salt environments, it is critical to understand the fundamental mechanisms in these changes. In this work, we will present how we utilized synchrotron X-ray nano-tomography to better understand the 3D morphological evolution of Ni, Cr, and their alloys in molten salt. The effects of temperature and additives in the salt on the morphological evolution will be discussed. At the higher temperature, a characteristic bicontinuous structure can form from molten salt dealloying a binary alloy. [1] This contrasts to the intergranular corrosion found in the same system reacted at a lower temperature. [2] Different additives in the salt were also found to alter the morphological changes of the alloys and can create planar corrosion, percolation dealloying, or redeposition. To complement the morphological studies by X-ray nano-tomography, a suite of X-ray and electron microscopy analyses were also carried out to better understand the chemical and structural (both short-and long-range ordering) evolution. Taking it as a multimodal approach, we will discuss how we couple the analysis from synchrotron operando X-ray absorption spectroscopy, diffraction, and imaging, as well as the multiscale imaging studies from both X-ray and electron microscopy. This work was supported as part of the Molten Salts in Extreme Environments (MSEE) Energy Frontier Research Center (EFRC), funded by the U.S. Department of Energy, Office of Science. References: [1] "Formation of three-dimensional bicontinuous structures via molten salt dealloying studied in real-time by in situ synchrotron X-ray nano-tomography" Xiaoyang Liu, Arthur Ronne*, Lin-Chieh Yu, Yang Liu, Mingyuan Ge, Cheng-Hung Lin, Bobby Layne, Phillip Halstenberg, Dmitry S. Maltsev, Alexander S. Ivanov, Stephen Antonelli, Sheng Dai, Wah-Keat Lee, Shannon M. Mahurin, Anatoly I. Frenkel, James F. Wishart, Xianghui Xiao & Yu-chen Karen Chen-Wiegart* Nature Communications (2021), DOI: 10.1038/s41467-021-23598-8 [2] "Visualizing time-dependent microstructural and chemical evolution during molten salt corrosion of Ni-20Cr model alloy using correlative quasi in situ TEM and in situ synchrotron X-ray nano-tomography" Kaustubh Bawane, Xiaoyang Liu, Ruchi Gakhar, Michael Woods, Mingyuan Ge, Xianghui Xiao, Wah-Keat Lee, Philip Halstenberg, Sheng Dai, Shannon Mahurin, Simon M. Pimblott, James F. Wishart, Yu-chen Karen Chen-Wiegart*, Lingfeng He* Corrosion Science (2021), DOI: 10.1016/j.corsci.2021.109962

Single atom catalysts have been found to exhibit superior selectivity over nanoparticulate catalysts for catalytic reactions such as hydrogenation due to their single-site nature. However, improved selectively is often accompanied by loss of activity and slow kinetics. Here we demonstrate that neighboring Pd single atom catalysts retain the high selectivity merit of sparsely isolated single atom catalysts, while the cooperative interactions between neighboring atoms greatly enhance the activity for hydrogenation of carbon-halogen bonds. Experimental results and computational calculations suggest that neighboring Pd atoms work in synergy to lower the energy of key meta-stable reactions steps, i.e., initial water desorption and final hydrogenated product desorption. The placement of neighboring Pd atoms also contribute to nearly exclusive hydrogenation of carbon-chlorine bond without altering any other bonds in organohalogens. The promising hydrogenation performance achieved by neighboring single atoms sheds light on a new approach for manipulating the activity and selectivity of single atom catalysts that are increasingly studied in multiple applications., Single atom catalysts have exhibited high selectivity for hydrogenation, yet improved selectively is often accompanied by loss of activity. Here the authors report that synergistic interactions of neighboring Pd single atoms lead to both high activity and selectivity for hydrodehalogenation catalysis.

We report synthetic strategies for installing platinum group metals (PGMs: Pd, Rh, Ir, and Pt) on a scorpionate-derived linker (TpmC*) within a metal-organic framework (MOF), both by room-temperature postsynthetic metalation and by direct solvothermal synthesis, with a wide range of metal loadings relevant for fundamental studies and catalysis. In-depth studies for the palladium adduct Pd(II)@Zr-TpmC* by density-functional-theory-assisted extended X-ray absorption fine structure spectroscopy reveals that the rigid MOF lattice enforces a close Pd(II)-Napical interaction between the bidentate palladium complex and the third uncoordinated pyrazole arm of the TpmC* ligand (Pd-Napical = 2.501 ± 0.067 A), an interaction that is wholly avoided in molecular palladium scorpionates.

Heterogeneous advanced oxidation processes (AOPs) allow for the destruction of aqueous organic pollutants via oxidation by hydroxyl radicals (•OH). However, practical treatment scenarios suffer from the low availability of short-lived •OH in aqueous bulk, due to both mass transfer limitations and quenching by water constituents, such as natural organic matter (NOM). Herein, we overcome these challenges by loading iron oxychloride catalysts within the pores of a ceramic ultrafiltration membrane, resulting in an internal heterogeneous Fenton reaction that can degrade organics in complex water matrices with pH up to 6.2. With •OH confined inside the nanopores (∼ 20 nm), this membrane reactor completely removed various organic pollutants with water fluxes of up to 100 L m-2 h-1 (equivalent to a retention time of 10 s). This membrane, with a pore size that excludes NOM (>300 kDa), selectively exposed smaller organics to •OH within the pores under confinement and showed excellent resiliency to representative water matrices (simulated surface water and sand filtration effluent samples). Moreover, the membrane exhibited sustained AOPs (>24 h) and could be regenerated for multiple cycles. Our results suggest the feasibility of exploiting ultrafiltration membrane-based AOP platforms for organic pollutant degradation in complex water scenarios.

We carried out a comprehensive study of the structural, electrical transport, thermal, and thermodynamic properties in ferrimagnetic ${\mathrm{Mn}}_{3}{\mathrm{Si}}_{2}{\mathrm{Te}}_{6}$ single crystals. Mn and Te $K$-edge x-ray absorption spectroscopy and synchrotron powder x-ray diffraction were measured to provide information on the local atomic environment and the average crystal structure. The dc and ac magnetic susceptibility measurements indicate a second-order paramagnetic to ferrimagnetic transition at ${T}_{c}\ensuremath{\sim}74$ K, which is further confirmed by the specific heat measurement. ${\mathrm{Mn}}_{3}{\mathrm{Si}}_{2}{\mathrm{Te}}_{6}$ exhibits semiconducting behavior along with a large negative magnetoresistance of $\ensuremath{-}87%$ at ${T}_{c}$ and a relatively high value of thermopower up to $\ensuremath{\sim}10$ mV/K at 5 K. Besides the rapidly increasing resistivity $\ensuremath{\rho}(T)$ and thermopower $S(T)$ below 20 K, the large discrepancy between the activation energy for resistivity ${E}_{\ensuremath{\rho}}$ and thermopower ${E}_{S}$ above 20 K indicates the polaronic transport mechanism. Furthermore, the thermal conductivity $\ensuremath{\kappa}(T)$ of ${\mathrm{Mn}}_{3}{\mathrm{Si}}_{2}{\mathrm{Te}}_{6}$ is notably rather low, comparable to ${\mathrm{Cr}}_{2}{\mathrm{Si}}_{2}{\mathrm{Te}}_{6}$, and is strongly suppressed in the magnetic field across ${T}_{c}$, indicating the presence of strong spin-lattice coupling, also similar with ${\mathrm{Cr}}_{2}{\mathrm{Si}}_{2}{\mathrm{Te}}_{6}$.