Co-Removal of Perfluorooctanoic Acid and Nitrate from Water by Coupling Pd Catalysis with Enzymatic Biotransformation.

PFAS (poly- and per-fluorinated alkyl substances) represent a large family of recalcitrant organic compounds that are widely used and pose serious threats to human and ecosystem health. Here, palladium (Pd0)-catalyzed defluorination and microbiological mineralization were combined in a denitrifying H2-based membrane biofilm reactor to remove co-occurring perfluorooctanoic acid (PFOA) and nitrate. The combined process, i.e., Pd-biofilm, enabled continuous removal of ∼4 mmol/L nitrate and ∼1 mg/L PFOA, with 81% defluorination of PFOA. Metagenome analysis identified bacteria likely responsible for biodegradation of partially defluorinated PFOA: Dechloromonas sp. CZR5, Kaistella koreensis, Ochrobacterum anthropic, and Azospira sp. I13. High-performance liquid chromatography-quadrupole time-of-flight mass spectrometry and metagenome analyses revealed that the presence of nitrate promoted microbiological oxidation of partially defluorinated PFOA. Taken together, the results point to PFOA-oxidation pathways that began with PFOA adsorption to Pd0, which enabled catalytic generation of partially or fully defluorinated fatty acids and stepwise oxidation and defluorination by the bacteria. This study documents how combining catalysis and microbiological transformation enables the simultaneous removal of PFOA and nitrate.

Synthesis of High Sn Content Ge1-x-ySixSny (0.1 < y < 0.22) Semiconductors on Si for MWIR Direct Band Gap Applications.

A systematic effort has been described to grow ternary Ge1-x-ySixSny semiconductors on silicon with high Sn concentrations spanning the 9.5-21.2% range. The ultimate goal is not only to produce direct band gap materials well into the infrared region of the spectrum but also to approach a critical concentration (yc) for which further additions of Si would decrease─rather than increase─the band gap. This counterintuitive behavior is expected as a result of the giant bowing parameter in the compositional dependence of the band gap associated with the presence of Si-Sn pairs. The growth approach in this study was based on a chemical vacuum deposition method that uses Si4H10, Ge3H8, and SnD4 or SnH4 as the sources of Si, Ge, and Sn, respectively. A fixed Si concentration near x = 0.05-0.07 was chosen to focus the exploration of the compositional space. A first family of samples was grown of Ge-buffered Si substrates. For Sn concentrations y < 0.12, it was found that the samples relaxed their mismatch strain in situ during growth, resulting in high Sn content films that had relatively low levels of strain and exhibited photoluminescence signals that demonstrated direct band gap behavior for the first time. The device potential of these materials was also demonstrated by fabricating a prototype photodiode with low dark currents. The optical studies suggest that the above-mentioned critical concentration is close to yc = 0.2. As the growth temperature was lowered in an effort to reach such values, Sn concentrations as high as y = 0.15 were obtained, but the films grew fully strained with compressive levels as high as 1.7%. To increase the Sn concentration beyond y = 0.15, a new strategy was adopted, in which the Ge buffer layer was eliminated, and the ternary alloy was grown directly on Si. The much higher lattice mismatch between the Ge1-x-ySixSny layer and the Si substrate caused strain relaxation right at the film/substrate interface, and the subsequent films grew with much lower levels of strain. This made it possible to lower the growth temperatures even further and achieve a comprehensive series of strained relaxed samples with tunable Sn concentrations as high as y = 0.21 (and beyond). The latter represent the highest Sn contents in crystalline Ge1-x-ySixSny attained to date and reach the desired yc = 0.2 range. The synthesized films exhibited significant thickness, allowing a thorough determination of composition, crystallinity, morphology, and bonding properties, indicating the formation of single-phase single-crystal alloys with random cubic structures. Further work will focus on optimizing the latter samples to explore the optical and electronic properties.

Vertical nanoscale strain-induced electronic localization in epitaxial La2/3Sr1/3MnO3 films with ZrO2 nanopillar inclusions.

Unusual electrical transport properties associated with weak or strong localization are sometimes found in disordered electronic materials. Here, we report experimental observation of a crossover of electronic behavior from weak localization to enhanced weak localization due to the spatial influence of disorder induced by ZrO2 nanopillars in (La2/3Sr1/3MnO3)1-x:(ZrO2)x (x = 0, 0.2, and 0.3) nanocomposite films. The spatial strain regions, identified by scanning transmission electron microscopy and high-resolution x-ray diffraction, induce a coexistence of two-dimentional (2D) and three-dimentional (3D) localization and switches to typical 2D localization with increasing density of ZrO2 pillars due to length scale confinement, which interestingly accords with enhancing vertically interfacial strain. Based on the excellent agreement of our experimental results with one-parameter scaling theory of localization, the enhanced weak localization exists in metal range close to the fixed point. These films provide a tunable experimental model for studying localization in particular the transition regime by appropriate choice of the second epitaxial phase.

A synergistic platform enables co-oxidation of halogenated organic pollutants without input of organic primary substrate.

While co-oxidation is widely used to biodegrade halogenated organic pollutants (HOPs), a considerable amount of organic primary substrate is required. Adding organic primary substrates increases the operating cost and also leads to extra carbon dioxide release. In this study, we evaluated a two-stage Reduction and Oxidation Synergistic Platform (ROSP), which integrated catalytic reductive dehalogenation with biological co-oxidation for HOPs removal. The ROSP was a combination of an H2-based membrane catalytic-film reactor (H2-MCfR) and an O2-based membrane biofilm reactor (O2-MBfR). 4-chlorophenol (4-CP) was used as a model HOP to evaluate the performance of ROSP. In the MCfR stage, zero-valent palladium nanoparticles (Pd0NPs) catalyzed reductive hydrodechlorination that converted 4-CP to phenol, with a conversion yield over 92%. In the MBfR stage, the phenol was oxidized and used as a primary substrate that supported the co-oxidation of residual 4-CP. Genomic DNA sequencing revealed that phenol produced from 4-CP reduction enriched bacteria having genes for functional enzymes for phenol biodegradation in the biofilm community. In the ROSP, over 99% of 60 mg/L 4-CP was removed and mineralized during continuous operation: Effluent 4-CP and chemical oxygen demand concentrations were below 0.1 and 3 mg/L, respectively. H2 was the only added electron donor to the ROSP, which means no extra carbon dioxide was produced by primary-substrate oxidation.

Reductive Dehalogenation of Herbicides Catalyzed by Pd0NPs in a H2-Based Membrane Catalyst-Film Reactor.

More food production required to feed humans will require intensive use of herbicides to protect against weeds. The widespread application and persistence of herbicides pose environmental risks for nontarget species. Elemental-palladium nanoparticles (Pd0NPs) are known to catalyze reductive dehalogenation of halogenated organic pollutants. In this study, the reductive conversion of 2,4-dichlorophenoxyacetic acid (2,4-D) was evaluated in a H2-based membrane catalyst-film reactor (H2-MCfR), in which Pd0NPs were in situ-synthesized as the catalyst film and used to activate H2 on the surface of H2-delivery membranes. Batch kinetic experiments showed that 99% of 2,4-D was removed and converted to phenoxyacetic acid (POA) within 90 min with a Pd0 surface loading of 20 mg Pd/m2, achieving a catalyst specific activity of 6.6 ± 0.5 L/g-Pd-min. Continuous operation of the H2-MCfR loaded with 20 mg Pd/m2 sustained >99% removal of 50 µM 2,4-D for 20 days. A higher Pd0 surface loading, 1030 mg Pd/m2, also enabled hydrosaturation and hydrolysis of POA to cyclohexanone and glycolic acid. Density functional theory identified the reaction mechanisms and pathways, which involved reductive hydrodechlorination, hydrosaturation, and hydrolysis. Molecular electrostatic potential calculations and Fukui indices suggested that reductive dehalogenation could increase the bioavailability of herbicides. Furthermore, three other halogenated herbicides─atrazine, dicamba, and bromoxynil─were reductively dehalogenated in the H2-MCfR. This study documents a promising method for the removal and detoxification of halogenated herbicides in aqueous environments.

Microwave synthesis of upconverting nanoparticles with bis(2-ethylhexyl) adipate.

Despite efforts to develop effective upconverting nanoparticle (UCNP) synthesis methods, there is still a need for approaches that are accessible and up-scalable while reproducibly providing fine control of UCNP size, crystallinity, and luminescence. This work presents a one-pot microwave-assisted strategy for synthesizing NaYF4:Yb3+/Er3 UCNPs. A premixed rare earth (RE) solution in oleic acid (OA) was used to enhance repeatability while testing various synthesis conditions. The stock solution aliquots were mixed with OA and bis(2-ethylhexyl) adipate (BEHA), a polycarboxylic ester with a high boiling point, high thermal stability, and moderately polar character that facilitated rapid microwave heating at an average rate (room temperature to 300 °C) up to 60 °C min-1. Combinations of BEHA concentration and high-temperature reaction time were identified for consistently producing cubic and hexagonal UCNPs with narrow size distributions in the tens and hundreds of nanometers. After washing, the resulting UCNPs were dispersible in aqueous media without further processing. This straightforward, accessible, and repeatable microwave-assisted synthesis method holds potential for scaling up the production of UCNPs with well-defined size and crystallinity.

Strong Ferromagnetism Achieved via Breathing Lattices in Atomically Thin Cobaltites.

Low-dimensional quantum materials that remain strongly ferromagnetic down to monolayer thickness are highly desired for spintronic applications. Although oxide materials are important candidates for the next generation of spintronics, ferromagnetism decays severely when the thickness is scaled to the nanometer regime, leading to deterioration of device performance. Here, a methodology is reported for maintaining strong ferromagnetism in insulating LaCoO3 (LCO) layers down to the thickness of a single unit cell. It is found that the magnetic and electronic states of LCO are linked intimately to the structural parameters of adjacent "breathing lattice" SrCuO2 (SCO). As the dimensionality of SCO is reduced, the lattice constant elongates over 10% along the growth direction, leading to a significant distortion of the CoO6 octahedra, and promoting a higher spin state and long-range spin ordering. For atomically thin LCO layers, surprisingly large magnetic moment (0.5 µB /Co) and Curie temperature (75 K), values larger than previously reported for any monolayer oxides are observed. The results demonstrate a strategy for creating ultrathin ferromagnetic oxides by exploiting atomic heterointerface engineering, confinement-driven structural transformation, and spin-lattice entanglement in strongly correlated materials.

Strain-Mediated High Conductivity in Ultrathin Antiferromagnetic Metallic Nitrides.

Strain engineering provides the ability to control the ground states and associated phase transition in epitaxial films. However, the systematic study of the intrinsic character and strain dependency in transition-metal nitrides remains challenging due to the difficulty in fabricating stoichiometric and high-quality films. Here the observation of an electronic state transition in highly crystalline antiferromagnetic CrN films with strain and reduced dimensionality is reported. By shrinking the film thickness to a critical value of ≈30 unit cells, a profound conductivity reduction accompanied by unexpected volume expansion is observed in CrN films. The electrical conductivity is observed surprisingly when the CrN layer is as thin as a single unit cell thick, which is far below the critical thickness of most metallic films. It is found that the metallicity of an ultrathin CrN film recovers from insulating behavior upon the removal of the as-grown strain by the fabrication of freestanding nitride films. Both first-principles calculations and linear dichroism measurements reveal that the strain-mediated orbital splitting effectively customizes the relatively small bandgap at the Fermi level, leading to an exotic phase transition in CrN. The ability to achieve highly conductive nitride ultrathin films by harnessing strain-control over competing phases can be used for utilizing their exceptional characteristics.

Exploiting Symmetry Mismatch to Control Magnetism in a Ferroelastic Heterostructure.

In the bulk, LaCoO_{3} (LCO) is a paramagnet, yet the low-temperature ferromagnetism (FM) is observed in tensile strained thin films, and its origin remains unresolved. Here, we quantitatively measured the distribution of atomic density and magnetization in LCO films by polarized neutron reflectometry (PNR) and found that the LCO layers near the heterointerfaces exhibit a reduced magnetization but an enhanced atomic density, whereas the film's interior (i.e., its film bulk) shows the opposite trend. We attribute the nonuniformity to the symmetry mismatch at the interface, which induces a structural distortion related to the ferroelasticity of LCO. This assertion is tested by systematic application of hydrostatic pressure during the PNR experiments. The magnetization can be controlled at a rate of -20.4% per GPa. These results provide unique insights into mechanisms driving FM in strained LCO films while offering a tantalizing observation that tunable deformation of the CoO_{6} octahedra in combination with the ferroelastic order parameter.

Nanoscale ferroelastic twins formed in strained LaCoO3 films.

The coexistence and coupling of ferroelasticity and magnetic ordering in a single material offers a great opportunity to realize novel devices with multiple tuning knobs. Complex oxides are a particularly promising class of materials to find multiferroic interactions due to their rich phase diagrams, and are sensitive to external perturbations. Still, there are very few examples of these systems. Here, we report the observation of twin domains in ferroelastic LaCoO3 epitaxial films and their geometric control of structural symmetry intimately linked to the material's electronic and magnetic states. A unidirectional structural modulation is achieved by selective choice of substrates having twofold rotational symmetry. This modulation perturbs the crystal field-splitting energy, leading to unexpected in-plane anisotropy of orbital configuration and magnetization. These findings demonstrate the use of structural modulation to control multiferroic interactions and may enable a great potential for stimulation of exotic phenomena through artificial domain engineering.

Spatially Resolved Large Magnetization in Ultrathin BiFeO3.

Here, a quantitative magnetic depth profile across the planar interfaces in BiFeO3 /La0.7 Sr0.3 MnO3 (BFO/LSMO) superlattices using polarized neutron reflectometry is obtained. An enhanced magnetization of 1.83 ± 0.16 µB /Fe in BFO layers is observed when they are interleaved between two manganite layers. The enhanced magnetic order in BFO persists up to 200 K. The depth dependence of magnetic moments in BFO/LSMO superlattices as a function of the BFO layer thickness is also explored. The results show the enhanced net magnetic moment in BFO from the LSMO/BFO interface extends 3-4 unit cells into BFO. The interior part of a thicker BFO layer has a much smaller magnetization, suggesting it still keeps the small canted AFM state. The results exclude charge transfer, intermixing, epitaxial strain, and octahedral rotations/tilts as dominating mechanisms for the large net magnetization in BFO. An explanation-one suggested by others previously and consistent with the observations-attributes the temperature dependence of the net magnetization of BFO to strong orbital hybridization between Fe and Mn across the interfaces. Such orbital reconstruction would establish an upper temperature limit for magnetic ordering of BFO.

Revealing the Reconstructed Surface of Li[Mn2]O4.

The spinel Li[Mn2]O4 is a candidate cathode for a Li-ion battery, but its capacity fades over a charge/discharge cycle of Li1-x[Mn2]O4 (0 < x < 1) that is associated with a loss of Mn to the organic-liquid electrolyte. It is known that the disproportionation reaction 2Mn(3+) = Mn(2+) + Mn(4+) occurs at the surface of a Mn spinel, and it is important to understand the atomic structure and composition of the surface of Li[Mn2]O4 in order to understand how Mn loss occurs. We report a study of the surface reconstruction of Li[Mn2]O4 by aberration-corrected scanning transmission electron microscopy. The atomic structure coupled with Mn-valence and the distribution of the atomic ratio of oxygen obtained by electron energy loss spectroscopy reveals a thin, stable surface layer of Mn3O4, a subsurface region of Li1+x[Mn2]O4 with retention of bulk Li[Mn2]O4. This observation is compatible with the disproportionation reaction coupled with oxygen deficiency and a displacement of surface Li(+) from the Mn3O4 surface phase. These results provide a critical step toward understanding how Mn is lost from Li[Mn2]O4, once inside a battery.

Tunability of exchange bias in Ni@NiO core-shell nanoparticles obtained by sequential layer deposition.

Films of magnetic Ni@NiO core-shell nanoparticles (NPs, core diameter d ≅ 12 nm, nominal shell thickness variable between 0 and 6.5 nm) obtained with sequential layer deposition were investigated, to gain insight into the relationships between shell thickness/morphology, core-shell interface, and magnetic properties. Different values of NiO shell thickness t(s) could be obtained while keeping the Ni core size fixed, at variance with conventional oxidation procedures where the oxide shell is grown at the expense of the core. Chemical composition, morphology of the as-produced samples and structural features of the Ni/NiO interface were investigated with x-ray photoelectron spectroscopy and microscopy (scanning electron microscopy, transmission electron microscopy) techniques, and related with results from magnetic measurements obtained with a superconducting quantum interference device. The effect of the shell thickness on the magnetic properties could be studied. The exchange bias (EB) field H(bias) is small and almost constant for ts up to 1.6 nm; then it rapidly grows, with no sign of saturation. This behavior is clearly related to the morphology of the top NiO layer, and is mostly due to the thickness dependence of the NiO anisotropy constant. The ability to tune the EB effect by varying the thickness of the last NiO layer represents a step towards the rational design and synthesis of core-shell NPs with desired magnetic properties.

Atomic scale studies of La/Sr ordering in colossal magnetoresistant La(2-2x)Sr(1+2x)Mn2O7 single crystals.

To date, it is unclear whether chemical order (or disorder) is in any way connected to double exchange, electronic phase separation, or charge ordering (CO) in manganites. In this work, we carry out an atomic resolution study of the colossal magnetoresistant manganite La(2-2x)Sr(1+2x)Mn2O7 (LSMO). We combine aberration-corrected electron microscopy and spectroscopy with spectroscopic image simulations, to analyze cation ordering at the atomic scale in real space in a number of LSMO single crystals. We compare three different compositions within the phase diagram: a ferromagnetic metallic material (x=0.36), an insulating, antiferromagnetic charge ordered (AF-CO) compound (x=0.5), which also exhibits orbital ordering, and an additional AF sample (x=0.56). Detailed image simulations are essential to accurately quantify the degree of chemical ordering of these samples. We find a significant degree of long-range chemical ordering in all cases, which increases in the AF-CO range. However, the degree of ordering is never complete nor can it explain the strongly correlated underlying ordering phenomena. Our results show that chemical ordering over distinct crystallographic sites is not needed for electronic ordering phenomena to appear in manganites, and cannot by itself explain the complex electronic behavior of LSMO.