XANES/EXAFS and quantum chemical study of the speciation of arsenic in the condensate formed in landfill gas processing: Evidence of the dominance of As-S species.

The XANES/EXAFS data and quantum chemical simulations presented in this study demonstrate several features of the chemistry of arsenic compounds found in the condensates and solids generated in landfill gas (LFG) processing carried out for renewable natural gas (RNG) production. The XANES data show the decrease in the position of the absorption edge of As atoms, similar to that characteristic for sulfur-containing As solutes and solids. The EXAFS data show that the As-O and As-S distances in these matrixes are similar to those in thioarsenates. Quantum-chemical calculations demonstrated the close agreement between the experimental and modeled As-S and As-O distances determined for a range of methylated and thiolated arsenic solutes. These calculations also showed that the increase of the number of the As-S bonds in the coordination shell of arsenic is accompanied by a consistent decrease of the charges of As atoms. This decrease is correlated with the number of the As-S bonds, in agreement with the trend observed in the XANES data. These results provide insight into the intrinsic chemistry and reactivity of As species present in LFG matrixes; they may be helpful for the development of treatment methods to control arsenic in these systems.

Prolonged lifespan of initial-anode-free lithium-metal battery by pre-lithiation in Li-rich Li2Ni0.5Mn1.5O4 spinel cathode.

Anode-free lithium metal batteries (AF-LMBs) can deliver the maximum energy density. However, achieving AF-LMBs with a long lifespan remains challenging because of the poor reversibility of Li+ plating/stripping on the anode. Here, coupled with a fluorine-containing electrolyte, we introduce a cathode pre-lithiation strategy to extend the lifespan of AF-LMBs. The AF-LMB is constructed with Li-rich Li2Ni0.5Mn1.5O4 cathodes as a Li-ion extender; the Li2Ni0.5Mn1.5O4 can deliver a large amount of Li+ in the initial charging process to offset the continuous Li+ consumption, which benefits the cycling performance without sacrificing energy density. Moreover, the cathode pre-lithiation design has been practically and precisely regulated using engineering methods (Li-metal contact and pre-lithiation Li-biphenyl immersion). Benefiting from the highly reversible Li metal on the Cu anode and Li2Ni0.5Mn1.5O4 cathode, the further fabricated anode-free pouch cells achieve 350 W h kg-1 energy density and 97% capacity retention after 50 cycles.

Zhang-Rice singlets state formed by two-step oxidation for triggering water oxidation under operando conditions.

The production of ecologically compatible fuels by electrochemical water splitting is highly desirable for modern industry. The Zhang-Rice singlet is well known for the superconductivity of high-temperature superconductors cuprate, but is rarely known for an electrochemical catalyst. Herein, we observe two steps of surface reconstruction from initial catalytic inactive Cu1+ in hydrogen treated Cu2O to Cu2+ state and further to catalytic active Zhang-Rice singlet state during the oxygen evolution reaction for water splitting. The hydrogen treated Cu2O catalyst exhibits a superior catalytic activity and stability for water splitting and is an efficient rival of other 3d-transition-metal catalysts. Multiple operando spectroscopies indicate that Zhang-Rice singlet is real active species, since it appears only under oxygen evolution reaction condition. This work provides an insight in developing an electrochemical catalyst from catalytically inactive materials and improves understanding of the mechanism of a Cu-based catalyst for water oxidation.

Metal Complex/ZnS-Modified Ni Foam as Magnetically Stirrable Photocatalysts: Roles of Redox Mediators and Carrier Dynamics Monitored by Operando Synchrotron X-ray Spectroscopy.

Magnetically stirrable photocatalysts binding the ZnS-decorated Ni foam with the metal complex cocatalyst as a redox mediator and light-absorbing composition were investigated. Loading metal complex can improve light absorption, surface hydrophilicity, interfacial charge migration, and H2 production activity. The variation of the metal valences of the composite photocatalysts in an operando environment (with sacrificial agent solution) with and without light irradiation was investigated by X-ray absorption near-edge structure (XANES) spectra and Fourier-transformed extended X-ray absorption fine structure (EXAFS) spectra to monitor the charge carrier dynamics of photocatalysis and explain how the macrocyclic Cu complex (CuC) acted as a redox mediator better than the Ni complex. The smaller valence difference of copper valence in ZS/CuC for dark and light states revealed that the Cu complex facilitates a reversible electron transfer between the ZnS photocatalyst and H+. Loading the Cu complex can improve the separation of photogenerated carriers by the redox couple of complexes, leading to a significantly improved photocatalytic H2 production activity of 8150 µmol h-1 g-1. The reactants can flow through these magnetically stirrable Ni foam-based photocatalysts by magnetic-field-driven stirring, which improves the contact between photocatalysts and the sacrificial agents. The operando synchrotron provides new insights for understanding the roles of redox mediators.

Interfacial Engineered Vanadium Oxide Nanoheterostructures Synchronizing High-Energy and Long-Term Potassium-Ion Storage.

Potassium ion hybrid capacitors (KICs) have drawn tremendous attention for large-scale energy storage applications because of their high energy and power densities and the abundance of potassium sources. However, achieving KICs with high capacity and long lifespan remains challenging because the large size of potassium ions causes sluggish kinetics and fast structural pulverization of electrodes. Here, we report a composite anode of VO2-V2O5 nanoheterostructures captured by a 3D N-doped carbon network (VO2-V2O5/NC) that exhibits a reversible capacity of 252 mAh g-1 at 1 A g-1 over 1600 cycles and a rate performance with 108 mAh g-1 at 10 A g-1. Quantitative kinetics analyses demonstrate that such great rate capability and cyclability are enabled by the capacitive-dominated potassium storage mechanism in the interfacial engineered VO2-V2O5 nanoheterostructures. The further fabricated full KIC cell consisting of a VO2-V2O5/NC anode and an active carbon cathode delivers a high operating voltage window of 4.0 V and energy and power densities up to 154 Wh kg-1 and 10 000 W kg-1, respectively, surpassing most state-of-the-art KICs.

Ligand-Promoted Cooperative Electrochemical Oxidation of Bio-Alcohol on Distorted Cobalt Hydroxides for Bio-Hydrogen Extraction.

Hydrogen is increasingly viewed as a game-changer in the clean energy sector. Renewable hydrogen production from water is industrialized by integrating water electrolysis and renewable electricity, but the current cost of water-born hydrogen remains high though. An ideal scenario would be to produce value-added chemicals along with hydrogen so the cost can be partially offset. Herein, facilitated bio-hydrogen extraction and biomass-derived chemical formation from sugar-derived 5-hydroxymethyfurfural (HMF) were achieved via the in-situ transformation of cobalt-bound electrocatalysts. The cyanide-bound cobalt hydroxide exhibited a low voltage at 1.55 V at 10 mA cm-2 for bio-hydrogen production, compared with an iridium catalyst (1.75 V). The interaction between the biomass intermediate and the cyanide ligand is suggested to be responsible for the improved activity.

Co-Induced Electronic Optimization of Hierarchical NiFe LDH for Oxygen Evolution.

Developing efficient and stable non-noble electrocatalysts for the oxygen evolution reaction (OER) remains challenging for practical applications. While nickel-iron layered double hydroxides (NiFe-LDH) are emerging as prominent candidates with promising OER activity, their catalytic performance is still restricted by the limited active sites, poor conductivity and durability. Herein, hierarchical nickel-iron-cobalt LDH nanosheets/carbon fibers (NiFeCo-LDH/CF) are synthesized through solvent-thermal treatment of ZIF-67/CF. Extended X-ray adsorption fine structure analyses reveal that the Co substitution can stabilize the Fe local coordination environment and facilitate the π-symmetry bonding orbital in NiFeCo-LDH/CF, thus modifying the electronic structures. Coupling with the structural advantages, including the largely exposed active surface sites and facilitated charge transfer pathway ensured by CF, the resultant NiFeCo-LDH/CF exhibits excellent OER activity with an overpotential of 249 mV at 10 mA cm-1 as well as robust stability over 20 h.

Tracking Charge Transfer to Residual Metal Clusters in Conjugated Polymers for Photocatalytic Hydrogen Evolution.

Semiconducting polymers are versatile materials for solar energy conversion and have gained popularity as photocatalysts for sunlight-driven hydrogen production. Organic polymers often contain residual metal impurities such as palladium (Pd) clusters that are formed during the polymerization reaction, and there is increasing evidence for a catalytic role of such metal clusters in polymer photocatalysts. Using transient and operando optical spectroscopy on nanoparticles of F8BT, P3HT, and the dibenzo[b,d]thiophene sulfone homopolymer P10, we demonstrate how differences in the time scale of electron transfer to Pd clusters translate into hydrogen evolution activity optima at different residual Pd concentrations. For F8BT nanoparticles with common Pd concentrations of >1000 ppm (>0.1 wt %), we find that residual Pd clusters quench photogenerated excitons via energy and electron transfer on the femto-nanosecond time scale, thus outcompeting reductive quenching. We spectroscopically identify reduced Pd clusters in our F8BT nanoparticles from the microsecond time scale onward and show that the predominant location of long-lived electrons gradually shifts to the F8BT polymer when the Pd content is lowered. While a low yield of long-lived electrons limits the hydrogen evolution activity of F8BT, P10 exhibits a substantially higher hydrogen evolution activity, which we demonstrate results from higher yields of long-lived electrons due to more efficient reductive quenching. Surprisingly, and despite the higher performance of P10, long-lived electrons reside on the P10 polymer rather than on the Pd clusters in P10 particles, even at very high Pd concentrations of 27000 ppm (2.7 wt %). In contrast, long-lived electrons in F8BT already reside on Pd clusters before the typical time scale of hydrogen evolution. This comparison shows that P10 exhibits efficient reductive quenching but slow electron transfer to residual Pd clusters, whereas the opposite is the case for F8BT. These findings suggest that the development of even more efficient polymer photocatalysts must target materials that combine both rapid reductive quenching and rapid charge transfer to a metal-based cocatalyst.

Structural and Electronic Optimization of MoS2 Edges for Hydrogen Evolution.

The activity and accessibility of MoS2 edge sites are critical to deliver high hydrogen evolution reaction (HER) efficiency. Here, a porous carbon network confining ultrasmall N-doped MoS2 nanocrystals (N-MoS2/CN) is fabricated by a self-templating strategy, which realizes synergistically structural and electronic modulations of MoS2 edges. Experiments and density functional theory calculations demonstrate that the N dopants could activate MoS2 edges for HER, while the porous carbon network could deliver high accessibility of the active sites from N-MoS2 nanocrystals. Consequently, N-MoS2/CN possesses superior HER activity with an overpotential of 114 mV at 10 mA cm-2 and excellent stability over 10 h, delivering one of best MoS2-based HER electrocatalysts. Moreover, this study opens a new venue for optimizing materials with enhanced accessible catalytic sites for energy-related applications.

Sandwich-Nanostructured n-Cu2O/AuAg/p-Cu2O Photocathode with Highly Positive Onset Potential for Improved Water Reduction.

An n-Cu2O layer formed a high-quality buried junction with p-Cu2O to increase the photovoltage and thus to shift the turn-on voltage positively. Mott-Schottky measurements confirmed that the improvement benefited from a positive shift in flat-band potential. The obtained extremely positive onset potential, 0.8 VRHE in n-Cu2O/AuAg/p-Cu2O, is comparable with measurements from water reduction catalysts. The AuAg alloy sandwiched between the homojunction of n-Cu2O and p-Cu2O improved the photocatalytic performance. This alloy both served as an electron relay and promoted electron-hole pair generation in nearby semiconductors; the charge transfer between n-Cu2O and p-Cu2O in the sandwich structure was measured with X-ray absorption spectra. The proposed sandwich structure can be considered as a new direction for the design of efficient solar-related devices.

Two-Dimensional Cobalt Phosphate Hydroxide Nanosheets: A New Type of High-Performance Electrocatalysts with Intrinsic CoO6 Lattice Distortion for Water Oxidation.

Despite the recent advances in electrochemical water splitting, developing cost-effective and highly efficient electrocatalysts for oxygen evolution reaction (OER) still remains a substantial challenge. Herein, two-dimensional cobalt phosphate hydroxides (Co5(PO4)2(OH)4) nanosheets, a unique stacking-disordered phosphate-based inorganic material, are successfully prepared via a facile and scalable method for the first time to serve as a superior and robust electrocatalyst for water oxidation. On the basis of the detailed characterization (e.g., X-ray absorption near-edge structure and X-ray photoelectron spectroscopy), the obtained nanosheets consist of special zigzag CoO6 octahedral chains along with intrinsic lattice distortion and excellent hydrophilicity, in which these factors contribute to the highly efficient performance of prepared electrocatalysts for OER. Specifically, Co5(PO4)2(OH)4 deposited on glassy carbon electrode (loading amount ≈0.553 mg cm-2) can exhibit an unprecedented overpotential of 254 mV to drive a current density of 10 mA cm-2 with a small Tafel slope of 57 mV dec-1 in alkaline electrolytes, which outperforms the ones of CO3(PO4)2 (370 mV) and Co(OH)2 (360 mV) as well as other advanced catalysts. Evidently, this work has opened a new pathway to the rational design of promising metal phosphate hydroxides toward the efficient electrochemical energy conversion.

Antimicrobial effects of zero-valent iron nanoparticles on gram-positive Bacillus strains and gram-negative Escherichia coli strains.

BACKGROUND: Zero-valent iron nanoparticles (ZVI NPs) have been used extensively for the remediation of contaminated soil and groundwater. Owing to their large active surface area, they serve as strong and effective reductants. However, the ecotoxicity and bioavailability of ZVI NPs in diverse ecological media have not been evaluated in detail and most studies have focused on non-nano ZVI or Fe0. In addition, the antimicrobial properties of ZVI NPs have rarely been investigated, and the underlying mechanism of their toxicity remains unknown. RESULTS: In the present study, we demonstrate that ZVI NPs exhibited significant toxicity at 1000 ppm against two distinct gram-positive bacterial strains (Bacillus subtilis 3610 and Bacillus thuringiensis 407) but not against two gram-negative strains (Escherichia coli K12 and ATCC11634). Specifically, ZVI NPs caused at least a 4-log and 1-log reductions in cell numbers, respectively, in the two Bacillus strains, whereas no change was detected in the two E. coli strains. X-ray photoelectron spectroscopy, X-ray absorption near-edge, and extended X-ray absorption fine structure spectra confirmed that Bacillus cells exposed to ZVI NPs contained mostly Fe2O3 with some detectable FeS. This finding indicated that Fe0 nanoparticles penetrated the bacterial cells, where they were subsequently oxidized to Fe2O3 and FeS. RedoxSensor analysis and propidium iodide (PI) staining showed decreased reductase activity and increased PI in both Bacillus strains treated with a high (1000 ppm) concentration of ZVI NPs. CONCLUSION: Taken together, these data show that the toxicity of ZVI NPs was derived from their oxidative properties, which may increase the levels of reactive oxygen species and lead to cell death.

The Antimicrobial Properties of Silver Nanoparticles in Bacillus subtilis Are Mediated by Released Ag+ Ions.

The superior antimicrobial properties of silver nanoparticles (Ag NPs) are well-documented, but the exact mechanisms underlying Ag-NP microbial toxicity remain the subject of intense debate. Here, we show that Ag-NP concentrations as low as 10 ppm exert significant toxicity against Bacillus subtilis, a beneficial bacterium ubiquitous in the soil. Growth arrest and chromosomal DNA degradation were observed, and flow cytometric quantification of propidium iodide (PI) staining also revealed that Ag-NP concentrations of 25 ppm and above increased membrane permeability. RedoxSensor content analysis and Phag-GFP expression analysis further indicated that reductase activity and cytosolic protein expression decreased in B. subtilis cells treated with 10-50 ppm of Ag NPs. We conducted X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses to directly clarify the valence and fine structure of Ag atoms in B. subtilis cells placed in contact with Ag NPs. The results confirmed the Ag species in Ag NP-treated B. subtilis cells as Ag2O, indicating that Ag-NP toxicity is likely mediated by released Ag+ ions from Ag NPs, which penetrate bacterial cells and are subsequently oxidized intracellularly to Ag2O. These findings provide conclusive evidence for the role of Ag+ ions in Ag-NP microbial toxicity, and suggest that the impact of inappropriately disposed Ag NPs to soil and water ecosystems may warrant further investigation.

ZnO Nanoparticles Affect Bacillus subtilis Cell Growth and Biofilm Formation.

Zinc oxide nanoparticles (ZnO NPs) are an important antimicrobial additive in many industrial applications. However, mass-produced ZnO NPs are ultimately disposed of in the environment, which can threaten soil-dwelling microorganisms that play important roles in biodegradation, nutrient recycling, plant protection, and ecological balance. This study sought to understand how ZnO NPs affect Bacillus subtilis, a plant-beneficial bacterium ubiquitously found in soil. The impact of ZnO NPs on B. subtilis growth, FtsZ ring formation, cytosolic protein activity, and biofilm formation were assessed, and our results show that B. subtilis growth is inhibited by high concentrations of ZnO NPs (≥ 50 ppm), with cells exhibiting a prolonged lag phase and delayed medial FtsZ ring formation. RedoxSensor and Phag-GFP fluorescence data further show that at ZnO-NP concentrations above 50 ppm, B. subtilis reductase activity, membrane stability, and protein expression all decrease. SDS-PAGE Stains-All staining results and FT-IR data further demonstrate that ZnO NPs negatively affect exopolysaccharide production. Moreover, it was found that B. subtilis biofilm surface structures became smooth under ZnO-NP concentrations of only 5-10 ppm, with concentrations ≤ 25 ppm significantly reducing biofilm formation activity. XANES and EXAFS spectra analysis further confirmed the presence of ZnO in co-cultured B. subtilis cells, which suggests penetration of cell membranes by either ZnO NPs or toxic Zn+ ions from ionized ZnO NPs, the latter of which may be deionized to ZnO within bacterial cells. Together, these results demonstrate that ZnO NPs can affect B. subtilis viability through the inhibition of cell growth, cytosolic protein expression, and biofilm formation, and suggest that future ZnO-NP waste management strategies would do well to mitigate the potential environmental impact engendered by the disposal of these nanoparticles.

Synthesis, characterization, and hydrogen storage enhancement of M2(BDC)2dabco with palladium-doped activated carbon.

Two bi-organic type metal organic frameworks (MOFs) such as Co2(BDC)2dabco and Zn2(BDC)2 dabco have been synthesized by hydrothermal method and characterized along with hydrogen adsorption. The hydrogen adsorption capacity of these MOFs was increased after doping by palladium-activated carbon. Co2(BDC)2dabco has cracked and folded thin film like surface while Zn2(BDC)2dabco has a brick-typed cubic structure with particle size about 10-15 microm identified by FE-SEM. The XRD patterns represents that both MOFs have the well crystalline structure. Nitrogen adsorption isotherms show that both structures have Type I adsorption isotherm with the BET specific surface area of 1,390 and 1,433 m2 g(-1) for Co2(BDC)2dabco and Zn2(BDC)2dabco, respectively. Pristine Co2(BDC)2dabco and Zn2(BDC)2dabco can store about 0.22 and 0.25 wt.% of H2 measured at 298 K and 32 bar. This capacity was greatly enhanced by doping palladium-activated carbon to 0.31 and 0.41 wt.%, respectively. Moreover, both structures were also characterized by XANES/EXAFS. EXAFS spectra indicate that Co2(BDC)2dabco has the Co--O bond distance of 2.030 A with the coordination number of 4.2 while Zn2(BDC)2dabco has 2.015 angstroms bond distance of Zn--O with the coordination number of 3.4.

Preparation, characterization, and hydrogen storage capacity of MIL-53 metal-organic frameworks.

Metal organic frameworks (MOFs) are considered as most promising candidate for hydrogen storage material for practical application. MIL-53(Cr) MOFs were synthesized from Cr(NO3)3 x 9H2O combined with terephthalic acid organic linker. MIL-53(Cr) MOFs are octahedral in shape and the particle size was around 10 microm identified by FE-SEM. The cleaning of the MOFs crystals with different solvents at different warm temperature were found effective and approved to increase the specific surface area and porosity of MIL-53(Cr) MOFs. The XRD patterns represented that MIL-53(Cr) MOFs had well crystalline structures. Nitrogen adsorption isotherms show that Mil-53(Cr) has approximately type-I isotherm with a highest BET specific surface area of 1946 m2 g(-1) after treated with hot methanol. Hydrogen adsorption study shows that this material can store 0.45 wt.% of H2 measured at 303 K and 32 bar. The pre-edge XANES spectra confirm the existence of Cr(III) in crystalline framework of MIL-53(Cr) and the sharp feature at 6007 eV in XANES spectra represents the dipole-allowed electron transition from 1s to 4p(xy). In addition, EXAFS spectra indicate that MIL-53(Cr) metal organic frameworks structure has the Cr-O bond distance of 1.96 angstroms with a coordination number of 5.4.