Mixed Network Former Effect in Ion-Conducting Alkali Borophosphate Glasses: Structure/Property Correlations in the System [M2O]1/3[(B2O3)x(P2O5)1–x]2/3 (M = Li, K, Cs)

Glasses in the system [M2O]1/3[(B2O3)x(P2O5)1–x]2/3(M = Li, K, Cs) (0.0 ≤ x ≤ 1.0) were prepared by standard melt-quenching procedures, and their physical properties were characterized by thermal analysis, density measurements, and impedance spectroscopy. Their atomic level structures were comprehensively characterized by Raman spectroscopy, by X-ray photoelectron spectroscopy (XPS), and by 11B, 31P, and 7Li as well as 133Cs high resolution solid state magic-angle-spinning (MAS) NMR techniques. 31P MAS NMR peak assignments were aided by the presence or absence of homonuclear indirect 31P–31P spin–spin interactions, “J-coupling”, as detected via refocused INADEQUATE techniques. Consistent speciations of the phosphate and borate network former components in terms of the various PnmB and BnmPunits, where n is the number of bridging oxygens (BOs) and m is the number of B or P units bonded to P or B, respectively, present in these glasses were derived from 11B MAS NMR, combined with both 31P MAS NMR and XPS line shape analyses, constrained by charge and mass balance considerations. The speciation of the BO species in the glassy network was quantified both by O 1s XPS and 11B{31P} rotational echo double resonance spectroscopy. Both experiments indicate a strong preference of heteroatomic B–O–P over homoatomic P–O–P and B–O–B linkages to the extent that close to the maximum number of possible B4–O–P linkages is formed. Further, the structural speciations of the borate and phosphate species, together with bond valence (BV) analyses of the charge redistribution on the various structural units, indicate that the alkali network modifier oxide is not proportionally shared between the two network former components B and P in these systems. Rather, the amounts and types of the various borate and phosphate species are found to be consistent with the negative charge brought in by the alkali modifier M2O being distributed more toward the phosphate structural units which are suggested to attract a larger concentration of network modifier species than predicted by the bulk composition. The experimental results obtained from these studies help in understanding the strongly nonlinear compositional dependence of the glass transition temperature and the ionic conductivity in terms of detailed atomic-level structural information. The emerging structural principles appear to be general to all of the alkali borophosphate glass systems, with the type of alkali ion network modifier producing only minor variations

Cu2ZnSnS4−Au Heterostructures: Toward Greener Chalcogenide- Based Photocatalysts

Chalcogenide-based semiconductor-metal heterostructures are interesting catalysts for solar-to-chemical energy conversion, but current compositions are impractical due to the relative toxicity and/or scarcity of their constituent elements. To address these concerns, Cu2ZnSnS4 (CZTS) emerged as an interesting alternative to other chalcogenide-based semiconductors; however, the fabrication of CZTS metal heterostructures remains unexplored. In this paper, we systematically explore four methods of synthesizing CZTS-Au heterostructures, specifically: reaction of CZTS nanorods with either a soluble molecular gold precursor (AuCl3) or preformed gold (Au) nanoparticles, each under thermal (heating in the dark) or photochemical reaction conditions (350 nm lamp illumination at room temperature). We find that using AuCl3 under thermal deposition conditions results in the most well-defined CZTS-Au heterostructures, containing \u3e99% surface-bound 2.1 ± 0.5 nm Au islands along the whole length of the nanorod. These CZTS-Au heterostructures are photocatalytically active, reducing the model compound methylene blue upon irradiation much more effectively than bare CZTS nanorods. We also demonstrate the removal of Au from the CZTS-Au heterostructures by amalgamation. These results open up a new area of greener, CZTS-based photocatalysts for solar-to-chemical energy conversion

Templated Synthesis and Chemical Behavior of Nickel Nanoparticles within High Aspect Ratio Silica Capsules

One-dimensional transition metal nanostructures are of interest in many magnetic and catalytic applications. Using a combination of wet chemical synthesis, optical (infrared), and structural characterization methods (powder X-ray diffraction, scanning and transmission electron microscopy), we have investigated four paths to access 1D nickel nanostructures: (1) direct chemical reduction of a self-assembled nickel-hydrazine coordination complex, (2) thermal decomposition of the silica encapsulated nickel-hydrazine complex, (3) treatment of the silica encapsulated nickel-hydrazine complex with sodium borohydride followed by thermal annealing, and (4) electroless nickel plating using silica encapsulated nickel seed particles. We find that only route 1, which does not require a silica template, results in the formation of nickel nanorods, albeit some particle aggregation is observed. Routes 2 and 3 result in the formation of isotropic nickel structures under a reducing atmosphere. Route 4 results in heterogeneous nucleation and growth of existing particles only when partial etching of the silica capsule occurs. Detailed examination of the encapsulated nickel particles allows studying the effect of silica surface silanols on the oxidation of encapsulated nickel particles, the presence of nanoparticle-silica support interactions, the sintering mechanism of nickel and nickel oxide particles, and the fate of boride impurities. Nickel/silica nanostructures are strongly magnetic at room temperature

Large Scale Nanoparticle Screening for Small Molecule Analysis in Laser Desorption Ionization Mass Spectrometry

Nanoparticles (NPs) have been suggested as efficient matrixes for small molecule profiling and imaging by laser-desorption ionization mass spectrometry (LDI-MS), but so far there has been no systematic study comparing different NPs in the analysis of various classes of small molecules. Here, we present a large scale screening of 13 NPs for the analysis of two dozen small metabolite molecules. Many NPs showed much higher LDI efficiency than organic matrixes in positive mode and some NPs showed comparable efficiencies for selected analytes in negative mode. Our results suggest that a thermally driven desorption process is a key factor for metal oxide NPs, but chemical interactions are also very important, especially for other NPs. The screening results provide a useful guideline for the selection of NPs in the LDI-MS analysis of small molecules

Mixed network former effect in ion-conducting Alkali borophosphate glasses: structure/property correlations in the system '['M IND. 2'O] IND. 1/3''['('B IND. 2''O IND. 3') IND. x''('P IND. 2''O IND. 5') IND. 1-x'] IND. 2/3' (M = 'LI', K, 'CS')

Glasses in the system '['M IND. 2'O] IND. 1/3''['('B IND. 2''O IND. 3') IND. x''('P IND. 2''O IND. 5') IND. 1-x'] IND. 2/3' (M = 'LI', K, 'CS') (0.0 'MENOR OU IGUAL' x 'MENOR OU IGUAL' 1.0) were prepared by standard melt-quenching procedures, and their physical properties were characterized by thermal analysis, density measurements, and impedance spectroscopy. Their atomic level structures were comprehensively characterized by Raman spectroscopy, by X-ray photoelectron spectroscopy (XPS), and by ' ANTPOT. 11 B', 'ANTPOT. 31 P', and 'ANTPOT. 7 LI' as well as 'ANTPOT. 133 CS' high resolution solid state magic-angle-spinning (MAS) NMR techniques. 'ANTPOT. 31 P' MAS NMR peak assignments were aided by the presence or absence of homonuclear indirect 'ANTPOT. 31 P'-'ANTPOT. 31 P' spin–spin interactions, "J-coupling", as detected via refocused INADEQUATE techniques. Consistent speciations of the phosphate and borate network former components in terms of the various 'P POT. n IND. mB' and 'B POT.n IND. mP' units, where n is the number of bridging oxygens (BOs) and m is the number of B or P units bonded to P or B, respectively, present in these glasses were derived from 'ANTPOT. 11 B' MAS NMR, combined with both 'ANTPOT. 31 P' MAS NMR and XPS line shape analyses, constrained by charge and mass balance considerations. The speciation of the BO species in the glassy network was quantified both by O 1s XPS and 'ANTPOT. 11 B'{'ANTPOT. 31 P'} rotational echo double resonance spectroscopy. Both experiments indicate a strong preference of heteroatomic B-O-P over homoatomic P-O-P and B-O-B linkages to the extent that close to the maximum number of possible 'B POT. 4'-O-P linkages is formed. Further, the structural speciations of the borate and phosphate species, together with bond valence (BV) analyses of the charge redistribution on the various structural units, indicate that the alkali network modifier oxide is not proportionally shared between the two network former components B and P in these systems. Rather, the amounts and types of the various borate and phosphate species are found to be consistent with the negative charge brought in by the alkali modifier 'M IND. 2'O being distributed more toward the phosphate structural units which are suggested to attract a larger concentration of network modifier species than predicted by the bulk composition. The experimental results obtained from these studies help in understanding the strongly nonlinear compositional dependence of the glass transition temperature and the ionic conductivity in terms of detailed atomic-level structural information. The emerging structural principles appear to be general to all of the alkali borophosphate glass systems, with the type of alkali ion network modifier producing only minor variations.Deutsche Forschungsgemeinschaft (SFB 458; Ionic Motion in Disordered Materials)National Science Foundation (DMR, Materials World Network NSFDMR 0701564

Preparation and Instability of Nanocrystalline Cuprous Nitride

Low-dimensional cuprous nitride (Cu3N) was synthesized by nitridation (ammonolysis) of cuprous oxide (Cu2O) nanocrystals using either ammonia (NH3) or urea (H2NCONH2) as the nitrogen source. The resulting nanocrystalline Cu3N spontaneously decomposes to nanocrystalline CuO in the presence of both water and oxygen from air at room temperature. Ammonia was produced in 60% chemical yield during Cu3N decomposition, as measured using the colorimetric indophenol method. Because Cu3N decomposition requires H2O and produces substoichiometric amounts of NH3\u3e, we conclude that this reaction proceeds through a complex stoichiometry that involves the concomitant release of both N2 and NH3. This is a thermodynamically unfavorable outcome, strongly indicating that H2O (and thus NH3 production) facilitate the kinetics of the reaction by lowering the energy barrier for Cu3N decomposition. The three different Cu2O, Cu3N, and CuO nanocrystalline phases were characterized by a combination of optical absorption, powder X-ray diffraction, transmission electron microscopy, and electronic density of states obtained from electronic structure calculations on the bulk solids. The relative ease of interconversion between these interesting and inexpensive materials bears possible implications for catalytic and optoelectronic applications

Search for new particles in events with energetic jets and large missing transverse momentum in proton-proton collisions at root s=13 TeV

A search is presented for new particles produced at the LHC in proton-proton collisions at root s = 13 TeV, using events with energetic jets and large missing transverse momentum. The analysis is based on a data sample corresponding to an integrated luminosity of 101 fb(-1), collected in 2017-2018 with the CMS detector. Machine learning techniques are used to define separate categories for events with narrow jets from initial-state radiation and events with large-radius jets consistent with a hadronic decay of a W or Z boson. A statistical combination is made with an earlier search based on a data sample of 36 fb(-1), collected in 2016. No significant excess of events is observed with respect to the standard model background expectation determined from control samples in data. The results are interpreted in terms of limits on the branching fraction of an invisible decay of the Higgs boson, as well as constraints on simplified models of dark matter, on first-generation scalar leptoquarks decaying to quarks and neutrinos, and on models with large extra dimensions. Several of the new limits, specifically for spin-1 dark matter mediators, pseudoscalar mediators, colored mediators, and leptoquarks, are the most restrictive to date.Peer reviewe

Probing effective field theory operators in the associated production of top quarks with a Z boson in multilepton final states at root s=13 TeV

Peer reviewe

Chemical and thermal stability of semiconductors

Utilization of solar energy continues to be a prominent topic in our society. The chemical and thermal stability of semiconductors is of great interest due to their widespread appeal and applicability as photoactive materials and in energy conversion devices. However, aside from surface chemistry studies and photo degradation (under continuous illumination), the general chemical reactivity and thermal stability of these materials is poorly understood. In this thesis, we use CdSe and CdS nanorods as model systems to investigate the behavior of II-VI semiconductor nanorods against various conditions of “extreme” chemical and physical stress (acids, bases, oxidants, reductants, heat). We demonstrate CdSe nanorods completely degrade in the presence of acids, but retain their structural and optical properties when subjected to basic or oxidative environments. Reductants, such as n-butyllithium, reduce CdSe to cadmium metal but hydrogen does not. Thermal treatment of both CdSe and CdS nanorods results in annealing, axial melting, and then coalescence of the particles. Axial melting did not depend on the type of inorganic material but on the ligand(s) coating the nanorod surface. We also explore the synthesis of copper nitride (Cu3N) nanocrystals by nitridation of Cu2O nanocrystals with either ammonia or urea. We characterize the structure, optical properties, and morphology of both oxide and nitride phases using structural, optical and computational methods. Upon exposure to moisture and air, Cu3N decomposes to CuO. Simple thermodynamics calculations explain that the observed concomitant release of NH3 and N2 is due to kinetic factors. Finally, we investigate the thermal stability of bulk methylammonium lead halide perovskites (CH3NH3PbX3 where X = I, Br, or Cl) to determine the decomposition products. Using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) coupled with a quadrupole mass spectrometer (QMS) and Fourier-Transform infrared (FT-IR) spectrometer, the decomposition temperature and evolved gas products are analyzed. Bulk iodide and bromide decompose by a similar route compared to that of the bulk chloride. For the iodide and bromide, ammonia and methyl halide evolve but the bromide evolves methylamine too. The chloride acts similar to reports in literature with evolution of methylamine and HCl. However, peaks for HCl overlap with the CH3 stretching of methylamine making it difficult to distinguish. Using this data and decomposition pathways, a way to achieve long-term thermal stability may be determined for methylammonium lead halide perovskites for solar cells.</p