A combined TEM and SAXS study of the growth and self-assembly of ultrathin Pt nanowires.

Ultrathin Pt nanowires possess high activity for various electrocatalytic applications. However, little work has focused on understanding their growth mechanisms. Herein, we utilize a combination of time-dependent,ex situtransmission electron microscopy (TEM) and small angle x-ray scattering (SAXS) techniques to observe the growth process in addition to associated surfactant-based interactions. TEM images indicate that initially nanoparticles are formed within 30 s; these small 'seed' particles quickly elongate to form ultrathin nanowires after 2 min. These motifs remain relatively unchanged in size and shape up to 480 min of reaction. Complementary SAXS data suggests that the initial nanoparticles, which are coated by a surfactant bilayer, arrange into abccsuperlattice. With increasing reaction time, thebcclattice disappears as the nanoparticles grow into nanowires, which then self-assemble into a columnar hexagonal structure in which the individual nanowires are covered by a CTAB monolayer. The hexagonal structure eventually degrades, thereby leading to the formation of lamellar stacking phases comprised of surfactant bilayers. To the best of our knowledge, this is the first time that SAXS has been used to monitor the growth and self-assembly of Pt nanowires. These insights can be used to better understand and rationally control the formation of anisotropic motifs of other metallic nanostructures.

Yttrium-based Double Perovskite Nanorods for Electrocatalysis.

Herein, we investigate the effect of the chemical composition of double perovskite nanorods on their versatile electrocatalytic activity not only as supports for the oxidation of small organic molecules but also as catalysts for the oxygen evolution reaction. Specifically, Y2CoMnO6 and Y2NiMnO6 nanorods with average diameters of 300 nm were prepared by a two-step hydrothermal method, in which the individual effects of synthetic parameters, such as the pH, annealing temperature, and precursor ratios on both the composition and morphology, were systematically investigated. When used as supports for Pt nanoparticles, Y2CoMnO6/Pt catalysts exhibited an electrocatalytic activity for the methanol oxidation reaction, which is 2.1 and 1.3 times higher than that measured for commercial Pt/C and Y2NiMnO6/Pt, respectively. Similarly, the Co-based catalyst support material displayed an ethanol oxidation activity, which is 2.3 times higher than both Pt/C and Y2NiMnO6/Pt. This clear enhancement in the activity for Y2CoMnO6 can largely be attributed to strong metal-support interactions, as evidenced by a downshift in the binding energy of the Pt 4f bands, measured by X-ray photoelectron spectroscopy (XPS), which is often correlated not only with a downshift in the d-band center but also to a decreased adsorption of poisoning adsorbates. Moreover, when used as catalysts for the oxygen evolution reaction, Y2CoMnO6 displayed a much greater activity as compared with Y2NiMnO6. This behavior can largely be attributed not only to a preponderance of comparatively more favorable oxidation states and electronic configurations but also to the formation of an active layer on the surface of the Y2CoMnO6 catalyst, which collectively gives rise to improved performance metrics and greater stability as compared with both IrO2 and Y2NiMnO6. Overall, these results highlight the importance of both the chemical composition and the electronic structure of double perovskites, especially when utilized in multifunctional roles as either supports or catalysts.

Probing the Physicochemical Behavior of Variously Doped Li4Ti5O12 Nanoflowers.

This study thoroughly investigated the synthesis of not only 4 triply-doped metal oxides but also 5 singly-doped analogues of Li4Ti5O12 for electrochemical applications. In terms of synthetic novelty, the triply-doped materials were fabricated using a relatively facile hydrothermal method for the first-time, involving the simultaneous substitution of Ca for the Li site, Ln (i.e., Dy, Y, or Gd) for the Ti site, and Cl for the O site. Based on XRD, SEM, and HRTEM-EDS measurements, the resulting materials, incorporating a relatively homogeneous and uniform dispersion of both the single and triple dopants, exhibited a micron-scale flower-like morphology that remained apparently undamaged by the doping process. Crucially, the surface chemistry of all of the samples was probed using XPS in order to analyze any nuanced changes associated with either the various different lanthanide dopants or the identity of the metal precursor types involved. In the latter case, it was observed that the use of a nitrate salt precursor versus that of a chloride salt enabled not only a higher lanthanide incorporation but also the potential for favorable N-doping, all of which promoted a concomitant increase in conductivity due to a perceptible increase in Ti3+ content. In terms of the choice of lanthanide system, it was observed via CV analysis that dopant incorporation generally (albeit with some notable exceptions, especially with Y-based materials) led to the formation of higher amounts of Ti3+ species within both the singly and triply-doped materials, which consequentially led to the potential for increased diffusivity and higher mobility of Li+ species with the possibility for enabling greater capacity within these classes of metal oxides.

Assessing the Catalytic Behavior of Platinum Group Metal-Based Ultrathin Nanowires Using X-ray Absorption Spectroscopy.

Ultrathin metal-based nanowires have excelled as electrocatalysts in small-molecule reactions, such as the oxygen reduction reaction (ORR), the methanol oxidation reaction (MOR), and the ethanol oxidation reaction (EOR), and have consistently outperformed analogous Pt/C standards. As such, a detailed understanding of the structural and electronic properties of ultrathin nanowires is essential in terms of understanding structure-property correlations, which are crucial in the rational design of ever more sophisticated electrocatalysts. X-ray absorption spectroscopy (XAS) represents an important and promising characterization technique with which to acquire unique insights into the electronic structure and the local atomic structure of nanomaterials. Herein, we discuss tangible examples of how both ex situ and in situ XAS experiments have been recently applied to probing the complex behavior of ultrathin nanowires used in electrocatalysis. Moreover, based on this precedence, we provide ideas about the future potential and direction of these ongoing efforts.

Reconciling structure prediction of alloyed, ultrathin nanowires with spectroscopy.

A number of complementary, synergistic advances are reported herein. First, we describe the 'first-time' synthesis of ultrathin Ru2Co1 nanowires (NWs) possessing average diameters of 2.3 ± 0.5 nm using a modified surfactant-mediated protocol. Second, we utilize a combination of quantitative EDS, EDS mapping (along with accompanying line-scan profiles), and EXAFS spectroscopy results to probe the local atomic structure of not only novel Ru2Co1 NWs but also 'control' samples of analogous ultrathin Ru1Pt1, Au1Ag1, Pd1Pt1, and Pd1Pt9 NWs. We demonstrate that ultrathin NWs possess an atomic-level geometry that is fundamentally dependent upon their intrinsic chemical composition. In the case of the PdPt NW series, EDS mapping data are consistent with the formation of a homogeneous alloy, a finding further corroborated by EXAFS analysis. By contrast, EXAFS analysis results for both Ru1Pt1 and Ru2Co1 imply the generation of homophilic structures in which there is a strong tendency for the clustering of 'like' atoms; associated EDS results for Ru1Pt1 convey the same conclusion, namely the production of a heterogeneous structure. Conversely, EDS mapping data for Ru2Co1 suggests a uniform distribution of both elements. In the singular case of Au1Ag1, EDS mapping results are suggestive of a homogeneous alloy, whereas EXAFS analysis pointed to Ag segregation at the surface and an Au-rich core, within the context of a core-shell structure. These cumulative outcomes indicate that only a combined consideration of both EDS and EXAFS results can provide for an accurate representation of the local atomic structure of ultrathin NW motifs.

Optimized Microwave-Based Synthesis of Thermally Stable Inverse Catalytic Core-shell Motifs for CO2 Hydrogenation.

The rational synthesis of Cu@TiO2 core@shell nanowire (NW) structures was thoroughly explored using a microwave-assisted method through the tuning of experimental parameters such as but not limited to (i) controlled variation in molar ratios, (ii) the effect of discrete Ti precursors, (iii) the method of addition of the precursors themselves, and (iv) time of irradiation. Uniform coatings were obtained using Cu/Ti molar ratios of 1:2, 1:1, 2:1, and 4:1, respectively. It should be noted that although relative molar precursor concentrations primarily determined the magnitude of the resulting shell size, the dependence was nonlinear. Moreover, additionally important reaction parameters, such as precursor identity, the means of addition of precursors, and the reaction time, were individually explored with the objective of creating a series of optimized reaction conditions. As compared with Cu NWs alone, it is evident that both of the Cu@TiO2 core-shell NW samples, regardless of pretreatment conditions, evinced much better catalytic performance, up to as much as 20 times greater activity as compared with standard Cu NWs. These results imply the significance of the Cu/TiO2 interface in terms of promoting CO2 hydrogenation, because TiO2 alone is known to be inert for this reaction. Furthermore, it is additionally notable that the N2 annealing pretreatment is crucial in terms of preserving the overall Cu@TiO2 core@shell structure. We also systematically analyzed and tracked the structural and chemical evolution of our catalysts before and after the CO2 reduction experiments. Indeed, we discovered that the core@shell wire motif was essentially maintained and conserved after this high-temperature reaction process, thereby accentuating the thermal stability and physical robustness of our as-prepared hierarchical motifs.

Synthesis, Characterization, and Stability Studies of Ge-Based Perovskites of Controllable Mixed Cation Composition, Produced with an Ambient Surfactant-Free Approach.

In this report, we have applied a facile, ligand-free, ambient synthesis protocol toward the fabrication of not only a series of lead-free Ge-based perovskites with the general formulation of MA1-x FA x GeI3 (where x was changed from 0, 0.25, 0.5, 0.75, to 1) but also CsGeI3. Specifically, our methodology for producing ABX3 systems is generalizable, regardless of the identity of either the A site cation or the X site halide ion. Moreover, it incorporates many advantages, including (i) the possibility of efficiently generating pure Ge-based perovskite particles of any desired chemical composition, (ii) the use of readily available, commercial precursors and comparatively lower toxicity solvents, (iii) the practicality of scale up, and (iv) the elimination of the need for any superfluous organic surface ligands or surfactants. In addition to providing mechanistic insights into their formation, we have examined the chemical composition, crystallite size, morphology, surface attributes, oxidation states, and optical properties of our as-prepared perovskites using a combination of diffraction, microscopy, and spectroscopy techniques. Specifically, we noted that the optical band gap could be reliably tuned as a function of chemical composition, via the identity of the A site cation. Moreover, we have probed their stability, not only under standard storage conditions but also, for the first time, when subjected to both e-beam- and X-ray-induced degradation, using cumulative data from sources such as synchrotron-based scanning hard X-ray microscopy. Importantly, of relevance for the potential practical incorporation of these Pb-free perovskites, our work has emphasized the possibility of controlling the chemical composition within Ge-based perovskites as a means of rationally tuning their observed band gaps and optical behavior.

Crystal structure of the co-crystal of 5-amino-isophthalic acid and 1,2-bis(pyridin-4-yl)ethene.

In the title 1:1 co-crystal, C12H10N2·C8H7NO4, the bi-pyridine moiety shows whole-mol-ecule disorder over two sets of sites in a 0.588 (3): 0.412 (3) ratio. In the crystal, the components form hydrogen-bonded sheets linked by N-H⋯O and O-H⋯N inter-actions, which stack along the a axis. A comparison to a related and previously published co-crystal of 5-amino-isophthalic acid and the shorter 4,4'-bipryidine is presented.