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.

Solution-Based, Anion-Doping of Li4 Ti5 O12 Nanoflowers for Lithium-Ion Battery Applications.

Solution-based, anionic doping represents a convenient strategy with which to improve upon the conductivity of candidate anode materials such as Li4 Ti5 O12 (LTO). As such, novel synthetic hydrothermally-inspired protocols have primarily been devised herein, aimed at the large-scale production of unique halogen-doped, micron-scale, three-dimensional, hierarchical LTO flower-like motifs. Although fluorine (F) doping has been explored, the use of chlorine (Cl) dopants is the primary focus here. Several experimental variables, such as dopant amount, lithium hydroxide concentration, and titanium butoxide purity, were probed and perfected. Furthermore, the Cl doping process did not damage the intrinsic LTO morphology. The analysis, based on interpreting a compilation of SEM, XRD, XPS, and TEM-EDS results, was used to determine an optimized dopant concentration of Cl. Electrochemical tests demonstrated an increased capacity via cycling of 12 % for a Cl-doped sample as compared with pristine LTO. Moreover, the Cl-doped LTO sample described in this study exhibited the highest discharge capacity yet reported at an observed rate of 2C for this material at 143mAh g-1 . Overall, these data suggest that the Cl dopant likely enhances not only the ion transport capabilities, but also the overall electrical conductivity of our as-prepared structures. To help explain these favorable findings, theoretical DFT calculations were used to postulate that the electronic conductivity and Li diffusion were likely improved by the presence of increased Ti3+ ion concentration coupled with widening of the Li migration channel.

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.

Synthesis, properties, and formation mechanism of Mn-doped Zn2SiO4 nanowires and associated heterostructures.

In this work, we have put forth a facile hydrothermal approach to synthesize an array of one-dimensional (1D) Mn-doped Zn2SiO4 nanostructures. Specifically, we have probed and correlated the effects of controllable reaction parameters such as the pH and Mn dopant concentrations with the resulting crystal structures and morphologies of the products obtained. Based upon our results, we find that careful tuning of the pH versus the Mn dopant level gives rise to opposite trends with respect to the overall size of the resulting one-dimensional nanostructures. Significantly, we have highlighted the role of the Mn dopant ion concentration as a potentially generalizable reaction parameter in solution-based synthesis for controlling morphology and hence, the observed optical behavior. Indeed, such a strategy can be potentially generalized to systems such as but not limited to Mn-doped ZnS, CdS, and CdSe quantum dots (QD), which, to the best of our knowledge, denote promising candidates for a variety of optoelectronic applications. Specifically, we have carefully optimized the synthesis conditions in order to generate a series of chemically well-defined Mn-doped Zn2SiO4 not only possessing Mn concentrations ranging from 3% to 8% but also characterized by highly crystalline, monodisperse wire-like motifs measuring ∼30 nm in diameter and ∼700 nm in length. Optically, the photoluminescence signals associated with the 1D series yielded a volcano-shaped relationship between PL intensities and the Mn dopant level. In additional experiments, we have immobilized CdSe quantum dots (QDs) onto the external surfaces of our as-synthesized Mn-doped Zn2SiO4 nanowires, in order to form novel composite heterostructures. The optical properties of the CdSe QD-Mn:Zn2SiO4 heterostructures have been subsequently examined. Our results have demonstrated the likely co-existence of both energy transfer and charge transfer phenomena between the two constituent components of our as-prepared composites. Specifically, when both components are photoexcited, both energy transfer and charge transfer were found to plausibly occur, albeit in opposite directions. When the CdSe QDs are excited alone for example, charge transfer probably takes place from the CdSe QDs to the dopant Mn2+ ions. We believe that our as-processed heterostructures are therefore promising as a tunable light-harvesting motif. Essentially, these materials have broadened the effective light absorption range for optical 'accessibility', not only through their incorporation of dopant-tunable Zn2SiO4 possessing complementary absorption properties to those of the QDs but also through their integration of CdSe QDs with size-tailorable optical behavior.

Chemical Strategies for Enhancing Activity and Charge Transfer in Ultrathin Pt Nanowires Immobilized onto Nanotube Supports for the Oxygen Reduction Reaction.

Multiwalled carbon nanotubes (MWNTs) represent a promising support medium for electrocatalysts, especially Pt nanoparticles (NPs). The advantages of using MWNTs include their large surface area, high conductivity, as well as long-term stability. Surface functionalization of MWNTs with various terminal groups, such as -COOH, -SH, and -NH2, allows for rational electronic tuning of catalyst-support interactions. However, several issues still need to be addressed for such systems. First, over the course of an electrochemical run, catalyst durability can decrease, due in part to metal NP dissolution, a process facilitated by the inherently high surface defect concentration within the support. Second, the covalent functionalization treatment of MWNTs adopted by most groups tends to lead to a loss of structural integrity of the nanotubes (NTs). To mitigate for all of these issues, we have utilized two different attachment approaches (i.e., covalent versus noncovalent) to functionalize the outer walls of pristine MWNTs and compared the catalytic performance of as-deposited ultrathin (<2 nm) 1D Pt nanowires with that of conventional Pt NPs toward the oxygen reduction reaction (ORR). Our results demonstrated that the electrochemical activity of Pt nanostructures immobilized onto functionalized carbon nanotube (CNT) supports could be dramatically improved by using ultrathin Pt nanowires (instead of NPs) with noncovalently (as opposed to covalently) functionalized CNT supports. Spectroscopic evidence corroborated the definitive presence of charge transfer between the metal catalysts and the underlying NT support, whose direction and magnitude are a direct function of (i) the terminal chemistry as well as (ii) the attachment methodology, both of which simultaneously impact upon the observed electrocatalytic performance. Specifically, the use of a noncovalent π-π stacking method coupled with a -COOH terminal moiety yielded the highest performance results, reported to date, for any similar system consisting of Pt (commercial NPs or otherwise) deposited onto carbon-based supports, a finding of broader interest toward the fabrication of high-performing electrocatalysts in general.

Probing charge transfer in a novel class of luminescent perovskite-based heterostructures composed of quantum dots bound to RE-activated CaTiO3 phosphors.

We report on the synthesis and structural characterization of novel semiconducting heterostructures composed of cadmium selenide (CdSe) quantum dots (QDs) attached onto the surfaces of novel high-surface area, porous rare-earth-ion doped alkaline earth titanate micron-scale spherical motifs, i.e. both Eu-doped and Pr-doped CaTiO3, composed of constituent, component nanoparticles. These unique metal oxide perovskite building blocks were created by a multi-pronged synthetic strategy involving molten salt and hydrothermal protocols. Subsequently, optical characterization of these heterostructures indicated a clear behavioral dependence of charge transfer in these systems upon a number of parameters such as the nature of the dopant, the reaction temperature, and particle size. Specifically, 2.7 nm diameter ligand-functionalized CdSe QDs were anchored onto sub-micron sized CaTiO3-based spherical assemblies, prepared by molten salt protocols. We found that both the Pr- and Eu-doped CaTiO3 displayed pronounced PL emissions, with maximum intensities observed using optimized lanthanide concentrations of 0.2 mol% and 6 mol%, respectively. Analogous experiments were performed on Eu-doped BaTiO3 and SrTiO3 motifs, but CaTiO3 still performed as the most effective host material amongst the three perovskite systems tested. Moreover, the ligand-capped CdSe QD-doped CaTiO3 heterostructures exhibited effective charge transfer between the two individual constituent nanoscale components, an assertion corroborated by the corresponding quenching of their measured PL signals.

Enhanced Performance of "Flower-like" Li4Ti5O12 Motifs as Anode Materials for High-Rate Lithium-Ion Batteries.

"Flower-like" motifs of Li4Ti5O12 were synthesized by using a facile and large-scale hydrothermal process involving unique Ti foil precursors followed by a short, relatively low-temperature calcination in air. Moreover, a detailed time-dependent growth mechanism and a reasonable reaction scheme were proposed to clearly illustrate and highlight the structural evolution and subsequent formation of this material. Specifically, the resulting "flower-like" Li4Ti5O12 microspheres consisting of thin nanosheets provide for an enhanced surface area and a reduced lithium-ion diffusion distance. The high surface areas of the exposed roughened, thin petal-like component nanosheets are beneficial for the interaction of the electrolyte with Li4Ti5O12 , which thereby ultimately provides for improved high-rate performance and favorable charge/discharge dynamics. Electrochemical studies of the as-prepared nanostructured Li4Ti5O12 clearly revealed their promising potential as an enhanced anode material for lithium-ion batteries, as they present both excellent rate capabilities (delivering 148, 141, 137, 123, and 60 mAh g(-1) under discharge rates of 0.2, 10, 20, 50, and 100 C, at cycles of 50, 55, 60, 65, and 70, respectively) and stable cycling performance (exhibiting a capacity retention of ≈97 % from cycles 10-100, under a discharge rate of 0.2 C, and an impressive capacity retention of ≈87 % by using a more rigorous discharge rate of 20 C from cycles 101-300).

Host-guest recognition-induced color change of water-soluble pillar[5]arene modified silver nanoparticles for visual detection of spermine analogues.

Silver nanoparticles with narrow size distribution were prepared in the presence of a water-soluble pillar[5]arene. They could be used for visual detection of spermine analogues in water through host-guest interactions.