69 research outputs found
Crystal growth and structural analysis of perovskite chalcogenide BaZrS and Ruddlesden-Popper phase BaZrS
Perovskite chalcogenides are gaining substantial interest as an emerging
class of semiconductors for optoelectronic applications. High quality samples
are of vital importance to examine their inherent physical properties. We
report the successful crystal growth of the model system, BaZrS and its
Ruddlesden-Popper phase BaZrS by flux method. X-ray diffraction
analyses showed space group of with lattice constants of = 7.056(3)
\AA\/, = 9.962(4) \AA\/, = 6.996(3) \AA\/ for BaZrS and
with = 7.071(2) \AA\/, = 7.071(2) \AA\/, = 25.418(5) \AA\/ for
BaZrS. Rocking curves with full-width-at-half-maximum of
0.011 for BaZrS and 0.027 for BaZrS were
observed. Pole figure analysis, scanning transmission electron microscopy
images and electron diffraction patterns also establish high quality of grown
crystals. The octahedra tilting in the corner-sharing octahedra network are
analyzed by extracting the torsion angles.Comment: 4 Figures, 2 Table
Pendant Hydrogen-Bond Donors in Cobalt Catalysts Independently Enhance CO_2 Reduction
The bioinspired incorporation of pendant proton donors into transition metal catalysts is a promising strategy for converting environmentally deleterious CO_2 to higher energy products. However, the mechanism of proton transfer in these systems is poorly understood. Herein, we present a series of cobalt complexes with varying pendant secondary and tertiary amines in the ligand framework with the aim of disentangling the roles of the first and second coordination spheres in CO_2 reduction catalysis. Electrochemical and kinetic studies indicate that the rate of catalysis shows a first-order dependence on acid, CO_2, and the number of pendant secondary amines, respectively. Density functional theory studies explain the experimentally observed trends and indicate that pendant secondary amines do not directly transfer protons to CO_2, but instead bind acid molecules from solution. Taken together, these results suggest a mechanism in which noncooperative pendant amines facilitate a hydrogen-bonding network that enables direct proton transfer from acid to the activated CO_2 substrate
Ideal Bandgap in a 2D Ruddlesden-Popper Perovskite Chalcogenide for Single-junction Solar Cells
Transition metal perovskite chalcogenides (TMPCs) are explored as stable,
environmentally friendly semiconductors for solar energy conversion. They can
be viewed as the inorganic alternatives to hybrid halide perovskites, and
chalcogenide counterparts of perovskite oxides with desirable optoelectronic
properties in the visible and infrared part of the electromagnetic spectrum.
Past theoretical studies have predicted large absorption coefficient, desirable
defect characteristics, and bulk photovoltaic effect in TMPCs. Despite recent
progresses in polycrystalline synthesis and measurements of their optical
properties, it is necessary to grow these materials in high crystalline quality
to develop a fundamental understanding of their optical properties and evaluate
their suitability for photovoltaic application. Here, we report the growth of
single crystals of a two-dimensional (2D) perovskite chalcogenide, Ba3Zr2S7,
with a natural superlattice-like structure of alternating double-layer
perovskite blocks and single-layer rock salt structure. The material
demonstrated a bright photoluminescence peak at 1.28 eV with a large external
luminescence efficiency of up to 0.15%. We performed time-resolved
photoluminescence spectroscopy on these crystals and obtained an effective
recombination time of ~65 ns. These results clearly show that 2D
Ruddlesden-Popper phases of perovskite chalcogenides are promising materials to
achieve single-junction solar cells.Comment: 4 Figure
Electronically Modified Cobalt Aminopyridine Complexes Reveal an Orthogonal Axis for Catalytic Optimization for CO₂ Reduction
The design of effective electrocatalysts for carbon dioxide reduction requires understanding the mechanistic underpinnings governing the binding, reduction, and protonation of CO₂. A critical aspect to understanding and tuning these factors for optimal catalysis revolves around controlling the electronic environments of the primary and secondary coordination sphere. Herein we report a series of para-substituted cobalt aminopyridine macrocyclic catalysts 2–4 capable of carrying out the electrochemical reduction of CO₂ to CO. Under catalytic conditions, complexes 2–4, as well as the unsubstituted cobalt aminopyridine complex 1, exhibit i_(cat)/i_p values ranging from 144 to 781. Complexes 2 and 4 exhibit a pronounced precatalytic wave suggestive of an ECEC mechanism. A Hammett analysis reveals that ligand modifications with electron-donating groups enhance catalysis (ρ < 0), indicative of positive charge buildup in the transition state. This trend also extends to the Co^(I/0) potential, where complexes possessing more negative E(CoI/0) reductions exhibit greater i_(cat)/i_p values. The reported modifications offer a synthetic lever to tune catalytic activity, orthogonal to our previous study of the role of pendant hydrogen bond donors
Proton-Assisted Reduction of CO_2 by Cobalt Aminopyridine Macrocycles
We report here the efficient reduction of CO_2 to CO by cobalt aminopyridine macrocycles. The effect of the pendant amines on catalysis was investigated. Several cobalt complexes based on the azacalix[4](2,6)pyridine framework with different substitutions on the pendant amine groups have been synthesized (R = H (1), Me (2), and allyl (3)), and their electrocatalytic properties were explored. Under an atmosphere of CO_2 and in the presence of weak Brønsted acids, large catalytic currents are observed for 1, corresponding to the reduction of CO_2 to CO with excellent Faradaic efficiency (98 ± 2%). In comparison, complexes 2 and 3 generate CO with TONs at least 300 times lower than 1, suggesting that the presence of the pendant NH moiety of the secondary amine is crucial for catalysis. Moreover, the presence of NH groups leads to a positive shift in the reduction potential of the Co^(I/0) couple, therefore decreasing the overpotential for CO_2 reduction
Pendant Hydrogen-Bond Donors in Cobalt Catalysts Independently Enhance CO_2 Reduction
The bioinspired incorporation of pendant proton donors into transition metal catalysts is a promising strategy for converting environmentally deleterious CO_2 to higher energy products. However, the mechanism of proton transfer in these systems is poorly understood. Herein, we present a series of cobalt complexes with varying pendant secondary and tertiary amines in the ligand framework with the aim of disentangling the roles of the first and second coordination spheres in CO_2 reduction catalysis. Electrochemical and kinetic studies indicate that the rate of catalysis shows a first-order dependence on acid, CO_2, and the number of pendant secondary amines, respectively. Density functional theory studies explain the experimentally observed trends and indicate that pendant secondary amines do not directly transfer protons to CO_2, but instead bind acid molecules from solution. Taken together, these results suggest a mechanism in which noncooperative pendant amines facilitate a hydrogen-bonding network that enables direct proton transfer from acid to the activated CO_2 substrate
Understanding the role of crystallographic shear on the electrochemical behavior of niobium oxyfluorides
The effects of shear planes in perovskite materials have been studied in order to identify their role in the electrochemical behavior of Li⁺ intercalation hosts. These planes modulate the structural stability and ionic transport pathways and therefore play an intimate role in the characteristics and performance of shear compounds. Herein, two Nb-based compounds, NbO₂F and Nb₃O₇F, were chosen as representative perovskite and shear derivatives respectively to investigate the role of crystallographic shear. A series of operando measurements, including X-ray diffraction and X-ray absorption spectroscopy, in conjunction with structural analysis, Raman spectroscopy, and detailed electrochemical studies identified the effect of shear planes. It was found that shear planes led to increased structural stability during Li⁺ (de)intercalation with shear layers being maintained, while perovskite layers were seen to degrade rapidly. However, disordering in the shear plane stacking introduced during delithiation ultimately led to poor capacity retention despite structural maintenance as Li⁺ diffusion channels are disrupted
Control of Emission Color with N-Heterocyclic Carbene (NHC) Ligands in Phosphorescent Three-Coordinate Cu(I) Complexes
A series of three phosphorescent mononuclear (NHC)–Cu(I) complexes were prepared and characterized. Photophysical properties were found to be largely controlled by the NHC ligand chromophore. Variation of the NHC ligand leads to emission colour tuning over 200 nm range from blue to red, and emission efficiencies of 0.16–0.80 in the solid state
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