Enhanced Light Emission through Symmetry Engineering of Halide Perovskites.

Metal-halide perovskites (MHPs) have attracted tremendous attention as active materials in optoelectronic devices. For light-emitting diode (LED) applications, nanostructuring of MHPs is considered to be inevitable, but its light-enhancement mechanism is still elusive because the particle (or grain) size is often beyond the quantum confinement regime. As motivated by the experimental finding that the nanostructuring can change the preferred crystalline symmetry from the orthorhombic phase to the high-symmetric cubic phase, we here investigated the carrier dynamics in various polymorphic phases of CsPbBr3 using ab initio quantum dynamics simulation. We found that the cubic phase shows a smaller inelastic phonon scattering than the orthorhombic phase; the suppression of the octahedral tilt minimizes the longitudinal Br fluctuation and helps disentangle the A-site cation dynamics from the nonadiabatic carrier dynamics. We thus anticipate that our present work will offer a material design principle to enhance the quantum yield of MHPs via symmetry engineering, which will help develop highly luminescent LED technology based on MHPs.

Intermetallic PtCu Nanoframes as Efficient Oxygen Reduction Electrocatalysts.

Nanoframe alloy structures represent a class of high-performance catalysts for the oxygen reduction reaction (ORR), owing to their high active surface area, efficient molecular accessibility, and nanoconfinement effect. However, structural and chemical instabilities of nanoframes remain an important challenge. Here, we report the synthesis of PtCu nanoframes constructed with an atomically ordered intermetallic structure (O-PtCuNF/C) showing high ORR activity, durability, and chemical stability. We rationally designed the O-PtCuNF/C catalyst by combining theoretical composition predictions with a silica-coating-mediated synthesis. The O-PtCuNF/C combines intensified strain and ligand effects from the intermetallic PtCu L11 structure and advantages of the nanoframes, resulting in superior ORR activity to disordered alloy PtCu nanoframes (D-PtCuNF/C) and commercial Pt/C catalysts. Importantly, the O-PtCuNF/C showed the highest ORR mass activity among PtCu-based catalysts. Furthermore, the O-PtCuNF/C exhibited higher ORR durability and far less etching of constituent atoms than D-PtCuNF/C and Pt/C, attesting to the chemically stable nature of the intermetallic structure.

Experimental and Density Functional Theory Corroborated Optimization of Durable Metal Embedded Carbon Nanofiber for Oxygen Electrocatalysis.

There is a growing need for widespread deployment of hydrogen and fuel cell technology for the realization of a sustainable energy landscape. However, due to the high price of platinum (Pt) catalysts, it is necessary to develop highly active and stable non-Pt oxygen reduction reaction (ORR) catalysts. Here, we describe a rational design of nonnoble metal-embedding and nitrogen-containing carbon nanofiber (M-CNF) catalysts. Using a combined experimental and computational approach, we establish an ORR activity volcano of M-CNF using the work function of the embedded metal as the descriptor. Near the top of the activity volcano, the embedded metal is further optimized by tuning the Fe1- xCo x alloy composition to simultaneously achieve high catalytic activity and durability. This work identifies the mechanistic importance of controlling the charge transfer between the metal and carbon layers, providing guidance for the design of non-Pt ORR catalysts using stable carbon-layer-protected metals.

Ultrafast charge transfer coupled with lattice phonons in two-dimensional covalent organic frameworks.

Covalent organic frameworks (COFs) have emerged as a promising light-harvesting module for artificial photosynthesis and photovoltaics. For efficient generation of free charge carriers, the donor-acceptor (D-A) conjugation has been adopted for two-dimensional (2D) COFs recently. In the 2D D-A COFs, photoexcitation would generate a polaron pair, which is a precursor to free charge carriers and has lower binding energy than an exciton. Although the character of the primary excitation species is a key factor in determining optoelectronic properties of a material, excited-state dynamics leading to the creation of a polaron pair have not been investigated yet. Here, we investigate the dynamics of photogenerated charge carriers in 2D D-A COFs by combining femtosecond optical spectroscopy and non-adiabatic molecular dynamics simulation. From this investigation, we elucidate that the polaron pair is formed through ultrafast intra-layer hole transfer coupled with coherent vibrations of the 2D lattice, suggesting a mechanism of phonon-assisted charge transfer.

A critical role of catalyst morphology in low-temperature synthesis of carbon nanotube-transition metal oxide nanocomposite.

The effect of the catalyst morphology on the growth of carbon nanotubes (CNT) on nanostructured transition metal oxides was investigated to study a novel low-temperature synthetic route to functional CNT-transition metal oxide nanocomposites. Among several nanostructured manganese oxides with various morphologies and structures, only exfoliated 2D nanosheets of layered MnO2 acted as an effective catalyst for the chemical vapor deposition of CNT at low temperatures of 400-500 °C, which emphasizes the critical role of the catalyst morphology in CNT growth. Heat treatment of the MnO2 nanosheets under a C2H2 flow induced the deposition of CNT, as well as a phase transition to a 2D ordered assembly of MnO nanoparticles. The resulting CNT-MnO nanocomposites displayed excellent functionalities in Li-ion electrodes with huge discharge capacities and good rate characteristics, which highlights the usefulness of the present method for studying functional CNT-metal oxide nanocomposites. Electron microscopy and density functional theory calculations propose a formation mechanism via the efficient adsorption of carbon on the MnO2 nanosheets followed by the surface diffusion of carbon. It is of prime importance that the substitution of Fe for layered MnO2 nanosheets remarkably improved the efficiency of the formation of CNT by enhancing the surface adsorption of carbon species. This is the first report of the efficient growth of CNT at a very low temperature of 400 °C. The universal merit of the 2D nanosheet morphology was confirmed by the successful synthesis of a CNT-TiO2 nanocomposite with exfoliated titanate nanosheets. The present study demonstrates that employing exfoliated transition metal oxide nanosheets as catalysts provides an efficient low-temperature synthetic route to functional CNT-transition metal oxide nanocomposites.

2D Covalent Metals: A New Materials Domain of Electrochemical CO2 Conversion with Broken Scaling Relationship.

Toward a sustainable carbon cycle, electrochemical conversion of CO2 into valuable fuels has drawn much attention. However, sluggish kinetics and a substantial overpotential, originating from the strong correlation between the adsorption energies of intermediates and products, are key obstacles of electrochemical CO2 conversion. Here we show that 2D covalent metals with a zero band gap can overcome the intrinsic limitation of conventional metals and metal alloys and thereby substantially decrease the overpotential for CO2 reduction because of their covalent characteristics. From first-principles-based high-throughput screening results on 61 2D covalent metals, we find that the strong correlation between the adsorption energies of COOH and CO can be entirely broken. This leads to the computational design of CO2-to-CO and CO2-to-CH4 conversion catalysts in addition to hydrogen-evolution-reaction catalysts. Toward efficient electrochemical catalysts for CO2 reduction, this work suggests a new materials domain having two contradictory properties in a single material: covalent nature and electrical conductance.

Probing Distinct Fullerene Formation Processes from Carbon Precursors of Different Sizes and Structures.

Fullerenes, cage-structured carbon allotropes, have been the subject of extensive research as new materials for diverse purposes. Yet, their formation process is still not clearly understood at the molecular level. In this study, we performed laser desorption ionization-ion mobility-mass spectrometry (LDI-IM-MS) of carbon substrates possessing different molecular sizes and structures to understand the formation process of fullerene. Our observations show that the formation process is strongly dependent on the size of the precursor used, with small precursors yielding small fullerenes and large graphitic precursors generally yielding larger fullerenes. These results clearly demonstrate that fullerene formation can proceed via both bottom-up and top-down processes, with the latter being favored for large precursors and more efficient at forming fullerenes. Furthermore, we observed that specific structures of carbon precursors could additionally affect the relative abundance of C60 fullerene. Overall, this study provides an advanced understanding of the mechanistic details underlying the formation processes of fullerene.