High‐entropy alloys combine multiple principal elements at a near equal fraction to form vast compositional spaces to achieve outstanding functionalities that are absent in alloys with one or two principal elements. Here, the prediction, synthesis, and multiscale characterization of 2D high‐entropy transition metal dichalcogenide (TMDC) alloys with four/five transition metals is reported. Of these, the electrochemical performance of a five‐component alloy with the highest configurational entropy, (MoWVNbTa)S2, is investigated for CO2 conversion to CO, revealing an excellent current density of 0.51 A cm−2 and a turnover frequency of 58.3 s−1 at ≈ −0.8 V versus reversible hydrogen electrode. First‐principles calculations show that the superior CO2 electroreduction is due to a multi‐site catalysis wherein the atomic‐scale disorder optimizes the rate‐limiting step of CO desorption by facilitating isolated transition metal edge sites with weak CO binding. 2D high‐entropy TMDC alloys provide a materials platform to design superior catalysts for many electrochemical systems.
To evaluate the role of planar defects in lead‐halide perovskites—cheap, versatile semiconducting materials—it is critical to examine their structure, including defects, at the atomic scale and develop a detailed understanding of their impact on electronic properties. In this study, postsynthesis nanocrystal fusion, aberration‐corrected scanning transmission electron microscopy, and first‐principles calculations are combined to study the nature of different planar defects formed in CsPbBr3 nanocrystals. Two types of prevalent planar defects from atomic resolution imaging are observed: previously unreported Br‐rich [001](210)∑5 grain boundaries (GBs) and Ruddlesden–Popper (RP) planar faults. The first‐principles calculations reveal that neither of these planar faults induce deep defect levels, but their Br‐deficient counterparts do. It is found that the ∑5 GB repels electrons and attracts holes, similar to an n–p–n junction, and the RP planar defects repel both electrons and holes, similar to a semiconductor–insulator–semiconductor junction. Finally, the potential applications of these findings and their implications to understand the planar defects in organic–inorganic lead‐halide perovskites that have led to solar cells with extremely high photoconversion efficiencies are discussed.
Lead–halide
perovskite nanocrystals (PNCs) have attracted
much attention in recent years due to their outstanding optical properties.
We report here a new chemical route for triggering postsynthetic growth
of CsPbBr3 PNCs at room temperature via intentional depletion
of stabilizing ligands, resulting in an immediate fusion growth of
the as-synthesized PNCs. Upon fusion, the CsPbBr3 PNCs
can grow from ca. 8 nm to ca. 60 nm in lateral dimensions in 48 h,
reaching about 14 nm in thickness. More importantly, it was found
that the fused PNCs have significantly enhanced optical properties.
They showed an exceptionally higher stability to photodegradation.
They also displayed sharper emission lines and a higher quantum yield,
contrary to the fact that the nanocrystals are much larger. The much-improved
optical properties are attributed to the Ruddlesden–Popper
(RP) planar faults formed during the fusion process and observed using
atomic resolution scanning transmission electron microscopy, which
are predicted to result in quantum confinement based on density-functional
theory calculations. The newly grown nanocrystals with RP defects
are expected to significantly improve light emission properties of
the PNCs and find applications in light-emitting diodes and other
optoelectronic devices.
Methylammonium
lead iodide (CH3NH3PbI3 or MAPbI3) perovskite is a promising new photovoltaic
material with high power conversion efficiency. However, its perovskite
phase with corner-connected PbI6 octahedra shows poor environmental
stability. More recently, MAPbI3 has been shown to be thermodynamically
unstable with a positive formation enthalpy. Here, using first-principles
density functional theory calculations, we predict a layered hexagonal
phase of MAPbI3 consisting of infinite chains of face-shared
PbI6 octahedra with P63
mc space-group symmetry to be thermodynamically the most
stable phase for a wide range of volume and temperature compared to
any of the experimentally observed perovskite phases with a different
tilt pattern of the corner-connected octahedra. The predicted hexagonal
phase is also dynamically stable without any soft phonon modes. The
change from corner to face-shared connectivity in the hexagonal phase
leads to a predicted band gap of 2.6 eV and a band structure that
favors highly anisotropic charge transport.
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