Epitaxial Growth of Two-Dimensional Nonlayered AuCrS2 Materials via Au-Assisted Chemical Vapor Deposition.

Two-dimensional (2D) nonlayered transition metal dichalcogenide (TMD) materials are emergent platforms for various applications from catalysis to quantum devices. However, their limited availability and nonstraightforward synthesis methods hinder our understanding of these materials. Here, we present a novel technique for synthesizing 2D nonlayered AuCrS2 via Au-assisted chemical vapor deposition (CVD). Our detailed structural analysis reveals the layer-by-layer growth of [AuCrS2] units atop an initial CrS2 monolayer, with Au binding to the adjacent monolayer of CrS2, which is in stark contrast with the well-known metal intercalation mechanism in the synthesis of many other 2D nonlayered materials. Theoretical calculations further back the crucial role of Cr in increasing the mobility of Au species and strengthening the adsorption energy of Au on CrS2, thereby aiding the growth throughout the CVD process. Additionally, the resulting free-standing nanoporous AuCrS2 (NP-AuCrS2) exhibits exceptional electrocatalytic properties for the hydrogen evolution reaction.

Morphology evolution of the aluminum surface in a fluorine-containing environment.

The interaction between aluminum (Al) and F and O atoms is essential to understand the etching process of Al and alumina (Al2O3) by fluorine-containing gases. In addition, it also has an influence on the corrosion behavior of Al devices, e.g., the Al collector in lithium-ion batteries operates in fluorine-containing electrolytes. However, the understanding of the structural evolution of the Al surface by fluorination at the atomistic level still remains elusive. Here, the thermodynamic and kinetic behaviors of F adatoms as well as co-adsorbed F and O adatoms on typical Al surfaces have been systematically investigated by combining density functional theory (DFT) calculations, canonical Monte Carlo (CMC) simulations and reactive molecular dynamics (RMD) simulations. The results of DFT calculations indicate that there is a repulsion (about 0.07 eV on Al(111) and Al(110), and 0.7 eV on Al(100)) between the first nearest neighboring (1NN) F adatoms, while an attraction of 0.14 eV on Al(111) exists within a 1NN F-O pair. CMC simulations reveal that the configurations of co-adsorbed F and O adatoms on the Al(111) surface at medium to low temperature (<600 K) and low total coverage (<0.2 monolayer, ML) have F adatoms dispersed in the hexagonal islands of O adatoms due to the attraction within the O-O and F-O pairs and the repulsion between F adatoms. As the coverage increases, the surface undergoes serious deformation. The average 1NN coordination numbers (1st CN) of O-to-O, F-to-O and F-to-F are six, three and zero, respectively. As the temperature increases, the interactions among adsorbates begin to be disrupted: the 1st CNs of O-to-O and F-to-O decrease, while that of F-to-F increases. The O-F hexagonal pattern remains until above the Al melting temperature (>1200 K). For F adatoms, both their migration on the surface and the penetration into the subsurface are easier than those of O adatoms, confirmed by both the DFT and RMD simulations. Our study on the co-adsorbates with opposite lateral interactions is instructive for understanding the thermal etching of Al and Al2O3 by fluorine-containing compounds.

Coverage-dependent adsorption and dissociation of H2O on Al surfaces.

The adsorption and dissociation of H2O on Al surfaces including crystal planes and nanoparticles (ANPs) are systematically investigated by using density functional theory (DFT) calculations. H2O adsorption strength follows the order ANPs > Al(110) > Al(111) > Al(100). Due to the smaller cluster deformation caused by the moderate H2O adsorption, the relative magnitude of H2O adsorption strength on ANPs and crystal planes is opposite to the trend of adatoms like O* and/or N*. The energy barrier for the decomposition of H2O into H* and OH* is larger on ANPs than on crystal planes, and it decreases with the increasing cluster size. Due to the competition between the hydrogen (H) bonding among water molecules and the interaction between the water molecules and the substrate, the adsorption strength of H2O first increases and then decreases with the increase of water coverage. Moreover, each H2O molecule can efficiently form up to two H bonds with two H2O molecules. As a result, H2O molecules tend to aggregate into cyclic structures rather than chains on Al surfaces. Furthermore, the dissociation energy barrier of H2O drops with the increasing water coverage due to the presence of H bonds. Our results provide insight into interactions between water and Al, which can be extended to understand the interaction between water and other metal surfaces.

Balanced Crystallinity and Nanostructure for SnS2 Nanosheets through Optimized Calcination Temperature toward Enhanced Pseudocapacitive Na+ Storage.

Sodium ion batteries (SIBs) are expected to take the place of lithium ion batteries (LIBs) as next-generation electrochemical energy storage devices due to the cost advantages they offer. However, due to the larger ion radius, the reaction kinetics of Na+ in anode materials is sluggish. SnS2 is an attractive anode material for SIBs due to its large interlayer spacing and alloying reactions with high capacity. Calcination is usually employed to improve the crystallinity of SnS2, which could affect the Na+ reaction kinetics, especially the pseudocapacitive storage. However, excessively high temperature could damage the well-designed nanostructure of SnS2. In this work, we uniformly grow SnS2 nanosheets on a Zn-, N-, and S-doped carbon skeleton (SnS2@ZnNS). To explore the optimal calcination temperature, SnS2@ZnNS is calcined at three typical temperatures (300, 350, and 400 °C), and the electrochemical performance and Na+ storage kinetics are investigated specifically. The results show that the sample calcined at 350 °C exhibited the best rate capacity and cycle performance, and the reaction kinetics analysis shows that the same sample exhibited a stronger pseudocapacitive response than the other two samples. This improved Na+ storage capability can be attributed to the enhanced crystallinity and the intact nanostructure.

A comparative DFT study of the oxidation of Al crystals and nanoparticles.

The thermodynamic and kinetic behaviors of O atoms on and in different Al nanoparticles (ANPs) and Al crystals have been systematically studied using first-principles calculations. The O adsorption strength on clean Al surfaces follows the order of (111) > ANPs > (110) > (100). The O adsorption strength on ANPs approaches that on Al(111) as the cluster size increases. The three-fold fcc-like sites on ANPs rather than the corner and edge sites are more favored by O* adatoms, which is due to the larger deformation energy related to the geometry change of ANPs when O is adsorbed at the corner and edge sites. The O adsorption behaviors on ANPs are different both from previous studies based on reactive force fields (ReaxFFs) and from those on transition metal clusters. The effective O-O interaction is short ranged (<5 Å), isotropic (in-plane and across-layer) and attractive over different Al surfaces and subsurfaces. The attraction is always about -0.1 eV per O pair at the first nearest neighboring (1NN) sites unless there is evident surface curvature or restructure. Due to the universal attraction, the O* adatoms either on the surface or in the subsurfaces prefer to form islands. In addition, any O diffusion away from the O islands will experience a much higher energy barrier than on a clear (sub)surface. Besides the most stable (Al2O3)n fragments, the metastable Al oxidation fragments are AlO4 monomers and (AlO2)n oligomers, which may be formed during intense oxidation and all feature tetra-coordinated Al and bi-coordinated O atoms.