Microstructure-Dependent Conformal Atomic Layer Deposition on 3D Nanotopography

The capability of atomic layer deposition (ALD) to coat conformally complex 3D nanotopography has been examined by depositing amorphous, polycrystalline, and single-crystal TiO₂ films over SnO₂ nanowires (NWs). Structural characterizations reveal a strong correlation between the surface morphology and the microstructures of ALD films. Conformal growth can only be rigorously achieved in amorphous phase with circular sectors developed at sharp asperities. Morphology evolution convincingly demonstrates the principle of ALD, i.e., sequential and self-limiting surface reactions result in smooth and conformal films. Orientation-dependent growth and surface reconstruction generally lead to nonconformal coating in polycrystalline and single-crystal films. Especially, an octagonal single-crystal TiO₂ shell was derived from a rectangular SnO₂ NW core, which was the consequence of both self-limited growth kinetics and surface reconstruction. Models were proposed to explain the conformality of ALD deposition over 3D nanostructures by taking account of the underlying microstructures. Besides the surface morphologies, the microstructures also have significant consequence to the surface electronic states, characterized by the broad band photoluminescence. The comparison study suggests that ALD process is determined by the interplay of both thermodynamic and kinetic factors

Origin of the Phase Transition in Lithiated Molybdenum Disulfide

Phase transitions and phase engineering in two-dimensional MoS₂ are important for applications in electronics and energy storage. By <i>in situ</i> transmission electron microscopy, we find that H-MoS₂ transforms to T-LiMoS₂ at the early stages of lithiation followed by the formation of Mo and Li₂S phases. The transition from H-MoS₂ to T-LiMoS₂ is explained in terms of electron doping and electron–phonon coupling at the conduction band minima. Both are essential for the development of two-dimensional semiconductor-metal contacts based on MoS₂ and the usage of MoS₂ as anode material in Li ion batteries

Direct Evidence of Lithium-Induced Atomic Ordering in Amorphous TiO₂ Nanotubes

In this paper, we report the first direct chemical and imaging evidence of lithium-induced atomic ordering in amorphous TiO₂ nanomaterials and propose new reaction mechanisms that contradict the many works in the published literature on the lithiation behavior of these materials. The lithiation process was conducted in situ inside an atomic resolution transmission electron microscope. Our results indicate that the lithiation started with the valence reduction of Ti⁴⁺ to Ti³⁺ leading to a Li_<i>x</i>TiO₂ intercalation compound. The continued intercalation of Li ions in TiO₂ nanotubes triggered an amorphous to crystalline phase transformation. The crystals were formed as nano-islands and identified to be Li₂Ti₂O₄ with cubic structure (<i>a</i> = 8.375 Å). The tendency for the formation of these crystals was verified with density functional theory (DFT) simulations. The size of the crystalline islands provides a characteristic length scale (∼5 nm) at which the atomic bonding configuration has been changed within a short time period. This phase transformation is associated with local inhomogeneities in Li distribution. On the basis of these observations, a new reaction mechanism is proposed to explain the first cycle lithiation behavior in amorphous TiO₂ nanotubes

Atomistic Insights into the Oriented Attachment of Tunnel-Based Oxide Nanostructures

Controlled synthesis of nanomaterials is one of the grand challenges facing materials scientists. In particular, how tunnel-based nanomaterials aggregate during synthesis while maintaining their well-aligned tunneled structure is not fully understood. Here, we describe the atomistic mechanism of oriented attachment (OA) during solution synthesis of tunneled α-MnO₂ nanowires based on a combination of <i>in situ</i> liquid cell transmission electron microscopy (TEM), aberration-corrected scanning TEM with subangstrom spatial resolution, and first-principles calculations. It is found that primary tunnels (1 × 1 and 2 × 2) attach along their common {110} lateral surfaces to form interfaces corresponding to 2 × 3 tunnels that facilitate their short-range ordering. The OA growth of α-MnO₂ nanowires is driven by the stability gained from elimination of {110} surfaces and saturation of Mn atoms at {110}-edges. During this process, extra [MnO_<i>x</i>] radicals in solution link the two adjacent {110} surfaces and bond with the unsaturated Mn atoms from both surface edges to produce stable nanowire interfaces. Our results provide insights into the controlled synthesis and design of nanomaterials in which tunneled structures can be tailored for use in catalysis, ion exchange, and energy storage applications

Sodium-Induced Reordering of Atomic Stacks in Black Phosphorus

While theoretical simulations predict contradictory results about how the intercalation of foreign metal atoms affects the order of atomic layers in black phosphorus (BP), no direct experimental visualization work has yet clarified this ambiguity. By in situ electrochemical sodiation of BP inside a high-resolution transmission electron microscope and first-principles calculations, we found that sodium intercalation induces a relative glide of ¹/₂ ⟨010⟩ {001}, resulting in reordering of atomic stacks from AB to AC in BP. The observed local amorphization in our experiments is triggered by lattice constraints. We predict that intercalation of sodium or other metal atoms introduces n-type carriers in BP. This potentially opens a new field for two-dimensional electronics based on BP

Sodium-Induced Reordering of Atomic Stacks in Black Phosphorus

While theoretical simulations predict contradictory results about how the intercalation of foreign metal atoms affects the order of atomic layers in black phosphorus (BP), no direct experimental visualization work has yet clarified this ambiguity. By in situ electrochemical sodiation of BP inside a high-resolution transmission electron microscope and first-principles calculations, we found that sodium intercalation induces a relative glide of ¹/₂ ⟨010⟩ {001}, resulting in reordering of atomic stacks from AB to AC in BP. The observed local amorphization in our experiments is triggered by lattice constraints. We predict that intercalation of sodium or other metal atoms introduces n-type carriers in BP. This potentially opens a new field for two-dimensional electronics based on BP

A Strategy for Synthesis of Nanosheets Consisting of Alternating Spinel Li₄Ti₅O₁₂ and Rutile TiO₂ Lamellas for High-Rate Anodes of Lithium-Ion Batteries

Ultrathin dual phase nanosheets consisting of alternating spinel Li₄Ti₅O₁₂ (LTO) and rutile TiO₂ (RT) lamellas are synthesized through a facile and scalable hydrothermal method, and the formation mechanism is explored. The thickness of constituent lamellas can be controlled exactly by adjusting the mole ratio of Li:Ti in the original reactants. Alternating insertion of the RT lamellas significantly improves the electrochemical performance of LTO nanosheets, especially at high charge/discharge rates. As anodes in lithium-ion batteries (LIBs), the dual phase nanosheet electrode with the optimized phase ratio can deliver stable discharge capacities of 178.5, 154.9, 148.4, 142.3, 138.2, and 131.4 mA h g^–1 at current densities of 1, 10, 20, 30, 40, and 50 C, respectively. Meanwhile, they inherit the excellent cyclic stability of pure spinel LTO and exhibit a capacity retention of 93.1% even after 500 cycles at 50 C. Our results indicate that the alternating nanoscaled lamella structure is a good alternative to facilitate the transfer of both the Li ions and electrons into the spinel LTO, giving rise to an excellent cyclability and fast rate performance. Therefore, the newly prepared carbon-free LTO-RT nanosheets with high safety provide a new opportunity to develop high-power anodes for LIBs

Twin Boundary-Assisted Lithium Ion Transport

With the increased need for high-rate Li-ion batteries, it has become apparent that new electrode materials with enhanced Li-ion transport should be designed. Interfaces, such as twin boundaries (TBs), offer new opportunities to navigate the ionic transport within nanoscale materials. Here, we demonstrate the effects of TBs on the Li-ion transport properties in single crystalline SnO₂ nanowires. It is shown that the TB-assisted lithiation pathways are remarkably different from the previously reported lithiation behavior in SnO₂ nanowires without TBs. Our in situ transmission electron microscopy study combined with direct atomic-scale imaging of the initial lithiation stage of the TB-SnO₂ nanowires prove that the lithium ions prefer to intercalate in the vicinity of the (101̅) TB, which acts as conduit for lithium-ion diffusion inside the nanowires. The density functional theory modeling shows that it is energetically preferred for lithium ions to accumulate near the TB compared to perfect neighboring lattice area. These findings may lead to the design of new electrode materials that incorporate TBs as efficient lithium pathways, and eventually, the development of next generation rechargeable batteries that surpass the rate performance of the current commercial Li-ion batteries

Discovering a First-Order Phase Transition in the Li–CeO₂ System

An in-depth understanding of (de)­lithiation induced phase transition in electrode materials is crucial to grasp their structure–property relationships and provide guidance to the design of more desirable electrodes. By operando synchrotron XRD (SXRD) measurement and Density Functional Theory (DFT) based calculations, we discover a reversible first-order phase transition for the first time during (de)­lithiation of CeO₂ nanoparticles. The Li_<i>x</i>CeO₂ compound phase is identified to possess the same fluorite crystal structure with FM3M space group as that of the pristine CeO₂ nanoparticles. The SXRD determined lattice constant of the Li_<i>x</i>CeO₂ compound phase is 0.551 nm, larger than that of 0.541 nm of the pristine CeO₂ phase. The DFT calculations further reveal that the Li induced redistribution of electrons causes the increase in the Ce–O covalent bonding, the shuffling of Ce and O atoms, and the jump expansion of lattice constant, thereby resulting in the first-order phase transition. Discovering the new phase transition throws light upon the reaction between lithium and CeO₂, and provides opportunities to the further investigation of properties and potential applications of Li_<i>x</i>CeO₂

Selective Ionic Transport Pathways in Phosphorene

Despite many theoretical predictions indicating exceptionally low energy barriers of ionic transport in phosphorene, the ionic transport pathways in this two-dimensional (2D) material has not been experimentally demonstrated. Here, using in situ aberration-corrected transmission electron microscopy (TEM) and density functional theory, we studied sodium ion transport in phosphorene. Our high-resolution TEM imaging complemented by electron energy loss spectroscopy demonstrates a precise description of anisotropic sodium ions migration along the [100] direction in phosphorene. This work also provides new insight into the effect of surface and the edge sites on the transport properties of phosphorene. According to our observation, the sodium ion transport is preferred in zigzag edge rather than the armchair edge. The use of this highly selective ionic transport property may endow phosphorene with new functionalities for novel chemical device applications