Nanowire Transformation by Size-Dependent Cation Exchange Reactions

The unique properties of nanostructured materials enable their transformation into complex, kinetically controlled morphologies that cannot be obtained during their growth. Solution-phase cation-exchange reactions can transform sub-10 nm nanocrystals/nanorods into varying compositions and superlattice structures; however, because of their small size, cation-exchange reaction rates are extremely fast, which limits control over the transformed products and possibilities for obtaining new morphologies. Nanowires are routinely synthesized via gas-phase reactions with 5−200 nm diameters, and their large aspect ratios allow them to be electrically addressed individually. To realize their full potential, it is desirable to develop techniques that can transform nanowires into tunable but precisely controlled morphologies, especially in the gas-phase, to be compatible with nanowire growth schemes. We report transformation of single-crystalline cadmium sulfide nanowires into composition-controlled ZnxCd(1−x)S nanowires, core−shell heterostructures, metal-semiconductor superlattices (Zn−ZnxCd(1−x)S), single-crystalline ZnS nanotubes, and eventually metallic Zn nanowires by utilizing size-dependent cation-exchange reaction along with temperature and gas-phase reactant delivery control. This versatile synthetic ability to transform nanowires offers new opportunities to study size-dependent phenomena at the nanoscale and tune their chemical/physical properties to design reconfigurable circuits

High-Performance One-Body Core/Shell Nanowire Supercapacitor Enabled by Conformal Growth of Capacitive 2D WS₂ Layers

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) have emerged as promising capacitive materials for supercapacitor devices owing to their intrinsically layered structure and large surface areas. Hierarchically integrating 2D TMDs with other functional nanomaterials has recently been pursued to improve electrochemical performances; however, it often suffers from limited cyclic stabilities and capacitance losses due to the poor structural integrity at the interfaces of randomly assembled materials. Here, we report high-performance core/shell nanowire supercapacitors based on an array of one-dimensional (1D) nanowires seamlessly integrated with conformal 2D TMD layers. The 1D and 2D supercapacitor components possess “one-body” geometry with atomically sharp and structurally robust core/shell interfaces, as they were spontaneously converted from identical metal current collectors via sequential oxidation/sulfurization. These hybrid supercapacitors outperform previously developed any stand-alone 2D TMD-based supercapacitors; particularly, exhibiting an exceptional charge–discharge retention over 30,000 cycles owing to their structural robustness, suggesting great potential for unconventional energy storage technologies

Hydrogen-Treated TiO₂ Nanorods Decorated with Bimetallic Pd–Co Nanoparticles for Photocatalytic Degradation of Organic Pollutants and Bacterial Inactivation

Herein, first, we synthesize a multifunctional photocatalyst via metal oxides loaded (Co/Pd) on acid-treated TiO2 nanorods (ATO) and further introduce hydrogen annealing treatment. The hydrogen annealing treatment introduces metal oxides converted into a bimetallic form and delays the photogenerated charge recombination process. Also, oxygen vacancies are formed due to the partial reduction of Ti4+ to Ti3+ sites. In addition, oxygen vacancies help to improve photocatalytic degradation and antibacterial activity. The hydrogen-treated photocatalyst (Pd(1)Co(1)/ATO (red)) demonstrates high degradation efficiencies of 99.63 and 99.90% (180 min) for orange II dye and BPA degradation, respectively, and an antibacterial activity of 97.00% (120 min) under one sun irradiation. In the photocatalytic removal of abiotic pollutants and live bacteria, the trapping experiment suggests that radical species (•O2– and •OH), assisted by photoinduced holes, are responsible for the high activities. The photoelectrochemical performance and time-resolved PL (TRPL) study illustrate that Pd(1)Co(1)/ATO (red) reveals superior photoelectrochemical charge separation (electron–hole), lower resistance, and shorter lifetime (τ1 = 0.40 ns) as a photocatalyst. Finally, plausible charge transport mechanisms are proposed for the photocatalytic degradation of organic dye and bacterial disinfection over the Pd(1)Co(1)/ATO (red) photocatalyst

Epitaxial Growth and Ordering of GeTe Nanowires on Microcrystals Determined by Surface Energy Minimization

We report self-assembly of highly aligned GeTe nanowires epitaxially grown on octahedral GeTe microcrystals in two well-defined directions by using one-step vapor transport process. The epitaxial relationship of nanowires with underlying microcrystals along with the growth orientations of nanowires were investigated in detail by electron microscopy combined with atomic unit cell models. We demonstrate that maximizing atomic planar density to minimize energy of the exposed surfaces is the determining factor that governs the unique growth characteristics of micro/nanostructures that evolve from three-dimensional octahedral microcrystals to tetrahedral bases to finally one-dimensional nanowires. The crystallographic understanding of structuring of crystalline nanomaterials obtained from this study will be critical to understand, predict, and control the growth orientation of nanostructures in three-dimensions

Robust Heteroepitaxial Growth of GaN Formulated on Porous TiN Buffer Layers

Gallium nitride (GaN) heteroepitaxial growth is widely studied as a semiconductor material due to its various benefits. Especially, development of a buffer layer between GaN and the substrate verifies to be an effective strategy to reduce high threading dislocation density. However, the buffer layer often impedes strong adhesion between the epilayer and foreign substrate because thermally induced residual stress often causes delamination of the epilayer during fabrication. Here, we developed a robust GaN heteroepitaxy employing a porous buffer layer formulated by hydride vapor phase epitaxy. A sufficiently low but completely coated thin Ti layer was deposited on the sapphire substrate, which led to a rough and porous TiN layer after nitridation. This porous structure enables the penetration of the GaN source into the porous structure, allowing GaN epitaxy initiation throughout the TiN layer. As a result, GaN crystal growth can fill the porous area during the GaN heteroepitaxy. Integrated visualization demonstrated that the voids were successfully removed by GaN infiltration, enabling the heteroepitaxial structure to show little deformation, confirmed by multiple indentations. Last, the void-free GaN heteroepitaxy with the porous TiN buffer layer displayed robust adhesion after delamination tests

Robust Heteroepitaxial Growth of GaN Formulated on Porous TiN Buffer Layers

Gallium nitride (GaN) heteroepitaxial growth is widely studied as a semiconductor material due to its various benefits. Especially, development of a buffer layer between GaN and the substrate verifies to be an effective strategy to reduce high threading dislocation density. However, the buffer layer often impedes strong adhesion between the epilayer and foreign substrate because thermally induced residual stress often causes delamination of the epilayer during fabrication. Here, we developed a robust GaN heteroepitaxy employing a porous buffer layer formulated by hydride vapor phase epitaxy. A sufficiently low but completely coated thin Ti layer was deposited on the sapphire substrate, which led to a rough and porous TiN layer after nitridation. This porous structure enables the penetration of the GaN source into the porous structure, allowing GaN epitaxy initiation throughout the TiN layer. As a result, GaN crystal growth can fill the porous area during the GaN heteroepitaxy. Integrated visualization demonstrated that the voids were successfully removed by GaN infiltration, enabling the heteroepitaxial structure to show little deformation, confirmed by multiple indentations. Last, the void-free GaN heteroepitaxy with the porous TiN buffer layer displayed robust adhesion after delamination tests

Robust Heteroepitaxial Growth of GaN Formulated on Porous TiN Buffer Layers

Gallium nitride (GaN) heteroepitaxial growth is widely studied as a semiconductor material due to its various benefits. Especially, development of a buffer layer between GaN and the substrate verifies to be an effective strategy to reduce high threading dislocation density. However, the buffer layer often impedes strong adhesion between the epilayer and foreign substrate because thermally induced residual stress often causes delamination of the epilayer during fabrication. Here, we developed a robust GaN heteroepitaxy employing a porous buffer layer formulated by hydride vapor phase epitaxy. A sufficiently low but completely coated thin Ti layer was deposited on the sapphire substrate, which led to a rough and porous TiN layer after nitridation. This porous structure enables the penetration of the GaN source into the porous structure, allowing GaN epitaxy initiation throughout the TiN layer. As a result, GaN crystal growth can fill the porous area during the GaN heteroepitaxy. Integrated visualization demonstrated that the voids were successfully removed by GaN infiltration, enabling the heteroepitaxial structure to show little deformation, confirmed by multiple indentations. Last, the void-free GaN heteroepitaxy with the porous TiN buffer layer displayed robust adhesion after delamination tests

Improved Interfacial Charge Transfer Dynamics and Onset Shift in Nanostructured Hematite Photoanodes via Efficient Ti⁴⁺/Sn⁴⁺ Heterogeneous Self-Doping Through Controlled TiO₂ Underlayers

We introduce a simple strategy to unintentional heterogeneous Ti4+/Sn4+ doping and surface passivation of hematite via TiO2 underlayers at high temperature quenching. The effects of the controlled TiO2 underlayer thickness and high temperature quenching process on the interfacial diffusion of Ti4+/Sn4+ and TiO2 passivation of hematite nanorod arrays have been carefully studied. The improved photoelectrochemical water oxidation performance of the TiO2 underlayered hematite nanorod photoanodes after high-temperature quenching (800 °C for 10 min) suggests enhanced interfacial Ti4+ diffusion, blocking of electron back transfer, and reduced interfacial charge recombination. The TiO2 underlayers led to more inclined growth of hematite (α-Fe2O3) nanorods on the fluorine-doped tin oxide (FTO) substrates. Ti4+ and Sn4+ diffusion and formation of the TiO2 passivation layer on the α-Fe2O3 surface are confirmed by HRTEM and X-ray photoelectron spectroscopy (XPS) analyses. As a result, the TU2 photoanode displayed higher donor density and enhanced photocurrent density of (1.45 mA·cm–2) than the pristine hematite photoelectrode (1.0 mA·cm–2). The improved photoelectrochemical performance of TU2 is attributed to the high separation efficiency of photoinduced carriers via TiO2 underlayer, the Ti4+/Sn4+ diffusion, and surface passivation of hematite at high-temperature annealing. The thickness of the TiO2 underlayer has great influence on the surface passivation as well as resistance on FTO/hematite interfaces than the diffused Sn

Robust Heteroepitaxial Growth of GaN Formulated on Porous TiN Buffer Layers

Gallium nitride (GaN) heteroepitaxial growth is widely studied as a semiconductor material due to its various benefits. Especially, development of a buffer layer between GaN and the substrate verifies to be an effective strategy to reduce high threading dislocation density. However, the buffer layer often impedes strong adhesion between the epilayer and foreign substrate because thermally induced residual stress often causes delamination of the epilayer during fabrication. Here, we developed a robust GaN heteroepitaxy employing a porous buffer layer formulated by hydride vapor phase epitaxy. A sufficiently low but completely coated thin Ti layer was deposited on the sapphire substrate, which led to a rough and porous TiN layer after nitridation. This porous structure enables the penetration of the GaN source into the porous structure, allowing GaN epitaxy initiation throughout the TiN layer. As a result, GaN crystal growth can fill the porous area during the GaN heteroepitaxy. Integrated visualization demonstrated that the voids were successfully removed by GaN infiltration, enabling the heteroepitaxial structure to show little deformation, confirmed by multiple indentations. Last, the void-free GaN heteroepitaxy with the porous TiN buffer layer displayed robust adhesion after delamination tests