Origin of Al Deficient Ti₂AlN and Pathways of Vacancy-Assisted Diffusion

To understand the origin of the Al deficient Ti₂AlN MAX phase observed in our experiments, the formation and the diffusion pathway of Al vacancy in Ti₂AlN have been calculated by density functional theory (DFT). Compared to Ti and N vacancies, Al vacancies require the lowest formation energies not only in the bulk but also at the top surface layer and the second surface layer. As a result, Ti₂AlN is calculated to be capable of accommodating Al vacancies in the supercell down to a substoichiometric Ti₂Al_0.75N while maintaining the MAX phase structure. After the vacancy formation, Al atom is calculated to diffuse along the (0001) plane preferentially via vacancy jump with an energy barrier of 0.80 eV, leading to Al surface segregation and subsequent desorption from Ti₂AlN at high temperatures

Defect Evolution Enhanced Visible-Light Photocatalytic Activity in Nitrogen-Doped Anatase TiO₂ Thin Films

Doping nitrogen (N) into TiO₂ is one of the promising ways to extend the photocatalytic activity into the visible-light range, enabling to harvest more solar energy. In this study, we realize a high concentration of N incorporated into the anatase TiO₂ films on indium tin oxide substrates. The band gap of TiO₂ with a high N substitutional doping is reduced to 1.91 eV, showing a much improved photocatalytic reactivity, as supported by the degrading methyl orange solution radiated with visible light. First-principles calculations further suggest that the form of dominant defects evolves from the substitution of N (N_O) to the coexistence of N_O and oxygen vacancies (O_V) when the N-doping concentration is increased, which leads to the reduction of band gap in the visible-light range and more delocalized charge distribution. Our results demonstrate a novel synthesis route that can realize a high concentration of N substitutional doping in TiO₂ films and provide an improved understanding of enhanced visible-light photocatalytic performance of N-doped TiO₂

Oxidation of Single Crystalline Ti₂AlN Thin Films between 300 and 900 °C: A Perspective from Surface Analysis

High temperature oxidation of 300 nm single crystalline Ti₂AlN MAX phase thin film deposited on MgO(111) substrate between 300 and 900 °C has been investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and mass spectrometry. As shown by XRD, Ti₂AlN remained structurally stable up to 700 °C, before it began to react with MgO substrate and ambient O₂ to form MgTi₂O₅ and MgAl₂O₄ at 900 °C. However, as revealed by XPS, oxidation of Ti₂AlN occurred at room temperature from its surface by forming TiO₂, TiN_<i>x</i>O_<i>y</i> and Al₂O₃ with surface enrichment of Al. This initial oxidation continued up to 300 °C, until Ti and Al in the surface layer (∼7.1 nm thick) have been completely oxidized into TiO₂ and Al₂O₃ at 500 °C, where Al in the subsurface preferentially diffused to the edges of the terraces and agglomerated into Al₂O₃ islands. At 700 °C and above, surface of Ti₂AlN lost its characteristic hexagonal terrace morphology by transforming into round islands as a result of high temperature oxidation. Mass spectrometry revealed that N in Ti₂AlN was released from the MAX thin film as N₂ and N₂O

Visible–Near-Infrared-Light-Driven Oxygen Evolution Reaction with Noble-Metal-Free WO₂–WO₃ Hybrid Nanorods

Understanding and manipulating the one half-reaction of photoinduced hole-oxidation to oxygen are of fundamental importance to design and develop an efficient water-splitting process. To date, extensive studies on oxygen evolution from water splitting have focused on visible-light harvesting. However, capturing low-energy photons for oxygen evolution, such as near-infrared (NIR) light, is challenging and not well-understood. This report presents new insights into photocatalytic water oxidation using visible and NIR light. WO₂–WO₃ hybrid nanorods were in situ fabricated using a wet-chemistry route. The presence of metallic WO₂ strengthens light absorption and promotes the charge-carrier separation of WO₃. The efficiency of the oxygen evolution reaction over noble-metal-free WO₂–WO₃ hybrids was found to be significantly promoted. More importantly, NIR light (≥700 nm) can be effectively trapped to cause the photocatalytic water oxidation reaction. The oxygen evolution rates are even up to around 220 (λ = 700 nm) and 200 (λ = 800 nm) mmol g^–1 h^–1. These results demonstrate that the WO₂–WO₃ material is highly active for water oxidation with low-energy photons and opens new opportunities for multichannel solar energy conversion

Strong (110) Texturing and Heteroepitaxial Growth of Thin Mo Films on MoS₂ Monolayer

Growth of textured and low-resistivity metallic seed layers for AlN-based piezoelectric films is of high importance for bulk acoustic wave resonator applications. Through optimization of Mo physical vapor deposition parameters, namely, the Ar flow rate, strong (110) texturing and low electrical resistivities (∼3 × 10–7 Ω m) were observed for 43 ± 3 nm thick Mo films on a CVD-grown MoS2 monolayer on c-Al2O3(0001) substrates. The strong texturing was attributed to the growth template effect of the monolayer MoS2 due to the presence of a local epitaxial relationship between (110)-Mo and (0001)-MoS2 (i.e., through MoS2(0001)[112̅0]||Mo(110)[1̅11] and/or MoS2(0001)­[112̅0]||Mo(110)[001]), coupled with an atomic-scale flatness of the MoS2 surface, which promotes layer-by-layer growth of the Mo film. The deposited Mo/MoS2 monolayer stack can also be easily peeled-off from the growth Al2O3(0001) substrate for possible subsequent transfers onto arbitrary substrates (e.g., SiO2/Si(001)) due to a weak van der Waals coupling at the MoS2 and Al2O3(0001) interface, facilitating vertical stacking strategies for monolithic integration of high quality and therefore high-performance, AlN-based piezoelectric devices and sensors on the Si platform

A Robust Hybrid Zn-Battery with Ultralong Cycle Life

Advanced batteries with long cycle life and capable of harnessing more energies from multiple electrochemical reactions are both fundamentally interesting and practically attractive. Herein, we report a robust hybrid zinc-battery that makes use of transition-metal-based redox reaction (M–O–OH → M–O, M = Ni and Co) and oxygen reduction reaction (ORR) to deliver more electrochemical energies of comparably higher voltage with much longer cycle life. The hybrid battery was constructed using an integrated electrode of NiCo₂O₄ nanowire arrays grown on carbon-coated nickel foam, coupled with a zinc plate anode in alkaline electrolyte. Benefitted from the M–O/M–O–OH redox reactions and rich ORR active sites in NiCo₂O₄, the battery has concurrently exhibited high working voltage (by M–O–OH → M–O) and high energy density (by ORR). The good oxygen evolution reaction (OER) activity of the electrode and the reversible M–O ↔ M–O–OH reactions also enabled smooth recharging of the batteries, leading to excellent cycling stabilities. Impressively, the hybrid batteries maintained highly stable charge–discharge voltage profile under various testing conditions, for example, almost no change was observed over 5000 cycles at a current density of 5 mA cm^–2 after some initial stabilization. With merits of higher working voltage, high energy density, and ultralong cycle life, such hybrid batteries promise high potential for practical applications

ZnO Nanorods with Low Intrinsic Defects and High Optical Performance Grown by Facile Microwave-Assisted Solution Method

Vertically aligned ZnO nanorods were grown at 90 °C by both microwave synthesis and traditional heated water bath method on Si (100) substrate with a precoated ZnO nanoparticle seed layer. A detailed comparison in the morphology, defects, and optical properties of the ZnO nanorods grown by the two methods across the pH range of 10.07–10.9 for microwave synthesis and conventional heated water bath method was performed using scanning electron microscopy, photoluminescence, and X-ray photoelectron spectroscopy. The results show that the microwave route leads to more uniformly distributed nanorods with a lower density of native defects of oxygen interstitials and zinc vacancies. The microwave synthesis presents a promising new approach of fabricating metal oxide nanostructures and devices toward green applications

Facile Synthesis of Vanadium-Doped Ni₃S₂ Nanowire Arrays as Active Electrocatalyst for Hydrogen Evolution Reaction

Ni₃S₂ nanowire arrays doped with vanadium­(V) are directly grown on nickel foam by a facile one-step hydrothermal method. It is found that the doping can promote the formation of Ni₃S₂ nanowires at a low temperature. The doped nanowires show excellent electrocatalytic performance toward hydrogen evolution reaction (HER), and outperform pure Ni₃S₂ and other Ni₃S₂-based compounds. The stability test shows that the performance of V-doped Ni₃S₂ nanowires is improved and stabilized after thousands of linear sweep voltammetry test. The onset potential of V-doped Ni₃S₂ nanowire can be as low as 39 mV, which is comparable to platinum. The nanowire has an overpotential of 68 mV at 10 mA cm^–2, a relatively low Tafel slope of 112 mV dec^–1, good stability and high Faradaic efficiency. First-principles calculations show that the V-doping in Ni₃S₂ extremely enhances the free carrier density near the Fermi level, resulting in much improved catalytic activities. We expect that the doping can be an effective way to enhance the catalytic performance of metal disulfides in hydrogen evolution reaction and V-doped Ni₃S₂ nanowire is one of the most promising electrocatalysts for hydrogen production

Effect of Extrinsically Introduced Passive Interface Layer on the Performance of Ferroelectric Tunnel Junctions

We report the effect of the top electrode/functional layer interface on the performance of ferroelectric tunnel junctions. Ex situ and in situ fabrication process were used to fabricate the top Pt electrode. With the ex situ fabrication process, one passive layer at the top interface would be induced. Our experimental results show that the passive interface layer of the ex situ devices increases the coercive voltage of the functional BaTiO₃ layer and decreases the tunneling current magnitude. However, the ex situ tunneling devices possess more than 1000 times larger ON/OFF ratios than that of the in situ devices with the same size of top electrode

Direct n- to p‑Type Channel Conversion in Monolayer/Few-Layer WS₂ Field-Effect Transistors by Atomic Nitrogen Treatment

We present a method for substitutional p-type doping in monolayer (1L) and few-layer (FL) WS₂ using highly reactive nitrogen atoms. We demonstrate that the nitrogen-induced lattice distortion in atomically thin WS₂ is negligible due to its low kinetic energy. The electrical characteristics of 1L/FL WS₂ field-effect transistors (FETs) clearly show an n-channel to p-channel conversion with nitrogen incorporation. We investigate the defect formation energy and the origin of p-type conduction using first-principles calculations. We reveal that a defect state appears near the Fermi level, leading to a shallow acceptor level at 0.24 eV above the valence band maximum in nitrogen-doped 1L/FL WS₂. This doping strategy enables a substitutional p-type doping in intrinsically n-type 1L/FL transition metal dichalcogenides (TMDCs) with tunable control of dopants, offering a method for realizing complementary metal-oxide-semiconductor FETs and optoelectronic devices on 1L/FL TMDCs by overcoming one of the major limits of TMDCs, that is, their n-type unipolar conduction