Unprecedented Insulator-to-Metal Transition Dynamics by Heterogeneous Catalysis in Pd-Sensitized Single Vanadium Oxide Nanowires

We report an unprecedented insulator-to-metal transition of the Pd-nanoparticle-decorated, single vanadium dioxide nanowires caused by the heterogeneous H₂–Pd catalytic reactions. Upon hydrogen gas exposure, the Pd–VO₂ heteronanostructure shows a remarkably large current increase (∼1000-fold) at a temperature 10 °C lower than the insulator-to-metal transition temperature in the bulk. This current increase occurs slowly over the duration of several seconds to several tens of seconds, depending on the Pd coverage, temperature, and hydrogen concentration. After hydrogen flow is shut off, the conductance is not immediately returned to the original value and it takes several hours, indicating that the atomic hydrogens, produced by the dissociative chemisorption on Pd, are incorporated into VO₂. From the electronic transport measurements and finite element analyses, we suggest that the slow transition is mainly due to the formation of alternating domains of metal and insulator regions of the VO₂ crystal lattice, produced by the generated heat via the exothermic catalytic process. The current response toward hydrogen gas is also found to be reversible after the heating process. This novel finding thus has significant implications for the effective engineering of the physicochemical properties of vanadium dioxide by a heterogeneous catalytic process

Unveiling a Surface Electronic Descriptor for Fe–Co Mixing Enhanced the Stability and Efficiency of Perovskite Oxygen Evolution Electrocatalysts

The influence of cation mixing on the oxygen evolution reaction (OER) activity of a LaxSr1–xCoyFe1–yO3 (LSCF) double perovskite is investigated using density functional theory (DFT) calculations. The O 2p band center (E2p) has a good linear relation with the binding energy of the OER intermediate species when the chemical composition is varied by only the x or y value, but this relation is insufficient for describing the nonmonotonic behavior over the entire x and y ranges. Based on the projected density of states and wavefunction analysis, the minority spin dxy electrons of surface layer metal atoms are significant due to their stability, where the antibonding states between dxy and the lattice oxygen p become occupied when Co atoms with one d electron more than Fe are present. Thus, by additionally considering the dxy band center, a surface electronic descriptor (E2p – 0.4Edxy) excellently describes the binding energy of the OER intermediates and the stability against oxygen-vacancy formation, which also explains the enhanced OER stability and efficient Fe–Co mixing. Our study unveils the key mechanism of the excellent OER performance and high stability of previously reported LSCF materials as well as provides heterostructure engineering guidance for optimal surface electronic structures

Self-Assembled and Highly Selective Sensors Based on Air-Bridge-Structured Nanowire Junction Arrays

We describe a strategy for creating an air-bridge-structured nanowire junction array platform that capable of reliably discriminating between three gases (hydrogen, carbon monoxide, and nitrogen dioxide) in air. Alternatively driven dual nanowire species of ZnO and CuO with the average diameter of ∼30 nm on a single substrate are used and decorated with metallic nanoparticles to form two-dimensional microarray, which do not need to consider the post fabrications. Each individual nanowires in the array form n–n, p–p, and p–n junctions at the micro/nanoscale on single substrate and the junctions act as electrical conducting path for carriers. The adsorption of gas molecules to the surface changes the potential barrier height formed at the junctions and the carrier transport inside the straight semiconductors, which provide the ability of a given sensor array to differentiate among the junctions. The sensors were tested for their ability to distinguish three gases (H₂, CO, and NO₂), which they were able to do unequivocally when the data was classified using linear discriminant analysis

Superaerophobic/Superhydrophilic Multidimensional Electrode System for High-Current-Density Water Electrolysis

Water electrolysis is emerging as a promising renewable-energy technology for the green production of hydrogen, which is a representative and reliable clean energy source. From economical and industrial perspectives, the development of earth-abundant non-noble metal-based and bifunctional catalysts, which can simultaneously exhibit high catalytic activities and stabilities for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), is critical; however, to date, these types of catalysts have not been constructed, particularly, for high-current-density water electrolysis at the industrial level. This study developed a heterostructured zero-dimensional (0D)–one-dimensional (1D) PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF)-Ni3S2 as a self-supported catalytic electrode via interface and morphology engineering. This unique heterodimensional nanostructure of the PBSCF-Ni3S2 system demonstrates superaerophobic/superhydrophilic features and maximizes the exposure of the highly active heterointerface, endowing the PBSCF-Ni3S2 electrode with outstanding electrocatalytic performances in both HER and OER and exceptional operational stability during the overall water electrolysis at high current densities (500 h at 500 mA cm–2). This study provides important insights into the development of catalytic electrodes for efficient and stable large-scale hydrogen production systems

Electrothermally Induced Highly Responsive and Highly Selective Vanadium Oxide Hydrogen Sensor Based on Metal–Insulator Transition

We report highly effective hydrogen gas detection based on the metal–insulator transition (MIT) by the electrothermally induced Pd-nanoparticles-decorated vanadium oxide (VO₂) nanowire prepared by the efficient and size-controllable growth method originating from V₂O₅ thin film driven by supercooled liquid nanodroplets. By irradiating a well-defined electron beam into the nanowires, we could significantly increase the conductivity up to four times with only a modest change in the semiconductor-to-metal transition temperature (<2 °C). When exposed to trace amounts of hydrogen gas in a single nanowire configuration, the enhanced conductivity gave rise to about a two times as fast transition to metallic phase even near room temperature (∼35 °C), by reaching much faster (∼3×) a critical current density at which the self-heating initiates. Consequently, we achieved the greatly shorter response time as well as lower operating temperature and voltage for the detection of hydrogen gas in a single VO₂ nanowire device, which can be attributed to the self-heating effect accelerated by the increase in the conductivity. The single nanowire sensor also shows the capability of detecting selectively hydrogen of different three gases (O₂, CO, and ethylene)

Self-Powering Gas Sensing System Enabled by Double-Layer Triboelectric Nanogenerators Based on Poly(2-vinylpyridine)@BaTiO₃ Core–Shell Hybrids with Superior Dispersibility and Uniformity

Current core–shell hybrids used in diverse energy-related applications possess limited dispersibility and film uniformity that govern their overall performances. Herein, we showcase superdispersible core–shell hybrids (P2VP@BaTiO3) composed of a poly(2-vinylpyridine) (P2VP) (5–20 wt %) and a barium titanate oxide (BaTiO3), maximizing dielectric constants by forming the high-quality uniform films. The P2VP@BaTiO3-based triboelectric nanogenerators (TENGs), especially the 10 wt % P2VP (P2VP10@BaTiO3)-based one, deliver significantly enhanced output performances compared to physically mixed P2VP/BaTiO3 counterparts. The P2VP10@BaTiO3-based double-layer TENG exhibits not only an excellent transferred charge density of 281.7 μC m–2 with a power density of 27.2 W m–2 but also extraordinary device stability (∼100% sustainability of the maximum output voltage for 54,000 cycles and ∼68.7% voltage retention even at 99% humidity). Notably, introducing the MoS2/SiO2/Ni-mesh layer into this double-layer TENG enables ultrahigh charge density of up to 1228 μC m–2, which is the top value reported for the TENGs so far. Furthermore, we also demonstrate a near-field communication-based sensing system for monitoring CO2 gas using our developed self-powered generator with enhanced output performance and robustness

Hierarchically Driven IrO₂ Nanowire Electrocatalysts for Direct Sensing of Biomolecules

Applying nanoscale device fabrications toward biomolecules, ultra sensitive, selective, robust, and reliable chemical or biological microsensors have been one of the most fascinating research directions in our life science. Here we introduce hierarchically driven iridium dioxide (IrO₂) nanowires directly on a platinum (Pt) microwire, which allows a simple fabrication of the amperometric sensor and shows a favorable electronic property desired for sensing of hydrogen peroxide (H₂O₂) and dihydronicotinamide adenine dinucleotide (NADH) without the aid of enzymes. This rational engineering of a nanoscale architecture based on the direct formation of the hierarchical 1-dimensional (1-D) nanostructures on an electrode can offer a useful platform for high-performance electrochemical biosensors, enabling the efficient, ultrasensitive detection of biologically important molecules

Self-Powering Gas Sensing System Enabled by Double-Layer Triboelectric Nanogenerators Based on Poly(2-vinylpyridine)@BaTiO₃ Core–Shell Hybrids with Superior Dispersibility and Uniformity

Current core–shell hybrids used in diverse energy-related applications possess limited dispersibility and film uniformity that govern their overall performances. Herein, we showcase superdispersible core–shell hybrids (P2VP@BaTiO3) composed of a poly(2-vinylpyridine) (P2VP) (5–20 wt %) and a barium titanate oxide (BaTiO3), maximizing dielectric constants by forming the high-quality uniform films. The P2VP@BaTiO3-based triboelectric nanogenerators (TENGs), especially the 10 wt % P2VP (P2VP10@BaTiO3)-based one, deliver significantly enhanced output performances compared to physically mixed P2VP/BaTiO3 counterparts. The P2VP10@BaTiO3-based double-layer TENG exhibits not only an excellent transferred charge density of 281.7 μC m–2 with a power density of 27.2 W m–2 but also extraordinary device stability (∼100% sustainability of the maximum output voltage for 54,000 cycles and ∼68.7% voltage retention even at 99% humidity). Notably, introducing the MoS2/SiO2/Ni-mesh layer into this double-layer TENG enables ultrahigh charge density of up to 1228 μC m–2, which is the top value reported for the TENGs so far. Furthermore, we also demonstrate a near-field communication-based sensing system for monitoring CO2 gas using our developed self-powered generator with enhanced output performance and robustness

Highly Branched RuO₂ Nanoneedles on Electrospun TiO₂ Nanofibers as an Efficient Electrocatalytic Platform

Highly single-crystalline ruthenium dioxide (RuO₂) nanoneedles were successfully grown on polycrystalline electrospun titanium dioxide (TiO₂) nanofibers for the first time by a combination of thermal annealing and electrospinning from RuO₂ and TiO₂ precursors. Single-crystalline RuO₂ nanoneedles with relatively small dimensions and a high density on electrospun TiO₂ nanofibers are the key feature. The general electrochemical activities of RuO₂ nanoneedles–TiO₂ nanofibers and Ru­(OH)₃-TiO₂ nanofibers toward the reduction of [Fe­(CN)₆]^3– were carefully examined by cyclic voltammetry carried out at various scan rates; the results indicated favorable charge-transfer kinetics of [Fe­(CN)₆]^3– reduction via a diffusion-controlled process. Additionally, a test of the analytical performance of the RuO₂ nanoneedles–TiO₂ nanofibers for the detection of a biologically important molecule, hydrogen peroxide (H₂O₂), indicated a high sensitivity (390.1 ± 14.9 μA mM^–1 cm^–2 for H₂O₂ oxidation and 53.8 ± 1.07 μA mM^–1 cm^–2 for the reduction), a low detection limit (1 μM), and a wide linear range (1–1000 μM), indicating H₂O₂ detection performance better than or comparable to that of other sensing systems