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.

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.

Recent Advances in Triboelectric Nanogenerators: From Technological Progress to Commercial Applications

Serious climate changes and energy-related environmental problems are currently critical issues in the world. In order to reduce carbon emissions and save our environment, renewable energy harvesting technologies will serve as a key solution in the near future. Among them, triboelectric nanogenerators (TENGs), which is one of the most promising mechanical energy harvesters by means of contact electrification phenomenon, are explosively developing due to abundant wasting mechanical energy sources and a number of superior advantages in a wide availability and selection of materials, relatively simple device configurations, and low-cost processing. Significant experimental and theoretical efforts have been achieved toward understanding fundamental behaviors and a wide range of demonstrations since its report in 2012. As a result, considerable technological advancement has been exhibited and it advances the timeline of achievement in the proposed roadmap. Now, the technology has reached the stage of prototype development with verification of performance beyond the lab scale environment toward its commercialization. In this review, distinguished authors in the world worked together to summarize the state of the art in theory, materials, devices, systems, circuits, and applications in TENG fields. The great research achievements of researchers in this field around the world over the past decade are expected to play a major role in coming to fruition of unexpectedly accelerated technological advances over the next decade.

Engineering the Local Atomic Configuration in 2H TMDs for Efficient Electrocatalytic Hydrogen Evolution

The introduction of heteroatoms is a widely employed strategy for electrocatalysis of transition metal dichalcogenides (TMDs). This approach activates the inactive basal plane, effectively boosting the intrinsic catalytic activity. However, the effect of atomic configurations incorporated within the TMDs' lattice on catalytic activity is not thoroughly understood owing to the lack of controllable synthetic approaches for highly doped TMDs. In this study, we demonstrate a facile approach to realizing heavily doped MoS₂ with a high doping concentration above 16% via intermediate-reaction-mediated chemical vapor deposition. As the V doping concentration increased, the incorporated V atoms coalesced in a manner that enabled both the basal plane activation and electrical conductivity enhancement of MoS₂. This accelerated the kinetics of the hydrogen evolution reaction (HER) through the reduced Gibbs free energy of hydrogen adsorption, as evidenced by experimental and theoretical analyses. Consequently, the coalesced V-doped MoS₂ exhibited superior HER performance, with an overpotential of 100 mV at 10 mA cm^-2, surpassing the pristine and single-atom-doped counterparts. This study provides an intriguing pathway for engineering the atomic doping configuration of TMDs to develop efficient 2D nanomaterial-based electrocatalysts.

Autonomous Resonance-Tuning Mechanism for Environmental Adaptive Energy Harvesting

An innovative autonomous resonance-tuning (ART) energy harvester is reported that utilizes adaptive clamping systems driven by intrinsic mechanical mechanisms without outsourcing additional energy. The adaptive clamping system modulates the natural frequency of the harvester's main beam (MB) by adjusting the clamping position of the MB. The pulling force induced by the resonance vibration of the tuning beam (TB) provides the driving force for operating the adaptive clamp. The ART mechanism is possible by matching the natural frequencies of the TB and clamped MB. Detailed evaluations are conducted on the optimization of the adaptive clamp tolerance and TB design to increase the pulling force. The energy harvester exhibits an ultrawide resonance bandwidth of over 30 Hz in the commonly accessible low vibration frequency range (<100 Hz) owing to the ART function. The practical feasibility is demonstrated by evaluating the ART performance under both frequency and acceleration-variant conditions and powering a location tracking sensor.

Batch Nanofabrication of Suspended Single 1D Nanoheaters for Ultralow-Power Metal Oxide Semiconductor-Based Gas Sensors

The demand for power-efficient micro-and nanodevices is increasing rapidly. In this regard, electrothermal nanowire-based heaters are promising solutions for the ultralow-power devices required in IoT applications. Herein, a method is demonstrated for producing a 1D nanoheater by selectively coating a suspended pyrolyzed carbon nanowire backbone with a thin Au resistive heater layer and utilizing it in a portable gas sensor system. This sophisticated nanostructure is developed without complex nanofabrication and nanoscale alignment processes, owing to the suspended architecture and built-in shadow mask. The suspended carbon nanowires, which are batch-fabricated using carbon-microelectromechanical systems technology, maintain their structural and functional integrity in subsequent nanopatterning processes because of their excellent mechanical robustness. The developed nanoheater is used in gas sensors via user-designable localization of the metal oxide semiconductor nanomaterials onto the central region of the nanoheater at the desired temperature. This allows the sensing site to be uniformly heated, enabling reliable and sensitive gas detection. The 1D nanoheater embedded gas sensor can be heated immediately to 250 °C at a remarkably low power of 1.6 mW, surpassing the performance of state-of-the-art microheater-based gas sensors. The presented technology offers facile 1D nanoheater production and promising pathways for applications in various electrothermal devices.

Phase-Tuned MoS2 and Its Hybridization with Perovskite Oxide as Bifunctional Catalyst: A Rationale for Highly Stable and Efficient Water Splitting

The efficient realization of bifunctional catalysts has immense opportunities in energy conversion technologies such as water splitting. Transition metal dichalcogenides (TMDs) are considered excellent hydrogen evolution catalysts owing to their hierarchical atomic-scale layered structure and feasible phase transition. On the other hand, for efficient oxygen evolution, perovskite oxides offer the best performance based on their rational design and flexible compositional structure. A unique way to achieve an efficient hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in a single-cell configuration is through the hybridization of TMDs with perovskite oxides to form a bifunctional electrocatalyst. Here, we report a simple yet effective strategy to inherently tune the intrinsic properties of a TMD based on MoS₂ and its hybridization with LaCoO₃ perovskite oxide to deliver enhanced electrocatalytic activity for both the HER and OER. Detailed Raman and XPS measurements highlighted a clear phase transformation of MoS₂ from a semiconducting to metallic phase by effectively tailoring the precursor compositions. Based on this, the morphological features yielded an interesting spherical flower-shaped nanostructure with vertically aligned petals of MoS₂ with increased surface-active edge sites suitable for the HER. Subsequent hybridization of nanostructured MoS₂ with LaCoO₃ provides a bifunctional catalytic system with an increased BET surface area of 33.4 m²/g for an overall improvement in water splitting with a low onset potential (HER: 242 mV and OER: 1.6 V @10 mA cm^-2) and Tafel slope (HER: 78 mV dec^-1; OER: 62.5 mV dec^-1). Additionally, the bifunctional catalyst system exhibits long-term stability of up to ∼400 h under continuous operation at a high current density of 50 mA cm^-2. These findings will pave the way for developing cost-effective and less complex bifunctional catalysts by simply and inherently tuning the influential material properties for full-cell electrochemical water splitting.

High rotational speed hand-powered triboelectric nanogenerator toward a battery-free point-of-care detection system

The timely biochemical detection of environmental pollutants or infectious disease is a predominant challenge for global health and people living in remote areas. However, the energy supply is still difficult for both the pretreatment and test steps, especially for diagnostics in resource-limited environments or outdoor point-of-care testing. Herein, we demonstrate a hand-powered triboelectric nanogenerator (TENG) system, which can simultaneously accomplish centrifugal pretreatment and analysis without an additional power supply. The complete separation of plasma from red blood cells can be achieved within 1.5 min at an operation frequency of 1 Hz. Besides, according to the impressive high rotational speed of 7500 rpm, the rotating mechanical energy can be efficiently recycled by the TENG to power different electronic devices, such as an electronic watch or thermometer. As a demonstration, the pretreatment of lake water and the detection of hydrogen peroxide contained in it has been realized. The combination of the system with different types of sensors will further promote its applications in multifarious biochemical detections. Moreover, this TENG system is effective, field-portable and ultra-low cost, and is promising for battery-free point-of-care diagnostic systems for outdoor or harsh environments.

A hand-powered TENG system has been developed to kill two birds with one stone, simultaneously realizing centrifugation and rotary mechanical energy harvesting. Then, centrifugation of lake water and detection of H₂O₂ in it has been realized.

Graphene Antiadhesion Layer for the Effective Peel-and-Pick Transfer of Metallic Electrodes toward Flexible Electronics

Owing to its exceptional physicochemical properties, graphene has demonstrated unprecedented potential in a wide array of scientific and industrial applications. By exploiting its chemically inert surface endowed with unique barrier functionalities, we herein demonstrate antiadhesive monolayer graphene films for realizing a peel-and-pick transfer process of target materials from the donor substrate. When the graphene antiadhesion layer (AAL) is inserted at the interface between the metal and the arbitrary donor substrate, the interfacial interactions can be effectively weakened by the weak interplanar van der Waals forces of graphene, enabling the effective release of the metallic electrode from the donor substrate. The flexible embedded metallic electrode with graphene AAL exhibited excellent electrical conductivity, mechanical durability, and chemical resistance, as well as excellent performance in flexible heater applications. This study afforded an effective strategy for fabricating high-performance and ultraflexible embedded metallic electrodes for applications in the field of highly functional flexible electronics.

Strategies for ultrahigh outputs generation in triboelectric energy harvesting technologies: from fundamentals to devices

Since 2012, a triboelectric nanogenerator (TENG) has provided new possibilities to convert tiny and effective mechanical energies into electrical energies by the physical contact of two objects. Over the past few years, with the advancement of materials' synthesis and device technologies, the TENGs generated a high instantaneous output power of several mW/cm², required to drive various self-powered systems. However, TENGs may suffer from intrinsic and practical limitations such as air breakdown that affect the further increase of the outputs. This article provides a comprehensive review of high-output TENGs from fundamental issues through materials to devices. Finally, we show some strategies for fabricating high-output TENGs.

Directional Ostwald Ripening for Producing Aligned Arrays of Nanowires

The remarkable electronic and mechanical properties of nanowires have great potential for fascinating applications; however, the difficulties of assembling ordered arrays of aligned nanowires over large areas prevent their integration into many practical devices. In this paper, we show that aligned VO₂ nanowires form spontaneously after heating a thin V₂O₅ film on a grooved SiO₂ surface. Nanowires grow after complete dewetting of the film, after which there is the formation of supercooled nanodroplets and subsequent Ostwald ripening and coalescence. We investigate the growth mechanism using molecular dynamics simulations of spherical Lennard-Jones particles, and the simulations help explain how the grooved surface produces aligned nanowires. Using this simple synthesis approach, we produce self-aligned, millimeter-long nanowire arrays with uniform metal-insulator transition properties; after their transfer to a polymer substrate, the nanowires act as a highly sensitive array of strain sensors with a very fast response time of several tens of milliseconds.

Morphology-Controlled Aluminum-Doped Zinc Oxide Nanofibers for Highly Sensitive NO2 Sensors with Full Recovery at Room Temperature

Room-temperature (RT) gas sensitivity of morphology-controlled free-standing hollow aluminum-doped zinc oxide (AZO) nanofibers for NO2 gas sensors is presented. The free-standing hollow nanofibers are fabricated using a polyvinylpyrrolidone fiber template electrospun on a copper electrode frame followed by radio-frequency sputtering of an AZO thin overlayer and heat treatment at 400 °C to burn off the polymer template. The thickness of the AZO layer is controlled by the deposition time. The gas sensor based on the hollow nanofibers demonstrates fully recoverable n-type RT sensing of low concentrations of NO2 (0.5 ppm). A gas sensor fabricated with Al2O3-filled AZO nanofibers exhibits no gas sensitivity below 75 °C. The gas sensitivity of a sensor is determined by the density of molecules above the minimum energy for adsorption, collision frequency of gas molecules with the surface, and available adsorption sites. Based on finite-difference time-domain simulations, the RT sensitivity of hollow nanofiber sensors is ascribed to the ten times higher collision frequency of NO2 molecules confined inside the fiber compared to the outer surface, as well as twice the surface area of hollow nanofibers compared to the filled ones. This approach might lead to the realization of RT sensitive gas sensors with 1D nanostructures.

Plasmonic gold nanoparticle-decorated BiVO4/ZnO nanowire heterostructure photoanodes for efficient water oxidation

To enhance the charge separation and kinetics of water oxidation using a BiVO₄ photoanode, a BiVO₄/ZnO nanowire heterostructure decorated with gold (Au) nanoparticles is fabricated as a photoanode for photoelectrochemical water splitting.

Photo-stimulated triboelectric generation

A new photo-stimulated triboelectric generation occurring between a metal-oxide and polyimide during friction was demonstrated. The output currents of the triboelectric nanogenerator were significantly enhanced, under light illumination, up to approximately 5 times depending on the wavelength of the light, providing a new route for energy harvesting devices as well as self-powered selective photodetectors.

Enhanced piezoresponse of highly aligned electrospun poly(vinylidene fluoride) nanofibers

Well-ordered nanostructure arrays with controlled densities can potentially improve material properties; however, their fabrication typically involves the use of complicated processing techniques. In this work, we demonstrate a uniaxial alignment procedure for fabricating poly(vinylidene fluoride) (PVDF) electrospun nanofibers (NFs) by introducing collectors with additional steps. The mechanism of the observed NF alignment, which occurs due to the concentration of lateral electric field lines around collector steps, has been elucidated via finite-difference time-domain simulations. The membranes composed of well-aligned PVDF NFs are characterized by a higher content of the PVDF β-phase, as compared to those manufactured from randomly orientated fibers. The piezoelectric energy harvester, which was fabricated by transferring well-aligned PVDF NFs onto flexible substrates with Ag electrodes attached to both sides, exhibited a 2-fold increase in the output voltage and a 3-fold increase in the output current as compared to the corresponding values obtained for the device manufactured from randomly oriented NFs. The enhanced piezoresponse observed for the aligned PVDF NFs is due to their higher β-phase content, denser structure, smaller effective radius of curvature during bending, greater applied strain, and higher fraction of contributing NFs.

Mechanically Robust, Stretchable Solar Absorbers with Submicron-Thick Multilayer Sheets for Wearable and Energy Applications

A facile method to fabricate a mechanically robust, stretchable solar absorber for stretchable heat generation and an enhanced thermoelectric generator (TEG) is demonstrated. This strategy is very simple: it uses a multilayer film made of titanium and magnesium fluoride optimized by a two-dimensional finite element frequency-domain simulation, followed by the application of mechanical stresses such as bending and stretching to the film. This process produces many microsized sheets with submicron thickness (∼500 nm), showing great adhesion to any substrates such as fabrics and polydimethylsiloxane. It exhibits a quite high light absorption of approximately 85% over a wavelength range of 0.2-4.0 μm. Under 1 sun illumination, the solar absorber on various stretchable substrates increased the substrate temperature to approximately 60 °C, irrespective of various mechanical stresses such as bending, stretching, rubbing, and even washing. The TEG with the absorber on the top surface also showed an enhanced output power of 60%, compared with that without the absorber. With an incident solar radiation flux of 38.3 kW/m², the output power significantly increased to 24 mW/cm² because of the increase in the surface temperature to 141 °C.

Robust nanogenerators based on graft copolymers via control of dielectrics for remarkable output power enhancement

A robust nanogenerator based on poly(tert-butyl acrylate) (PtBA)-grafted polyvinylidene difluoride (PVDF) copolymers via dielectric constant control through an atom-transfer radical polymerization technique, which can markedly increase the output power, is demonstrated. The copolymer is mainly composed of α phases with enhanced dipole moments due to the π-bonding and polar characteristics of the ester functional groups in the PtBA, resulting in the increase of dielectric constant values by approximately twice, supported by Kelvin probe force microscopy measurements. This increase in the dielectric constant significantly increased the density of the charges that can be accumulated on the copolymer during physical contact. The nanogenerator generates output signals of 105 V and 25 μA/cm2, a 20-fold enhancement in output power, compared to pristine PVDF-based nanogenerator after tuning the surface potential using a poling method. The markedly enhanced output performance is quite stable and reliable in harsh mechanical environments due to the high flexibility of the films. On the basis of these results, a much faster charging characteristic is demonstrated in this study.

Unidirectional growth of single crystalline β-Na0.33V2O5and α-V2O5nanowires driven by controlling the pH of aqueous solution and their electrochemical performances for Na-ion batteries

Single crystalline β-Na_0.33V₂O₅and α-V₂O₅nanowires were prepared with pH controlled precursors.

Parallel Aligned Mesopore Arrays in Pyramidal-Shaped Gallium Nitride and Their Photocatalytic Applications

Parallel aligned mesopore arrays in pyramidal-shaped GaN are fabricated by using an electrochemical anodic etching technique, followed by inductively coupled plasma etching assisted by SiO2 nanosphere lithography, and used as a promising photoelectrode for solar water oxidation. The parallel alignment of the pores of several tens of micrometers scale in length is achieved by the low applied voltage and prepattern guided anodization. The dry etching of single-layer SiO2 nanosphere-coated GaN produces a pyramidal shape of the GaN, making the pores open at both sides and shortening the escape path of evolved gas bubbles produced inside pores during the water oxidation. The absorption spectra show that the light absorption in the UV range is ∼93% and that there is a red shift in the absorption edge by 30 nm, compared with the flat GaN. It also shows a remarkable enhancement in the photocurrent density by 5.3 times, compared with flat GaN. Further enhancement (∼40%) by the deposition of Ni was observed due to the generation of an electric field, which increases the charge separation ratio.

Gate-Tunable Spin Exchange Interactions and Inversion of Magnetoresistance in Single Ferromagnetic ZnO Nanowires

Electrical control of ferromagnetism in semiconductor nanostructures offers the promise of nonvolatile functionality in future semiconductor spintronics. Here, we demonstrate a dramatic gate-induced change of ferromagnetism in ZnO nanowire (NW) field-effect transistors (FETs). Ferromagnetism in our ZnO NWs arose from oxygen vacancies, which constitute deep levels hosting unpaired electron spins. The magnetic transition temperature of the studied ZnO NWs was estimated to be well above room temperature. The in situ UV confocal photoluminescence (PL) study confirmed oxygen vacancy mediated ferromagnetism in the studied ZnO NW FET devices. Both the estimated carrier concentration and temperature-dependent conductivity reveal the studied ZnO NWs are at the crossover of the metal-insulator transition. In particular, gate-induced modulation of the carrier concentration in the ZnO NW FET significantly alters carrier-mediated exchange interactions, which causes even inversion of magnetoresistance (MR) from negative to positive values. Upon sweeping the gate bias from -40 to +50 V, the MRs estimated at 2 K and 2 T were changed from -11.3% to +4.1%. Detailed analysis on the gate-dependent MR behavior clearly showed enhanced spin splitting energy with increasing carrier concentration. Gate-voltage-dependent PL spectra of an individual NW device confirmed the localization of oxygen vacancy-induced spins, indicating that gate-tunable indirect exchange coupling between localized magnetic moments played an important role in the remarkable change of the MR.

Optical design of ZnO-based antireflective layers for enhanced GaAs solar cell performance

A series of hierarchical ZnO-based antireflection coatings with different nanostructures (nanowires and nanosheets) is prepared hydrothermally, followed by means of RF sputtering of MgF2 layers for coaxial nanostructures. Structural analysis showed that both ZnO had a highly preferred orientation along the 〈0001〉 direction with a highly crystalline MgF2 shell coated uniformly. However, a small amount of Al was present in nanosheets, originating from Al diffusion from the Al seed layer, resulting in an increase of the optical bandgap. Compared with the nanosheet-based antireflection coatings, the nanowire-based ones exhibited a significantly lower reflectance (∼2%) in ultraviolet and visible light wavelength regions. In particular, they showed perfect light absorption at wavelength less than approximately 400 nm. However, a GaAs single junction solar cell with nanosheet-based antireflection coatings showed the largest enhancement (43.9%) in power conversion efficiency. These results show that the increase of the optical bandgap of the nanosheets by the incorporation of Al atoms allows more photons enter the active region of the solar cell, improving the performance.

Visible Color Tunable Emission in Three-Dimensional Light Emitting Diodes by MgO Passivation of Pyramid Tip

We demonstrated visible color tunable three-dimensional (3D) pyramidal light emitting diodes by depositing the MgO on and near the tip of the pyramid as an insulating layer. Here, we show that the degradation of the materials (i.e., p-GaN) crystallinity and the built-in electric field due to the nanoscale geometry of the tip region is responsible for the large leakage current observed in LEDs. Confocal scanning electroluminescence microscopy images clearly showed that the intensity of the light emitted out of the side facet of the pyramid is much higher than that of the light extracted out of the tip surface, indicating that the MgO layer prohibited the carrier injection to the MQWs layer, suppressing the leakage occurring at or near the tip region of the pyramids. The color range of the LEDs can be also tuned by using the MgO layer, a blue-shift by 10.3 nm in the wavelength. This technique is simple and scalable, providing a promising solution for developing 3D pyramidal LEDs with low leakage current and controllable light emission.

Correction: Electrospun ion gel nanofibers for flexible triboelectric nanogenerator: electrochemical effect on output power

Correction for 'Electrospun ion gel nanofibers for flexible triboelectric nanogenerator: electrochemical effect on output power' by Byeong Uk Ye et al., Nanoscale, 2015, 7, 16189-16194.

Electrospun ion gel nanofibers for flexible triboelectric nanogenerator: electrochemical effect on output power

A simple fabrication route for ion gel nanofibers in a triboelectric nanogenerator was demonstrated. Using an electrospinning technique, we could fabricate a large-area ion gel nanofiber mat. The triboelectric nanogenerator was demonstrated by employing an ion gel nanofiber and the device exhibited an output power of 0.37 mW and good stability under continuous operation.

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.

Structural Evolution of Chemically-Driven RuO2 Nanowires and 3-Dimensional Design for Photo-Catalytic Applications

Growth mechanism of chemically-driven RuO2 nanowires is explored and used to fabricate three-dimensional RuO2 branched Au-TiO2 nanowire electrodes for the photostable solar water oxidation. For the real time structural evolution during the nanowire growth, the amorphous RuO2 precursors (Ru(OH)3 · H2O) are heated at 180 (°)C, producing the RuO2 nanoparticles with the tetragonal crystallographic structure and Ru enriched amorphous phases, observed through the in-situ synchrotron x-ray diffraction and the high-resolution transmission electron microscope images. Growth then proceeds by Ru diffusion to the nanoparticles, followed by the diffusion to the growing surface of the nanowire in oxygen ambient, supported by the nucleation theory. The RuO2 branched Au-TiO2 nanowire arrays shows a remarkable enhancement in the photocurrent density by approximately 60% and 200%, in the UV-visible and Visible region, respectively, compared with pristine TiO2 nanowires. Furthermore, there is no significant decrease in the device's photoconductance with UV-visible illumination during 1 day, making it possible to produce oxygen gas without the loss of the photoactvity.

Highly Stretchable 2D Fabrics for Wearable Triboelectric Nanogenerator under Harsh Environments

Highly stretchable 2D fabrics are prepared by weaving fibers for a fabric-structured triboelectric nanogenerator (FTENG). The fibers mainly consist of Al wires and polydimethylsiloxane (PDMS) tubes with a high-aspect-ratio nanotextured surface with vertically aligned nanowires. The fabrics were produced by interlacing the fibers, which was bonded to a waterproof fabric for all-weather use for fabric-structured triboelectric nanogenerator (FTENG). It showed a stable high-output voltage and current of 40 V and 210 μA, corresponding to an instantaneous power output of 4 mW. The FTENG also exhibits high robustness behavior even after 25% stretching, enough for use in smart clothing applications and other wearable electronics. For wearable applications, the nanogenerator was successfully demonstrated in applications of footstep-driven large-scale power mats during walking and power clothing attached to the elbow.

Ruthenium based nanostructures driven by morphological controls as efficient counter electrodes for dye-sensitized solar cells

We introduce a facile approach to use ruthenium dioxide (RuO₂) and ruthenium (Ru) nanostructures as effective counter electrodes instead of using platinum (Pt) for dye-sensitized solar cells (DSSCs).

RuO2–ReO3 composite nanofibers for efficient electrocatalytic responses

The electrochemical responses of RuO₂–ReO₃ electrospun composite nanofibers were remarkably enhanced depending on the amount of ReO₃ increased.

Conductance control in VO2 nanowires by surface doping with gold nanoparticles

The material properties of semiconductor nanowires are greatly affected by electrical, optical, and chemical processes occurring at their surfaces because of the very large surface-to-volume ratio. Precise control over doping as well as the surface charge properties has been demonstrated in thin films and nanowires for fundamental physics and application-oriented research. However, surface doping behavior is expected to differ markedly from bulk doping in conventional semiconductor materials. Here, we show that placing gold nanoparticles, in controlled manner, on the surface of an insulating vanadium dioxide nanowire introduces local charge carriers in the nanowire, and one could, in principle, completely and continuously alter the material properties of the nanowire and obtain any intermediate level of conductivity. The current in the nanowire increased by nearly 3 times when gold nanoparticles of 10(11) cm(-2) order of density were controllably placed on the nanowire surface. A strong quadratic space-charge limited (SCL) transport behavior was also observed from the conductance curve suggesting the formation of two-dimensional (2D) electron-gas-like confined layer in the nanowire with adsorbed Au NPs. In addition to stimulating scientific interest, such unusual surface doping phenomena may lead to new applications of vanadium dioxide-based electronic, optical, and chemical sensing nanodevices.

Hydrophobic sponge structure-based triboelectric nanogenerator

Hydrophobic sponge structure-based triboelectric nanogenerators using an inverse opal structured film for sustainable energy harvesting over a wide range of humid atmosphere have been successfully demonstrated. The output voltage and current density reach a record value of 130 V and 0.10 mA cm(-2) , respectively, giving over 10-fold power enhancement, compared with the flat film-based triboelectric nanogenerator.

ZnO nanowire-based antireflective coatings with double-nanotextured surfaces

High-aspect-ratio nanotextured surfaces with different morphologies (straight, core-shell type, and core-branch type nanowires) are prepared by a hydrothermal method of ZnO nanowires, followed by means of RF sputtering for core-shell type nanowires and e-beam evaporation for branch-type nanowires. The structural analysis showed that the MgO has highly preferred orientation along the ⟨111⟩ and ⟨200⟩ direction, respectively, and the crystalline continuity between the ZnO and MgO layers were also showed. Compared with ZnO nanowires, the MgO/ZnO samples drastically suppress broad and omnidirection reflection, which ascribes to the refractive-index modulation along the lateral direction of nanowires growth as well as the vertical direction. It was also shown that morphology could have a substantial influence on the antireflection property. These results suggest that double-nanotextured surface is one of the promising structures for antireflective surfaces without fine control in nanowire morphology.

A near single crystalline TiO2 nanohelix array: enhanced gas sensing performance and its application as a monolithically integrated electronic nose

We present high performance gas sensors based on an array of near single crystalline TiO(2) nanohelices fabricated by rotating oblique angle deposition (OAD). The combination of large surface-to-volume ratio, extremely small size (<30 nm) comparable to the Debye length, a near single crystallinity of TiO(2) nanohelices, together with the unique top-and-bottom electrode configuration hugely improves the H(2)-sensing performance, including ∼10 times higher response at 50 ppm, approximately a factor of 5 lower detection limit, and much faster response time than the conventional TiO(2) thin film devices. Beyond such remarkable performance enhancement, the excellent compatibility of the OAD method compared with the conventional micro-fabrication technology opens a new avenue for monolithic integration of high-performance chemoresistive sensors to fabricate a simple, low cost, reliable, yet fully functional electronic nose and multi-functional smart chips for in situ environmental monitoring.

Unprecedented colorimetric responses of polydiacetylenes driven by plasma induced polymerization and their patterning applications

In this communication, we report that polydiacetylenes (PDAs) can be alternatively prepared via a plasma induced polymerization process.

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.

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 (IrO(2)) 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 (H(2)O(2)) 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-connected and habitually tilted piezoelectric nanorod array

We fabricated a self-connected and habitually tilted ZnO nanorod (NR) array, which is free of any patterning process for the connection of the NRs and is easily bent by a normal force. The vertically well-aligned ZnO NRs were grown by a strain relaxation process on MgO-buffered C-plane sapphire, and the remaining epitaxial ZnO wetting layer acted as a self-connecting layer of NRs. The epitaxial ZnO film on the step-terrace structured substrate caused the tilting angle from the surface normal direction (~0.2°) to match the step between the ZnO film and MgO-buffered C-plane sapphire, resulting in easy bending of the ZnO NRs by normal force. The unsymmetrical strain between the tensile and compressive stressed region in the habitually tilted ZnO NRs caused a gradient in the piezoelectric potential, resulting in an electrical field along the lateral direction of NR growth, resulting in the control of the current direction and level to be about 0.1 μA/cm(2) at 2 kgf normal force.

Tin-oxide-nanowire-based electronic nose using heterogeneous catalysis as a functionalization strategy

An electronic nose (e-nose) strategy is described based on SnO(2) nanowire arrays whose sensing properties are modified by changing their operating temperatures and by decorating some of the nanowires with metallic nanoparticles. Since the catalytic processes occurring on the metal nanoparticles depend on the identity of the metal, decorating the semiconducting nanowires with various metal nanoparticles is akin to functionalizing them with chemically specific moieties. Other than the synthesis of the nanowires, all other steps in the fabrication of the e-nose sensors were carried out using top-down microfabrication processes, paving the way to a useful strategy for making low cost, nanowire-based e-nose chips. The sensors were tested for their ability to distinguish three reducing gases (H(2), CO, and ethylene), which they were able to do unequivocally when the data was classified using linear discriminant analysis. The discriminating ability of this e-nose design was not impacted by the lengths or diameters of the nanowires used.

Growth of metal oxide nanowires from supercooled liquid nanodroplets

Nanometer-sized liquid droplets formed at temperatures below the bulk melting point become supercooled as they grow through Ostwald ripening or coalescence and can be exploited to grow nanowires without any catalyst. We used this simple approach to synthesize a number of highly crystalline metal oxide nanowires in a chemical or physical vapor deposition apparatus. Examples of nanowires made in this way include VO(2), V(2)O(5), RuO(2), MoO(2), MoO(3), and Fe(3)O(4), some of which have not been previously reported. Direct evidence of this new mechanism of nanowire growth is found from in situ 2-dimensional GISAXS (grazing incidence small angle X-ray scattering) measurements of VO(2) nanowire growth, which provides quantitative information on the shapes and sizes of growing nanowires as well as direct evidence of the presence of supercooled liquid droplets. We observe dramatic changes in nanowire growth by varying the choice of substrate, reflecting the influence of wetting forces on the supercooled nanodroplet shape and mobility as well as substrate-nanowire lattice matching on the definition of nanowire orientation. Surfaces with defects can also be used to pattern the growth of the nanowires. The simplicity of this synthesis concept suggests it may be rather general in its application.

Pd-sensitized single vanadium oxide nanowires: highly responsive hydrogen sensing based on the metal-insulator transition

Exceptionally sensitive hydrogen sensors were produced using Pd-nanoparticle-decorated, single vanadium dioxide nanowires. The high-sensitivity arises from the large downward shift in the insulator to metal transition temperature following the adsorption on and incorporation of atomic hydrogen, produced by dissociative chemisorption on Pd, in the VO(2), producing approximately 1000-fold current increases. During a rapid initial process, the insulator to metal transition temperature is decreased by >10 degrees C even when exposed to trace amounts of hydrogen gas. Subsequently, hydrogen continues to diffuse into the VO(2) for several hours before saturation is achieved with only a modest change in the insulator to metal transition temperature but with a significant increase in the conductivity. The two time scales over which H-related processes occur in VO(2) likely signal the involvement of two distinct mechanisms influencing the electronic structure of the material one of which involves electron-phonon coupling pursuant to the modification of the vibrational normal modes of the solid by the introduction of H as an impurity.

Polarized surface-enhanced Raman spectroscopy from molecules adsorbed in nano-gaps produced by electromigration in silver nanowires

Uniform nanogaps were produced in silver nanowires previously fabricated by AC electrodeposition in porous anodic alumina templates and released by dissolving the oxide matrix. Configuring the nanowires with electrodes and passing current through them created nanogaps by electromigration. Uniform nanogaps approximately perpendicular to the axes of the nanowires were produced with widths in the range approximately 17 to approximately 40 nm. Gaps as small as 5 nm but with less uniform geometries could also be produced in this manner. A heuristic model, in which electromigrative and diffusional effects are included, is used to relate the size of the nanogap produced to the local resistance of the region in the nanowire where the gap ultimately forms. Intense surface-enhanced Raman spectra were observed from Rhodamine 6G adsorbed on the nanowire in the gap. For gaps that uniformly divided the nanowire across its width, the surface enhanced Raman spectroscopy intensity was maximum when the electric vector of the exciting light was oriented across the gap, as predicted by electromagnetic enhancement calculations.

Polarization-dependent surface-enhanced Raman scattering from a silver-nanoparticle-decorated single silver nanowire

Single silver nanowires produced by DC electrodeposition in porous anodic alumina templates were surface-functionalized with the bifunctional molecule 4-aminobenzenthiol (ABT) then exposed to aqueous silver nanoparticles resulting in a silver nanoparticle-decorated silver nanowire. The polarization dependent surface-enhanced Raman (SERS) signal from this system showed significant intensity anisotropy when measured at a midsection of the nanowire, where the largest SERS intensities were observed when the incident light was polarized perpendicular to the nanowire's long axis but was almost isotropic near the tip of the nanowire. The observed effects were accounted for in terms of the electromagnetic fields concentrated in the collection of hot spots created through the ABT-linker-driven nanoparticle-nanowire self-assembly process.