A Transparent Nanopatterned Chemiresistor: Visible-Light Plasmonic Sensor for Trace-Level NO2 Detection at Room Temperature.

The highly selective detection of trace gases using transparent sensors at room temperature remains challenging. Herein, transparent nanopatterned chemiresistors composed of aligned 1D Au-SnO2 nanofibers, which can detect toxic NO2 gas at room temperature under visible light illumination is reported. Ten straight Au-SnO2 nanofibers are patterned on a glass substrate with transparent electrodes assisted by direct-write, near-field electrospinning, whose extremely low coverage of sensing materials (≈0.3%) lead to the high transparency (≈93%) of the sensor. The sensor exhibits a highly selective, sensitive, and reproducible response to sub-ppm levels of NO2 , and its detection limit is as low as 6 ppb. The unique room-temperature NO2 sensing under visible light emanates from the localized surface plasmonic resonance effect of Au nanoparticles, thereby enabling the design of new transparent oxide-based gas sensors without external heaters or light sources. The patterning of nanofibers with extremely low coverage provides a general strategy to design diverse compositions of gas sensors, which can facilitate the development of a wide range of new applications in transparent electronics and smart windows wirelessly connected to the Internet of Things.

Superionic Halogen-Rich Li-Argyrodites Using In Situ Nanocrystal Nucleation and Rapid Crystal Growth.

Although several crystalline materials have been developed as Li-ion conductors for use as solid electrolytes in all-solid-state batteries (ASSBs), producing materials with high Li-ion conductivities is time-consuming and cost-intensive. Herein, we introduce a superionic halogen-rich Li-argyrodite (HRLA) and demonstrate its innovative synthesis using ultimate-energy mechanical alloying (UMA) and rapid thermal annealing (RTA). UMA with a 49 G-force milling energy provides a one-pot process that includes mixing, glassification, and crystallization, to produce as-milled HRLA powder that is ∼70% crystallized; subsequent RTA using an infrared lamp increases this crystallinity to ∼82% within 25 min. Surprisingly, this HRLA exhibits the highest Li-ion conductivity among Li-argyrodites (10.2 mS cm-1 at 25 °C, cold-pressed powder compact) reported so far. Furthermore, we confirm that this superionic HRLA works well as a promising solid electrolyte without a decreased intrinsic electrochemical window in various electrode configurations and delivers impressive cell performance (114.2 mAh g-1 at 0.5 C).

Conversion Reaction Mechanism of Ultrafine Bimetallic Co-Fe Selenides Embedded in Hollow Mesoporous Carbon Nanospheres and Their Excellent K-Ion Storage Performance.

Potassium-ion batteries (KIBs) are considered as promising alternatives to lithium-ion batteries owing to the abundance and affordability of potassium. However, the development of suitable electrode materials that can stably store large-sized K ions remains a challenge. This study proposes a facile impregnation method for synthesizing ultrafine cobalt-iron bimetallic selenides embedded in hollow mesoporous carbon nanospheres (HMCSs) as superior anodes for KIBs. This involves loading metal precursors into HMCS templates using a repeated "drop and drying" process followed by selenization at various temperatures, facilitating not only the preparation of bimetallic selenide/carbon composites but also controlling their structures. HMCSs serve as structural skeletons, conductive templates, and vehicles to restrain the overgrowth of bimetallic selenide particles during thermal treatment. Various analysis strategies are employed to investigate the charge-discharge mechanism of the new bimetallic selenide anodes. This unique-structured composite exhibits a high discharge capacity (485 mA h g-1 at 0.1 A g-1 after 200 cycles) and enhanced rate capability (272 mA h g-1 at 2.0 A g-1 ) as a promising anode material for KIBs. Furthermore, the electrochemical properties of various nanostructures, from hollow to frog egg-like structures, obtained by adjusting the selenization temperature, are compared.

Investigation of Binary Metal (Ni, Co) Selenite as Li-Ion Battery Anode Materials and Their Conversion Reaction Mechanism with Li Ions.

Highly efficient anode materials with novel compositions for Li-ion batteries are actively being researched. Multicomponent metal selenite is a promising candidate, capable of improving their electrochemical performance through the formation of metal oxide and selenide heterostructure nanocrystals during the first cycle. Here, the binary nickel-cobalt selenite derived from Ni-Co Prussian blue analogs (PBA) is chosen as the first target material: the Ni-Co PBA are selenized and partially oxidized in sequence, yielding (NiCo)SeO3 phase with a small amount of metal selenate. The conversion mechanism of (NiCo)SeO3 for Li-ion storage is studied by cyclic voltammetry, in situ X-ray diffraction, ex situ X-ray photoelectron spectroscopy, in situ electrochemical impedance spectroscopy, and ex situ transmission electron microscopy. The reversible reaction mechanism of (NiCo)SeO3 with the Li ions is described by the reaction: NiO + CoO + xSeO2 + (1 - x)Se + (4x + 6)Li+ + (4x + 6)e- ↔ Ni + Co + (2x + 2)Li2 O + Li2 Se. To enhance electrochemical properties, polydopamine-derived carbon is uniformly coated on (NiCo)SeO3 , resulting in excellent cycling and rate performances for Li-ion storage. The discharge capacity of C-coated (NiCo)SeO3 is 680 mAh g-1 for the 1500th cycle when cycled at a current density of 5 A g-1 .

Synthesis Process of CoSeO3 Microspheres for Unordinary Li-ion Storage Performances and Mechanism of Their Conversion Reaction with Li ions.

Multicomponent materials with various double cations have been studied as anode materials of lithium-ion batteries (LIBs). Heterostructures formed by coupling different-bandgap nanocrystals enhance the surface reaction kinetics and facilitate charge transport because of the internal electric field at the heterointerface. Accordingly, metal selenites can be considered efficient anode materials of LIBs because they transform into metal selenide and oxide nanocrystals in the first cycle. However, few studies have reported synthesis of uniquely structured metal selenite microspheres. Herein, synthesis of high-porosity CoSeO3 microspheres is reported. Through one-pot oxidation at 400 °C, CoSex -C microspheres formed by spray pyrolysis transform into CoSeO3 microspheres showing unordinary cycling and rate performances. The conversion mechanism of CoSeO3 microspheres for lithium-ion storage is systematically studied by cyclic voltammetry, in situ X-ray diffraction and electrochemical impedance spectroscopy, and transmission electron microscopy. The reversible reaction mechanism of the CoSeO3 phase from the second cycle onward is evaluated as CoO + xSeO2 + (1 - x)Se + 4(x + 1)Li+ + 4( x + 1)e- ↔ Co + (2x + 1)Li2 O + Li2 Se. The CoSeO3 microspheres show a high reversible capacity of 709 mA h g-1 for the 1400th cycle at a current density of 3 A g-1 and a high reversible capacity of 526 mA h g-1 even at an extremely high current density of 30 A g-1 .

Synthesis and Characterization of CuO Nanodisks for High-Sensitive and Selective Ethanol Gas Sensor Applications.

Herein, the fabrication and characterization of highly sensitive and selective ethanol gas sensor based on CuO nanodisks is reported. The CuO nanodisks were synthesized by facile hydrothermal process and detailed characterization revealed the well-crystallinity, high-purity and high density growth of the prepared material. To fabricate the ethanol gas sensor, the prepared nanodisks were coated on alumina substrate. The fabricated sensor exhibited high-sensitivity and the recorded gas response (resistance-ratio), response time (τ res) and recovery time (τ recov) were 6.2, 119 and 35 s, respectively for 100 ppm of C2H5OH at 300 °C. Further, the fabricated sensor shows high selectivity towards ethanol gas compared to H2 and CO gases.

A New Strategy for Humidity Independent Oxide Chemiresistors: Dynamic Self-Refreshing of In2 O3 Sensing Surface Assisted by Layer-by-Layer Coated CeO2 Nanoclusters.

The humidity dependence of the gas sensing characteristics of metal oxide semiconductors has been one of the greatest obstacles for gas sensor applications during the last five decades because ambient humidity dynamically changes with the environmental conditions. Herein, a new and novel strategy is reported to eliminate the humidity dependence of the gas sensing characteristics of oxide chemiresistors via dynamic self-refreshing of the sensing surface affected by water vapor chemisorption. The sensor resistance and gas response of pure In2 O3 hollow spheres significantly change and deteriorate in humid atmospheres. In contrast, the humidity dependence becomes negligible when an optimal concentration of CeO2 nanoclusters is uniformly loaded onto In2 O3 hollow spheres via layer-by-layer (LBL) assembly. Moreover, In2 O3 sensors LBL-coated with CeO2 nanoclusters show fast response/recovery, low detection limit (500 ppb), and high selectivity to acetone even in highly humid conditions (relative humidity 80%). The mechanism underlying the dynamic refreshing of the In2 O3 sensing surfaces regardless of humidity variation is investigated in relation to the role of CeO2 and the chemical interaction among CeO2 , In2 O3 , and water vapor. This strategy can be widely used to design high performance gas sensors including disease diagnosis via breath analysis and pollutant monitoring.

Monolayer Co3 O4 Inverse Opals as Multifunctional Sensors for Volatile Organic Compounds.

Monolayers of periodic porous Co3 O4 inverse opal (IO) thin films for gas-sensor applications were prepared by transferring cobalt-solution-dipped polystyrene (PS) monolayers onto sensor substrates and subsequent removal of the PS template by heat treatment. Monolayer Co3 O4 IO thin films having periodic pores (d≈500 nm) showed a high response of 112.9 to 5 ppm C2 H5 OH at 200 °C with low cross-responses to other interfering gases. Moreover, the selective detection of xylene and methyl benzenes (xylene+toluene) could be achieved simply by tuning the sensor temperature to 250 and 275 °C, respectively, so that multiple gases can be detected with a single chemiresistor. Unprecedentedly high ethanol response and temperature-modulated control of selectivity with respect to ethanol, xylene, and methyl benzenes were attributed to the highly chemiresistive IO nanoarchitecture and to the tuned catalytic promotion of different gas-sensing reactions, respectively. These well-ordered porous nanostructures could have potential in the field of high-performance gas sensors based on p-type oxide semiconductors.

Design of Highly Selective Gas Sensors via Physicochemical Modification of Oxide Nanowires: Overview.

Strategies for the enhancement of gas sensing properties, and specifically the improvement of gas selectivity of metal oxide semiconductor nanowire (NW) networks grown by chemical vapor deposition and thermal evaporation, are reviewed. Highly crystalline NWs grown by vapor-phase routes have various advantages, and thus have been applied in the field of gas sensors over the years. In particular, n-type NWs such as SnO2, ZnO, and In2O3 are widely studied because of their simple synthetic preparation and high gas response. However, due to their usually high responses to C2H5OH and NO2, the selective detection of other harmful and toxic gases using oxide NWs remains a challenging issue. Various strategies-such as doping/loading of noble metals, decorating/doping of catalytic metal oxides, and the formation of core-shell structures-have been explored to enhance gas selectivity and sensitivity, and are discussed herein. Additional methods such as the transformation of n-type into p-type NWs and the formation of catalyst-doped hierarchical structures by branch growth have also proven to be promising for the enhancement of gas selectivity. Accordingly, the physicochemical modification of oxide NWs via various methods provides new strategies to achieve the selective detection of a specific gas, and after further investigations, this approach could pave a new way in the field of NW-based semiconductor-type gas sensors.

Electrochemical properties of yolk-shell-structured Zn-Fe-S multicomponent sulfide materials with a 1:2 Zn/Fe molar ratio.

Yolk-shell-structured Zn-Fe-S multicomponent sulfide materials with a 1:2 Zn/Fe molar ratio were prepared applying a sulfidation process to ZnFe2O4 yolk-shell powders. The Zn-Fe-S powders had mixed sphalerite (Zn,Fe)S and hexagonal FeS crystal structures. The discharge capacities of the Zn-Fe-S powders sulfidated at 350 °C at a constant current density of 500 mA g(-1) for the first, second, and fiftieth cycles were 1098, 912, and 913 mA h g(-1), respectively. The powders exhibited a high discharge capacity of 602 mA h g(-1) even at the high current density of 10 A g(-1). The synergistic effect of yolk-shell structure and multicomponent composition improved the electrochemical properties of Zn-Fe-S powders.

A new concept for obtaining SnO2 fiber-in-tube nanostructures with superior electrochemical properties.

Tin oxide (SnO2 ) nanotubes with a fiber-in-tube structure have been prepared by electrospinning and the mechanism of their formation has been investigated. Tin oxide-carbon composite nanofibers with a filled structure were formed as an intermediate product, which were then transformed into SnO2 nanotubes with a fiber-in-tube structure during heat treatment at 500 °C. Nanofibers with a diameter of 85 nm were found to be located inside hollow nanotubes with an outer diameter of 260 nm. The prepared SnO2 nanotubes had well-developed mesopores. The discharge capacities of the SnO2 nanotubes at the 2nd and 300th cycles at a current density of 1 A g(-1) were measured as 720 and 640 mA h g(-1), respectively, and the corresponding capacity retention measured from the 2nd cycle was 88 %. The discharge capacities of the SnO2 nanotubes at incrementally increased current densities of 0.5, 1.5, 3, and 5 A g(-1) were 774, 711, 652, and 591 mA h g(-1), respectively. The SnO2 nanotubes with a fiber-in-tube structure showed superior cycling and rate performances compared to those of SnO2 nanopowder. The unique structure of the SnO2 nanotubes with a fiber@void@tube configuration improves their electrochemical properties by reducing the diffusion length of the lithium ions, and also imparts greater stability during electrochemical cycling.

Pure and palladium-loaded Co3O4 hollow hierarchical nanostructures with giant and ultraselective chemiresistivity to xylene and toluene.

Pure and palladium-loaded Co3O4 hollow hierarchical nanostructures consisting of nanosheets have been prepared by solvothermal self-assembly. The nanostructures exhibited an ultrahigh response and selectivity towards p-xylene and toluene. The responses (resistance ratio) of the palladium-loaded Co3O4 hollow hierarchical nanostructures to 5 ppm of p-xylene and toluene were as high as 361 and 305, respectively, whereas the selectivity values (response ratios) towards p-xylene and toluene over interference from ethanol were 18.1 and 16.1, respectively. We attributed the giant response and unprecedented high selectivity towards methylbenzenes to the abundant adsorption of oxygen by Co3O4, the high chemiresistive variation in the Co3O4 nanosheets (thickness≈11 nm), and the catalytic promotion of the specific gas-sensing reaction. The morphological design of the p-type Co3O4 nanostructures and loading of the palladium catalyst have paved a new way to monitoring the most representative indoor air pollutants in a highly selective, sensitive, and reliable manner.

One-pot synthesis of Pd-loaded SnO(2) yolk-shell nanostructures for ultraselective methyl benzene sensors.

A continuous, single-step, and large-scale preparation of Pd-catalyst-loaded SnO2 yolk-shell spheres is demonstrated. These nanostructures show an unusually high response and selectivity to methyl benzenes, such as xylene and toluene, with very low cross-responses to various interfering gases, making them suitable for precise monitoring of indoor air quality.

Large scale production of yolk-shell ß-tricalcium phosphate powders, and their bioactivities as novel bone substitutes.

This paper proposes the production of yolk-shell structured ß-tricalcium phosphate (ß-TCP) powders using a spray-drying method, suitable for commercial scale production. Spray-dried precursor powders, consisting of calcium-phosphate salts and each of the various carbon source materials, are combusted in an oxygen atmosphere to obtain a yolk-shell structure. Only dextrin among the carbon source materials investigated shows promise in the production of ß-TCP yolk-shell powders. By evaluating their apatite-forming capacity in simulated body fluid, the outstanding bioactivity of ß-TCP yolk-shell powders is confirmed: numerous acicular and newly formed hydroxyl carbonate apatite crystals cover the entire ß-TCP surface after a single day of soaking. These crystals are observed on both the outer and inner surfaces of the shells, and on the outer surface of the core, which is encouraging for its potential use as a bone grafting material.

Electronic sensitization of the response to C2H5OH of p-type NiO nanofibers by Fe doping.

Pure and 0.18-13.2 at.% Fe-doped NiO nanofibers were prepared by electrospinning and their gas sensing characteristics and microstructural evolution were investigated. The responses ((Rg - Ra)/Ra, where Rg is the resistance in gas and Ra is the resistance in air) to 5 ppm C2H5OH, toluene, benzene, p-xylene, HCHO, CO, H2, and NH3 at 350-500 ° C were significantly enhanced by Fe doping of the NiO nanofibers, while the responses of pure NiO nanofibers to all the analyte gases were very low ((Rg - Ra)/Ra = 0.07-0.78). In particular, the response to 100 ppm C2H5OH was enhanced up to 217.86 times by doping of NiO nanofibers with 3.04 at.% Fe. The variation in the gas response was closely dependent upon changes in the base resistance of the sensors in air. The enhanced gas response of Fe-doped NiO nanofibers was explained in relation to electronic sensitization, that is, the increase in the chemoresistive variation due to the decrease in the hole concentration induced by Fe doping.

Exclusive detection of volatile aromatic hydrocarbons using bilayer oxide chemiresistors with catalytic overlayers.

The accurate detection and identification of volatile aromatic hydrocarbons, which are highly toxic pollutants, are essential for assessing indoor and outdoor air qualities and protecting humans from their sources. However, real-time and on-site monitoring of aromatic hydrocarbons has been limited by insufficient sensor selectivity. Addressing the issue, bilayer oxide chemiresistors are developed using Rh-SnO2 gas-sensing films and catalytic CeO2 overlayers for rapidly and cost-effectively detecting traces of aromatic hydrocarbons in a highly discriminative and quantitative manner, even in gas mixtures. The sensing mechanism underlying the exceptional performance of bilayer sensor is systematically elucidated in relation to oxidative filtering of interferants by the CeO2 overlayer. Moreover, CeO2-induced selective detection is validated using SnO2, Pt-SnO2, Au-SnO2, In2O3, Rh-In2O3, Au-In2O3, WO3, and ZnO sensors. Furthermore, sensor arrays are employed to enable pattern recognition capable of discriminating between aromatic gases and non-aromatic interferants and quantifying volatile aromatic hydrocarbon classifications.

Highly Sensitive and Selective Real-Time Breath Isoprene Detection using the Gas Reforming Reaction of MOF-Derived Nanoreactors.

Real-time breath isoprene sensing provides noninvasive methods for monitoring human metabolism and early diagnosis of cardiovascular diseases. Nonetheless, the stable alkene structure and high humidity of the breath hinder sensitive and selective isoprene detection. In this work, we derived well-defined Co3O4@polyoxometalate yolk-shell structures using a metal-organic framework template. The inner space, including highly catalytic Co3O4 yolks surrounded by a semipermeable polyoxometalate shell, enables stable isoprene to be reformed to reactive intermediate species by increasing the gas residence time and the reaction with the inner catalyst. This sensor exhibited selective isoprene detection with an extremely high chemiresistive response (180.6) and low detection limit (0.58 ppb). The high sensing performance can be attributed to electronic sensitization and catalytic promotion effects. In addition, the reforming reaction of isoprene is further confirmed by the proton transfer reaction-quadrupole mass spectrometry analysis. The practical feasibility of this sensor in smart healthcare applications is exhibited by monitoring muscle activity during the workout.

MOF-Based Chemiresistive Gas Sensors: Toward New Functionalities.

The sensing performances of gas sensors must be improved and diversified to enhance quality of life by ensuring health, safety, and convenience. Metal-organic frameworks (MOFs), which exhibit an extremely high surface area, abundant porosity, and unique surface chemistry, provide a promising framework for facilitating gas-sensor innovations. Enhanced understanding of conduction mechanisms of MOFs has facilitated their use as gas-sensing materials, and various types of MOFs have been developed by examining the compositional and morphological dependences and implementing catalyst incorporation and light activation. Owing to their inherent separation and absorption properties and catalytic activity, MOFs are applied as molecular sieves, absorptive filtering layers, and heterogeneous catalysts. In addition, oxide- or carbon-based sensing materials with complex structures or catalytic composites can be derived by the appropriate post-treatment of MOFs. This review discusses the effective techniques to design optimal MOFs, in terms of computational screening and synthesis methods. Moreover, the mechanisms through which the distinctive functionalities of MOFs as sensing materials, heterostructures, and derivatives can be incorporated in gas-sensor applications are presented.

Highly sensitive and selective trimethylamine sensor using one-dimensional ZnO-Cr2O3 hetero-nanostructures.

Highly selective and sensitive detection of trimethylamine (TMA) was achieved by the decoration of discrete p-type Cr(2)O(3) nanoparticles on n-type ZnO nanowire (NW) networks. Semielliptical Cr(2)O(3) nanoparticles with lateral widths of 3-8 nm were deposited on ZnO NWs by the thermal evaporation of CrCl(2) at 630 °C, while a continuous Cr(2)O(3) shell layer with a thickness of 30-40 nm was uniformly coated on ZnO NWs at 670 °C. The response (R(a)/R(g): R(a), resistance in air; R(g), resistance in gas) to 5 ppm TMA of Cr(2)O(3)-decorated ZnO NWs was 17.8 at 400 °C, which was 2.4 times higher than that to 5 ppm C(2)H(5)OH and 4.3-8.4 times higher than those to 5 ppm p-xylene, NH(3), benzene, C(3)H(8), toluene, CO, and H(2). In contrast, both pristine ZnO and ZnO (core)-Cr(2)O(3) (shell) nanocables (NCs) showed comparable responses to the different gases. The highly selective and sensitive detection of TMA that was achieved by the deposition of semielliptical Cr(2)O(3) nanoparticles on ZnO NW networks was explained by the catalytic effect of Cr(2)O(3) and the extension of the electron depletion layer via the formation of p-n junctions.

Selective detection of NO2 using Cr-doped CuO nanorods.

CuO nanosheets, Cr-doped CuO nanosheets, and Cr-doped CuO nanorods were prepared by heating a slurry containing Cu-hydroxide/Cr-hydroxide. Their responses to 100 ppm NO(2), C(2)H(5)OH, NH(3), trimethylamine, C(3)H(8), and CO were measured. For 2.2 at% Cr-doped CuO nanorods, the response (R(a)/R(g), R(a): resistance in air, R(g): resistance in gas) to 100 ppm NO(2) was 134.2 at 250 °C, which was significantly higher than that of pure CuO nano-sheets (R(a)/R(g) = 7.5) and 0.76 at% Cr-doped CuO nanosheets (R(a)/R(g) = 19.9). In addition, the sensitivity for NO(2) was also markedly enhanced by Cr doping. Highly sensitive and selective detection of NO(2) in 2.2 at% Cr-doped CuO nanorods is explained in relation to Cr-doping induced changes in donor density, morphology, and catalytic effects.