Ethanol Gas Detection Using a Yolk-Shell (Core-Shell) α-Fe2O3 Nanospheres as Sensing Material

Highly selective ethanol vapour sensing materials for a new generation of gas sensors based on molecularly imprinted polymers

A simple gas sensor consisting of a molecularly imprinted polymer-carbon nanotube composite cast onto a screen-printed electrode has been developed with extremely high selectivity for ethanol vapour over methanol vapour. Ethanol gas sensors typically display selectivity for ethanol over methanol in the range 2-4 times, while the mean ratio of ethanol to methanol response observed with the described device was 672. This selectivity was achieved under ambient conditions. Additionally, the molecularly imprinted polymer was produced using reagents previously applied in the development of a device selective for methanol, with only the template being changed. This demonstrates the versatility of molecular imprinting and provides a foundation for their greater integration into future gas sensors.

Electrochemical sensors based on the composite of reduced graphene oxide and a multiwalled carbon nanotube-modified glassy carbon electrode for simultaneous detection of hydroquinone, dopamine, and uric acid

Using a simple drop-casting technique, we successfully fabricated a sensitive electrochemical sensor based on the composite of reduced graphene oxide (RGO) and multiwalled carbon nanotubes (MWCNT) deposited on the surface of a glassy carbon electrode (GCE) for individual and simultaneous measurements of hydroquinone (HQ), dopamine (DA), and uric acid (UA). The nanocomposite of RGO/MWCNT was further characterized in terms of its structural properties, surface morphology, and topography using Raman, FT-IR spectroscopy, SEM, HRTEM, and AFM. Then, the proposed sensor for simultaneous measurement of HQ, DA, and UA based on RGO/MWCNT-modified GCE was investigated for its electrochemical behavior and electroanalytical performances using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), and differential pulse voltammetry (DPV). In addition, the composition ratio between RGO and MWCT was 1 : 1 showing the highest electrochemical response for simultaneous detection of HQ, DA, and UA. Owing to the synergistic effect between RGO and MWCNT leading to excellent conductivity properties, the proposed sensor exhibited improved electrochemical response at pH 7 toward the oxidation processes of HQ, DA, and UA on the surface of modified electrode. The proposed sensor demonstrated three well-defined anodic peaks of these analytes with their linear concentrations ranges of 3.0-150.0 μM for HQ, 4.0-100.0 μM for DA, and 2.0-70.0 μM for UA. The limit of detection values for the simultaneous detection of HQ, DA, and UA were found as follows 0.400 ± 0.014, 0.500 ± 0.006, and 0.300 ± 0.016 μM, respectively. The additional features of this proposed sensor are high reproducibility and stability for the simultaneous detection of HQ, DA, and UA with negligible interference effect from interferents such as Mg2+, K+, Cl-, ascorbic acid, and glucose. An acceptable percentage of recovery was also shown by this sensor for simultaneous measurements of HQ, DA, and UA using 6 samples of human urine. In summary, the RGO/MWCNT nanocomposite has been shown to be a promising platform for rapid, simple, and reliable determination of simultaneous measurements of HQ, DA, and UA in practical applications.

Constructing magnetic/dielectric dual loss and phonon/electron thermal carriers γ-Al2O3-based yolk-shell microspheres to collaboratively advance microwave absorption and heat conduction

The severe electromagnetic (EM) interference and overheating issues in 5G/6G electric devices increasingly heighten the need for developing multifunctional materials with large heat conduction (HC) and high EM wave (EMW) absorption. Here, a series of γ-Al2O3-based yolk-shell microspheres (γ-AlOOH, γ-Al2O3, γ-Al2O3@C, γ-Al2O3@Fe3O4@C, and γ-Al2O3@FeAl2O4@Fe@C YSMSs) as multifunctional fillers are investigated for the simultaneous improvement in the HC and EMW absorption of γ-Al2O3-based composites. Using γ-AlOOH YSMSs as precursors produced from a hydrothermal method, the γ-Al2O3-based YSMSs were synthesized via an annealing route or soaking-annealing route; their phases, textures, and compositions were finely adjusted by changing the Al3+/Fe3+ molar ratio (β) and annealing temperature (Ta). Results show that the thermal transfers in the γ-Al2O3-based YSMSs are promoted by the synergic effect of phonons and electrons when they are utilized as thermally conductive fillers. Comparatively, the γ-Al2O3@FeAl2O4@Fe@C YSMSs formed at β = 8 : 2 and Ta = 700 °C exhibit a high HC of 1.84-3.29 W m-1 K-1 in a loading amount of 5-40%, exceeding those of not merely γ-Al2O3, γ-AlOOH, γ-Al2O3@C, and γ-Al2O3@Fe3O4@C YSMSs but also most previously reported fillers. Furthermore, the γ-Al2O3@Fe3O4@C YSMSs exhibit prominent EMW absorption properties with a large ABW/d of 4.49 GHz mm-1 (just 30% loading), superior to most other Al2O3-based absorbers. Such excellent EMW absorption could be explained by magnetic/dielectric dual loss and significant cavity and interfacial effects caused by yolk-shell structures. In conclusion, this work inspires the development of yolk-shell structures with magnetic/dielectric dual loss and phonon/electron thermal carriers as high-performance bifunctional materials with exceptional heat conduction and EMW absorption.

Block Copolymer-Templated, Single-Step Synthesis of Transition Metal Oxide Nanostructures for Sensing Applications

The synthesis of transition metal oxide nanostructures, thanks to their high surface-to-volume ratio and the resulting large fraction of surface atoms with high catalytic activity, is of prime importance for the development of new sensors and catalytic materials. Here, we report an economical, time-efficient, and easily scalable method of fabricating nanowires composed of vanadium, chromium, manganese, iron, and cobalt oxides by employing simultaneous block copolymer (BCP) self-assembly and selective sequestration of metal-organic acetylacetonate complexes within one of the BCP blocks. We discuss the mechanism and the primary factors that are responsible for the sequestration and conformal replication of the BCP template by the inorganic material that is obtained after the polymer template is removed. X-ray photoelectron spectroscopy (XPS) and powder X-ray diffraction (PXRD) studies indicate that the metal oxidation state in the nanowires produced by plasma ashing the BCP template closely matches that of the precursor complex and that their structure is amorphous, thus requiring high-temperature annealing in order to sinter them into a crystalline form. Finally, we demonstrate how the developed nanowire array fabrication scheme can be used to rapidly pattern a multilayered iron oxide nanomesh, which we then used to construct a prototype volatile organic compound sensor.

A dimethyl disulfide gas sensor based on nanosized Pt-loaded tetrakaidecahedralα-Fe2O3nanocrystals

Surface modification by employing precious metals is one of the most effective ways to improve the gas-sensing performance of metal oxide semiconductors. Pureα-Fe₂O₃nanoparticles and Pt-modifiedα-Fe₂O₃nanoparticles were prepared sequentially using a rather simple hydrothermal synthesis and impregnation method. Compared with the originalα-Fe₂O₃nanomaterials, the Pt-α-Fe₂O₃nanocomposite sensor shows a higher response value (R_a/R_g = 58.6) and a shorter response/recovery time (1 s/168 s) to 100 ppm dimethyl disulfide (DMDS) gas at 375 °C. In addition, it has better selectivity to DMDS gas with the value of more than 9 times higher than the other target gases at 375 °C. This study indicates that the Pt-α-Fe₂O₃nanoparticle sensor has good prospects and can be used as a low-cost and effective DMDS gas sensor.

MOF-Derived Mesoporous and Hierarchical Hollow-Structured In2O3-NiO Composites for Enhanced Triethylamine Sensing

It remains a challenge to design and fabricate high-performance gas sensors using metal-organic framework (MOF)-derived metal oxide semiconductors (MOS) as sensing materials due to the structural damage during the annealing process. In this study, the mesoporous In₂O₃-NiO hollow spheres consisting of nanosheets were prepared via a solvothermal reaction and subsequent cation exchange. More importantly, the transformation of Ni-MOF into In/Ni-MOF through exchanging the Ni²⁺ ion with In³⁺ ion can prevent the destruction of the porous reticular skeleton and hierarchical structure of Ni-MOF during calcination. Thus, the mesoporous In₂O₃-NiO hollow composites possess high porosity and large specific surface area (55.5 m² g^-1), which can produce sufficient permeability pathways for volatile organic compound (VOCs) molecules, maximize the active sites, and enhance the capacity of VOC capture. The mesoporous In₂O₃-NiO-based sensors exhibit enhanced triethylamine (TEA) sensing performance (S = 33.9-100 ppm) with distinct selectivity, good long-term stability, and lower detection limit (500 ppb) at 200 °C. These results can be attributed to the mesoporous hollow hierarchical structure and p-n junction of In₂O₃-NiO. The preparation concept mentioned in this work may provide a versatile platform applicable to various mesoporous composite sensing material-based hollow structures.

Improving the CO and CH4 Gas Sensor Response at Room Temperature of α-Fe2O3(0001) Epitaxial Thin Films Grown on SrTiO3(111) Incorporating Au(111) Islands

In this work, the functional character of complex α-Fe2O3(0001)/SrTiO3(111) and Au(111) islands/α-Fe2O3(0001)/SrTiO3(111) heterostructures has been proven as gas sensors at room temperature. Epitaxial Au islands and α-Fe2O3 thin film are grown by pulsed laser deposition on SrTiO3(111) substrates. Intrinsic parameters such as the composition, particle size and epitaxial character are investigated for their influence on the gas sensing response. Both Au and α-Fe2O3 layer show an island-type growth with an average particle size of 40 and 62 nm, respectively. The epitaxial and incommensurate growth is evidenced, confirming a rotation of 30° between the in-plane crystallographic axes of α-Fe2O3(0001) structure and those of SrTiO3(111) substrate and between the in-plane crystallographic axes of Au(111) and those of α-Fe2O3(0001) structure. α-Fe2O3 is the only phase of iron oxide identified before and after its functionalization with Au nanoparticles. In addition, its structural characteristics are also preserved after Au deposition, with minor changes at short-range order. Conductance measurements of Au(111)/α-Fe2O3(0001)/SrTiO3(111) system show that the incorporation of epitaxial Au islands on top of the α-Fe2O3(0001) layer induces an enhancement of the gas-sensing activity of around 25% under CO and 35% under CH4 gas exposure, in comparison to a bare α-Fe2O3(0001) layer grown on SrTiO3(111) substrates. In addition, the response of the heterostructures to CO gas exposure is around 5–10% higher than to CH4 gas in each case.

A Gas Sensor With Fe2O3 Nanospheres Based on Trimethylamine Detection for the Rapid Assessment of Spoilage Degree in Fish

A spherical iron oxide precursor was prepared using a solvothermal method, and then treated thermally at 400°C to obtain α-Fe₂O₃ nanoparticles. The structures and morphology of the as-obtained products were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The results showed that the diameter of the α-Fe₂O₃ nanoparticles was approximately 500 nm. In addition, we formed the α-Fe₂O₃ nanoparticles into a thick film as a gas sensor and performed a gas sensing test. When the working temperature was set at 250°C, the α-Fe₂O₃ nanoparticle displayed very good selectivity and high sensitivity for trimethylamine (TMA). The minimum detection was as low as 1 ppm, and the response value for 100 ppm TMA gas was 27.8. Taken together, our findings illustrated that the α-Fe₂O₃ nanoparticles could be used as a gas-sensitive material to test the freshness of fish.

SnO2-ZnO-Fe2O3 tri-composite based room temperature operated dual behavior ammonia and ethanol sensor for ppb level detection

In this paper, we present a novel room temperature (RT) operated SnO2-ZnO-Fe2O3 based tri-composite analyte sensor with dual behavior having detection ability of up to ∼1 ppb with a substantial % response (R) to detect ammonia and ethanol vapors. The tri-composite is synthesized via a sol-gel spin coating technique and characterized using X-ray diffraction (XRD) for structural analysis. Fourier transform infrared spectroscopy (FTIR) and Raman results are used to confirm tri-composite formation. Further, field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM) results are used for examining the detailed surface morphology and structural and topographical characteristics of the tri-composite. The sensing characteristics are monitored from 1 ppb to 50 ppm for ammonia detection and 1 ppb to 25 ppm for ethanol detection at RT (∼27 °C) under ∼45% relative humidity (RH) conditions. This dual sensing behavior (based on change in resistance under ammonia and ethanol exposure) of the sensor is used to differentiate and detect the presence of ammonia (resistance decreases) and ethanol (resistance increases) with high %R within a few seconds. In addition, the sensor showed excellent sensing characteristics under moist conditions (up to 85% RH) and outstanding reproducibility, and was found to be highly stable, selective and specific towards the target analytes. This work not only reports a RT operated ppb level ammonia and ethanol sensor, but also explores the novel SnO2-ZnO-Fe2O3 tri-composite along with a scientific approach towards multi-composite nanostructures to develop analyte sensors.

A microcube-like hierarchical heterostructure of α-Fe2O3@α-MoO3 for trimethylamine sensing

A rational design and directional synthesis of well-defined porous metal oxide heterostructures remains a challenge for the fabrication of new gas sensors with enhanced sensing properties. Herein, a novel hierarchical heterostructure of α-Fe2O3@α-MoO3 microcubes was prepared by applying a self-sacrifice template strategy, in which a porous metal-organic framework (MOF) was used as the precursor with the partial insertion of OH- and MoO42- and extraction of [Fe(CN)6]34- followed by heating treatment. Benefiting from the porous microcubic structural characteristic, the α-Fe2O3@α-MoO3 heterostructures exhibited excellent selectivity and high sensitivity for triethylamine gas with the highest response value (Ra/Rg) of 18.64 under the operating temperature of 240 °C. The experimental results also revealed that the material showed a fast response and recovery time (12 s/106 s) and excellent repeatability for at least 20 days. This feasible synthetic strategy may provide a versatile and controlled way to prepare other polymetal oxide-based heterostructures with improved sensing performance.

Low-Temperature Carbon Dioxide Gas Sensor Based on Yolk-Shell Ceria Nanospheres

Monitoring carbon dioxide (CO₂) levels is extremely important in a wide range of applications. Although metal oxide-based chemoresistive sensors have emerged as a promising approach for CO₂ detection, the development of efficient CO₂ sensors at low temperature remains a challenge. Herein, we report a low-temperature hollow nanostructured CeO₂-based sensor for CO₂ detection. We monitor the changes in the electrical resistance after CO₂ pulses in a relative humidity of 70% and show the high performance of the sensor at 100 °C. The yolk-shell nanospheres have not only 2 times higher sensitivity but also significantly increased stability and reversibility, faster response times, and greater CO₂ adsorption capacity than commercial ceria nanoparticles. The improvements in the CO₂ sensing performance are attributed to hollow and porous structure of the yolk-shell nanoparticles, allowing for enhanced gas diffusion and high specific surface area. We present an easy strategy to enhance the electrical and sensing properties of metal oxides at a low operating temperature that is desirable for practical applications of CO₂ sensors.

Optimization and gas sensing properties of Au nanoparticle modified α-Fe2O3 nanodisk structures for highly sensitive acetone detection

Au nanoparticle (Au NP) modified α-Fe₂O₃ nanodisk structures are obtained using a facile hydrothermal method and annealing based surface treatment.

Hollow-sphere iron oxides exhibiting enhanced cycling performance as lithium-ion battery anodes

Hollow-sphere Fe2O3 is synthesized as a lithium-ion battery anode. Current densities for the initial material activation are important, related to electrode stability during cycling. The as-prepared anodes are able to retain 92% capacity after 1000 cycles at 1 A g-1. The full-cells assembled with Fe2O3 anodes and LiFePO4 cathodes exhibit good electrochemical properties.

Single-Crystal α-Fe2O3 with Engineered Exposed (001) Facet for High-Rate, Long-Cycle-Life Lithium-Ion Battery Anode

Designing electrode materials with engineered exposed facets provides a novel strategy to improve their electrochemical properties. However, the controllability of the exposed facet remains a daunting challenge, and a deep understanding of the correlation between exposed facet and Li⁺-transfer behavior has been rarely reported. In this work, single-crystal α-Fe₂O₃ hexagonal nanosheets with an exposed (001) facet are prepared with the assistance of aluminum ions through a one-step hydrothermal process, and structural characterizations reveal an Al³⁺-concentration-dependent-growth mechanism for the α-Fe₂O₃ nanosheets. Furthermore, such α-Fe₂O₃ nanosheets, when used as lithium-ion battery anodes, exhibit high specific capacity (1261.3 mAh g^-1 at 200 mA g^-1), high rate capability (with a reversible capacity of approximately 605 mAh g^-1 at 10 A g^-1), and excellent cyclic stability (with a capacity of over 900 mAh g^-1 during 500 cycles). The superior electrochemical performance of α-Fe₂O₃ nanosheets is attributed to the pseudocapacitive behavior, Al-doping in the α-Fe₂O₃ structure, and improved Li⁺-transfer property across the (001) facet, as elucidated by first-principles calculations based on density functional theory. These results reveal the underlying mechanism of Li⁺ transfer across different facets and thus provide insights into the understanding of the excellent electrochemical performance.

Controllable Synthesis of Zn-Doped α-Fe2O3 Nanowires for H2S Sensing

One-dimensional Zn-doped α-Fe₂O₃ nanowires have been controllably synthesized by using the pure pyrite as the source of Fe element through a two-step synthesis route, including the preparation of Fe source solution by a leaching process and the thermal conversion of the precursor solution into α-Fe₂O₃ nanowires by the hydrothermal and calcination process. The microstructure, morphology, and surface composition of the obtained products were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. It was found that the formation process of α-Fe₂O₃ is significantly influenced by the introduction of Zn²⁺. The gas sensing measurements indicated that the sensor based on 1% Zn-doped α-Fe₂O₃ nanowires showed excellent H₂S sensing properties at the optimum operating temperature of 175 °C. Notably, the sensor showed a low H₂S detection limit of 50 ppb with a sensor response of 1.5. Such high-performance sensing would be ascribed to the one-dimensional structure and high specific surface area of the prepared 1% Zn-doped α-Fe₂O₃ nanowires, which can not only provide a large number of surface active sites for the adsorption and reaction of the oxygen and H₂S molecules, but also facilitate the diffusion of the gas molecules towards the entire sensing materials.

The Synthesis of the Pomegranate-Shaped α-Fe2O3 Using an In Situ Corrosion Method of Scorodite and Its Gas-Sensitive Property

The release of hazardous gas increases with the development of industry. The research of gas-sensitive materials has attracted attention. Nanoscale iron oxide (α-Fe₂O₃) is one of the research hotspots of gas-sensitive materials because it is a cheap, non-toxic semiconductor material. In this study, pomegranate-shaped α-Fe₂O₃ was synthesized using an in situ corrosion method of scorodite. Spherical-shaped α-Fe₂O₃ nanoparticles were included in the octahedral shells. The forming process of the structure was analyzed by a variety of measurements. The shell was formed first through the deposition of Fe(OH)₃, which was produced by hydrolyzing scorodite. Then, the corrosion was continued and Fe(OH)₃ precipitation was produced below the shell. The particles aggregated and formed spheres. The pomegranate-shaped α-Fe₂O₃ was formed when the scorodite was hydrolyzed completely. The gas-sensing properties of α-Fe₂O₃ were investigated. The results showed that pomegranate-shaped α-Fe₂O₃ was responsive to a variety of gases, especially xylene. The value of R_a/R_g was 67.29 at 340 °C when the concentration of xylene was 1000 ppm. This indicated the pomegranate-shaped α-Fe₂O₃ has potential application as a xylene gas sensor.

Grain-Boundary-Induced Drastic Sensing Performance Enhancement of Polycrystalline-Microwire Printed Gas Sensors

The development of materials with high efficiency and stable signal output in a bent state is important for flexible electronics. Grain boundaries provide lasting inspiration and a promising avenue for designing advanced functionalities using nanomaterials. Combining bulk defects in polycrystalline materials is shown to result in rich new electronic structures, catalytic activities, and mechanical properties for many applications. However, direct evidence that grain boundaries can create new physicochemical properties in flexible electronics is lacking. Here, a combination of bulk electrosensitive measurements, density functional theory calculations, and atomic force microscopy technology with quantitative nanomechanical mapping is used to show that grain boundaries in polycrystalline wires are more active and mechanically stable than single-crystalline wires for real-time detection of chemical analytes. The existence of a grain boundary improves the electronic and mechanical properties, which activate and stabilize materials, and allow new opportunities to design highly sensitive, flexible chemical sensors.

Metal-Organic Framework-Assisted Construction of TiO2/Co3O4 Highly Ordered Necklace-like Heterostructures for Enhanced Ethanol Vapor Sensing Performance

In this work, we report a metal-organic framework (MOF)-assisted strategy to synthesize necklace-like TiO₂/Co₃O₄ nanofibers with highly ordered heterostructures via a facile approach including electrospinning and subsequent calcination. Polycrystalline TiO₂ nanofibers and Co₃O₄ nanocages are consummately interconnected to form a highly ordered heterogeneous nanostructure, which can be of benefit for precisely accommodating the interface resistance of the p-n heterojunctions and the future realization of improved material performance. The ethanol-gas-sensing investigation showed that TiO₂/Co₃O₄ nanofiber sensors exhibited a strong ethanol response ( R_air/ R_gas -1 = 16.7 @ 150 ppm) and a low operating temperature of 150 °C. The sensing enhancement mechanism of the TiO₂/Co₃O₄ nanofibers is related to the formation of heterojunctions at interfaces and the high catalytic activity of MOF-derived Co₃O₄. Furthermore, this versatile method is a promising approach to constructing ordered heterostructures and extending the MOF-based heterogeneous materials toward wide applications.

Density Gradient Strategy for Preparation of Broken In2O3 Microtubes with Remarkably Selective Detection of Triethylamine Vapor

Tubule-like structured metal oxides, combined with macroscale pores onto their surfaces, can fast facilitate gas-accessible diffusion into the sensing channels, thus leading a promoted utilization ratio of sensing layers. However, it generally remains a challenge for developing a reliable approach to prepare them. Herein, this contribution describes a density gradient strategy for obtaining broken In₂O₃ microtubes from the In₂O₃ products prepared using a chemical conversion method. These In₂O₃ microtubes hold a diameter about 1.5 μm with many broken regions and massive ultrafine nanopores onto their surfaces. When employed as a sensing element for detection of triethylamine (TEA) vapor, these broken In₂O₃ microtubes exhibited a significant response toward TEA at 1-100 ppm and the lowest detected concentration can reach 0.1 ppm. In addition, an excellent selectivity of the sensor to TEA was also displayed, though upon exposure of other interfering vapors, including ammonia, methanol, ethanol, isopropanol, acetone, toluene, and hydrogen. Such promoted sensing performances toward TEA were ascribed to the broken configuration (superior gas permeability and high utilization ratio), one-dimensional configuration with less agglomerations, and low bond energy for C-N in a TEA molecule.

Synergy between nanomaterials and volatile organic compounds for non-invasive medical evaluation

This article is an overview of the present and ongoing developments in the field of nanomaterial-based sensors for enabling fast, relatively inexpensive and minimally (or non-) invasive diagnostics of health conditions with follow-up by detecting volatile organic compounds (VOCs) excreted from one or combination of human body fluids and tissues (e.g., blood, urine, breath, skin). Part of the review provides a didactic examination of the concepts and approaches related to emerging sensing materials and transduction techniques linked with the VOC-based non-invasive medical evaluations. We also present and discuss diverse characteristics of these innovative sensors, such as their mode of operation, sensitivity, selectivity and response time, as well as the major approaches proposed for enhancing their ability as hybrid sensors to afford multidimensional sensing and information-based sensing. The other parts of the review give an updated compilation of the past and currently available VOC-based sensors for disease diagnostics. This compilation summarizes all VOCs identified in relation to sickness and sampling origin that links these data with advanced nanomaterial-based sensing technologies. Both strength and pitfalls are discussed and criticized, particularly from the perspective of the information and communication era. Further ideas regarding improvement of sensors, sensor arrays, sensing devices and the proposed workflow are also included.

Dual Role of Multiroom-Structured Sn-Doped NiO Microspheres for Ultrasensitive and Highly Selective Detection of Xylene

Sn-doped NiO multiroom spheres with unique microreactor morphology were prepared by facile ultrasonic spray pyrolysis of a solution containing tin oxalate, nickel nitrate, and dextrin and subsequent heat treatment. The multiroom structure was formed by phase segregation between the molten metal source and liquidlike dextrin and sequent decomposition of dextrin during spray pyrolysis, which played the dual roles of enhancing gas response and selectivity. The response (resistance ratio) of the Sn-doped NiO multiroom spheres to 1 ppm p-xylene was as high as 65.4 at 300 °C, which was 50.3 and 9.0 times higher than those of pure NiO multiroom spheres and Sn-doped NiO dense spheres, respectively. In addition, the Sn-doped NiO multiroom sensors showed a high selectivity to xylene. The unprecedented high response that enables the sensing of sub-ppm xylene was explained by the high gas accessibility of the multiroom structures and the Sn-doping-induced change in oxygen adsorption as well as the charge carrier concentration, whereas the high xylene selectivity was attributed to the decomposition/re-forming of xylene into smaller or more active species within the unique multiroom structure of Sn-doped NiO microreactors characterized by high catalytic activities. The multiroom oxide spheres can be used as a new and generalized platform to design high-performance gas sensors.

Coordination Polymer-Derived Multishelled Mixed Ni-Co Oxide Microspheres for Robust and Selective Detection of Xylene

Multishell, stable, porous metal-oxide microspheres (Ni-Co oxides, Co₃O₄ and NiO) have been synthesized through the amorphous coordination polymer-based self-templated method. Both oxides of Ni and Co show poor selectivity to xylene, but the composite phase has substantial selectivity (e.g., S_xylene/ S_ethanol = 2.69) and remarkable sensitivity (11.5-5 ppm xylene at 255 °C). The short response and recovery times (6 and 9 s), excellent humidity-resistance performance (with coefficient of variation = 11.4%), good cyclability, and long-term stability (sensitivity attenuation of ∼9.5% after 30 days and stable sensitivity thereafter) all show that this composite is a competitive solution to the problem of xylene sensing. The sensing performances are evidently due to the high specific surface area and the nano-heterostructure in the composite phase.

Volatile Organic Compound Gas-Sensing Properties of Bimodal Porous α-Fe2O3 with Ultrahigh Sensitivity and Fast Response

Porous solid with multimodal pore size distribution provides plenty of advantages including large specific surface area and superior mass transportation to achieve high gas-sensing performances. In this study, α-Fe₂O₃ nanoparticles with bimodal porous structures were prepared successfully through a nanocasting pathway, adopting the bicontinuous 3D cubic symmetry mesoporous silica KIT-6 as the hard template. Its structure and morphology were characterized by X-ray diffraction, nitrogen adsorption-desorption, transmission electron microscopy, and so on. Furthermore, the gas sensor fabricated from this material exhibited excellent gas-sensing performance to several volatile organic compounds (acetone, ethyl acetate, isopropyl alcohol, n-butanol, ethanol, and methanol), such as ultrahigh sensitivity, rapid response speed (less than 10 s) and recovery time, good reproducibility, as well as stability. These would be associated with the desirable pore structure of the material, facilitating the molecules diffusion toward the entire sensing surface, and providing more active sensing sites for analytical gas.

The Morphologies of the Semiconductor Oxides and Their Gas-Sensing Properties

Semiconductor oxide chemoresistive gas sensors are widely used for detecting deleterious gases due to low cost, simple preparation, rapid response and high sensitivity. The performance of gas sensor is greatly affected by the morphology of the semiconductor oxide. There are many semiconductor oxide morphologies, including zero-dimensional, one-dimensional, two-dimensional and three-dimensional ones. The semiconductor oxides with different morphologies significantly enhance the gas-sensing performance. Among the various morphologies, hollow nanostructures and core-shell nanostructures are always the focus of research in the field of gas sensors due to their distinctive structural characteristics and superior performance. Herein the morphologies of semiconductor oxides and their gas-sensing properties are reviewed. This review also proposes a potential strategy for the enhancement of gas-sensing performance in the future.

Controllable Evolution of Dual Defect Zni and VO Associate-Rich ZnO Nanodishes with (0001) Exposed Facet and Its Multiple Sensitization Effect for Ethanol Detection

Building an effective way for finding the role of surface defects in gas sensing property remains a big challenge. In the present work, we synthesized the ZnO nanodishes (NDs) and first explored the formation process of rich electron donor surface defects by means of studying mechanism for the ZnO NDs synthesis. The test results revealed that ZnO-6, added by 6 mmol Zn powder, had the best gas-sensing properties with the excellent selectivity to ethanol than the others. Specially, the ZnO-6 sensor exhibited the best response (about 49) to 100 ppm ethanol at 230 °C among four as-synthesized samples, while noncustomized ZnO was only 28. It was mainly caused by the following two reasons: the exposure of target (0001) crystal facet and rich electron donor surface defects zinc interstitial (Zn_i) and oxygen vacancy (V_O). As a guide, the formation process of surface defects was revealed by an ideal defect model. By the small-angle XRD and TEM patterns, we could conclude that ZnO NDs, changing stoichiometric ratio, increased the content of Zn_i by adding Zn powder, while excessive Zn powder promoted the growth of c axis of ZnO NDs in the self-assembly engineering. Besides, a depletion model has been provided to explain how the surface defects work on the sensors and the complex mechanism of gas sensing performance. These findings will develop the application of ZnO-based gas sensor in health and security.

Enhanced Gas Sensitivity and Sensing Mechanism of Network Structures Assembled from α-Fe2O3 Nanosheets with Exposed {104} Facets

Network structures assembled from α-Fe₂O₃ nanosheets with exposed {104} facets were successfully prepared by heating Fe(NO₃)₃ solution containing polyvinylpyrrolidone (PVP) in air. The α-Fe₂O₃ nanosheet-based network structures demonstrate significantly higher response to ethanol and triethylamine than α-Fe₂O₃ commercial powders. The excellent sensing performances can be ascribed to the exposed (104) facet terminated with Fe atoms. A concept of the unsaturated Fe atoms serving as the sensing reaction active sites is thus proposed, and the sensing reaction mechanism is described at the atomic and molecular level for the first time in detail. The concept of the surface metal atoms with dangling bonds serving as active sites can deepen understanding of the sensing and other catalytic reaction mechanisms and provides new insight into the design and fabrication of highly efficient sensing materials, catalysts, and photoelectronic devices.

Wafer-Scale Synthesized MoS2/Porous Silicon Nanostructures for Efficient and Selective Ethanol Sensing at Room Temperature

This paper presents the performance of a highly selective ethanol sensor based on MoS₂-functionalized porous silicon (PSi). The uniqueness of the sensor includes its method of fabrication, wafer scalability, affinity for ethanol, and high sensitivity. MoS₂ nanoflakes (NFs) were synthesized by sulfurization of oxidized radio-frequency (RF)-sputtered Mo thin films. The MoS₂ NFs synthesis technique is superior in comparison to other methods, because it is chip-scalable and low in cost. Interdigitated electrodes (IDEs) were used to record resistive measurements from MoS₂/PSi sensors in the presence of volatile organic compound (VOC) and moisture at room temperature. With the effect of MoS₂ on PSi, an enhancement in sensitivity and a selective response for ethanol were observed, with a minimum detection limit of 1 ppm. The ethanol sensitivity was found to increase by a factor of 5, in comparison to the single-layer counterpart levels. This impressive response is explained on the basis of an analytical resistive model, the band gap of MoS₂/PSi/Si, the interface formed between MoS₂ and PSi, and the chemical interaction of the vapor molecules and the surface. This two-dimensional (2D) composite material with PSi paves the way for efficient, highly responsive, and stable sensors.

Hollow ZnSnO3 Cubes with Controllable Shells Enabling Highly Efficient Chemical Sensing Detection of Formaldehyde Vapors

In structural hierarchy, inherently hollow nanostructured materials preferentially possessing high surface area demand attention due to their alluring sensing performances. However, the activity of hollow and structural hierarchy nanomaterials generally remains suboptimal due to their hollow space structure and large lateral size, which greatly hamper and limit the availability of inner space active sites. Here, hollow ZnSnO₃ cubes with a controllable interior structure were successfully prepared through a simple and low-cost coprecipitation approach followed with a calcination process. The solid-, single-, double-, and multishelled ZnSnO₃ hollow cubes could be selectively tailored by repeated addition of alkaline solution. The multishelled architecture displayed outstanding sensing properties for formaldehyde vapors due to large specific surface area, less agglomerations, abundant interfaces, thin shells, and high proportion porous structure, which act synergistically to facilitate charge transfer and promote target gas adsorption.

Design and Synthesis of Cu@CuS Yolk-Shell Structures with Enhanced Photocatalytic Activity

Non-spherical Cu@CuS yolk-shell structures are successfully obtained using Cu₂O cube templates in a process combining rapid surface sulfidation followed by disproportionation of the Cu₂O core upon treatment with a hydrochloric acid solution. By employing the above method, Cu@CuS yolk-shell structures with different morphologies, including octahedral, truncated octahedral, and cuboctahedral shapes, can be synthesized. The void space within the hollow structures provides a unique confined space, where the metallic copper present in the core of a shell can be protected from agglomeration and oxidation. Furthermore, the presence of metal copper in these hollow structures contributes to improvement in the photocatalytic properties of these materials. The application of these Cu@CuS structures indeed shows clearly improved photocatalytic performance.

Preparation of 2D α-Fe2O3 platelets via a hydrothermal heterogeneous growth approach and study of their magnetic properties

(001)-exposed α-Fe₂O₃ platelets can be synthesized via a hydrothermal process by producing BaFe₁₂O₁₉ nuclei as cores for heterogeneous growth.

General fabrication and enhanced VOC gas-sensing properties of hierarchically porous metal oxides

A facile and general route to synthesize hierarchically porous metal oxides, along with their noble-metal modification, is reported. The lowest detection limit achieved for formaldehyde is 10 ppb, much less than the indoor limit (60 ppb).

New insights and perspectives into biological materials for flexible electronics

Materials based on biological materials are becoming increasingly competitive and are likely to be critical components in flexible electronic devices.

Highly Selective Xylene Sensor Based on NiO/NiMoO4 Nanocomposite Hierarchical Spheres for Indoor Air Monitoring

Xylene is a hazardous volatile organic compound, which should be measured precisely for monitoring of indoor air quality. The selective detection of ppm-level xylene using oxide semiconductor chemiresistors, however, remains a challenging issue. In this study, NiO/NiMoO₄ nanocomposite hierarchical spheres assembled from nanosheets were prepared by hydrothermal reaction, and the potential of sensors composed of these nanocomposites to selectively detect xylene gas was investigated. The sensors based on the NiO/NiMoO₄ nanocomposite hierarchical spheres exhibited high responses (maximum resistance ratio =101.5) to 5 ppm p-xylene with low cross-responses (resistance ratios <30) to 5 ppm toluene, benzene, C₂H₅OH, CH₃COCH₃, HCHO, CO, trimethylamine, and NH₃. In contrast, a sensor based on pure NiO hierarchical spheres exhibited negligibly low responses to all 9 analyte gases. The gas-sensing mechanism underlying the high selectivity and response to xylene in the NiO/NiMoO₄ nanocomposite hierarchical spheres is discussed in relation to the catalytic promotion of the xylene-sensing reaction by synergistic combination between NiO and NiMoO₄, gas-accessible hierarchical morphology, and electronic sensitization by Mo addition. Highly selective detection of xylene can pave the road toward a new solution for precise monitoring of indoor air pollution.

Yolk@Shell Nanoarchitectures with Bimetallic Nanocores-Synthesis and Electrocatalytic Applications

In the present paper, we demonstrate a versatile approach for the one-pot synthesis of metal oxide yolk@shell nanostructures filled with bimetallic nanocores. This novel approach is based on the principles of hydrophobic nanoreactor soft-templating and is exemplified for the synthesis of various AgAu_NP@tin-rich ITO (AgAu@ITO_TR) yolk@shell nanomaterials. Hydrophobic nanoreactor soft-templating thereby takes advantage of polystyrene-block-poly(4-vinylpiridine) inverse micelles as two-compartment nanoreactor template, in which the core and the shell of the micelles serve as metal and metal oxide precursor reservoir, respectively. The composition, size and number of AuAg bimetallic nanoparticles incorporated within the ITO_TR yolk@shell can easily be tuned. The conductivity of the ITO_TR shell and the bimetallic composition of the AuAg nanoparticles, the as-synthesized AuAg_NP@ITO_TR yolk@shell materials could be used as efficient electrocatalysts for electrochemical glucose oxidation with improved onset potential when compared to their gold counterpart.

Wide bandgap mesoporous hematite nanowire bundles as a sensitive and rapid response ethanol sensor

In this study, α-Fe2O3 nanowires were synthesized using mesoporous SBA-15 silica as the hard templates with the nanocasting method, and then mesoporous α-Fe2O3 nanowire bundles (NWBs) were separated from the well-dispersed α-Fe2O3 nanowires (NWs) by the centrifugation technique. Both samples were characterized by x-ray diffraction, transmission electron microscopy (TEM), nitrogen adsorption/desorption isotherm and UV-vis spectra. All results indicated that the α-Fe2O3 NWBs with mesoporous structure presented a higher BET surface area (95 m(2) g(-1)) and wider bandgap (2.08 eV) than those of α-Fe2O3 NWs (32 m(2) g(-1) and 1.91 eV). The bandgap of α-Fe2O3 NWBs was in accordance with the bulk α-Fe2O3, while the BET surface area was much higher. The results from the gas-sensing measurement revealed that the α-Fe2O3 NWBs based gas sensor exhibited a high sensitivity of 21.7, fast response-recovery of 7.5 s and 1 s, and good selectivity to ethanol at 340 °C. The sensitivity (21.7) for ethanol of α-Fe2O3 NWBs was much better than that of the α-Fe2O3 NWs (12.2), which should be attributed to the higher BET surface area and wider bandgap of α-Fe2O3 NWBs.

Hybrid Co3O4/SnO2 Core-Shell Nanospheres as Real-Time Rapid-Response Sensors for Ammonia Gas

Novel hybrid Co3O4/SnO2 core-shell nanospheres have been effectively realized by a one-step hydrothermal, template-free preparation method. Our strategy involves a simple fabrication scheme that entails the coating of natural cross-link agents followed by electrostatic interaction between the positive charges of Sn and Co ions and the negative charge of glutamic acid. The core-shell architecture enables novel flexibility of gas sensor surfaces compared to commonly used bulk materials. The highly efficient charge transfer and unique structure are key to ensuring the availability of high response and rapid-response speed. It demonstrates how hybrid core-shell nanospheres can be used as an advance function material to fabricate electrical sensing devices that may be useful as gas sensors.

Selectively enhanced sensing performance for oxidizing gases based on ZnO nanoparticle-loaded electrospun SnO2 nanotube heterostructures

In this work, we present gas sensors based on ZnO nanoparticle-loaded electrospun SnO₂ nanotube (ZnO/SnO₂) n–n heterostructures (HSs) synthesized by electrospinning combined with facile thermal decomposition.

The synthesis and fast ethanol sensing properties of core–shell SnO2@ZnO composite nanospheres using carbon spheres as templates

A special core–shell SnO₂@ZnO composite nanosphere with hollow and porous structures shows excellent ethanol sensing properties.