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Approximately one in 10 people worldwide suffer from irritable bowel syndrome (IBS). Thus, early diagnosis and simple monitoring of chronic diseases will help to improve the patient's quality of life. Herein, we report a chemiresistive-type gas sensor for exhaled hydrogen detection that was fabricated by combining hierarchically porous SnO2 nano-architectures and PdO. The unique structured PdO-functionalized SnO2 hydrogen sensor exhibited excellent selectivity (response ratio > 5.2), high response (resistance ratio = 6.3), extremely fast sensing kinetics (response speed: 4.0 s and recovery period: 36.2 s), and low detection limit (0.096 ppm) under high humidity (relative humidity 80%). These exceptional sensing properties toward hydrogen were investigated from multiple perspectives, including effective gas diffusion through multimodal pores, changes in charge-carrier concentration in p–n heterojunctions, and the catalytic effect of PdO. The proposed sensor has a high potential for practical high-performance hydrogen sensor applications and will offer new possibilities for a simple disease diagnosis based on breath analysis. [ABSTRACT FROM AUTHOR]

The precise detection of trace concentrations of ethylene, a plant hormone gas, is essential for controlling plant health and status. However, owing to the simple chemical structure and low reactivity of ethylene, the highly sensitive and exclusive detection of ethylene has been a significant challenge, despite the strong demand from the agricultural industry. Herein, we report a novel bilayer ethylene gas sensor consisting of a Pd-doped V2O5–TiO2 yolk–shell catalytic overlayer and an In2O3 sensing layer. The coating of 2 wt% Pd-doped V2O5–TiO2 on the In2O3 sensing film remarkably enhances the ethylene response and selectivity over a wide range of operating temperatures (300–400 °C). This is attributed to the reforming of ethylene into more reactive acetaldehyde and the filtering of interference gas into non-reactive forms using the unique Pd–V2O5–TiO2 catalytic overlayer for heterogeneous Wacker oxidation promoted by the yolk–shell sphere structure. To the best of our knowledge, this paper presents the first report on the design of a catalytic overlayer that converts less-reactive ethylene into more-reactive species. The outstanding capability of the unique bilayer sensor proposed in this study enables the monitoring of the ripeness of fruits and correlates well with the results of proton transfer reaction-quadrupole mass spectrometry (PTR-QMS). [ABSTRACT FROM AUTHOR]

Bimetallic Fe/Ni-based metal-organic frameworks (MOFs) with different Fe/Ni ratios were coated on TiO2 nanorods (NRs), and the performances of the heterojunction photoanodes in photoelectrochemical water splitting were investigated. The bandgaps and band positions of the MOFs could be modulated by changing the ratio of the Fe and Ni components. An ideal band alignment was achieved between the TiO2 NRs and bimetallic MOFs with an optimum ratio of [Fe]/[Ni] = 0.25/0.75, which allowed efficient light absorption and charge separation. The coating of NH2−MIL(Fe)−88 layer on the TiO2 NRs decreased the photocurrent density by 33%. In comparison, TiO2/NH2−MIL(Ni)−88 showed a modest improvement in photocurrent density (0.85 mA·cm−2 at 1.23 V vs. a reversible hydrogen electrode (RHE)). When bimetallic NH2−MIL(Fe0.25Ni0.75)−88 was coated on the TiO2 NRs, the photocurrent density reached 1.56 mA·cm−2, which was an efficiency enhancement of 3.2 times. The mechanism underlying high photoelectrochemical performance was investigated. [ABSTRACT FROM AUTHOR]

Oxide chemiresistors have mostly been used to detect reactive gases such as ethanol, acetone, formaldehyde, nitric dioxide, and carbon monoxide. However, the selective and sensitive detection of volatile aromatic compounds such as benzene, toluene, and xylene, which are extremely toxic and harmful, using oxide chemiresistors remains challenging because of the molecular stability of benzene rings containing chemicals. Moreover, the performance of the sensing materials is insufficient to detect trace concentration levels of volatile aromatic compounds, which lead to harmful effects on human beings. Here, the strategies for designing highly selective and sensitive volatile aromatic compound gas sensors using oxide chemiresistors were suggested and reviewed. Key approaches include the use of thermal activation, design of sensing materials with high catalytic activity, the utilization of catalytic microreactors and bilayer structures with catalytic overlayer, and the pretreatment of analyte gases or post analysis of sensing signals. In addition, future perspectives from the viewpoint of designing sensing materials and sensor structures for high-performance and robust volatile aromatic compounds gas sensors are provided. Finally, we discuss possible applications of the sensors and sensor arrays. [ABSTRACT FROM AUTHOR]

Porous CuBr films are the most promising platform for the exclusive detection of NH3 but their simple and cost-effective fabrication without employing a high-temperature process remains elusive. Herein, we report the facile and low-temperature preparation of porous CuBr films by a solution oxidation method, and their unprecedentedly high NH3 sensing properties. The porous CuBr films are prepared by coating Cu thin films with different thicknesses (10–100 nm) on sensor substrates by direct current sputtering, immersion of Cu thin films in a CuBr2 solution (0.001–0.1 M) for a controlled time (10–300 s), and washing of the substrate with methanol. Among the CuBr film sensors, the sensor fabricated by immersing the 30 nm-thick Cu film in 0.01 M CuBr2 solution for 120 s (30n-120s sensor) exhibits the highest NH3 response (resistance ratio = 211), which is the highest level compared with those reported in the literature. Moreover, the 30n-120s sensor exhibits ultrahigh NH3 selectivity, extremely low detection limit (∼25 ppb), excellent reversibility, and long-term stability at room temperature. The sensing film of the 30n-120s sensor has a porous structure consisting of loosely percolated CuBr particles, whereas the other films have a dense structure with tightly percolated particles or, conversely, an island configuration composed of nearly discrete particles. The superior NH3 sensing properties of the sensor are discussed in terms of the porosity and particle connectivity of the film. The high performance NH3 sensor can be used to realize flexible and even wearable NH3 detectors for emerging applications such as environmental monitoring, food freshness assessment, and breath analysis for disease diagnosis. [ABSTRACT FROM AUTHOR]

Formaldehyde, a probable carcinogen, is a ubiquitous indoor pollutant, but its highly selective detection has been a long-standing challenge. Herein, a chemiresistive sensor that can detect ppb-level formaldehyde in an exclusive manner at room temperature is designed. The TiO2 sensor exhibits under UV illumination highly selective detection of formaldehyde and ethanol with negligible cross-responses to other indoor pollutants. The coating of a mixed matrix membrane (MMM) composed of zeolitic imidazole framework (ZIF-7) nanoparticles and polymers on TiO2 sensing films removed ethanol interference completely by molecular sieving, enabling an ultrahigh selectivity (response ratio > 50) and response (resistance ratio > 1,100) to 5 ppm formaldehyde at room temperature. Furthermore, a monolithic and flexible sensor is fabricated successfully using a TiO2 film sandwiched between a flexible polyethylene terephthalate substrate and MMM overlayer. Our work provides a strategy to achieve exclusive selectivity and high response to formaldehyde, demonstrating the promising potential of flexible gas sensors for indoor air monitoring. Formaldehyde, a probable carcinogen, is a ubiquitous indoor pollutant, but its ultraselective detection has been a long-standing challenge. Here, the authors develop a chemiresistive sensor that can detect ppb-level formaldehyde in an exclusive manner at room temperature. [ABSTRACT FROM AUTHOR]

Water poisoning has been a long-standing problem in oxide semiconductor gas sensors. Herein, for the first time, we report that pure and Pr-doped Ce4W9O33 provide humidity-independent gas sensing characteristics. Ce4W9O33 and Pr-doped Ce4W9O33 powders with a porous structure have been successfully prepared by ultrasonic spray pyrolysis and subsequent annealing at low temperature (600 °C). Interestingly, these p-type oxide semiconductors exhibited nearly the same gas sensing characteristics at 300 °C regardless of humidity variation, whereas pure WO3 showed a significant decrease of sensor resistance and gas response when the atmosphere is changed from dry to relative humidity 80%. Furthermore, Ce4W9O33-based sensors showed highly selective and sensitive detection of ppm-level trimethylamine (TMA). Moisture-endurant gas sensing characteristics were discussed in relation to surface regeneration through the hydroxyl scavenging reaction assisted by abundant Ln3+ (Ln = Ce, Pr) in (Ce1−xPrx)4W9O33 (x = 0–0.3) and TMA selectivity was explained by the acid–base interaction between the analyte gas and sensing material. Phase-pure ternary or quaternary oxides with a decreased oxidation state of lanthanide components provide a new and general strategy to design humidity-independent gas sensors with new functionality. [ABSTRACT FROM AUTHOR]

Solar fuel production, water splitting, and CO2 reduction by sunlight‐assisted catalytic reactions, are attractive and environmentally sustainable approaches used to generate energy. Since many different parameters, including energy band structures, electronic conductivity, surface area, porosity, catalytic activity, and stability of photocatalytic materials, determine the photocatalytic reaction, a single photocatalytic material is often insufficient to fulfill all the requirements. Hybridization to complement the limitations of two or more component materials can provide a viable solution. Particularly, hybridization with metal organic frameworks (MOFs), a new class of materials with excellent controllability of topology, surface area, porosity, morphology, band structure, electrical conductivity, and composition, enables the on‐demand design of a myriad of high‐performance photocatalysts. Moreover, hybrids formed by MOF‐derived materials inherit the distinctive merits from the MOF and offer further diversification for hybrid photocatalysts. Here, the rational design of MOF‐based hybrid photocatalysts for solar fuel production is discussed. The synthetic strategies of diverse MOF‐based hybrids, the key physicochemical parameters of hybrids to determine photocatalytic and photoelectrochemical reactions, and the mechanisms underlying the synergistic enhancement of solar fuel production are reviewed. Moreover, remaining challenges and future perspectives are addressed. [ABSTRACT FROM AUTHOR]

Volatile aromatic compounds are major air pollutants, and their health impacts should be assessed accurately based on the concentration and composition of gas mixtures. Herein, novel bilayer sensors consisting of a SnO2 sensing layer and three different xRh‐TiO2 catalytic overlayers (x = 0.5, 1, and 2 wt%) are designed for the new functionalities such as the selective detection, discrimination, and analysis of benzene, toluene, and p‐xylene. The 2Rh‐TiO2/SnO2 bilayer sensor shows a high selectivity and response toward ppm‐ and sub‐ppm‐levels of benzene over a wide range of sensing temperatures (325–425 °C). An array of 0.5Rh‐, 1Rh‐, and 2Rh‐TiO2/SnO2 sensors exhibits discrimination and composition analyses of aromatic compounds. The conversion of gases into more active species at moderate catalytic activation and the complete oxidation of gases into non‐reactive forms by excessive catalytic promotion are proposed as the reasons behind the enhancement and suppression of analyte gases, respectively. Analysis using proton transfer reaction‐quadrupole mass spectrometer (PTR‐QMS) is performed to verify the above proposals. Although the sensing characteristics exhibit mild moisture interference, bilayer sensors with systematic and tailored control of gas selectivity and response provide new pathways for monitoring aromatic air pollutants and evaluating their health impacts. [ABSTRACT FROM AUTHOR]

Volatile aromatic compounds are major air pollutants, and their health impacts should be assessed accurately based on the concentration and composition of gas mixtures. Herein, novel bilayer sensors consisting of a SnO2 sensing layer and three different xRh‐TiO2 catalytic overlayers (x = 0.5, 1, and 2 wt%) are designed for the new functionalities such as the selective detection, discrimination, and analysis of benzene, toluene, and p‐xylene. The 2Rh‐TiO2/SnO2 bilayer sensor shows a high selectivity and response toward ppm‐ and sub‐ppm‐levels of benzene over a wide range of sensing temperatures (325–425 °C). An array of 0.5Rh‐, 1Rh‐, and 2Rh‐TiO2/SnO2 sensors exhibits discrimination and composition analyses of aromatic compounds. The conversion of gases into more active species at moderate catalytic activation and the complete oxidation of gases into non‐reactive forms by excessive catalytic promotion are proposed as the reasons behind the enhancement and suppression of analyte gases, respectively. Analysis using proton transfer reaction‐quadrupole mass spectrometer (PTR‐QMS) is performed to verify the above proposals. Although the sensing characteristics exhibit mild moisture interference, bilayer sensors with systematic and tailored control of gas selectivity and response provide new pathways for monitoring aromatic air pollutants and evaluating their health impacts. [ABSTRACT FROM AUTHOR]

Water poisoning, the dependence of gas‐sensing characteristics on moisture, in oxide chemiresistors remains a long‐standing challenge. Various approaches are explored to mitigate water poisoning but they are often accompanied by significant deterioration of sensing capabilities such as gas response deterioration, gas selectivity alteration, and sensor resistance increase up to unmeasurable levels. Herein, a novel sensor design with a moisture‐blocking Tb4O7 overlayer is suggested as a facile and universal strategy to remove moisture poisoning without sacrificing intrinsic sensing properties. A submicrometer‐thick coating of Tb4O7 overlayer on In2O3 sensors effectively eliminates the humidity dependence of the gas‐sensing characteristics without significantly altering the gas response, selectivity, and sensor resistance. Furthermore, the general validity of the water‐blocking effect using the Tb4O7 overlayer is confirmed in diverse gas sensors using SnO2, ZnO, and Pd/SnO2. The negligible moisture interference of the bilayer sensor is explained in terms of the hydrophobic nature of the Tb4O7 overlayer and the prevention of formation of the OH radical by the interaction between Tb4O7 and In2O3. A universal solution to design diverse humidity‐independent gas sensors with different gas selectivities can open up new pathways toward building accurate and robust gas sensors with new functionalities and high‐performance artificial olfaction. [ABSTRACT FROM AUTHOR]

A highly selective and sensitive detection of the plant hormone ethylene, particularly at low concentrations, is essential for controlling the growth, development, and senescence of plants, as well as for ripening of fruits. However, this remains challenging because of the non‐polarity and low reactivity of ethylene. Herein, a strategy for detecting ethylene at a sub‐ppm‐level is proposed by using oxide semiconductor chemiresistors with a nanoscale oxide catalytic overlayer. The SnO2 sensor coated with the nanoscale catalytic Cr2O3 overlayer exhibits rapid sensing kinetics, good stability, and an unprecedentedly high ethylene selectivity with exceptional gas response (Ra/Rg − 1, where Ra represents the resistance in air and Rg represents the resistance in gas) of 16.8 at an ethylene concentration of 2.5 ppm at 350 °C. The sensing mechanism underlying the ultraselective and highly sensitive ethylene detection in the unique bilayer sensor is systematically investigated with regard to the location, configuration, and thickness of the catalytic Cr2O3 overlayer. The mechanism involves the effective catalytic oxidation of interfering gases into less‐ or non‐reactive species, without limiting the analyte gas transport. The sensor exhibits a promising potential for achieving a precise quantitative assessment of the ripening of five different fruits. [ABSTRACT FROM AUTHOR]

Heterostructured nanocomposites comprising transition metal compounds (TMCs) with different bandgaps are attractive due to their excellent electrochemical performances. Candidates that combine various cations and anions are actively researched. Herein, it is demonstrated for the first time that nickel hydroxy chloride, once transformed into a heterostructured nanocomposite during the initial cycle, can be used as a new anode material for lithium-ion storage. In particular, the reaction mechanism for lithium-ion storage with a metal hydroxy chloride as the anode is demonstrated through various analyses for the first time. The model compound, nickel hydroxy chloride (Ni(OH)Cl), prepared by a one-pot hydrothermal method, is used to investigate the detailed conversion mechanism in Li-ion storage. Through systemically analyzed results, it is demonstrated that Ni(OH)Cl is transformed into Ni(OH)2 and NiCl2 after one cycle and that the layered Ni(OH)2/NiCl2 nanocomposite heterointerface reacts with Li ions from the second cycle onward. Flower-like Ni(OH)Cl microspheres display extremely high and stable cycling performance (1236 mA h g−1 for the 150th cycle at a current density of 0.2 A g−1) and outstanding rate capability (232 mA h g−1) at an extremely high current density of 30 A g−1. [ABSTRACT FROM AUTHOR]

To organize one-dimensional (1D) metal oxides into highly ordered and controllable architectures on the required regions remains challenging. Herein, we report for the first time the facile, versatile, and on-demand fabrication of metal oxide patterns comprising 1D nanofibers via near-field electrospinning (NFES), which have a wide variety of potential applications in sensors, optoelectronic circuits, and functional nanoelectronics. Grids, diamonds, and hexagrams of In2O3, Co3O4, and NiO nanofibers are first demonstrated, and the underlying mechanisms for fiber formation are systematically investigated with respect to the experimental parameters of NFES. Furthermore, we propose the nano-architectures as a novel gas sensing platform that exhibits an unprecedentedly high gas response (resistance ratio, ST = 239, T: trimethylamine) and selectivity (STSE−1 > 7, E: ethanol) to 5 ppm trimethylamine compared with thin film counterparts (ST = 24, STSE−1 ≈ 1). The research provides a vital breakthrough to fabricate metal oxide nano-architectures of 1D nanofibers and new platforms to design next-generation functional nanodevices for a wide range of emerging applications. [ABSTRACT FROM AUTHOR]

In spite of the attractive electrical properties of metal oxide nanowires, it is difficult to tune their surface states, notably the ionic adsorbents and oxygen vacancies, both of which can cause instability, degradation, and the irreproducibility or unrepeatable changes of the electrical characteristics. In order to control the surface states of the nanowires, electron beams were locally irradiated onto the channels of metal oxide nanowire field effect transistors. This high energy electron beam irradiation changed the electrical properties of the individual metal oxide nanowires, due to the removal of the negative adsorbents (O2-, O-). The detachment of the ionic adsorbents changes the charge states of the nanowires, resulting in the enhancement of the electrical conductance in n-type nanowires (ZnO, SnO2) and the degradation of the conductance in p-type nanowires (CuO). By investigating the changes in the electrical properties of nanowire devices in air or vacuum, with or without exposure to electron beams, the roles of the physisorbed water molecules or chemisorbed oxygen molecules can be independently understood. Unlike the electron beam irradiation, the vacuum enhanced the conductance of both n-type (ZnO, SnO2) and p-type (CuO) nanowires, due to the release of charges caused by the detachment of the polarized water molecules that were screening them from the surface of the nanowires, irrespective of the major carrier type. The electron beam irradiation technique has the potential to locally modulate the charge carriers in electronic nanowire devices, and the changes could be maintained with proper passivation for the long-term preservation of the device characteristics. [ABSTRACT FROM AUTHOR]

Multishell structured metal oxide microspheres with considerable structural stabilities during repeated cycling and higher volumetric capacities than those of hollow structured microspheres are popular as anode materials for lithium-ion batteries. However, the long-term cycling and rate performances of multishell metal oxide microspheres for lithium-ion storage require further improvement. Herein, novel and unique structured microspheres (Fe3O4–GC) with multishells comprising graphitic carbon (GC)-coated Fe3O4 hollow nanopowders are successfully synthesized. The reduction of pitch-infiltrated Fe2O3 yolk–shell microspheres yields metallic Fe yolk–shell microspheres filled with pitch-derived carbon. Oxidation yields yolk–shell Fe3O4–GC microspheres with multishells comprising hollow nanopowders coated with GC via the selective decomposition of amorphous carbon and oxidation of metallic Fe. Metallic Fe nanocrystals transform into hollow Fe3O4 nanopowders due to nanoscale Kirkendall diffusion. The primary hollow Fe3O4 nanopowders comprising yolk–shell microspheres and GC layer formed by pitch-derived carbon decrease the distance for lithium-ion diffusion and improve the electrical conductivity resulting in outstanding cycling and rate performances of Fe3O4–GC, respectively. The discharge capacity of Fe3O4–GC microspheres for the 1000th cycle at a current density of 2.0 A g−1 is 1018 mA h g−1, and the microspheres exhibit a high reversible capacity of 649 mA h g−1 even at 20 A g−1. [ABSTRACT FROM AUTHOR]

Ultrahigh gas selectivity and response for ppb levels of xylene are achieved by doping Pt in CoCr2O4 hollow spheres, whereas the addition of Au or Pd is less effective or even deteriorated the performance. [ABSTRACT FROM AUTHOR]

As an effective alternative to simple binary oxide chemiresistors, polynary oxides with excellent tunability of the composition and hetero-interfaces are considered as promising material platforms for designing highly selective and sensitive gas sensors. In this study, ternary spinel CoCr2O4 hollow spheres and CoCr2O4–Cr2O3 mixed oxide hollow spheres were prepared via one-pot ultrasonic spray pyrolysis using solutions with different cation compositions (i.e., [Cr]/[Co] = 2, 3, and 4), and their gas-sensing characteristics were investigated. The pure CoCr2O4 hollow spheres exhibited an unusually high response to 5 parts per million (ppm) of p-xylene (ratio of resistance to gas and air = 61.4), with negligible cross-responses to 5 ppm of ethanol, toluene, benzene, trimethylamine, ammonia, formaldehyde, and carbon monoxide. When CoCr2O4–Cr2O3 hollow spheres with discrete Cr2O3 nanoclusters were formed using a spray solution with a [Cr]/[Co] ratio of 3, the xylene response was enhanced to 144.1, which allows the sensitive and selective detection of sub-ppm level p-xylene. The unprecedentedly high xylene selectivity and response in the present study are explained by the gas-accessible hollow morphology, the unique catalytic activity of the ternary and mixed oxides, and the enhanced chemiresistivity due to the formation of a heterojunction between CoCr2O4 and Cr2O3. The novel ternary oxide-based gas sensors with excellent xylene-sensing performance can be used in indoor air-monitoring applications. [ABSTRACT FROM AUTHOR]

We demonstrate that the Li-ion conductivity can be improved by adding a certain amount of Li (x = 0.25–0.5) as a charge carrier to the composition of glass-ceramic Li7+xP3S11. Structural analysis clarified that the structural changes caused by the ratio of ortho-thiophosphate tetrahedra PS43− and pyro-thiophosphate ditetrahedra P2S74− affect the Li-ion conductivity. The ratio of PS43− and P2S74− varies depending on x and the highest Li-ion conductivity (2.5 × 10−3 S cm−1) at x = 0.25. All-solid-state LiNi0.8Co0.15Al0.05O2/Li7.25P3S11/In-metal cell exhibits the discharge capacity of 106.2 mAh g−1. This ion conduction enhancement from excess Li is expected to contribute to the future design of sulfide-type electrolytes. [ABSTRACT FROM AUTHOR]

Reduced graphene oxide (rGO) and MoS2, the most representative two-dimensional (2D) materials, are receiving significant attention for the fabrication of sensing devices owing to their high surface area, many abundant sites, and excellent mechanical flexibility. Herein, we demonstrated chemoresistive humidity sensors with fast response, excellent selectivity, and ultrahigh sensitivity based on 2D rGO and 2D MoS2 hybrid composites (RGMSs). The RGMSs were fabricated by simple ultrasonication without the addition of additives and additional heating. Compared to pristine rGO, the RGMS exhibited a 200 times higher response to humidity at room temperature. The significant enhancement in the sensing performance of the composite was attributed to electronic sensitization due to p–n heterojunction formation and porous structures between rGO and MoS2. The synergistic combination of rGO and MoS2 could be applied to construct a flexible humidity sensor. The sensing performance of an RGMS flexible device remains the same before and after bending; this indicates that the use of rGO and MoS2 is a viable new and simple strategy to realize a humidity sensor for use in wearable electronics. [ABSTRACT FROM AUTHOR]

Semiconductor gas sensors using metal oxides, carbon nanotubes, graphene-based materials, and metal chalcogenides have been reviewed from the viewpoint of the sensitive, selective, and reliable detection of exhaled biomarker gases, and perspectives/strategies to realize breath analysis on a chip for disease diagnosis are discussed based on the concurrent design of high-performance sensing materials and miniaturized pretreatment components. Carbon-based sensing materials that show relatively high responses to NO and NH3 at low or mildly raised temperatures can be applied to the diagnosis of asthma and renal disease. Halitosis can be diagnosed by employing sensing or additive materials such as CuO and Mo that have high chemical affinities for H2S, while catalyst-loaded metal oxide nanostructure sensors or their arrays have been used to diagnose diabetes via the selective detection of acetone or by pattern recognition of sensor signals. For the ultimate miniaturization of a breath-analysis system into a tiny chip, preconditioning that includes preconcentration, dehumidification, and flow sensing needs to be either improved through the design of gas/moisture adsorbents or removed/simplified through the design of highly sensitive sensing materials that are less impervious to interference from humidity and temperature. Moreover, an abundant sensing library needs to be provided for the diagnosis of diseases (e.g. lung cancer) that are associated with multiple biomarker gases and for finding new methods to diagnose other diseases. For this aim, p-type oxide semiconductors with high catalytic activities, as well as combinatorial approaches, can be considered for the development of sensing materials that detect less-reactive large molecules, and high-throughput screening, respectively. Selectivity for a specific biomarker gas will simplify the system further. Breath analysis on a tiny chip using semiconductor chemiresistors with ultralow power consumption that is connected to the ‘Internet of Things’ will pave new roads for disease diagnosis and patient monitoring. [ABSTRACT FROM AUTHOR]

This study aims to develop an anode catalyst for a solid oxide fuel cell (SOFC) using electroless nickel plating. We have proposed a new method for electroless plating of Ni metal on yttria-stabilized zirconia (YSZ) particles. We examine the uniformity of the Ni layer on the plated core-shell powder, in addition to the content of Ni and the reproducibility of the plating. We have also evaluated the carbon deposition rate and characteristics of the SOFC anode catalyst. To synthesize Ni-plated YSZ particles, the plated powder is heat-treated at 1200 °C. The resultant particles, which have an average size of 50 μm, were subsequently used in the experiment. The size of the Ni particles and the Ni content both increase with increasing plating temperature and plating time. The X-ray diffraction pattern reveals the growth of Ni particles. After heat-treatment, Ni is oxidized to NiO, leading to the co-existence of Ni and NiO; NiP is also observed due to the presence of phosphorous in the plating solution. Following heat treatment for 1 h at 1200 °C, Ni is mostly oxidized to NiO. The carbon deposition rate of the reference YSZ powder is ~135%, while that of the Ni-plated YSZ is 1%-6%. [ABSTRACT FROM AUTHOR]

Chemoresistive gas sensors based on two-dimensional (2D) materials including graphene-based materials have attracted significant research interest owing to their potential use in next-generation technologies including the Internet of Things (IoT). The functionalization of 2D materials is considered as a key strategy to achieve superior gas sensing properties such as high selectivity, high sensitivity, and reversible response and recovery, because it can modulate the chemical and electrical properties of 2D materials for more efficient gas sensing. Herein, we present a facile solution process and the room temperature gas sensing properties of chemically fluorinated graphene oxide (CFGO). The CFGO sensors exhibit improved sensitivity, selectivity, and reversibility upon exposure to NH3 with a significantly low theoretical detection limit of ∼6 ppb at room temperature in comparison to NO2 sensing properties. The effect of fluorine doping on the sensing mechanism is examined by first-principles calculations based on density functional theory. The calculations reveal that the fluorine dopant changes the charge distribution on the oxygen containing functional groups in graphene oxide, resulting in the preferred selective adsorption and desorption of NH3 molecules. We believe that the remarkable NH3 sensing properties of CFGO and investigation by first-principles calculations would enlarge the possibility of functionalized 2D materials for practical gas sensing applications such as the IoT. [ABSTRACT FROM AUTHOR]

Ultra-selective and sensitive detection of carcinogenic benzene vapor with negligible interferences from other major indoor pollutants is not only critical but also challenging because the BTX gases (benzene, toluene, and xylene) are generally less reactive to a majority of n-type oxide semiconductor gas sensors and the similar chemical structures of BTX gases hamper their discrimination by chemiresistive variation. Through this paper, we propose a new strategy to detect sub-ppm-level benzene vapor in a highly selective manner using oxide semiconductor chemiresistors. A Pd-loaded SnO2 yolk–shell sensing film coated with a thin catalytic Co3O4 overlayer showed an ultrahigh response (resistance ratio = 88) to 5 ppm benzene with negligibly low cross-responses to the other representative and ubiquitous indoor pollutants such as toluene, xylene, HCHO, CO, and ethanol. The response to benzene vapor was significantly enhanced by reforming of highly stable benzene into more active and smaller species. The reforming was synergistically assisted by the Co3O4 catalytic overlayer and sensing layer consisting of Pd-loaded SnO2 micro-reactors, while the cross-responses to the other indoor pollutants became low because of the catalytic oxidation of the gases into less- or non-reactive species. This method will pave a new way to the precise monitoring of critically harmful benzene in both indoor and outdoor atmospheres. [ABSTRACT FROM AUTHOR]

Monolayers of periodic porous Co3O4 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 Co3O4 IO thin films having periodic pores ( d≈500 nm) showed a high response of 112.9 to 5 ppm C2H5OH 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. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) molybdenum disulfide (MoS2) has been attracting rapidly increasing interest for application in chemoresistive gas sensors owing to its moderate band gap energy and high specific surface area. However, the mechanism of chemoresistive sensing via the adsorption and desorption of gas molecules and the influence of the shape of 2D materials are not well understood yet. Herein we investigate the oxygen sensing behavior of MoS2 microflakes and nanoparticles prepared by mechanical and liquid exfoliation, respectively. Liquid-exfoliated MoS2 nanoparticles with an increased number of edge sites present high and linear responses to a broad range of oxygen concentrations (1–100%). The energetically favorable oxygen adsorption sites, which are responsible for reversible oxygen sensing, are identified by first-principles calculations based on density functional theory. This study serves as a proof-of-concept for the gas sensing mechanism depending on the surface configuration of 2D materials and broadens the potential of 2D MoS2 in gas sensing applications. [ABSTRACT FROM AUTHOR]

Pd@In2O3 yolk–shell nanoparticles (NPs) were synthesized by a simple solution route using Pd@C core–shell NPs as template and applied for gas sensing. A glucose-assisted hydrothermal method was used for the synthesis of Pd@C core–shell NPs. Pd@In2O3 yolk–shell NPs were formed after calcination (450 °C for 3h) of Pd@C core–shell NPs containing indium precursor. In the Pd@In2O3 yolk–shell geometry, about 50–70 nm Pd NPs were present at the periphery of an In2O3 shell (10–20 nm thickness). The In2O3 shell was composed of ∼10 nm primary particles. The role of Pd NPs in gas sensing behavior of In2O3 has been investigated. The loading of In2O3 with Pd NPs improved the response for reducing gases, but reduced the response for oxidizing gases. The response of Pd@In2O3 yolk–shell NPs to ethanol was approximately 14 times higher than that of pure In2O3 hollow nanospheres at 350 °C. However, no response was recorded for NO2 for Pd@In2O3 as compared to In2O3 (resistance ratio Rs = 2.50) at 350 °C. The maximum response of Pd@In2O3 yolk–shell NPs to 5 ppm ethanol was 159.02 at 350 °C, which was approximately 2.5 times higher than those for other interfering gases (NO2, p-xylene, trimethylamine, HCHO, CO and H2). The effect of humidity on the gas sensing characteristics of Pd@In2O3 yolk–shell NPs suggested that the present sensor can be used to detect ppm-level ethanol even in highly humid atmosphere (80% RH). The improved gas sensing performance of Pd@In2O3 yolk–shell NPs was attributed to catalytic activity of Pd NPs as well as hollow spaces that allowed the accessibility of Pd NPs to gas molecules. [ABSTRACT FROM AUTHOR]

The article Heterojunction between bimetallic metal-organic framework and TiO2: Band-structure engineering for effective photoelectrochemical water splitting, written by Ji Won Yoon et al., was erroneously originally published electronically on the publisher's internet portal (currently SpringerLink) on 17 May 2022 with the Acknowledgements. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The detection of nitrogen dioxide, NO2, is currently the subject of extensive scientific and technological research, motivated by its deleterious impact on the environment and on human health and safety. However, detecting trace levels of NO2 gas in a timely, sensitive, and selective manner remains challenging, while the mechanisms governing selective NO2 sensing are still unclear. In this work, a monolayer α-Fe2O3 inverse opal (IO) film with single-crystalline 3-fold rotocenters is firstly synthesized in situ on a substrate using a sacrificial template and evaluated as a sensitive NO2 chemiresistor. Interestingly, the Fe2O3 macroporous film manifests abnormal sensing behavior with reversible transitions from p- to n-type sensing as a function of the NO2 concentration (C) and working temperature (T). Based on more familiar phase diagrams, a binary (T–C) transition diagram has been created in terms of the gas sensing response, which can be directly used to design and control the p–n transitions. Further investigations also show that such abnormal p–n transitions do not occur if a different sensing material (e.g. SnO2 IO) is used, or on exposure to gases other than NO2. The porous Fe2O3 sensor is capable of detecting trace levels of NO2, as low as 10 ppb, and shows good stability. Finally, the mechanisms underlying the unusual NO2 sensing transitions are described based on the nature of the materials and the well-known Lennard-Jones model. Reversible switching from p- to n-type sensing and the associated transition diagram carry great potential for the recognition and sensitive detection of trace levels of NO2. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 Ag-1 were measured as 720 and 640 mAhg-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 Ag-1 were 774, 711, 652, and 591 mAhg-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. [ABSTRACT FROM AUTHOR]

In this study, for the first time, polymeric precursors have been used in the preparation of yolk-shell powders using a large-scale spray drying process. An esterification reaction between the carboxyl group of citric acid and the hydroxyl group of ethylene glycol inside the droplet produced organic polymers during the drying process of the droplet. During the spray drying process, the polymeric precursors enabled the formation of multi-shell cobalt oxide yolk-shell powders with superior electrochemical properties. The maximum number of shells of the particles in the yolk-shell powders post-treated at 300, 400, and 500 °C were six, five, and four, respectively. The initial discharge capacities of the cobalt oxide yolk-shell powders post-treated at 300, 400, and 500 °C were 1,188, 1,331, and 1,110 mAh·g, and their initial charge capacities were 868, 1,005, and 798 mAh·g, respectively. The discharge capacities of the powders post-treated at 300, 400, and 500 °C after 100 cycles were 815, 958, and 670 mAh·g, respectively, and their corresponding capacity retentions measured after the first cycles were 92%, 93%, and 82%, respectively. The pure phase CoO yolk-shell powders post-treated at 400 °C had low charge transfer resistance and high lithium-ion diffusion rate. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]