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The decoration of graphene quantum dots enhances the NO2 gas sensing performance of SnO2 nanodomes by utilizing both electrical and chemical sensitization effects, leading to an impressive reduction in the theoretical detection limit.

Cadmium-based quantum dot nanocrystals are promising optoelectronic materials due to their core/shell structure, excellent properties, and fascinating potential applications in LED, display, and photovoltaics. However, those materials are very sensitive to moisture, temperature, UV light, and polar solvents, and this greatly hinders their practical applications. Here, highly luminescent CdSeS/ZnS quantum dots-polystyrene composite beads with uniform morphology are prepared via a simple swelling–shrinking strategy. This process is carried out only in nonpolar toluene and hexane without the addition of any polar reagents. In addition, a Si-Al oxidation layer is coated on the hybrid beads for further protection. As a result, the as-prepared composite beads not only retain high luminescence but also exhibit superior water, temperature, and UV-O3-resistant properties.

White-emitting hexagonal boron nitride (h-BN) nanophosphors were synthesized via pyrolysis (1200 °C). The nanoparticle phosphors were prepared by chemical mixed molecular adduction in which the sta...

With the increasing demands of indoor air quality monitoring, highly sensitive and selective gas sensor arrays consisting of metal oxide semiconductors have been increasingly studied. As an effective strategy to obtain the desired performance, noble metal functionalization is frequently chosen owing to its target-specific sensing mechanisms. However, the lack of a comprehensive analysis on the sensing mechanisms of different combinations of metal oxides and noble metals limit the versatility of these sensor arrays for use in selective gas sensor arrays. In this study, we fabricate a 3 × 3 gas sensor array to obtain a reliable comparison of the sensing mechanism through the use of three metal oxides—WO3, SnO2 and NiO—in three different nanostructure forms: a thin film and dome-like nanostructures with and without Au nanoparticle (NP) decoration. The responses of the sensor arrays to four target gases (CH3COCH3, C6H5CH3, NH3, and H2S) are generally enhanced by the dome-like nanostructure and show distinctively enhanced responses when the sensors are decorated with Au NPs. Moreover, the dome-like SnO2 with Au NPs shows an increase of up to 121 times for C6H5CH3 compared with the pristine SnO2 thin film. Additionally, the sensitization effects of Au NPs depend on the types of metal oxides and gases, which improve the gas discrimination capability by diversifying the gas selectivity of the sensor units in the array. The prepared sensor array can distinguish four target gases by principal component analysis (PCA). The contributions of different enhancement mechanisms, which are dependent on the gases and metal oxides, are investigated by comparing the activation energy of each gas response with and without Au. The comparison among the chosen gases reveals that the decoration of Au NPs is effective for C6H5CH3 and NH3 regardless of the type of metal oxide. Among the metal oxides, the effects of Au NPs on gas responses are in the order of SnO2, NiO, and WO3, in which the energy level difference between the Au NPs and metal oxide are 0.2, 0.1, and −0.6 eV, respectively. These results confirm that the dominant enhancement mechanism of Au NPs for each combination of gases and metal oxide is a decrease in activation energy. Therefore, this study provides a systematic understanding of the sensitization mechanism of Au NPs on metal oxides toward gases for the fabrication of selective gas sensor arrays.

For the last several years, indoor air quality monitoring has been a significant issue due to the increasing time portion of indoor human activities. Especially, the early detection of volatile organic compounds potentially harmful to the human body by the prolonged exposure is the primary concern for public human health, and such technology is imperatively desired. In this study, highly porous and periodic 3D TiO

Objective: To investigate the associations between hematologic parameters and obesity in children and adolescents.Methods: A total of 7,997 subjects (4,259 boys, 3,738 girls) aged 10–18 years was enrolled and hematologic parameters, including WBC, RBC, Hb, Hct, and platelet levels, were recorded and compared against body mass index (BMI) classified into normal-weight, overweight, and obesity groups.Results : The obesity group had significantly higher mean levels of WBC (7.16 vs. 6.16 (x103/mm3), p6/mm3), ppp3/mm3), ppppppConclusion: Higher BMI is associated with elevated WBC, RBC, Hb, Hct and platelet counts in children and adolescents. Because higher hematologic parameters are potential risk factors for obesity-related morbidity, more attention should be paid to evaluating and interpreting hematologic parameters in children and adolescents with obesity

To date, chemoresistive gas sensors based on metal oxide semiconductors (MOS) have been the most attractive sensor types for the practical application. However, with the emerging concept of Internet of Everything, high operating temperatures over 300 °C of the gas sensors based on MOS must be reduced to achieve low power consumption and overcome a limited battery capacity of mobile devices. The 2-dimensional materials like MoS2 have been, therefore, one of the most recently studied materials for gas sensors with capability of operation at significantly lower temperatures. However, lacking selectivity toward various target gas species limited their application to gas sensors. Herein, we investigated the effects of noble metals (Pd and Au) decoration on the gas sensing properties of MoS2 thin films. Due to the electronic sensitization of the noble metal catalysts and the formation of Pd hydride, overall gas responses and selectivity were significantly improved toward tests gas species. These results provide clear understandings on the effects of the surface noble metal catalysts on gas sensing properties of MoS2.

In order to develop high performance chemoresistive gas sensors for Internet of Everything applications, low power consumption should be achieved due to the limited battery capacity of portable devices. One of the most efficient ways to reduce power consumption is to lower the operating temperature to room temperature. Herein, we report superior gas sensing properties of SnS2 nanograins on SiO2 nanorods toward NO2 at room temperature. The gas response is as high as 701% for 10 ppm of NO2 with excellent recovery characteristics and the theoretical detection limit is evaluated to be 408.9 ppb at room temperature, which has not been reported for SnS2-based gas sensors to the best of our knowledge. The SnS2 nanograins on the template used in this study have excessive sulfur component (Sn:S = 1:2.33) and exhibit p-type conduction behavior. These results will provide a new perspective of nanostructured two-dimensional materials for gas sensor applications on demand.

Vertically ordered hematite nanotubes decorated with Au nanoparticles are synthesized by a facile electrochemical anodization method and self-agglomeration of Au films. Au decoration significantly enhances the response of hematite nanotubes to acetone at 350 °C and the response to acetone is 2.4 times higher than the response to ethanol, the counterpart target gas. The Au nanoparticles decorated hematite nanotubes exhibit unprecedentedly high and selective responses to acetone at 350 °C with 50% relative humidity condition (calibrated at room temperature, 25 °C). The enhanced gas sensing properties in a humid condition were attributed to the spillover effect by Au nanoparticles and high porosity of hematite nanotubes with a large surface-to-volume ratio. These results indicate that Au nanoparticles decorated hematite nanotubes are promising for use in high quality sensor materials for breath analyzers to diagnose diabetes mellitus.

Objective: To investigate investigated the association of hemoglobin (Hb) concentration, hematocrit (Hct) and blood pressure (BP) in children and adolescents.Methods: The study population consisted of a total 7,950 subjects (4,229 boys, 3,721 girls) aged 10 to 18 years who participated in the Korea National Health and Nutrition Examination Surveys conducted between 2007 and 2017.Results: The prevalence of hypertension is estimated about 19.19% (21.51% for boys and 16.5% for girls). The prevalence of obesity is estimated about 9.59% (10.5% for boys and 8.6% for girls). Hb and Hct level tended to increase with the degree of obesity and blood pressure. The systolic BP (SBP) and diastolic BP (DBP) have positively correlated with Hb and Hct levels in both sex. In the multiple regression analysis, the Hb and Hct presented positive association with SBP and DBP after adjustment for for age, BMI SDS, alcohol consumption, smoking physical activity, rural residence, household income, diagnosis of T2DM, hypertension, and dyslipidemia.Conclusion: Hemoglobin and hematocrit levels were positively associated with SBP and DBP, in children and adolescent, aged 10- 18 year.

One of the well‐known strategies for achieving high‐performance light‐activated gas sensors is to design a nanostructure for effective surface responses with its geometric advances. However, no study has gone beyond the benefits of the large surface area and provided fundamental strategies to offer a rational structure for increasing their optical and chemical performances. Here, a new class of UV‐activated sensing nanoarchitecture made of highly periodic 3D TiO2, which facilitates 55 times enhanced light absorption by confining the incident light in the nanostructure, is prepared as an active gas channel. The key parameters, such as the total 3D TiO2 film and thin‐shell thicknesses, are precisely optimized by finite element analysis. Collectively, this fundamental design leads to ultrahigh chemoresistive response to NO2 with a theoretical detection limit of ≈200 ppt. The demonstration of high responses with visible light illumination proposes a future perspective for light‐activated gas sensors based on semiconducting oxides., The high‐performance light‐activated NO2 gas sensors using 3D TiO2 nanostructures are achieved by the unique light harvesting properties of the nanoporous structures. The light scattering effects from optical maze‐like nanostructures and numerous intermediate energy states from atomic layer deposition of the film electronically activate 3D TiO2 and enhance its gas sensing performance under UV illumination.

In this era of the Internet of Things, the development of innovative sensors has rapidly accelerated with that of nanotechnology to accommodate various demands for smart applications. The practical use of three-dimensional (3D) nanostructured materials breaks several limitations of conventional sensors, including the large surface-to-volume ratio, precisely tunable pore size and porosity, and efficient signal transduction of 3D geometries. This review provides an in-depth discussion on recent advances in chemical and mechanical sensors based on 3D nanostructures, which are rationally designed and manufactured by advanced 3D nanofabrication techniques that consider structural factors (e.g., porosity, periodicity, and connectivity). In particular, we focus on a proximity-field nanopatterning technique that specializes in the production of periodic porous 3D nanostructures that satisfy the structural properties universally required to improve the performance of various sensor systems. State-of-the-art demonstrations of high-performance sensor devices such as supersensitive gas sensors and wearable strain sensors realized through designed 3D nanostructures are summarized. Finally, challenges and outlooks related to nanostructures and nanofabrication for the practical application of 3D nanostructure-based sensor systems are proposed.

Barium tungstate (BaWO4) powders with various sintering temperatures, and BaWO4:Dy3+ phosphor samples with concentrations of different rare-earth (RE) activator ions (Dy3+, Sm3+, Tb3+) were prepared through co-precipitation. The structural, morphological, and photoluminescent characteristics of barium tungstate phosphors depend on the concentration of RE ions. The crystallographic characteristics of the synthesized BaWO4 were analyzed using X-ray diffraction (XRD) patterns. The size and shape of the crystalline particles were estimated based on images measured with a field emission scanning electron microscope (FE-SEM). As the sintering temperature of the BaWO4 particles increased from 400 °C to 1000 °C, the size of the particles gradually increased and showed a tendency to clump together. In the sample doped with 7 mol % Dy3+ ions, the intensity of all emission bands reached their maximum. The emission spectra of the RE3+-doped BaWO4 powders by excitation at 325 nm were composed of yellow (Dy3+), red (Sm3+), and green (Tb3+) band at 572, 640, and 544 nm. This indicates that most of the RE3+ ions absorbed the position without reversal symmetry in the BaWO4 lattice. These results propose that strong emission intensity and tunable color for the phosphors can be accomplished by rare-earth doped host with an suitable quantity. In addition, the phosphor thin films, having high transparency from aqueous colloidal solutions, were deposited on banknotes, and it is considered whether it is suitable for anti-counterfeiting applications.

The realization of high‐contrast modulation in optically transparent media is of great significance for emerging mechano‐responsive smart windows. However, no study has provided fundamental strategies for maximizing light scattering during mechanical deformations. Here, a new type of 3D nanocomposite film consisting of an ultrathin (≈60 nm) Al2O3 nanoshell inserted between the elastomers in a periodic 3D nanonetwork is proposed. Regardless of the stretching direction, numerous light‐scattering nanogaps (corresponding to the porosity of up to ≈37.4 vol%) form at the interfaces of Al2O3 and the elastomers under stretching. This results in the gradual modulation of transmission from ≈90% to 16% at visible wavelengths and does not degrade with repeated stretching/releasing over more than 10 000 cycles. The underlying physics is precisely predicted by finite element analysis of the unit cells. As a proof of concept, a mobile‐app‐enabled smart window device for Internet of Things applications is realized using the proposed 3D nanocomposite with successful expansion to the 3 × 3 in. scale., Mechano‐responsive smart windows are developed using stretchable 3D hybrid nanocomposites. The inserted periodic oxide nanoshell in the elastomer offers formation of light scattering air gaps upon stretching, and this provides high‐contrast optical modulation. It is demonstrated that smart window devices can be prepared for self‐responding to the surrounding environments and temporary projection screens by integrating with Internet of Things technology.

The utilization of edge sites in two-dimensional materials including transition-metal dichalcogenides (TMDs) is an effective strategy to realize high-performance gas sensors because of their high catalytic activity. Herein, we demonstrate a facile strategy to synthesize the numerous edge sites of vertically aligned MoS2 and larger surface area via SiO2 nanorod (NRs) platforms for highly sensitive NO2 gas sensor. The SiO2 NRs encapsulated by MoS2 film with numerous edge sites and partially vertical-aligned regions synthesized using simple thermolysis process of [(NH4)2MoS4]. Especially, the vertically aligned MoS2 prepared on 500 nm thick SiO2 NRs (500MoS2) shows approximately 90 times higher gas-sensing response to 50 ppm NO2 at room temperature than the MoS2 film prepared on flat SiO2, and the theoretical detection limit is as low as ∼2.3 ppb. Additionally, it shows reliable operation with reversible response to NO2 gas without degradation at an operating temperature of 100 °C. The use of the proposed fa...

Recently, two-dimensional (2D) materials have attracted attention for gas sensor fields due to their unique properties such as high surface to volume ratio and numerous active sites. In particular, reduced graphene oxide (rGO) and MoS2 are one of the most promising materials for humidity sensor due to the oxygen containing functional groups on the surface of rGO and dangling bonds at the edge site of MoS2. Herein, we present 2D rGO/2D MoS2 hybrid composites (MS-GOs) synthesized by hydrothermal method. rGO and MoS2 are mixed with different molar ratios and drop-casted on SiO2 substrate with Pt interdigitated electrode. At an optimized molar ratio of rGO/MoS2, the sensor device exhibits high sensitivity, good selectivity, rapid response and recovery, and good linearity for humidity sensing. The enhanced sensing properties are attributed to the p-n junction between rGO and MoS2. Our work provides an efficient way for realizing high-performance and fast responding humidity sensors utilizing 2D-2D heterojunction materials based chemoresistive humidity sensors for use in diverse applications.

We report a micromachined H2 sensor that is composed of a Pt micro-heater, low-stress insulating layer (SiO2/SiNx/SiO2), Pt-interdigitated electrodes, and gas sensing materials. Three types of Pt micro-heater are designed as function of electrode width, and their thermal properties are systematically analyzed by finite element modeling FEM with infrared camera. The power consumptions when the surface temperature reached 150, 200, 250, and 300 °C are calculated to approximately 33, 48, 67 and 85 mW, respectively. The response of the PdO nanoparticles-decorated TiO2 thin films to H2 is much higher than those of other gases such as CH4 and CO at 200 °C (48 mW). Further, the response time is reduced to approximately 3 s. The enhancement of gas sensing properties is related to well-designed micro-heater and catalytic effects of PdO nanoparticles such as electronic and chemical sensitization. These results suggest that the PdO nanoparticles-decorated TiO2 thin film, namely MEMS-based H2 sensors are very promising for use in IoT application to improve the quality of human’s life.

Chemiresistive electronic nose (CEN) composed of villi-like nanostructures (VLNs) of SnO2 and WO3, and Au-functionalized VLNs was fabricated by applying electron-beam evaporation in a glancing angle deposition mode. The VLNs-based CEN with a back-heater (212 °C) shows high responses with low detection limits of parts per billion (ppb)-levels for NO and NH3 vapors at 80% relative humidity atmosphere. The enhanced sensitivities in a high humidity condition turn out to be attributed to the spillover effect by the Au nanoparticles and a large surface-to-volume ratio in porous VLNs. Employing Au NPs on VLNs leads to the increase of O− ions via the spillover effect which impedes the adsorption of water molecules, maintaining the enhanced responses against environmental humidity. Consequently, high responses for NO and NH3 vapors maintain even in the high humidity condition. Herein, with the principal component analysis (PCA), we demonstrate highly selective detection of NO and NH3 vapors against C2H5OH, CO, C7H8, C6H6, and CH3COCH3 vapors. These results open up wide applications of the VLNs-based CEN as an inexpensive and non-invasive diagnostic tool for asthma and kidney disorder.

We present a simple and facile method for producing high-performance hydrogen (H 2 ) sensors based on vertically ordered metal-oxide nanorods with a Pd films on a 4-inch SiO 2 /Si substrate by a glancing-angle deposition. Firstly, optimal density of nanorods was formed by changing an incident angle of vapor flux. Secondly, nanogaps between each nanorod were precisely controlled by manipulating thickness of Pd films. At room temperature in ambient air, 15-nm-thick Pd-coated SiO 2 nanorods showed the rapid on-off switches. The average response time was approximately 2.8 s (the longest response time: 5 s), and the recovery time was less than 1 s for 2%–0.8% H 2 . For 20-nm-thick Pd-coated SiO 2 nanorods, detection of limit was reduced to 10 ppm due to semi-on-off operation. The reproducibility of our approaches was investigated by fabricating the Pd-coated SnO 2 nanorods. They also exhibited the high H 2 sensing performance as Pd-coated SiO 2 nanorods. We strongly believe that high H 2 sensing performance of Pd nanogap controlled metal oxide nanorods provides a new perspective for room-temperature H 2 switches and sensors based on H 2 -induced lattice expansion.

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.

One of the well-known pathways toward low power consuming chemoresistive gas sensors is the utilization of 2-dimensional materials. Especially, transition metal dichalcogenides (TMDs), which are usually atomically thin semiconductors, have a notable characteristic of their highly reactive edge sites. The edge sites of TMDs having high d-orbital electron density can serve as highly favorable chemically active sites for direct interaction with target gas molecules. In this study, WS2 was synthesized on highly porous SiO2 nanorods template to have numerous edge-exposed WS2 flakes in a limited active area taking advantage of 1-dimensional nanostructures with extremely high surface-to-volume ratio. The fabricated WS2 on 1D nanostructures exhibited a gas response of 151.2 % toward 5 ppm NO2, which has not been reported in performance-wise at room temperature to the best of the author’s knowledge. Density functional theory calculations theoretically supported the highly sensitive and selective NO2 detection with a theoretical detection limit of 13.726 ppb.

Semiconducting gas sensors that operated at room-temperature (around 25 °C) have been studied to monitor target gases. Furthermore, a self-powered gas sensor with no reliance on a battery or external power source was highly demanded to effectively reduce the power consumption and apply for practical environments. In this study, we designed the self-powered gas sensors by connecting ionic-activated semiconducting sensors, a promising candidate for room-temperature operation, with an oval-shaped piezoelectric generator, and light-emitting diode alarm. The effect of the divided voltage from the energy generator to the sensor on the sensing performance was analyzed. The self-powered sensor has an extremely low detection limit (4.32 ppb NO2) with rapid and constant recovery (6 s) regardless of the NO2 concentration. Additionally, the practical applicability was confirmed by the light-emitting diode alarms according to the NO2 concentration in the atmosphere.

We report the highly sensitive and selective downsized gas sensors for the IoT application. Sensing areas of Pt-interdigitated electrodes (IDEs) were varied to investigate the relation between sensing materials and electrodes. In2O3 nanocolumns were deposited on the pre-patterned Pt IDEs using glancing angle deposition (GLAD). The effect of the interface resistance between electrodes and sensing materials, and the intergrain resistance between nanocolumns is analyzed by linear regression at different sensing area and incident angle of GLAD. In2O3 (angle: 85°, sensing area: 0.3 mm × 0.3 mm) nanocolumns with double Schottky barriers show the highest response and selectivity with fast response time of 10 s to VOCs among the samples fabricated in this study. The analysis reveals that the intrinsic response of In2O3 (angle: 85°, sensing area: 0.3 mm × 0.3 mm) nanocolumns are dominantly affected by intergrain resistance, resulting in a high response. Our demonstration for the fundamental aspect of downsizing gas sensor makes an important contribution to the chemical sensor field with broad interest.

Vanadium dioxide (VO 2 ) as strong correlated oxide has been intensively studied due to the unique properties accompanied by metal-insulator transition (MIT) near room temperature. Their extrinsic electrical and optical transformations on MIT have been greatly attracted in many potential applications. In this study, we propose to apply an intrinsic electrical property of VO 2 in metallic phase as doping agent of chemiresistive sensor. Self-doped nanocoulmnar vanadium oxides (SNVO) thin films are simply fabricated by glancing angle deposition without any template and chemical additives. The nanocolumnar VO 2 thin films were deposited by e-beam evaporator, and then the SNVO with residual VO 2 in V 2 O 5 nanocolums were synthesized by an air atmosphere annealing, which are composed of dominant oxidized V 2 O 5 which plays a sensing role on the surface and the residual VO 2 with self-doping effect inside nanocolumns. Especially at low temperature (150 °C), SNVO sensor shows an ultra-high performance (R gas /R air > 100) to NO 2 (5 ppm) gas. It is supposed that the residual VO 2 which shows metallic properties above Mott transition temperature (∼67 °C) acts as an intrinsic electron donor by pseudo-free electron, which helps adsorption of oxygen ions on the surface and extends the depletion region of few nanometers of narrow necks between nanocolumns.

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.

The development of high performance gas sensors that operate at room temperature has attracted considerable attention. Unfortunately, the conventional mechanism of chemiresistive sensors is restricted at room temperature by insufficient reaction energy with target molecules. Herein, novel strategy for room temperature gas sensors is reported using an ionic-activated sensing mechanism. The investigation reveals that a hydroxide layer is developed by the applied voltages on the SnO2 surface in the presence of humidity, leading to increased electrical conductivity. Surprisingly, the experimental results indicate ideal sensing behavior at room temperature for NO2 detection with sub-parts-per-trillion (132.3 ppt) detection and fast recovery (25.7 s) to 5 ppm NO2 under humid conditions. The ionic-activated sensing mechanism is proposed as a cascade process involving the formation of ionic conduction, reaction with a target gas, and demonstrates the novelty of the approach. It is believed that the results presented will open new pathways as a promising method for room temperature gas sensors.

In order to develop high performance chemoresistive gas sensors for Internet of Everything applications, low power consumption should be achieved due to the limited battery capacity of portable devices. One of the most efficient ways to reduce power consumption is to lower the operating temperature to room temperature. Herein, we report superior gas sensing properties of SnS

Morphological evolution accompanying a surface roughening and preferred orientation is an effective way to realize a high-performance gas sensor because of its significant potential as a chemical catalyst through chemical potentials and atomic energy states. In this work, we investigated a heterojunction of double-side-W-decorated NiO nanoigloos fabricated through radio frequency sputtering and a soft-template method. Interestingly, a morphological evolution characterized by a pyramidal rough surface and the preferred orientation of the (111) plane was observed upon decorating the bare NiO nanoigloos with W. The underlying mechanism of the morphological evolution was precisely demonstrated based on the van der Drift competitive growth model originating from the oxygen transport and chemical strain in the lattice. The gas sensing properties of W-decorated NiO show an excellent NO2 response and selectivity when compared to other gases. In addition, high response stability was evaluated under interference ga...

To take advantage of nanostructures beyond their large surface area for light‐activated gas sensing, in article number 2001883, Seokwoo Jeon, Young‐Seok Shim, Ho Won Jang, and co‐workers introduce a new class of UV‐activated sensing of rationally designed highly periodic 3D TiO(2). The 3D TiO(2) facilitates 55 times enhanced light absorption and ultra‐sensitive NO(2) sensing properties with a detection limit of ∼200 ppt at room temperature. [Image: see text]

Electroplating has been favored to date as a surface treatment technology in various industries in the development of semiconductors, automobiles, ships, and steel due to its advantages of being a simple, solution-based process, with low cost and high throughput. Recently, classical electroplating has been reborn as an advanced manufacturing process for functional materials by combining it with unconventional optical three-dimensional (3D) nanofabrication techniques capable of generating polymer templates with high-resolution 3D periodic nanostructures. The bottom-up filling behavior of electroplating rising from a seed layer makes it possible to densely fill the nanoporous network of the template with heterogeneous inorganic materials. At this time, understanding and optimizing the process parameters (e.g., additive, current density, type of current waveform, etc.) of electroplating is critical for defect control. In addition, since electroplating is generally performed near room temperature, unlike other thin film deposition techniques, structural damage to the polymer template by heat during electroplating is almost negligible. Based on the excellent compatibility of electroplating and optical 3D nanofabrication, innovative functional materials with 3D periodic nanostructures targeting electrochemical or energy-related applications have been created. In this mini review, a strategy for producing functional materials with 3D periodic nanostructures through a templating process will be covered, and the recent cases of successful applications to electrodes for energy storage devices, electrocatalysts, and thermoelectric materials will be summarized. We will also discuss technical issues that need to be considered in the process to improve the quality of the resulting functional materials with 3D nanoarchitectures.

Vertically ordered hematite nanotubes are considered to be promising photoactive materials for high-performance water-splitting photoanodes. However, the synthesis of hematite nanotubes directly on conducting substrates such as fluorine-doped tin oxide (FTO)/glass is difficult to be achieved because of the poor adhesion between hematite nanotubes and FTO/glass. Here, we report the synthesis of hematite nanotubes directly on FTO/glass substrate and high-performance photoelectrochemical properties of the nanotubes with NiFe cocatalysts. The hematite nanotubes are synthesized by a simple electrochemical anodization method. The adhesion of the hematite nanotubes to the FTO/glass substrate is drastically improved by dipping them in nonpolar cyclohexane prior to postannealing. Bare hematite nanotubes show a photocurrent density of 1.3 mA/cm(2) at 1.23 V vs a reversible hydrogen electrode, while hematite nanotubes with electrodeposited NiFe cocatalysts exhibit 2.1 mA/cm(2) at 1.23 V which is the highest photocurrent density reported for hematite nanotubes-based photoanodes for solar water splitting. Our work provides an efficient platform to obtain high-performance water-splitting photoanodes utilizing earth-abundant hematite and noble-metal-free cocatalysts.

Detection of gas-phase chemicals finds a wide variety of applications, including food and beverages, fragrances, environmental monitoring, chemical and biochemical processing, medical diagnostics, and transportation. One approach for these tasks is to use arrays of highly sensitive and selective sensors as an electronic nose. Here, we present a high performance chemiresistive electronic nose (CEN) based on an array of metal oxide thin films, metal-catalyzed thin films, and nanostructured thin films. The gas sensing properties of the CEN show enhanced sensitive detection of H2S, NH3, and NO in an 80% relative humidity (RH) atmosphere similar to the composition of exhaled breath. The detection limits of the sensor elements we fabricated are in the following ranges: 534 ppt to 2.87 ppb for H2S, 4.45 to 42.29 ppb for NH3, and 206 ppt to 2.06 ppb for NO. The enhanced sensitivity is attributed to the spillover effect by Au nanoparticles and the high porosity of villi-like nanostructures, providing a large surface-to-volume ratio. The remarkable selectivity based on the collection of sensor responses manifests itself in the principal component analysis (PCA). The excellent sensing performance indicates that the CEN can detect the biomarkers of H2S, NH3, and NO in exhaled breath and even distinguish them clearly in the PCA. Our results show high potential of the CEN as an inexpensive and noninvasive diagnostic tool for halitosis, kidney disorder, and asthma.

Semiconductor gas sensors based on metal oxide are widely used in a number of applications, from health and safety to energy effi-ciency and emission control. Nanomaterials including nanowires, nanorods, and nanoparticles have dominated the research focus in thisfield owing to their large number of surface sites that facilitate surface reactions. Recently, metal oxide hollow structures using soft tem-plates have been developed owing to their high sensing properties with large-area uniformity. Here, we provide a brief overview of metaloxide hollow structures and their gas-sensing properties from the aspects of template size, morphology, and additives. In addition, a gas-sensing mechanism and perspectives are presented. Keywords: Gas sensor, Soft-template method, Nanostructure, Hollow hemisphere, Metal decoration 1. INTRODUCTION The functional convergence of the Internet with radio frequencyidentification, sensors, and smart objects led to the era of theInternet of Things (IoT), which supplies and accesses all real-world information [1]. Sensor technology is the most importantfactor in IoT because sensors offer enormous amounts ofinformation for many types of environment. Accordingly, varioussensors such as gas, pressure, illumination, tilt, temperature, andmotion sensors have been heavily studied to more preciselymeasure and monitor changes in environments [2]. In particular,diverse applications of the gas sensor in broad fields including thedetection of air pollutants and gaseous hazards, homeland security,medical diagnosis, and fuel combustion control have acceleratedstudies for high-performance gas sensors [3-7]. Gas sensors for applications in the IoT should meet specialrequirements such as low cost, miniaturized size, ease ofintegration with electronic circuits, and high sensing performance[8]. Various gas sensors including optical, electrochemical, surfaceacoustic wave, and semiconductor metal oxide sensors have beenextensively studied to fulfill the needs of the IoT [9]. In particular,owing to their simplicity in operation, low cost, flexibility inproduction, small size, and easy integration with electroniccircuits, semiconductor gas sensors based on metal oxides arevery promising as sensing elements for IoT [10-12]. Generally, the gas-sensing properties of metal oxide aredetermined by three basic factors: utility factor, transducerfunction, and receptor function [13]. The first factor is related togas diffusion (porous structure), and the second factor is related tothe mechanism of electron transport between adjacent crystals(neck control). The receptor function is related to the ability of theoxide surface to interact with target gases (metal additives anddecoration). Over the past decade, metal oxide nanostructuressuch as nanoparticles, nanowires, nanotubes, and nanofibers withmetal catalysts have significantly improved the three basic factors,leading to high-performance gas sensors [14]. However, sincethese nanostructures are commonly synthesized using wetchemical methods or transfer methods, developing versatile andreproducible fabrication processes has remained a challenge [15]. An alternative method for achieving highly sensitive metaloxide gas sensors is a soft-template method consisting of polymermicrospheres, which is effective in fabricating long-range orderedsubmicron hollow structures of various metal oxides [16,17]. Inspite of their potential advantages, no review focused on the gas-sensing properties of metal oxide nanostructures fabricated by thesoft-template method has been published to the best of the

Sustainable and efficient conversion of solar energy to transportable green energy and storable fuels, hydrogen, represents a solution to the energy crisis and reduces the consumption of fossil fuels, which are mainly responsible for the rise in global temperature. Solar water splitting using semiconductors, such as silicon, is promising to satisfy the global energy demand by producing hydrogen molecules. However, the solar to hydrogen conversion efficiency of a silicon photoelectrode is suppressed by overpotential, high reflectance and/or instability in liquid electrolytes. Herein, we report the synthesis of multifunctional solution-processed TiO2 nanorods on a 4-inch p-silicon wafer with controllable heights and diameters for highly efficient water splitting photocathodes. The solution-processed passivation layer of TiO2 nanorods reduces the overpotential of the silicon photocathode due to its catalytic properties. The TiO2 NRs also dramatically improves the light absorption of silicon due to the antireflective ability of the nanorods. The reflectance of silicon is decreased from 37.5% to 1.4% and enhances the saturated photocurrent density. The Pt-decorated (1–2.5 nm diameter) TiO2 nanorods/p-Si photocathodes show a short circuit current density of up to 40 mA cm−2, an open circuit voltage ∼440 mV and incident photon to current conversion efficiency of >90% using 0.5 M H2SO4 electrolyte with simulated 1 sun irradiation. The heterostructure photocathodes are stable for more than 52 h without noticeable degradation and an ideal regenerative cell efficiency of 2.5% is achieved.

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.

We demonstrate that wafer-scale, transferable, and transparent thin-film catalysts based on MoS2, which consists of cheap and earth abundant elements, can provide a low onset potential of 1 mA cm−2 at 0.17 V versus a reversible hydrogen electrode and the high photocurrent density of 24.6 mA cm−2 at 0 V for a p-type Si photocathode. c-Domains with vertically stacked (100) planes in the transferable 2H-MoS2 thin films, which are grown via a thermolysis method, act as active sites for the hydrogen evolution reaction, and photogenerated electrons are efficiently transported through the n-MoS2/p-Si heterojunction.

Barium tungstate (BaWO4) powders with various sintering temperatures, and BaWO4:Dy3+ phosphor samples with concentrations of different rare-earth (RE) activator ions (Dy3+, Sm3+, Tb3+) were prepared through co-precipitation. The structural, morphological, and photoluminescent characteristics of barium tungstate phosphors depend on the concentration of RE ions. The crystallographic characteristics of the synthesized BaWO4 were analyzed using X-ray diffraction (XRD) patterns. The size and shape of the crystalline particles were estimated based on images measured with a field emission scanning electron microscope (FE-SEM). As the sintering temperature of the BaWO4 particles increased from 400 °C to 1000 °C, the size of the particles gradually increased and showed a tendency to clump together. In the sample doped with 7 mol % Dy3+ ions, the intensity of all emission bands reached their maximum. The emission spectra of the RE3+-doped BaWO4 powders by excitation at 325 nm were composed of yellow (Dy3+), red (Sm3+), and green (Tb3+) band at 572, 640, and 544 nm. This indicates that most of the RE3+ ions absorbed the position without reversal symmetry in the BaWO4 lattice. These results propose that strong emission intensity and tunable color for the phosphors can be accomplished by rare-earth doped host with an suitable quantity. In addition, the phosphor thin films, having high transparency from aqueous colloidal solutions, were deposited on banknotes, and it is considered whether it is suitable for anti-counterfeiting applications.