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Dihydrogen sulfide (H2S) gas has a flammable nature and is one of the most toxic and dangerous gases. Even small concentrations can be fatal to humans. Herein, we investigated the H2S gas-sensing features of commercial pristine cerium oxide (CeO2 ) and gadolinium (Gd)-doped CeO2 (GDC) nanoparticles. First, the sensing materials were well-characterized using various methods including X-ray photoelectron spectroscopy, transmission electron microscopy and X-ray diffraction to gain insight into their chemical composition, morphology, phases, and crystallinity, respectively. In the next step, gas sensors were fabricated using a top electrode (Au/Ti) configuration. Preliminary H2S-gas-sensing studies revealed that GDC gas sensor had a superior gas response to H2S gas than the pristine CeO2 gas sensor at 350°C. The responses of the pristine CeO2 gas sensor to 20 ppm H2S gas was 1.542, while the response of the GDC gas sensor to the aforementioned H2S concentration was 3.489. In addition, the GDC sensor exhibited good selectivity to H2S gas among C2H5OH, C7H8 and NH3 gases. Also, we investigated the response of the sensor in up to 60% relative humidity. The enhanced response of the GDC gas sensor to H2S gas was mainly related to the formation of oxygen defects as a result of Gd-doping in CeO2 . Also, good selectivity to H2S was related to the sensing temperature, the higher reactivity of H2S relative to other gases and the small bond energy of H-SH. This study demonstrates the promising ability of Gd-doping to enhance the H2S gas-sensing characteristics of CeO2 , which can be applied to other similar systems based on semiconducting metal oxides.

Three-dimensional flower-like tungsten oxide nanoplates sequentially doped with Co at different concentrations were fabricated using a single solvothermal process. The response of the nanostructure of the n-type tungsten oxide used for sensing ethanol gas, which is a reducing gas, changed sequentially according to the process temperature and doping concentration of Co. This response cannot be attributed to a single reason, but it can be regarded as a result of the combination of several complicated direct and indirect phenomena such as (1) point, line, and cross-section defects on the surface, (2) change in initial resistance corresponding to doping concentration, (3) catalytic effect of dopant, and (4) difference in energy level at heterojunctions. Therefore, unlike in the case of other gas sensing methods that depend on a main cause or mechanism, it might be meaningful in the case of ethanol gas sensing to analyse each cause and predict the results of different combinations.

The urchin-like nanoparticles of semiconducting oxides have been attracting considerable attention as they can significantly improve reactivity owing to their enlarged surface areas. Herein, we report a simple and scalable synthesis method of ZnO nanomaterials via a microwave-assisted drying process. Commercial ZnO powders (~300 nm) and expanded graphite were used as starting materials to prepare urchin-like ZnO nanoparticles with nanoscale legs (diameter ~20 nm) by triggering the rapid phase transformation, which was induced by a repeated microwave irradiation in an alumina crucible for a few minutes. The possible mechanisms to trigger this morphological and dimensional change are described in detail. The high-yield production and superior reproducibility completed in shortened processing times enable this synthetic route to be widely applicable for the massive production of nanostructured oxide particles.

In this study, reduced graphene oxide (RGO) nanosheets were synthesized and then functionalized with Pt nanoparticles (NPs) for gas sensing studies at room temperature. Pt-functionalized RGOs (Pt-RGOs) were irradiated by a 2 MeV electron beam with different doses of 100 and 500 kGy (Pt-RGO-100 and Pt-RGO-500) to study the role of electron beam irradiation (EBI) on properties and NO2 gas sensing characteristics. Different characterization techniques verified the generation of structural defects after EBI. Additionally, NO2 sensing tests at room temperature revealed that the Pt-RGO-100 sensor had the highest response to NO2 gas as well as the shortest response time. We surmise that the improved sensing signal with respect to NO2 gas for Pt-RGO-100 sensor was related to the role of oxygen functional groups. However, decreases in the response for Pt-RGO-500 sensor were related to the creation of reactive oxygen gases that could be adsorbed on the active defects of the RGO by the interaction of electron beams with oxygen species in the ambient air, leading to a decrease of the effective defect sites and decreases of the sensing response. This work suggests progress towards an improved performance of RGO-based gas sensors that can work at low temperature, by using EBI as a clean tool applied at room temperature with Pt-functionalization.

Assuming that many of the materials produced in a non-equilibrium state remain unchanged, an extended space-energy conservation law was proposed based on the existing energy conservation law. In the present study, by analyzing the well developed equilibrium binary phase diagram of iron (Fe) - carbon (C), we show that energy non-equilibrium microstructures can appear as a part of the equilibrium between the space energy and the mass energy. The correlation between these two energies is objectively and logically explained via (1) one-to-one correspondences between the equilibrium and non-equilibrium phases based on the binary Fe-C phase diagram and (2) the heat-treated Fe-C phases with the spatial energy represented by temperature. Additionally, we found that the morphological and microstructural changes in non-equilibrium states could be consistently explained using the extended energy law as a major premise. This suggests that material factors such as size, distribution, and the shape of materials, which appear to have no energy transfer, are all formed to balance the energy equilibrium with the spatial energy surrounding the materials. Thus, an extended energy conservation law, which can control mass through space or vice versa, can provide a comprehensive logical framework for analyzing various unsolved physicochemical phenomena. (Received May 28, 2020; Accepted June 17, 2020)

In this work, a low-temperature (100 °C) gas sensor based on heterojunctions of porous Si and SnO2 nanowires (NWs) is presented. Porous Si was obtained from p-Si wafers by electrochemical etching, and SnO2 NWs were fabricated by a vapor-liquid-solid route. Different characterization techniques were used to verify the formation of porous Si/SnO2 NW heterojunctions. H2S gas sensing results showed enhanced gas sensing performance of the porous Si/SnO2 NW sensor in comparison with that of a porous Si sensor. The reasons for such enhancement are discussed in detail. This study demonstrates the promising effects of SnO2 NWs in combination with porous Si to realize low-temperature H2S gas sensors that are highly compatible with existing Si processing technology.

To attain high life standards, it is important to develop high-performance non-toxic gas sensors for public safety, environmental pollutant control, industrial processes, etc. Because reports on single element semiconductor-coated semiconducting metal oxides for sensing applications are rare, we synthesized SnO2 nanowires and coated them with a 5 nm-thick or 10 nm-thick Si layer for H2S gas sensing studies. SnO2 nanowires were successfully synthesized using a highly pure metallic Sn powder at high temperature in a tube furnace by the vapor-liquid-solid method and Si was deposited on the nanowires by the sputtering technique. The desired morphology and composition of the synthesized nanowires were confirmed by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy. Moreover, the gas sensing characteristics of pristine and Si-coated SnO2 nanowires toward H2S, CO, H2, C6H6, C2H5OH and C6H7 gases were investigated. The sensing results revealed a good response to H2S at the optimum operational temperature of 100 °C. Notably, Si-coated SnO2 nanowire sensors showed a better response to H2S than pristine SnO2 nanowires. The mechanism of H2S sensing is discussed in detail here. This study shows that the Si coating on the SnO2 nanowire enhances its sensing performance and decreases the sensing temperature required for H2S gas detection. (Received May 3, 2019; Accepted September 7, 2019)

A new gas sensor working in room temperature, which is compatible with silicon fabrication technology is presented. Porous silicon nanowires (NWs) were synthesized by metal-assisted chemical etching method and then TeO₂ NWs branches were attached to their stem by thermal evaporation of Te powders in the presence of air. Afterwards TeO₂ branched porous Si NWs were functionalized by Pt via sputtering followed by low temperature thermal annealing. Scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy collectively confirmed successful formation of TeO₂ branched porous Si NWs functionalized by Pt nanoparticles. Their gas sensing properties in the presence of CO, C6H6 and C7H8 were tested at room temperature, for Si wafer, pristine porous Si NWs, pristine TeO₂ branched porous Si NWs, and Pt functionalized TeO₂ branched porous Si NWs sensors. Pt functionalized TeO₂ branched porous Si NWs have higher responses to all tested gases than the other sensors. The origin of high response is discussed in detail. This new room temperature gas sensor can open a new aperture for development of gas sensors with minimum energy consumption which are compatible with silicon fabrication technology.

Branched nanowires (NWs) are a novel class of composite materials with increased surface area relative to their pristine one-dimensional counterparts. Accordingly, they are good choice for gas sensing studies. In this study, p-n, TeO2-branched SnO2 NWs were produced by a two-step catalyst-assisted vapor-liquid-solid (VLS) growth technique for gas sensing studies. First, SnO2 NWs were synthesized from highly pure Sn powders, and TeO2 branches were subsequently added. The fabricated samples were well characterized in terms of morphology, crystallinity, and chemical composition. Gas sensing results exhibited the enhanced NO2 sensing capability of TeO2 branched SnO2 NW sensors relative to pristine SnO2 NWs. In particular, the maximum responses (Rg/Ra) of pristine and TeO2 branched SnO2 sensors to 10 ppm NO2 were 6.34 and 10.25, respectively. Furthermore, dynamics of TeO2 branched sensor at the optimal temperature was faster. Superior sensing properties of TeO2 branched SnO2 NWs were related to the high surface area of the branched sensors and creation of p-n heterojunctions on the surfaces of this sensor. We believe that branching is a good way to realize gas sensors for practical usages.

A simple yet powerful flame chemical vapor deposition technique is proposed that allows free control of the surface morphology, microstructure, and composition of existing materials with regard to various functionalities within a short process time (in seconds) at room temperature and atmospheric pressure as per the requirement. Since the heat energy is directly transferred to the material surface, the redox periodically converges to the energy dynamic equilibrium depending on the energy injection time; therefore, bidirectional transition between the semiconductor/metal is optionally available. To demonstrate this, a variety of Sn-based particles were created on preformed SnO2 nanowires, and this has been interpreted as a new mechanism for the response and response times of gas-sensing, which are representative indicators of the most surface-sensitive applications and show one-to-one correspondence between theoretical and experimental results. The detailed technologies derived herein are clearly influential in both research and industry.

In view of the various qualifications required of materials nowadays, efforts to change the characteristics of inherent materials have continued. However, most material conversion techniques that have been used in the past, such as alloy design and doping effect, cannot overcome the limitation that properties are only added to the original characteristics of pristine materials. Therefore, herein, we introduced a new material design technique, a so-called “Molecular Group System”, which is completely different from existing methods. Since whole-set-systems are considered one-energetic-unit-system, either only the merits of the constituent elements can be emphasized or new materials completely different from the raw ones can be synthesized. In this study, block-stacking bottom-up approach was employed to form a one group system from SnO2, SnOx, Sn, and graphene powders, and a binder, using high-energy irradiation. Then, we discussed theoretical verifications such as SnO2-reduction and Sn-channeling.

A novel gas sensor fabricated from ZnO-shelled Si nanowires (SiNWs) is presented. After coating a thin layer of Au on the surfaces of Si NWs, ZnO layers were formed on the surfaces of p-SiNWs by thermal evaporation of Zn powders and a subsequent oxidation process. Microscopic analysis confirmed the successful formation of ZnO-Si core-shell NWs with Au nanoparticles present on the shell surface. The gas sensing performance of the gas sensor fabricated using the p-Si/n-ZnO core-shell NWs was evaluated for various gases. The sensor exhibited outstanding response and selectivity to H2S gas. The gas sensing mechanism was evaluated in detail and attributed to various factors, including the formation of ZnO/Si and Au/ZnO heterojunctions and the chemical attraction between ZnO and Au. The results demonstrate a new sensing material for H2S detection in various fields that can be easily incorporated into Si-based devices.

The unexpectedly large outbreak of Middle East respiratory syndrome in South Korea in 2015 was initiated by an infected traveler and amplified by several "superspreading" events. Previously, we reported the emergence and spread of mutant Middle East respiratory syndrome coronavirus bearing spike mutations (I529T or D510G) with reduced affinity to human receptor CD26 during the outbreak. To assess the potential association of spike mutations with superspreading events, we collected virus genetic information reported during the outbreak and systemically analyzed the relationship of spike sequences and epidemiology. We found sequential emergence of the spike mutations in 2 superspreaders. In vivo virulence of the mutant viruses seems to decline in human patients, as assessed by fever duration in affected persons. In addition, neutralizing activity against these 2 mutant viruses in serum samples from mice immunized with wild-type spike antigen were gradually reduced, suggesting emergence and wide spread of neutralization escapers during the outbreak.

Branched nanowires (b-NWs) have been used for advanced applications because of their nanoscale dimensions, high surface area, and scalable synthetic methods. Herein, for the first time, we report the fabrication of Au-functionalized ZnO-branched SnO2 NWs for enhanced gas sensing application. As-fabricated NWs were structurally and morphologically investigated by X-ray diffraction, scanning electron microscopy as well as transmission electron microscopy to reveals the crystallinity and morphology of NWs. By comparing the gas sensing properties of Au-functionalized b-NWs, b-NWs, and pristine SnO2 NWs gas sensors, in terms of their sensitivity and cross-sensitivity towards NO2, we demonstrate the superior sensing properties of Au-functionalized b-NWs sensors. In particular the Au-functionalized b-NWs sensor showed a response (Rg/Ra) of 13.07 to 10 ppm NO2 gas which was higher than those of b-NWs, and pristine SnO2 NWs gas sensors. In addition to the special architecture of sensors (i.e. ZnO b-NWs on SnO2 NWs), which produces a large number of hetero- and homojunctions (a branching effect), Au acts as a catalyst to further improve the sensing characteristics of this particular sensor (a functionalization effect). According to the obtained results, we believe that metal oxide gas sensors based on noble metal-functionalized b-NWs, which use a facile synthesis procedure and produce enhanced sensing performance, will play a vital role in the future of the sensor industry.

We report the electrical and optical characteristics of SnO2 thin films irradiated by microwaves (MWs) and grown using atomic layer deposition in a commercial MW oven operating at a frequency of 2.45 GHz. The properties of the MW-irradiated SnO2 thin films were compared with those of the as-deposited SnO2 thin films. After MW irradiation, the conductivity and transparency of the thin films were enhanced. In addition, the samples irradiated for 5 min showed optimal carrier concentration, Hall mobility, resistivity, and transmittance values of 1.5 × 1020 cm−3, 4.6 cm2/V s, 8 × 10−3 Ω cm, and 95.77%, respectively. The improved properties of the MW-irradiated samples were attributed mainly to the formation of an oxygen vacancy in the SnO2 lattice during MW irradiation. Our results can be applied for the fabrication of pure SnO2-based transparent conductive oxides; these oxides are generally doped with other elements.

An unstable energy-unbalanced state such as superheating or supercooling is often unexpectedly observed because a factor of energy depends not only on the temperature but is a product of temperature (T) and entropy (S). Thus, at the same temperature, if the entropy is different, the total energy of the system can be different. In such cases, the temperature-change-rate cannot match the entropy-change-rate, which results in a hysteresis curve for the temperature/entropy relationship. Due to the difference between the temperature- and entropy-change-rates, properties of a material, such as the boiling and freezing points, can be extended from point to area. This study confirmed that depending on the heating rate, tin powders exhibit different melting points. Given the contemporary reinterpretation of many energy-non-equilibrium phenomena that have only been discussed on the basis of temperature, this study is expected to contribute to the actual expansion of scientific/engineering applications.

Scrub typhus is an acute febrile disease caused by Orientia tsutsugamushi infection. Despite the wide range of approaches explored during the last seventy years, an effective prophylactic vaccine is not yet available. Here, we developed a novel recombinant antigen derived from conserved regions of 56 kDa type-specific antigen (TSA56), a major outer membrane protein responsible for genetic heterogeneity and antigenicity, and evaluated it as a protective vaccine antigen. Our findings demonstrate that immunization with conserved blocks of TSA56 (cTSA56) not only provides protective immunity against lethal challenges with the homologous genotype, but also confers significantly better protection against heterologous genotypes than TSA56. Adoptive transfer of CD4+ or CD8+ T cells from immunized mice provided significantly enhanced protection against lethal challenge, whereas immune B cells failed to do so, indicating that cellular immunity against the conserved epitopes plays a protective role. Moreover, immunization with a 10-mer peptide mixture, screened from CD8+ T cell epitopes within the conserved region of TSA56, provided enhanced protection against lethal challenge with O. tsutsugamushi. Therefore, this novel recombinant antigen is a promising candidate for scrub typhus vaccine against a wide range of O. tsutsugamushi genotypes.

Breakthrough process technologies have been introduced that can increase the chemical sensitivity of an interface at which reactions occur without significantly altering the physico-chemical properties of the material. Such an interfacial treatment method is based on amorphous-carbon as a base so that fluids can be deposited, and the desired thickness and quality of the deposition can be ensured irrespective of the interface state of the material. In addition, side effects such as diffusion and decreasing strength at the interface can be avoided. This is simpler than existing vacuum-based deposition technology and it has an unmatched industrial advantage in terms of economics, speed, accuracy, reliability, accessibility, and convenience. In particular, this amorphous-carbon interface treatment technology has been demonstrated to improve gas-sensing characteristics of NO2 at room temperature.

We report the gas-sensing properties of pristine and SnS2 functionalized TeO2 nanowires (NWs). TeO2 NWs were synthesized by a vapor–liquid–solid growth method, and SnS2 functionalization was performed using an atomic layer deposition technique followed by thermal treatment. Structural and morphological analyses verified the formation of pristine and SnS2 functionalized TeO2 NWs with desired composition, phase, and morphology. Interestingly, sensing results showed that the pristine TeO2 NW gas sensor had better sensing properties relative to the SnS2 functionalized TeO2 NW gas sensor. An underlying sensing mechanism is explained in detail, and reasons for the decrease of sensing performance with the SnS2 functionalized TeO2 NW sensor was attributed to the coverage of TeO2 surface by the SnS2 nanoparticles.

We present a highly sensitive and selective NO2 sensor based on Bi2O3 branched SnO2 nanowires (NWs). SnO2 NWs were first synthesized by a vapor-liquid-solid method, were coated with an Au layer, and Bi2O3 branches were grown on their stems by the same procedure used for pure Bi powders. The fabricated sensor showed a high response (Rg/Ra) of 56.92 to 2 ppm of NO2 gas at an optimal temperature. Furthermore, its response to other interfering gases such as ethanol, acetone, toluene, and benzene, was less than 1.55, which demonstrated excellent selectivity of the sensor towards NO2 gas. For comparison and to better understand the sensing mechanism, a pristine SnO2 NWs sensor was also tested. The superior sensing properties of the branched NW sensor relative to the pristine sensor were mainly attributed to the high surface area of the sensor resulting from Bi2O3 branching, as well as the formation of homo-and heterojunctions (Bi2O3-SnO2). In addition, several factors including the presence of Au contributed to the excellent selectivity to NO2 gas. Based on the results obtained in this work, we believe that the present sensor with an easy fabrication method, along with its high sensitivity and selectivity towards NO2, can be used for the detection of NO2 gas in real applications.

We present a dual sensitization gas sensor for selective detection of either H2S or C2H5OH gases, based on ZnO/CuO nanoparticle (NP)-decorated MWCNTs. First, Cu-Zn layers of three different thickness (3, 6, 9 nm) were successfully deposited on the surfaces of CNTs by a sputtering process and subsequently converted to their corresponding oxides by thermal annealing. The gas sensing characteristics of metal oxide decorated-MWCNTs were studied in the presence of three gases, namely NO2, H2S, and C2H5OH. The best sensing properties were obtained when the Cu-Zn deposition layer was 6 nm thick. The optimized sensor showed extraordinary responses to both H2S and C2H5OH gases at working temperatures of 100 °C and 200 °C, respectively. Therefore, selectivity was tuned by selection of the working temperature. The sensing mechanisms of the metal oxide-decorated CNTs sensors are discussed in detail. The approach described, namely co-decoration of the surfaces of MWCNTs with different metal oxides, will be of great utility to researchers who wish to fabricate dual sensitive gas sensors.

Dipeptidyl peptidase 4 (DPP4) is a receptor for MERS-CoV. The soluble form of DPP4 (sDPP4) circulates systematically and can competitively inhibit MERS-CoV entry into host cells. Here, we measured the concentration of sDPP4 in the plasma and sputa of 14 MERS-CoV-infected patients of various degrees of disease severity. The concentration of sDPP4 in the plasma of MERS patients (474.76 ± 108.06 ng/ml) was significantly lower than those of healthy controls (703.42 ± 169.96 ng/ml), but there were no significant differences among the patient groups. Interestingly, plasma levels of IL-10 and EGF were negatively and positively correlated with sDPP4 concentrations, respectively. The sDPP4 levels in sputa were less than 300 ng/ml. Viral infection was inhibited by 50% in the presence of more than 8000 ng/ml of sDPP4. Therefore, sDPP4 levels in the plasma of MERS patients are significantly reduced below the threshold needed to exert an antiviral effect against MERS-CoV infection., Highlights • sDPP4, a soluble form of MERS-CoV receptor, in plasma is reduced in MERS patients. • IL-10 and EGF in plasma are negatively and positively correlated with sDPP4. • sDPP4 levels of MERS patients are not sufficient to exert an antiviral effect.

In this study, we synthesized porous (porosity: ~16%, average pore size: ~60 nm) ZnO nanosheets (thickness: ~80 nm) using a conventional solvothermal method to investigate NO2 gas sensing properties. Porous ZnO nanosheets triggered the detection of NO2 gas with high sensitivity. Responses of 2.93 – 0.5 ppm and 74.68 – 10 ppm NO2 gas at 200 °C were observed in the porous ZnO nanosheet-based gas sensor. In addition, improved sensing properties with high selectivity to NO2 gas, reasonable stability, and high response even in the presence of water vapor molecules were obtained. We found that the enhanced NO2 gas response of the porous ZnO nanosheet-based gas sensor was due to the synergetic effects of the high surface area, ZnO/ZnO homojunctions, and structural defects. We developed a highly sensitive NO2 gas sensor with improved reliability using morphologically engineered ZnO, which was prepared via a simple and scalable chemical-synthesis route.

In this work, we fabricated gas sensors based on p-type NiO/n-type ZnO composite nanofibers (NFs), which could selectively detect oxidizing and reducing gas molecules. The p-type NiO/n-type ZnO composite NFs with both hetero- and homojunctions were successfully synthesized, and their sensing performances for oxidizing and reducing gases were systematically investigated with different composition ratios of p-type NiO and n-type ZnO. Interestingly, for oxidizing and reducing gases, the 0.5NiO-0.5ZnO NFs (nominal composition) exhibited an excellent gas response to oxidizing gases such as NO2 and SO2, whereas the 0.8NiO-0.2ZnO NFs (nominal composition) showed good selectivity for reducing gases such as C3H6O, C2H5OH, and NH3. In addition, we also examined the NO2 and CO sensing performance under a humid atmosphere to confirm the role of the NiO component, which possesses high affinity for water molecules, in p-type NiO/n-type ZnO composite NFs. We discussed the correlation between variations in composition and sensing performances with respect to gas sensing behavior, selectivity, and humidity effect for oxidizing and reducing gases. The results reveal that we successfully imparted selectivity for oxidizing and reducing gas molecules to the p-type NiO/n-type ZnO composite NFs by adjusting the composition ratio of NiO/ZnO.

Pristine and Al-doped SnO2 nanoparticles with different amounts of Al dopant were synthesized using a conventional hydrothermal process. The existing SnO2 exhibited simple interstitial physical bonding (SnO2–Al (1:0.16)) with the Al dopant; however, with an increase in the Al concentration, the bonding changed to substitutional chemical bonding (SnO2–Al (1:0.33)). We found that this crystal structural change is strongly interrelated with surface reactivity; the optimized Al-doped SnO2 nanoparticles-based sensor exhibited a significantly improved response of 17.38 – 20 ppm H2S gas with a response time of 35 s. The enhanced gas response was related to the high surface area of the optimal gas sensor (BET surface area = 78.087 m3/g) as well as the beneficial effects of Al doping. It is highlighted that this simple technique of engineering the bonding characteristics can be widely applied to other semiconducting metal oxides.

In this study, a peculiar configuration for a gas sensor consisting of vertically aligned silicon nanowires (VA-Si NWs) synthesized by metal-assisted chemical etching (MACE) is reported. Si NWs were prepared via a facile MACE method and subsequent thermal annealing. Etching was performed by generation of silver nanoparticles (Ag NPs) and subsequent etching in HF/H2O2 aqueous solution; the growth conditions were optimized by changing the process parameters. Highly vertically oriented arrays of Si NWs with a straight-line morphology were obtained, and a top–top electrode configuration was applied. The VA-Si NW gas sensor showed good sensing performance, and the VA-Si NWs exhibited a remarkable response (Rg/Ra = 11.5 ∼ 17.1) to H2 gas (10–50 ppm) at 100 °C which was the optimal working temperature. The formation mechanism and gas sensing mechanism of VA-Si NWs are described. The obtained results can suggest new approaches to making inexpensive, versatile, and portable sensors based on Si NWs having a novel top–top electrode structure that are fully compatible with well-developed Si technologies.

In this study, multi-walled carbon nanotubes (MWCNTs) were decorated by the Cu2O/CuO nanoparticles for gas sensing. For this purpose, Cu layer with different thicknesses of 3, 6, and 9 nm was coated on multi-walled carbon nanotubes using sputtering technique for different times of 1, 2, and 3 min at 25°C, respectively, followed by annealing at 500°C to produce isolated Cu2O/CuO islands. The synthesized products were fully characterized and their expected morphology, chemical composition and phases were confirmed. The gas sensors were fabricated and the optimal sensing temperatures for H2S sensing was found to be relatively low (150 °C). With control of the size of the Cu2O/CuO nanoparticles, very high sensor response ((Rg – Ra)/Ra x 100) of about 1244 % to 1 ppm H2S gas was obtained, with response time and recovery time of 219 and 77 s, respectively. Since sensor response, response time, and recovery time became significantly higher, shorter, and shorter, respectively, by the Cu2O/CuO functionalization, the associated mechanisms were explained in regard to the selective sensitivity to H2S gas. In fact, Cu2O/CuO decorations not only increased the effective surface area for sensing studies, but also they acted as effective catalyst for H2S gas. By considering the energy bands, among the multi-walled carbon nanotubes-comprising interfaces, the multi-walled carbon nanotubes/CuS heterointerfaces were effective in the enhancing the sensing behavior.

The typical methods used to improve the sensing properties of metal oxide nanowire-based gas sensors include decoration of a metal catalyst on the surface of nanowires or generation of surface defects. In this study, we suggest a process for preparing a Pd metal catalyst surrounded in SnO2-x with a large number of structural defects on the surface of SnO2 nanowires for gas sensing applications. Initially, SnO2 nanowires were prepared using a simple vapor-liquid-solid method and Pd-embedded SnO2-x-decoration was achieved by flame chemical vapor deposition. Gas sensing studies demonstrated the promising effects of Pd-embedded SnO2-x-decoration. The underlying sensing mechanism was studied in detail and explained in terms of generation of oxygen defects, catalytic activity of Pd, and formation of heterojunctions in the sensing material. The results of the present study clearly demonstrate the usefulness of this simple and fast approach to significantly enhance the gas sensing properties of metal oxide nanowires.

Complex metal oxides such as functionalized branched nanowires (NWs) are a new category of nanocomposites with unique properties for gas sensing applications. In the present work, we studied the gas sensing properties of Cr 2 O 3 -functionalized ZnO-branched SnO 2 NWs, inspired by their high surface area and numerous resistive points. These NWs were studied and compared with pristine SnO 2 NWs and ZnO-branched SnO 2 NWs. To prepare the functionalized NWs, ZnO-branched SnO 2 NWs were sputter-coated with a Cr layer and then annealed at 500 °C to produce isolated Cr 2 O 3 NPs on the ZnO branches. Results of X-ray diffraction, scanning electron microscopy, and lattice-resolved transmission electron microscopy collectively showed that Cr 2 O 3 -functionalized ZnO-branched SnO 2 NWs were successfully formed. Cr 2 O 3 functionalization was found to greatly improve the sensors’ response to NO 2 gas. Furthermore, the sensors’ good selectivity was demonstrated by testing them in the presence of various interfering gases. The underlying gas sensing mechanisms are discussed herein in detail. We believe that the Cr 2 O 3 -functionalized ZnO-branched SnO 2 NWs reported herein are promising sensors for the highly sensitive and selective detection of NO 2 gas.

Microwave (MW) irradiation has obtained extensive importance in the field of synthesis and treatment of nanoparticles, because of its faster, cleaner and cost effectiveness than the other conventional and wet chemical methods. In this study, ZnO/graphene nanocomposites were prepared and subsequently post-treated by MW irradiation. Responses of the MW irradiated ZnO/graphene nanocomposites sensors were tested towards various gases including NO2, ethanol, acetone, toluene and CO and the results were compared with those of pristine ZnO and ZnO/graphene sensors without MW irradiation. It was demonstrated that the MW irradiated sensor had much higher response particularly to NO2 gas along with superior selectivity and shorter response/recovery times in comparison to unirradiated ZnO/graphene and pristine ZnO sensors. The possible underlying mechanism of this behavior is discussed in detail, mainly in terms of the MW-induced surface defects and the generation of finer ZnO nanoparticles. The results obtained demonstrated the beneficial effect of MW irradiation for enhancing the NO2-gas sensing behavior of ZnO/graphene nanocomposites, opening a new door not only to a novel synthesis of semiconductors/graphene nanocomposites, but also to a cost-effective way of improving their sensing capabilities.

We obtained extremely high and selective sensitivity to NO2 gas by fabricating graphene–SnO2 nanocomposites using a commercial microwave oven. Structural characterization revealed that the products corresponded to agglomerated structures of graphene and SnO2 particles, with small secondary SnOx (x ≤ 2) nanoparticles deposited on the surfaces. The overall oxygen atomic ratio was decreased with the appearance of an SnOx (x < 2) phase. By the microwave treatment of graphene–SnO2 nanocomposites, with the graphene promoting efficient transport of the microwave energy, evaporation and redeposition of SnOx nanoparticles were facilitated. The graphene–SnO2 nanocomposites exhibited a high sensor response of 24.7 for 1 ppm of NO2 gas, at an optimized temperature of 150 °C. The graphene–SnO2 nanocomposites were selectively sensitive to NO2 gas, in comparison with SO2, NH3, and ethanol gases. We suggest that the generation of SnOx nanoparticles and the SnOx phase in the matrix results in the formation of SnO2/SnO2 ho...

In this work, ZnO-decorated multi-walled carbon nanotube (MWCNT) nanocomposites prepared using an in-situ method involving the thermal evaporation of Zn powders in the presence of MWCNTs. The gas sensing characteristics of the MWCNT/ZnO nanocomposites are studied, and results for the material characterizations for the synthesized nanocomposites confirm the formation of well-distributed ZnO nanoparticles onto MWCNTs, creating MWCNT/ZnO nanocomposites. The gas sensing properties of the MWCNT/ZnO nanocomposite gas sensor, such as response, cross-sensitivity, and response-recovery time, are investigated and compared with a bare MWCNT sensor. The decoration of ZnO nanoparticles greatly improves the gas sensing properties of bare MWCNTs. We discussed the possible mechanisms for the enhancement of sensing capabilities. The results suggest that decoration of n-type semiconducting oxide materials, such as ZnO in the form of nanoparticles, is a promising strategy for improvement of gas sensing properties of p-MWCNTs.

We report a novel method to significantly improve the C 6 H 6 -sensing performance of Si nanowires through the combination of TeO 2 branches and Pd sensitization. The morphological investigation revealed that TeO 2 branches were densely formed on the stem nanowires (NWs). The sensor responses of the Pd-functionalized branched NWs exhibited superior sensor responses of 55.19 to C 6 H 6 gas. In particular, Pd nanoparticles enhanced the sensor response to C 6 H 6 gas most efficiently, increasing the sensor response by 173.5%. Possible mechanisms for the sensing of the Pd-decorated branched nanowires will be associated with resistance modulation along the branch TeO 2 nanowires (including catalytic Pd effects), at the networked homojunctions between the branch TeO 2 nanowires, at the boundaries of the TeO 2 nanograins, and at the Pd/TeO 2 heterojunctions.

TeO 2 nanowires with diameters ranging from a few tens to hundreds of nanometers and lengths of a few tens of micrometers were prepared by thermal evaporation of Te powder with a Co catalyst. Initially, bead-like γ-TeO 2 nanoparticles on the surface of pre-formed thick TeO 2 nanowires were heterogeneously formed. Subsequently, uniform thin TeO 2 nanowires without the γ-TeO 2 phase were homogeneously synthesized by changing only the inlet N 2 gas flow rates and keeping other process parameters constant. The morphological and microstructural evolution of the two different nanomaterials was examined by X-ray diffraction, energy-dispersive X-ray spectrometry, and scanning and transmission electron microscopy. The optical properties of the nanomaterials were also investigated using photoluminescence spectroscopy. The emission peaks in the PL measurement without the supply of N 2 gas showed a synergetic effect between (1) bead-like orthorhombic γ-TeO 2 nanoparticles that show two specific emission bands at 388 nm and 590 nm; and (2) uniform tetragonal TeO 2 nanowires that show a characteristic emission band at 440 nm. In contrast, the pure tetragonal TeO 2 nanostructures synthesized under a N 2 gas flow rate of 2 standard liters per minute flow rate revealed a red-shifted emission band at 450 nm compared to heterogeneous structures of the same composition. The origins of the growth and emission mechanisms in the different TeO 2 structures are also discussed.