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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.

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.

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.

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 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.

We prepared nanocomposites of n-SnO2/p-Co3O4 for application in chemical sensors. In order to fabricate p-Co3O4-decorated n-SnO2 nanowires, we sputtered a Co layer and subsequently annealed the material in air ambient. Characterization revealed that crystalline cubic Co3O4 with a tubular-like structure was attached to the surface of SnO2 core nanowires. We carried out sensing tests at 573 K in at NO2 gas concentrations ranging between 2 and 10 ppm. The sensor response was increased both by adding the Co3O4 layer and also by decreasing the thickness of the Co3O4 layer from 19.2 to 6.4 nm. We proposed possible mechanisms to explain the enhanced sensor properties obtained by Co3O4-functionalization. Co3O4-functionalized SnO2 nanowires exhibited a higher sensor response than pristine nanowires, not only due to the heterostructure-induced depletion of n-SnO2 region but also due to the surface effects of Co3O4. The generation of hole-accumulated Co3O4 layer in case of thicker-layered nanowires will decrease the sensor response. We demonstrated that Co3O4-functionalized SnO2 nanowire sensors can be used as gas sensors at very low concentrations.

We fabricated gas sensors by decorating amorphous carbon layers on the surfaces of SnO2 nanowires. Pretreatment and functionalization were not required for the synthesis of the SnO2-based nanocomposite, requiring only short few-second processing times. A sensing mechanism is proposed to explain the room-temperature (24 °C) operation of the gas sensor. The amorphous carbon not only increased the surface area, but also provided electronic effects improving the NO2 gas sensing likely by supplying electrons to the SnO2 and/or changing the conducting channel width inside the SnO2 by carrier transfer. The optimized gas sensor, having high response and high selectivity, can be utilized for room-temperature NO2 gas sensing applications.

Until now, the synthesis of cationic vanadate nanofibers/conducting polymers composites have required template such as surfactants and electrochemical equipment for the coating of conducting polymers. The cationic vanadate nanofibers have been synthesized by complex methods to overcome the low electrical conductivity of bulk vanadium oxide (V2O5). However, in some cases a reaction time of at least 10 h is needed. In this study, for the first time, Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) intercalated ammonium vanadate ((NH4)2V6O16∙1.5H2O) nanofiber (AVNF) composites were synthesized in 4 h using a simple sonochemical method for NH3 gas sensing studies. In addition, chemical composition, crystal morphologies and structures changes of the nanofiber composites according to the intercalation of ammonium cation and PSS:PEDOT were investigated. The NH3 gas sensing results reveal that the fabricated PEDOT:PSS-AVNF composite sensor had higher sensitivity and shorter response time to NH3 gas than V2O5, PEDOT, AVNF, and PEDOT-AVNF sensors at room temperature (24 °C). The enhanced NH3 sensitivity was mainly attributed to enhanced electrical conductivity (4.5 × 10−2 S. cm-1), high surface area, and p-p heterojunctions formed between the PEDOT:PSS and AVNF. The results obtained in this research demonstrate the beneficial effects of intercalating PEDOT:PSS in AVNF for NH3 gas sensing.

We present novel research devoted to the synthesis of SnO2-ZrO2 core-shell nanowires (C-S NWs) and investigation of their NO2 gas sensing properties. ZrO2 shell layer was deposited on networked SnO2 NWs. The ZrO2 shell thickness was varied using four different numbers of atomic layer deposition (ALD) cycles (50, 100, 150 and 200). Structural, morphological and chemical composition of the synthesized products were confirmed by different characterization techniques. The gas sensors showed an optimal sensing temperature at a relatively low temperature (≤ 150 °C). NO2 sensing results showed a strong dependence of response on the thickness of the ZrO2 shell, or equivalently the number of ALD cycles of ZrO2. The optimal sensor with a ZrO2 shell deposited at 150 ALD cycles (24.1 nm thick) exhibited a high response of (Rg/Ra) 24.7–10 ppm NO2 gas and revealed a good selectivity to NO2 gas. Also, with an increase in the thickness of the ZrO2 shell, the negative influence of humidity on the sensor response to NO2 was significantly decreased. The sensing mechanism involves NO2 removing electrons from the ZrO2 shell. The dependence of responses to shell thickness was explained based on two different regimes, i.e., the surface-electron-limiting regime and adsorbing species-limiting regime. The results obtained in this study can be used for further exploration of the sensing properties of ZrO2 as a novel shell material.