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Carbon nanotubes (CNTs)@MnOOH core–shell nanocomposites, in which the CNTs core is uniformly coated with a shell of MnOOH nanoflakes, were successfully synthesized using a facile hydrothermal method. The appearance of sodium dodecyl benzene sulfonate (SDBS) plays a key role in forming the unique CNTs@MnOOH core–shell. A high dielectric loss tangent value was observed for the samples with/without SDBS. Significantly, the CNTs@MnOOH core–shell nanocomposites exhibit an electromagnetic shielding effectiveness of 13–15 dB in the frequency range of 8–18 GHz, which can be attributed to the improved absorption loss, resulting from the increased electrical conductivity, and the improved impedance matching of the core–shell structure. Moreover, the increasing interfaces between the MnOOH and CNTs favor the electromagnetic attenuation performance. Our findings strongly confirm that the CNTs@MnOOH core–shell nanocomposites can be considered as a potential candidate for electromagnetic applications.

The aim of this paper is to develop easily manufactured and highly sensitive gas sensors for VOCs (volatile organic compounds) detection. Macro-/mesoporous 5 at% Al-doped ZnO (AZO) and Ag-AZO composite powders were prepared by a one-step solution combustion method and used to fabricate gas sensors. Both powders showed the same macro-/mesoporous morphology but have different grain sizes and separated silver phase due to the fast growth of ZnO and the reduction of Ag+. The capability of Ag-AZO was investigated for VOCs detection, including n-butanol, methanol, acetone, ethanol, isopropanol and formaldehyde. It is found that the added Ag greatly increases the gas response towards different VOCs. Particularly 2.5 at% Ag-AZO shows the most superior gas sensing performance to 100 ppm VOCs at an operating temperature of 240 °C, which is also the optimum operating temperature of other sensors. The results may be attributed to the synergistic effects of these doped and composite elements, the amount of adsorbed oxygen and the macro-/mesoporous morphology.

Here, pristine and WO3 decorated TiO2 nanoparticles were synthesized by a one-step hydrothermal method without the use of a surfactant or template and used to fabricate gas sensors. Various techniques were employed for the characterization of the structure and morphology of the as-prepared products. The gas-sensing characteristics of the fabricated sensors were investigated for various concentrations of xylene at different temperatures. At a low operation temperature of 160 °C, the sensors possess an excellent gas response, selectivity, linear dependence, low detection limitation, and repeatability as well as long-term stability. In particular, for the high gas response of the 10.0 mol% WO3 decorated TiO2 nanoparticles based sensor, its response reaches 92.53 for 10 ppm xylene, which is much higher than that of the pristine TiO2 based sensor. And the detection limit is 1 ppm. Those values demonstrate the potential of using WO3 decorated TiO2 nanoparticles for xylene gas detection, particularly with low concentration xylene. Apart from this, the mechanism related to the advanced properties was also investigated and presented.

SnO2 microspheres in a composite with Pt nanoparticles (0, 0.5, 1.5, 2.5, 5.0 mol% Pt loading) were synthesized by a solvothermal method. The crystal structure, morphology, and specific surface area were thoroughly characterized. It is found that the Pt–SnO2 nanocomposites consist of a large amount of small spheres with average diameters up to hundreds of nanometers, and every small sphere is composed of numerous primary nanocrystallites with an average size of about 8 nm. Compared with the pristine SnO2, the presence of Pt nanoparticles has no influence on the growth behavior of the SnO2 microspheres. The gas sensors based SnO2 microspheres in a composite with Pt nanoparticles not only show a lower operating temperature and immensely enhanced responses, but also exhibit a faster response and recovery speeds and remarkable stability to methanol, especially the 5.0 mol% Pt–SnO2 nanocomposite. The gas sensor based on the 5.0 mol% Pt–SnO2 nanocomposite exhibits a response value of 190.88 to 100 ppm methanol at a low operating temperature of 80 °C, while the gas sensor based on pristine SnO2 only displayed a response value of 19.38 at an operating temperature of 200 °C. The reasonable explanation of the gas-sensing performance enhancement for the gas sensors based on Pt–SnO2 nanocomposites is attributed to the strong spillover effect of the Pt nanoparticles and the electronic interaction between Pt nanoparticles and SnO2 microspheres, both of which promoted the low temperature gas-sensing performance.

ZnO nanoparticles with high crystallinity and several nanometers in size were synthesized by a low-temperature solvothermal route from zinc acetate dihydrate (Zn(CH3COO)2·2H2O), potassium hydroxide (KOH) and methanol (CH3OH). The structural and the morphological characterizations of the ZnO nanoparticles were performed by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and N2-sorption isotherms. The obtained nanoparticles are highly crystalline wurtzite-type ZnO with a uniform near-spherical shape and an average particle size estimated to be 8.4 ± 1.3 nm. Such a small particle size and slight agglomeration are attributed to the use of methanol, which acts as both a solvent and an inhibitor of growth and agglomeration. The as-synthesized ZnO nanoparticles were directly used as a gas sensing material toward n-butanol gas. Such a designed sensor device exhibits several advantages such as a high and fast response, short recovery time, and good stability toward n-butanol gas. At the optimal operating temperature (320 °C), its gas response toward 500 ppm n-butanol is 805 and the response and recovery times are 22 and 6 seconds, respectively.

Ternary oxide Zn2SnO4 was introduced to a rod-like nanostructured SnO2 gas sensor for formaldehyde detection by a facile one-step hydrothermal synthesis. The effects of the Zn2SnO4 additive on the structure, morphology and gas-sensing property of SnO2 were investigated in this study. It was confirmed that control of the Zn amounts in the precursor solution was effective in realizing well-developed one- and two-dimensional coexisting structured SnO2–Zn2SnO4 (SnZn) nanocomposites. The gas sensing properties of the resulting SnZn composites to HCHO vapor were tested. The results showed that the presence of Zn2SnO4 species in SnO2 powders could effectively enhance electrical conductivity, reduce optimal operating temperature and improve the gas response of the sensors. The composite exhibited the highest response towards HCHO in the case of 35 at% Zn2SnO4 nanoplates coupling with hierarchical branched structures of SnO2 nanorods (SnZn35) at a relatively lower operating temperature of 162 °C. The good gas-sensing performance of the SnZn35 composite can be ascribed to the smaller particle size, the larger surface area and the more absorbed Ox− species, which all are favorable for gas diffusion and sensing reactions. This work renders great potential in the fabrication of gas sensors using a binary–ternary oxide composite, which can be further applied in indoor pollution detection.

Porous NiO nanosheets were self-grown on an alumina tube with a pair of Au electrodes connected by platinum wires via a simple solution combustion synthesis. A cubic NiO phase was obtained by a mixed solution of an oxidizer of nickel nitrate and a fuel of ethylene glycol (EG) at 400 °C. The phases and the morphologies of the materials self-grown on an alumina tube were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results showed that the alumina tube was entirely covered by NiO nanosheets with several micrometers in thickness. The NiO nanosheets on the surface of the tube were assembled by a large number of nanoparticles of irregular shapes and pores with different sizes. The electronic and gas-sensing characteristics of the self-grown porous NiO nanosheets for volatile organic compound (VOC) vapours (ethanol, acetone, methanol, and formaldehyde) were investigated. The resistance of the sensor directly based on the self-grown NiO dramatically drops from 100–240 °C, and then slightly decreases with further increasing temperature to about 28 kΩ at 400 °C. The sensor based on the self-grown NiO exhibits low detection limit, fast response and recovery and wide dynamic range detection to VOC vapours, especially ethanol, at the respectively optimal operating temperatures.

Ag–ZnO heterostructure nanoparticles were synthesized by a one-step solvothermal route from zinc acetate dihydrate (Zn(CH3COO)2·2H2O), silver nitrate (AgNO3), potassium hydroxide (KOH) and methanol (CH3OH). The structure, morphology, component and optical properties were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, UV-vis spectroscopy and photoluminescence spectroscopy, respectively. The results show that highly crystalline wurtzite-type ZnO nanoparticles matrices with an average grain size of 7.4 nm are obtained. Highly crystalline metallic Ag nanoparticles are observed on the ZnO matrix with a good combination. The absorption spectra of the Ag–ZnO heterostructure nanoparticles show the existence of a special two-absorption-region (strong UV-light and weak visible-light at 421 nm). The intensities of photoluminescence in the visible light region have a regular decrease with the increase in the load amount of Ag. The photoactivity of the as-synthesized samples was tested by measuring the degradation of azo dye Congo red (CR) under visible light irradiation. And it is found that both ZnO nanoparticles and Ag–ZnO heterostructured nanoparticles have better photocatalytic efficiency than commercial TiO2 (P-25), and an appropriate loading amount of Ag nanoparticles can significantly enhance the photocatalytic efficiency. The photodegradation mechanism as well as enhancement of the photoactivity in the presence of silver nanoparticles is further investigated. The experimental results indicate the potential of using Ag–ZnO heterostructured nanoparticles for degradation of Congo red dye.

Uniform, hollow NiO microspheres constituted from nanosheets were synthesized by a simple water bath method through a SiO2 spheres template-assisted approach and applied in an efficient gas sensor towards volatile organic compound (VOC) vapors. The structural characterizations reveal that sub-micrometer NiO hollow microspheres (350–400 nm) were formed by assembling NiO nanosheets of 20–40 nm thickness. The gas responses to the five reductive gases, including isopropanol, acetone, methanol, ethanol and formaldehyde, at a low concentration of 50 ppm are 11.3, 9.9, 8.2, 7.7 and 5.0, respectively. NiO hollow microspheres showed higher responses for VOCs compared to other NiO nanostructures previously reported in the literature. The gas sensor based on NiO hollow microspheres shows a high response and fast response and recovery towards VOCs, making it a promising candidate for a practical detector of VOCs. The improved performance of NiO hollow microspheres was attributed to hollow spaces that offer a high surface-to-volume ratio and an intrinsically large specific surface area of 167.31 m2 g 1, leading to an improved surface activity. It was suggested that a layer of adsorbed oxygen over the NiO surface possibly decreases the thickness of the space charge layer in VOC gases and leads to an increase in resistance and therefore response.

This article describes a new option for butane detection: W-doped TiO2 nanoparticles with high sensitivity and fast response/recovery toward butane, which were obtained from a simple, non-aqueous sol–gel route. The structure, morphology, surface chemical state and specific surface area were analyzed by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectrum (XPS) and N2-sorption isotherm, respectively. The obtained products are anatase-type TiO2 with a small grain size (7.5 ± 1.4 nm) and a high specific surface area (181.15 m2 g−1). Tungsten element presents in the +6 oxidation state. The resistance–temperature measurements indicate that tungsten dopant leads to the decrease in resistance. The as-prepared pure and W-doped TiO2 nanoparticles were used to fabricate gas sensor devices. Gas response toward 3000 ppm butane is increased from 6 to 17.8 through the doping of 5% tungsten. Meanwhile, the response and recovery time toward 3000 ppm butane are as fast as 2 and 12 s, respectively. Moreover, the sensor also possesses low detection limit, good linear dependence, good repeatability and long-term stability, indicating the potential of using W-doped TiO2 nanoparticles for butane gas detection. In addition, a possible mechanism for the enhanced sensitivity of W-doped TiO2 nanoparticles toward butane is also offered.

A coral-like bimodal macro-/mesoporous ZnO was prepared rapidly by self-sustained decomposition of a metal complex. The achieved ZnO powders demonstrated excellent gas-sensing properties for detecting ethanol and acetone because the abundant macropores provided open entrances and channels; whilst the mesopores guaranteed a large contact area for target gases.