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The manner how nano building blocks assemble into hierarchical architectures exerts a tremendous influence on gas-sensing performance of the metal oxides. Here, we focus on tuning the 2D SnO 2 nanosheets into 3D hierarchical nanoflowers by manipulating the presence of NaOH, and investigate their gas-sensing functionalities. We find that the blooming SnO 2 nanoflowers assembled by ultrathin nanosheets (∼50 nm) are shrunk into semi-blooming state, and that the semi-blooming nanoflowers based sensor shows enhanced gas-sensing performance towards the ethanol, which is attributed mainly to the confined effect due to numerous nano or micro reaction rooms by keeping oxygen and ethanol molecules to complete gas-sensing reactions. While the semi-blooming nanoflowers turn into ordered mesoporous via thermally removing the periodically arranged polyvinyl pyrrolidone micelles, their gas-sensing performance is found to be improved dramatically, indicating that sufficient amount of gas diffusion is crucial to gas-sensing properties rather than the fast gas diffusion speed. As a final verification, we fabricate the sensors using the mesoporous semi-blooming SnO 2 nanoflowers and successfully monitor the existence of beer by a simple integrated device, making it a promising candidate in detecting drunk driving. [ABSTRACT FROM AUTHOR]

Quasi-one-dimensional (1D) heterostructures act as promising gas-sensing materials due to their high surface-to-volume ratio, large depletion area, and synergistic behaviors of the two components that contribute to dramatic change in resistance. To date, the noble metals and p-type metal oxides, a component of the heterostructure systems, have been extensively applied as good catalysts owing to their excellent selectivity and sensitivity to analyte gases. In particular, much effort has been devoted to the fabrication and processing of nanoscale quasi-1D metal-oxide-based heterostructural materials for a variety of practical applications especially as gas sensors. In this review, we first introduce the two-step preparation process of various quasi-1D nanostructures and then discuss how the primary parameters, e.g. the amount of activated adsorption sites, the gas diffusion ability, and the coverage the secondary materials, can have an impact on the gas-sensing performances. We finally review gas-sensing mechanism in terms of the species of heterojunctions, such as the Schottky-junction, p – n or n – p junction, and n – n or p – p junction. [ABSTRACT FROM AUTHOR]

NO 2 is an oxidizing gas, which is considered to be the most dangerous and most toxic gas that should be controlled. Because of the advantages of high sensitivity, selectivity, stability and fast response/recovery speed, the chemical resistance sensor is used as a NO 2 gas sensor. This article reviews the NO 2 gas sensor based on metal oxides. Firstly, the hazards of NO 2 and the major disadvantages of chemiresistive sensors based on metal oxides were discussed. Then, different sensitive materials of chemical resistance sensors were discussed, including metal oxides (MO), conductive polymers and carbon-based materials. Finally, the NO 2 gas sensors based on metal oxide (including ZnO, NiO, CuO, SnO 2 , WO 3) were discussed from the aspects of morphology, preparation method, modification, doping and composite. [ABSTRACT FROM AUTHOR]

Abstract: We have synthesized undoped and Ti-doped SnO2 spheres with a facile yet efficient hydrothermal approach and investigated their microstructures and gas sensing response to carbon monoxide, focusing especially on the impact of Ti doping. We find that gas response of SnO2 is enhanced significantly by doping, the origin of which is clarified in light of a proposed adsorption model. Through a combination of systematic measurements with density-functional-theory calculations, we discuss the enhancement mechanism of the gas-sensing function arising from the doping. [Copyright &y& Elsevier]

Abstract: Using a simple hydrothermal method, the pristine and Nb doped TiO2 is prepared, and their microstructures and gas-sensing responses to the harmful volatile organic compounds are investigated with a special focus on the impact of Nb additive. We find that the gas response of TiO2 is enhanced significantly by doping Nb, which is understood in theory upon proposed adsorption models. Combining experimental measurements with first-principles calculations, the working mechanism underlying such improvement in gas-sensing functions by the Nb additive is discussed. [Copyright &y& Elsevier]

Abstract: The microstructure of M-doped SnO2 (M=Cr3+, Cu2+ and Pd2+) prepared by the sol–gel method and their gas-sensing performance were investigated. In particular, we focus on the effects of metallic ions on the hydrogen sensing behavior of the SnO2-based sensor. It is found that hydrogen gas response of SnO2 can be enhanced evidently by adding Pd2+, while such effect from Cr3+ and Cu2+ exhibits somewhat slight. A theoretical study based on first principles calculation shows that SnO2–Pd (110) surface enable adsorb more H2 gas and receive larger electrons from adsorbed H2 molecule, thereby holding the potential for the improvement of gas response to hydrogen. [Copyright &y& Elsevier]

• Synthesis of heterostructure NiO-ZnO nanodisks. • Utilization of NiO-ZnO nanodisks as gas sensor. • Fabrication of high sensitive and low-concentration sulfur dioxide gas sensor based on NiO-ZnO nanodisks. This paper reports the synthesis and characterization of NiO-ZnO nanodisks synthesized through a facile hydrothermal method. The morphological characterizations confirmed the formation of well-defined nanodisks in high density with an average thickness of 60 ± 5 nm. The x-ray diffraction analysis confirmed the well incorporation of the NiO into the matrix of ZnO crystal and the average crystallite size for the NiO-ZnO nanodisks was found to be 39.71 nm. The compositional analysis revealed the formation of pure NiO-ZnO nanostructures. The synthesized NiO-ZnO nanodisks were used as electrode materials to fabricate Sulfur dioxide (SO 2) gas sensors and the gas sensor response and recovery times were systematically analyzed with respect to the operating temperatures and the SO 2 gas concentration. The observed sensor gas response, response time and recovery time of the fabricated NiO-ZnO nanodisks based gas sensor were 16.25, 52 s and 41 s, respectively, towards 20 ppm SO 2 gas at an optimized temperature of 240 °C. Thus, it is believed that the NiO-ZnO nanodisks could be a promising candidate for the fabrication of efficient gas sensors towards toxic and hazardous gases. [ABSTRACT FROM AUTHOR]

Metal oxide semiconductor (MOS)-based core-shell nanostructures with the unique structural and synthetic superiority have been universally applied for the chemiresistive gas sensors. We summarize the recent work involving MOS@MOS and noble metal@MOS core-shell nanostructures and further relate the improved performance to the work function-induced electron transfer through the N-N, N-P or Schottky junction. Generally, the performance can be enhanced if electrons transfer from the shell to the core and the heterostructure shows the same N- or P- type behavior to the shell MOS. Besides, the shell thickness is regarded to play a significant role in the performance modulation including the N-to-P (or P-to-N) conversion. The utilization of noble metals can sometimes change energy barriers and reaction routes of gas sensing, especially the adoption of Pd in detecting H 2. Moreover, alloying which can prevent the oxidation of noble metals, and constructing more complicated multi-shell nanostructures are both discussed as the potential development directions in this field. [Display omitted] [ABSTRACT FROM AUTHOR]

The detection of decomposed components is one of the important means of condition monitoring of SF 6 gas insulation equipment, among which chemical sensing is a kind of method with great potential. In this paper, two-dimensional Ti 3 C 2 T x was used as the sensing material substrate, and WO 3 -Ti 3 C 2 T x with different WO 3 doping concentrations were prepared by a one-step hydrothermal method. These materials were used to detect SO 2 , one of the decomposition products of SF 6. The results show that WO 3 -Ti 3 C 2 T x has a response of 12.5 % to 50 ppm SO 2 at room temperature(20℃), and the response time and recovery time are 54 s and 47 s, respectively. At the same time, WO 3 -Ti 3 C 2 T x has an excellent selectivity and long-term stability. Therefore, it is believed that WO 3 -Ti 3 C 2 T x can be used as an excellent material for SO 2 gas sensing devices. • Three-dimensional porous WO 3 /Ti3C2Tx synthesized by one-step hydrothermal method. • The sensor exhibits high sensitivity, rapid response and recovery to low-concentration SO2 in room temperature(20℃). • Good linear response for a wide range of SO 2 gas concentrations. [ABSTRACT FROM AUTHOR]

Here, we reported the synthesis of pure CoSn(OH) 6 and Fe-doped CoSn(OH) 6 nanocubes for gas sensing application by hydrothermal method. The extensive characterizations determined the crystal structure, morphology and chemical composition of the samples, such as, XRD, SEM, XPS, etc. Compared with the pure CoSn(OH) 6 , the gas sensing performance of 5Fe-CoSn(OH) 6 to N-butyl alcohol is improved significantly. The 5Fe-CoSn(OH) 6 based sensor presented high gas response of 60.309 toward 30 ppm N-butyl alcohol at relatively low operating temperature of 200 °C. Moreover, the 5Fe-CoSn(OH) 6 based sensor also exhibited excellent stability, selectivity and humidity resistance to N-butyl alcohol. The gas sensing properties of 5Fe-CoSn(OH) 6 is greatly improved due to the large specific surface area, narrow band gap, sufficient oxygen vacancies and quickly separation efficiency of electrons-hole pairs. The results demonstrate that the doping of Fe3+ is an effective way to improve the gas sensing properties of CoSn(OH) 6 sensors to detect N-butyl alcohol. [Display omitted] • The pure CoSn(OH)6 and Fe-doped CoSn(OH)6 were successfully synthesized by a hydrothermal method. • The sensing properties of the pure CoSn(OH)6 and Fe-doped CoSn(OH)6 to N-butyl alcohol were investigated for the first time. • 5Fe-CoSn(OH)6 exhibits the most remarkable gas sensing performance to N-butyl alcohol at 200 ℃. • The doping of Fe3 + is a promising approach for achieving an excellent gas sensing performance for the development of CoSn(OH)6 gas sensors to detect N-butyl alcohol. [ABSTRACT FROM AUTHOR]

In this work, we designed and prepared CuO-loaded In 2 O 3 /CeO 2 nanofiber heterojunctions, which are promising for trace CO gas detection. We characterized the materials, confirmed the morphology of the nanofibers and structure of the p-n heterojunctions, and analyzed the effect of morphology and structure on the gas sensing performance. In gas sensing tests, the sensor is found to show a low optimum operating temperature of 70 °C, with a limit of detection that reaches as low as 50 ppb. At concentrations ranging from 50 ppb to 10 ppm, the sensor shows a sensitive and linear response for CO gas. In particular, the sensor shows a response (R a /R g) of 11.7–10 ppm CO gas, and a response/recovery time of 125 s/12 s for 2 ppm CO gas. In addition, we tested the selectivity, humidity response and long-term stability of the gas sensors. Finally, based on the structure of the materials, we propose a mechanism for the enhanced CO gas sensing performance. • CuO-loaded In 2 O 3 /CeO 2 nanofibers were synthesized for fabricating superior CO gas sensors. • CuO was loaded onto the surface of nanofibers to form p-n junctions, leading to enhanced sensing properties. • The CO gas sensor shows a low optimum operating temperature of 70 °C, with a limit of detection as low as 50 ppb. • Combined with the experimental results, the mechanism for the enhanced performance of the gas sensor is discussed. [ABSTRACT FROM AUTHOR]

In this work, we designed and prepared CuO-loaded In 2 O 3 /CeO 2 nanofiber heterojunctions, which are promising for trace CO gas detection. We characterized the materials, confirmed the morphology of the nanofibers and structure of the p-n heterojunctions, and analyzed the effect of morphology and structure on the gas sensing performance. In gas sensing tests, the sensor is found to show a low optimum operating temperature of 70 °C, with a limit of detection that reaches as low as 50 ppb. At concentrations ranging from 50 ppb to 10 ppm, the sensor shows a sensitive and linear response for CO gas. In particular, the sensor shows a response (R a /R g) of 11.7–10 ppm CO gas, and a response/recovery time of 125 s/12 s for 2 ppm CO gas. In addition, we tested the selectivity, humidity response and long-term stability of the gas sensors. Finally, based on the structure of the materials, we propose a mechanism for the enhanced CO gas sensing performance. • CuO-loaded In 2 O 3 /CeO 2 nanofibers were synthesized for fabricating superior CO gas sensors. • CuO was loaded onto the surface of nanofibers to form p-n junctions, leading to enhanced sensing properties. • The CO gas sensor shows a low optimum operating temperature of 70 °C, with a limit of detection as low as 50 ppb. • Combined with the experimental results, the mechanism for the enhanced performance of the gas sensor is discussed. [ABSTRACT FROM AUTHOR]

Wurtzite zero dimensional ZnS nanospheres and random-shaped nanoparticles successfully synthesized via a low temperature hydrothermal route. The formation of nanostructures is attributed due to PVP and thiourea and discussed in details. The as-prepared nanostructures were characterized by XRD, EDS, SEM and HRTEM. Based on experimental results, a plausible growth mechanism is also proposed along with crystal structure of ZnS. The gas–sensing properties of as–prepared ZnS products are tested using efficient substrate sensors via an intelligent gas testing system. Both at room and 100–400 °C temperature, high response and selectivity towards formaldehyde than other gases is observed. Comparative studies of ZnS nanostructures for different concentrations of formaldehyde gas and definite sensor-stability for different gases is studied, as well. [ABSTRACT FROM AUTHOR]

In this work, the pure and Ni-doped Bi 2 WO 6 are successfully synthesized by a one-step hydrothermal method. We apply the various techniques, such as X-ray diffraction (XRD), Scanning Electron Microscope (SEM),Transmission Electron Microscope (TEM), Ultraviolet–visible spectroscopy (UV–vis) and photo luminescence (PL) et al., to characterize the structure, morphology, chemical compositions, optical properties and crystal defects of the prepared samples. Compared with pure Bi 2 WO 6 and other Ni-doped Bi 2 WO 6 , the 4 at% Ni-Bi 2 WO 6 sensor exhibits the highest gas response (31.2), fast response-recovery characteristic, good stability and unique selectivity to 30 ppm n-butanol at 325 ℃. The enhanced gas sensing mechanism could be attributed to its unique hierarchical hydrangea-like structure, the large specific surface area, the low band gap energy, high electron separation efficiency and abundant oxygen vacancies. We have fabricated the pure Bi 2 WO 6 and Ni-doped Bi 2 WO 6 successfully. The as-fabricated 4 at% Ni-doped Bi 2 WO 6 gas sensor shows superior gas-sensing performance toward n-butanol. [Display omitted] • The pure and Ni-doped Bi 2 WO 6 were successfully synthesized by hydrothermal method and the 4 at% Ni-Bi 2 WO 6 exhibits hydrangea-like morphology with hierarchical structures. • The Ni-doped Bi 2 WO 6 gas sensor presented a largely enhanced gas sensing properties to n-butanol. • The 4 at% Ni-doped Bi 2 WO 6 gas sensor showed excellent gas sensing properties to n-butanol. • The introducing Ni into Bi 2 WO 6 can increase oxygen vacancies and improve high electron-hole pairs separation efficiency. [ABSTRACT FROM AUTHOR]

Abstract: We report the synthesis of a hierarchical nanostructure of hollow and porous ZnO nanospheres with a high specific surface area as a novel sensing material to toxic formaldehyde by a simple template-free hydrothermal technique in organic solution. We demonstrate that the liquid mixture ratio and hydrothermal time play a pivotal role in forming such unique morphology and propose a growth mechanism of Ostwald ripening coupled with grain rotation induced grain coalescence. Comparison investigations reveal that yttrium allows resistance reduction of sensors and enhances significantly gas-sensing performances of ZnO nanospheres toward the formaldehyde over the commonly used undecorated ZnO nanoparticles. Such hollow, porous, and yttrium functionalized ZnO nanospheres could therefore serve as hybrid functional materials for chemical gas sensors. The results represent an advance of hierarchical nanostructures in enhancing further the functionality of gas sensors, and the facile method presented could be applicable to many other sensing materials. [Copyright &y& Elsevier]

Abstract: We report a successful synthesis of ZnO nanoparticles, nanosheets and nanoflowers via a simple hydrothermal process, and investigate comprehensively their gas-sensing performances. Of all the nanostructures, nanoflowers are found to show the most superior gas-sensing properties, e.g., highest gas response, shortest response and recovery time, excellent selectivity, and good repeatability and stability, which are attributed to their unique three-dimensional hierarchical structures with the largest specific surface area arising from remarkable amount of petals and pores. Further, the sodium citrate is found to be the key to producing such unique flower-like architecture, which can be understood upon the nucleation and self-assembly of building blocks of ZnO. Such development of the hierarchical architectures may open up an avenue to further enhance the gas-sensing performances of ZnO nanostructures for the on-site detection of the gases of interest. [Copyright &y& Elsevier]

The Ni-SnO 2 /g-C 3 N 4 composite exhibits unique hierarchical structure, which presents the gas response of 142.5 toward 10 ppm acetone at 125 °C. [Display omitted] • Hierarchical Ni-doped SnO 2 /g-C 3 N 4 nanocomposites are successfully synthesized. • The highly ordered Ni-doped SnO 2 nanosheets in situ grew onto g-C 3 N 4 lamina. • Ni-doped SnO 2 /g-C 3 N 4 presented the gas response of 142.5 toward 10 ppm acetone at 125 °C. • Introduction of g-C 3 N 4 formed g-C 3 N 4 -SnO 2 heterojunction and resulted in sharp resistance change. • Doping of Ni produced more oxygen vacancies for SnO 2. The development of metal oxides semiconductor-based gas sensors for the accuracy detection of acetone in human exhaled breath is highly desirable for diabetes diagnosis. Herein, we use calcining and hydrothermal method to grow highly ordered Ni-doped SnO 2 nanosheets onto g-C 3 N 4 lamina. The prepared Ni-SnO 2 /g-C 3 N 4 composite presents high gas response of 142.5 (Resistance ratio, defined as S = R air /R gas) toward 10 ppm acetone at the optimized operating temperature of 125 °C, under the environment temperature of 25 °C, 80 %RH. Moreover, the Ni-SnO 2 /g-C 3 N 4 based senor also maintains fast response–recovery time, good stability and extraordinary humidity resistance for detecting acetone, including the theoretical detection limit (LOD) as low as 1.38 ppb. The greatly improved gas sensing properties of the Ni-SnO 2 /g-C 3 N 4 composite can be due to the highly ordered hierarchical structure, Ni dopant and SnO 2 /g-C 3 N 4 heterojunctions. We believe this novel Ni-SnO 2 /g-C 3 N 4 nanocomposite promises to be a potential gas sensing material for diabetes diagnosis. [ABSTRACT FROM AUTHOR]