•As we know, there has rarely been research about the gas sensing properties of Zr-doped In2O3. In this work, we synthesized and characterized Zr-doped In2O3 with a hard template method and investigated the gas sensing properties of the sensor based on Zr-doped In2O3.•Zr-doping makes the operating resistance rather low (23 kΩ). Besides, the operating temperature is very low (75 °C), too. It means this material has the potential in the field of portable device due to its low energy cost.•The detection limit is down to 20 ppb of NO2.Ordered mesoporous Zr-doped In2O3 and undoped In2O3 nanostructures were synthesized via nanocasting method which is an easy, repeatable and friendly route. Zr incorporation might lead to In2O3 lattice deformation without destroying the original crystal structure and increased chemical adsorbed oxygen species. Gas sensors based on undoped and Zr-doped In2O3 were fabricated and their NO2 gas sensing properties were tested. The sensor performance was improved by Zr-doped strategy. The Zr-doped In2O3 based sensor showed good response (169) toward 1 ppm NO2 at the operating temperature of 75 °C with low resistance of 23 kΩ and the detection limit was 20 ppb. Such favorable sensing performances endowed mesoporous Zr-doped In2O3 with a potential application in the field of gas sensor. The enhanced sensing properties were mainly attributed to its mesoporous microstructures, increased chemical adsorbed oxygen and limited grain size by Zr-doping.

Pt-loaded mesoporous WO3 was fabricated by nanocasting method. Mesoporous structure provided ordered tunnel which was convenient for gas diffusion and the large specific surface area which could offer more active sites. The noble metal (Pt) improved the catalytic efficiency which played crucial role in enhancing the performance of the gas sensor. The obtained materials were characterized by X-ray diffraction (XRD), Brunauer-Emmet-Teller (BET), Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Characterization indicated that the synthesized materials had ordered mesoporous structure with excellent crystallinity and the pore size was about 10.6 nm. Static test system was employed to measure ammonia sensing properties for the as-prepared samples. The sensor based on Pt-loaded WO3 presented higher sensitivity, quicker response-recovery rates, excellent repeatability and selectivity. It indicated that the Pt-loaded mesoporous WO3 was a potential ammonia gas sensor material.

In this work, we described gas sensors based on the materials composed of hierarchical flower-likeIn2O3 and reduced graphene oxide (rGO), which were fabricated by a facile one-step hydrothermal method. The rGO-In2O3 composites exhibited enhanced sensing performance towards NO2 through comparison with the pure In2O3 sample. The operating temperature can be tuned by the percentage of rGO in the composites. The sensor based on 5 wt% rGO-In2O3 could work at room temperature with a high response value to 1 ppm NO2. 3 wt% rGO-In2O3 composite was adopted for the ultra-sensitivity gas sensor owing to its extremely low limit of detection of 10 ppb with rapid response time to NO2. The sensor also exhibited excellent selectivity and stability. The ultra-sensitivity of rGO-In2O3 should be related to synergistic effect of the hierarchical structure of In2O3 and the presence of rGO in the composites, which provided enhanced surface area and local p-n heterojunctions in rGO/In2O3 composites.Download high-res image (98KB)Download full-size image

•The rGO/ZnO hybrids with flower-like ZnO and flexible rGO sheets were obtained.•The response of rGO/ZnO to 50 ppb NO2 was enhanced seven times than that of ZnO.•The hierarchical rGO/ZnO hybrids can detect NO2 as low as 5 ppb.•The enhanced response was attributed to the local p-n heterojunctions in hybrids.Hierarchical rGO/ZnO hybrids with a flower-like morphology of ZnO and flexible rGO sheets were synthesized by a facile solution-processed method. The structures and morphologies of the hybrids were investigated by different kinds of techniques, including X-ray diffraction, field-emission electron scanning microscopy, transmission electron microscopy, and energy dispersive spectroscopy. The gas sensing properties of hierarchical rGO/ZnO hybrids toward nitrogen dioxide were studied via a static system. The response of rGO/ZnO hybrids to 50 ppb NO2 was 12, which was seven times higher than that of pristine ZnO at 100 °C. The limit of detection could be achieved as low as 5 ppb. The enhanced sensor response was attributed to the presence of local p-n heterojunctions between rGO sheets and hierarchical structure of ZnO.

The ordered mesoporous Ni-doped In2O3 and undoped In2O3 nanostructures have been synthesized via a nanocasting method, which is an easy, repeatable and friendly route. The structure of the as-prepared product was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), nitrogen physisorption and scanning electron microscopy (SEM). The results of XRD and TEM revealed the ordered structure of undoped and Ni-doped In2O3. The wide-angle XRD of the samples revealed that Ni incorporation may lead to lattice deformation without destroying the original crystal structure. Gas sensors based on undoped and Ni-doped In2O3 were fabricated and their gas sensing properties were tested. The Ni-doped In2O3 based sensor showed excellent selectivity toward NO2 at the operating temperature of 58 °C and the detection limit was 10 ppb. The response of the sensor based on mesoporous Ni-doped In2O3 was nearly 4 times higher than that of the sensor based on mesoporous undoped In2O3.

•ZnFe2O4 with ordered mesoporous structure has been successfully prepared by hard template (KIT-6).•The mesoporous ZnFe2O4 possesses the higher surface specific area and uniform pore size distribution.•The mesoporous ZnFe2O4 has been applied in gas sensor which has an outstanding sensing performance towards acetone.•We expect that the mesoporous ZnFe2O4 would be an excellent candidate for acetone gas sensing.Mesoporous ZnFe2O4 with ordered tunnel and large specific surface area has been successfully prepared through hard template (mesoporous silica KIT-6). The necessary characterizations such as X-ray diffraction (XRD), Brunauer-Emmet-Teller (BET), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) have been carried out to investigate the obtained material. ZnFe2O4 with mesoporous structure presents excellent crystallinity and periodic mesostructure. The sensing properties of gas sensor based on ZnFe2O4 mesoporous material towards acetone has been investigated. The sensor presents outstanding sensitivity, selectivity and longtime stability. This indicates that the obtained mesostructured ZnFe2O4 through hard template method is a potential acetone gas sensor material.

Hierarchical Sn3O4 nanoflowers, assembled from single-crystalline Sn3O4 nanosheets, were synthesized by a facile one-step hydrothermal route without any template. The crystal structure and phase purity of the Sn3O4 were investigated by X-ray diffraction (XRD). Morphologies and structures were analyzed by field-emission electron scanning microscopy (FESEM), and transmission electron microscopy (TEM), which indicated that flower-like Sn3O4 had an average size of about 700 nm and the surface of two-dimensional was smooth. BET results revealed the high surface area of the products (34.5 m2/g). The gas-sensing properties of flower-like Sn3O4 toward ethanol were investigated. Significantly, the sensor exhibited low detection limit and good repeatability to ethanol at the optimal operating temperature of 225 °C.

Ordered mesoporous WO3 and Ag-loaded mesoporous WO3 have been synthesized by using three-dimensional cubic KIT-6 as a hard template. A series of material characterization methods were employed to characterize the as-synthesized materials above and gas sensing properties were measured. It showed that the as-obtained ordered mesoporous structure materials have excellent crystallite and the pore sizes were about 9 nm which matched with the result of BET results. Furthermore, the NO2 sensing properties of the sensors based on mesoporous WO3 and different molar ratios of Ag-loaded (0.2%, 0.5% and 1.0%) mesoporous WO3 were detected by utilizing a static test system, respectively. The sensor using 0.5% Ag-loaded mesoporous WO3 exhibited much higher response, shorter response and recovery time and excellent selectivity to NO2 comparing with the sensors those based on other Ag-loaded ratio mesoporous WO3 and the pure mesoporous WO3.

•Fabrication of Ag2O/SnO2-sensing materials with ordered mesoporous structures and high surface areas.•Direct heating of the gas senor with an embedded heater.•Introduction of pulse current to provide two operating temperatures for the gas sensor.•High H2S-sensing response of a pulse-driven gas sensor.Ordered mesoporous Ag2O/SnO2 was synthesized via nanocasting method using hexagonal mesoporous SBA-15 as template. As-prepared Ag2O/SnO2 samples were characterized by X-ray diffraction, nitrogen adsorption–desorption, transmission electron microscopy, energy-dispersive X-ray analysis, and X-ray photoelectron spectroscopy. A microspheric directly heated gas sensor based on the mesoporous Ag2O/SnO2 was fabricated, and its gas sensing performance was investigated. Results indicated that the sensor based on mesoporous Ag2O/SnO2 exhibited excellent selectivity, high response, and good stability to H2S at 100 °C. A pulse-driving method was then introduced to enhance the sensitivity to H2S. Under pulse-driving, the response of the sensor to 300 ppb H2S was 5.7, which was approximately two times higher than that under constant current, and the limit of detection was improved to 50 ppb. The high-sensing performance of the sensor was attributed to the composition and structure of mesoporous Ag2O/SnO2 and the pulse-driven mode.

A facile one-step hydrothermal method for a novel discoid crystal of rutile SnO2 modified by reduced graphene oxide (rGO) is reported in this work. X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were performed to characterize the structure and morphology of the SnO2/rGO composites. Uniform discoid rutile SnO2 monocrystal with a diameter of approximately 100 nm and a center thickness of 40 nm was anchored on both sides of rGO nanosheets. The SnO2/rGO composite exhibited preferential detection toward NO2 with high response, good selectivity and reproducibility. The response of the sensor to 1 ppm NO2 at 75 °C was nearly one order of magnitude higher than that of SnO2, and the detection limit was improved to 50 ppb. The improved response was discussed and the gas sensing mechanism was established.

Au-loaded mesoporous WO3 was synthesized by nanocasting method. Mesoporous structure provided a large specific surface area, and the noble metal (Au) improved the catalytic efficiency. The above two characteristics played crucial roles in enhancing the performance of the gas sensors. The as-synthesized materials were deduced by X-ray diffraction (XRD), Brunauer-Emmet-Teller (BET), Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The obtained materials showed ordered mesoporous structure with excellent crystallinity and the pore size was about 10.8 nm which matched with Barrett-Joyner-Halenda analysis. Static test system was employed to measure volatile organic compounds (VOCs) (such as methanol, ethanol, isopropanol and n-butanol) sensing properties for the sensors using mesoporous WO3 and different weight ratios of Au-loaded (0.2%, 0.5% and 1.0%) mesoporous WO3. The sensors loaded with Au exhibited much higher sensitivity and selectivity to n-butanol in this work. Furthermore, Au-loaded materials showed lower operating temperature.

•Pure and Cu-doped flower-like In2O3 microspheres have been successfully synthesized.•The sensing properties to NO2 have been significantly improved by Cu doping.•The 1.0 mol% Cu-doped sample exhibited excellent sensing properties to NO2 at 60 °C.Pure and Cu-doped hierarchical flower-like In2O3 microspheres constructed from numerous nanosheets have been successfully synthesized via a facile and efficient solvothermal route combined with the subsequent thermal treatment. Various techniques, including X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) were employed to acquire the crystalline and morphological information of the as-obtained samples. XRD measurement results apparently revealed that the lattice constants of doped products were slightly smaller than that of the pure products owing to Cu incorporation. Gas sensing performances of the sensor devices fabricated from undoped and Cu-doped In2O3 were systematically investigated. It was demonstrated that the Cu-doping significantly improved the response to NO2. For example, sensors based on Cu-doped In2O3 (1.0 mol%) give a response of about 1800–400 ppb NO2, which was about 14.5 times higher than sensors based on primary In2O3 microstructures. The excellent and enhanced NO2 sensing performances of Cu-doped In2O3 were associated to its novel hierarchical structure and the incorporation of Cu ions.

Urchinlike CuO modified by reduced graphene oxide (rGO) was synthesized by a one-pot microwave-assisted hydrothermal method. The as-prepared composites were characterized using various characterization methods. A humidity sensor based on the CuO/rGO composites was fabricated and tested. The results revealed that the sensor based on the composites showed much higher impedance than pure CuO. Compared with the sensors based on pristine rGO and CuO, the sensor fabricated with the composites exhibited relatively good humidity-sensing performance in terms of response time and response value. The humidity-sensing mechanism was also briefly introduced. The enlargement of the impedance and improvement of the humidity-sensing properties are briefly explained by the Schottky junction theory.Keywords: humidity sensor; microwave-assisted hydrothermal; reduced graphene oxide; Schottky junction theory; urchinlike CuO;

Monodisperse WO3 hierarchical spheres were successfully synthesized via a microwave assisted hydrothermal method and a subsequent annealing process. The synthesis was performed using peroxopolytungstic acid as a precursor in the presence of sodium sulfate. The effects of hydrothermal reaction time on the microstructure were also investigated and discussed. It is found that the reaction time influences the morphologies in terms of particle sizes and dispersities. A gas sensor based on the as-prepared WO3 was fabricated and tested. The results revealed that the sensor showed relatively good selectivity and repeatability to acetone vapour. When the acetone concentration was in the range of 100 to 1000 ppm, the relationship between the response and the acetone concentration exhibited a good linearity.

The flower-like WO3 structures were successfully synthesized by a simple calcination of W18O49 nanowires obtained by a microwave-assisted solvothermal method. The as-prepared products were characterized by field emission scanning electron microscopy, X-ray powder diffraction, transmission electron microscopy, and nitrogen adsorption and desorption measurements. The results indicated that the building blocks of the flower-like structure were intercrossing single crystalline WO3 nanorods with diameter of about 30–40 nm and length of about 300–400 nm. The gas sensing properties of as-prepared products to NO2 and acetone were investigated. It was found that the sensor based on WO3 nanostructures exhibited excellent selectivity and high sensitivity toward NO2 at optimum temperature of 90 °C, giving a response of about 42 to 40 ppb NO2. Significantly, operating at 300 °C, the sensor's response and recovery time to 100 ppm acetone was only 1 s and 6 s, respectively.

•Pure and Cu-doped flower-like In2O3 microspheres have been successfully synthesized.•The sensing properties to NO2 have been significantly improved by Cu doping.•The 1.0 mol% Cu-doped sample exhibited excellent sensing properties to NO2 at 60 °C.Pure and Cu-doped hierarchical flower-like In2O3 microspheres constructed from numerous nanosheets have been successfully synthesized via a facile and efficient solvothermal route combined with the subsequent thermal treatment. Various techniques, including X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) were employed to acquire the crystalline and morphological information of the as-obtained samples. XRD measurement results apparently revealed that the lattice constants of doped products were slightly smaller than that of the pure products owing to Cu incorporation. Gas sensing performances of the sensor devices fabricated from undoped and Cu-doped In2O3 were systematically investigated. It was demonstrated that the Cu-doping significantly improved the response to NO2. For example, sensors based on Cu-doped In2O3 (1.0 mol%) give a response of about 1800–400 ppb NO2, which was about 14.5 times higher than sensors based on primary In2O3 microstructures. The excellent and enhanced NO2 sensing performances of Cu-doped In2O3 were associated to its novel hierarchical structure and the incorporation of Cu ions.