•Highly sensitive mixed potential type gas sensor based on YSZ and CdMoO4-SE was first developed for detection of acetone.•The device utilizing CdMoO4-SE exhibited the highest response of −133.5 mV to 100 ppm acetone at 625 °C.•The sensitivity of the fabricated sensor to acetone in the concentration range of 5–300 ppm was −84 mV/decade.•The sensor also showed good repeatability, selectivity, moisture resistance and stability.A highly sensitive mixed potential type gas sensor based on stabilized zirconia (YSZ) and CdMoO4 sensing electrode (SE) was developed and used for detection of acetone at 625 °C. By comparing the sensing performance for different devices fabricated, the sensor utilizing CdMoO4-SE exhibited the highest response value (−133.5 mV) to 100 ppm acetone at 625 °C, and even could achieve low detection limit of 500 ppb at 625 °C. The sensor attached with CdMoO4-SE displayed high sensitivity of −84 mV/decade to acetone in the range of 5–300 ppm at 625 °C. The present device also showed good repeatability, selectivity to certain deleterious gases, moisture resistance and acceptable drifts in 10 days measured period at 625 °C, demonstrating great potential for practical application in acetone sensing detection. Additionally, the sensor involving mixed potential mechanism was proposed and further clarified by polarization curve.

•The YSZ-based mixed-potential type gas sensor using CoTa2O6-SE was developed for NO2 detection at high temperature.•The device utilizing CoTa2O6-SE sintered at 1000 °C gave the highest response of 93 mV to 100 ppm NO2 at 650 °C.•The sensitivity of the fabricated sensor to NO2 in the concentration range of 5–500 ppm was 80 mV/decade.•The fabricated sensor also exhibited good selectivity, stability and slight effect of oxygen and moisture at 650 °C.A mixed-potential type NO2 sensor based on stabilized-zirconia (YSZ) solid electrolyte and novel trirutile CoTa2O6 sensing electrode (SE) prepared through the sol-gel method was developed at high temperature. The effect of CoTa2O6 sensing electrode material calcinated at different temperatures on NO2 sensing property was mainly investigated and the device attached with CoTa2O6-SE sintered at 1000 °C gave the highest response of 93 mV to 100 ppm NO2 and fast response and recovery times at 650 °C. The response of the sensor utilizing CoTa2O6-SE annealed at 1000 °C displayed segmentally linear relationship to the logarithm of NO2 concentration in the ranges of 0.5–5 ppm and 5–500 ppm, which the sensitivities were 12 and 80 mV/decade, respectively. Meanwhile, the present fabricated device also showed good reproducibility, selectivity, long-term stability and slight effect of oxygen and moisture at 650 °C. Furthermore, the mixed potential sensing mechanism proposed was discussed quantitatively and further verified by polarization curve measurement.

The NO2 sensing performance of a stabilized zirconia (YSZ)-based mixed potential type gas sensor utilizing a Co3V2O8 sensing electrode (SE) was improved by the addition of a noble metal. Among the different types of noble metal (Au, Pt, Pd and Rh), the sensor attached with Co3V2O8-SE loaded with Rh exhibited a noticeable improvement in NO2 response and the maximum response value was obtained when the loading mass fraction of Rh was 3 wt%. Results showed that the response for the sensor utilizing 3 wt% Rh/Co3V2O8-SE was 113.5 mV to 50 ppm NO2 and the sensitivity to 10–300 ppm NO2 was 85 mV per decade at the operating temperature of 650 °C, which were enhanced by 77.5 mV and 39 mV per decade compared to those of a sensor attached with Co3V2O8-SE, respectively. It is noteworthy that the response for each of sensor displayed a good linear relationship to the logarithm of NO2 concentration in the ranges of 10–300 ppm at 650 °C. Additionally, the sensor attached with 3 wt% Rh/Co3V2O8-SE also exhibited a low detection limit of 500 ppb and good selectivity to NO2 at 650 °C. The improvement of sensing characteristics for a sensor using 3 wt% Rh/Co3V2O8-SE may be attributed to enhanced electrochemical catalytic reaction activity to NO2 and a mixed potential mechanism was further verified by polarization curve.

The NO2 sensing performance of a stabilized zirconia (YSZ)-based mixed potential type gas sensor utilizing a Co3V2O8 sensing electrode (SE) was improved by the addition of a noble metal. Among the different types of noble metal (Au, Pt, Pd and Rh), the sensor attached with Co3V2O8-SE loaded with Rh exhibited a noticeable improvement in NO2 response and the maximum response value was obtained when the loading mass fraction of Rh was 3 wt%. Results showed that the response for the sensor utilizing 3 wt% Rh/Co3V2O8-SE was 113.5 mV to 50 ppm NO2 and the sensitivity to 10–300 ppm NO2 was 85 mV per decade at the operating temperature of 650 °C, which were enhanced by 77.5 mV and 39 mV per decade compared to those of a sensor attached with Co3V2O8-SE, respectively. It is noteworthy that the response for each of sensor displayed a good linear relationship to the logarithm of NO2 concentration in the ranges of 10–300 ppm at 650 °C. Additionally, the sensor attached with 3 wt% Rh/Co3V2O8-SE also exhibited a low detection limit of 500 ppb and good selectivity to NO2 at 650 °C. The improvement of sensing characteristics for a sensor using 3 wt% Rh/Co3V2O8-SE may be attributed to enhanced electrochemical catalytic reaction activity to NO2 and a mixed potential mechanism was further verified by polarization curve.

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 paper, a novel amperometric CO sensor using Nafion and Pt/C composite electrodes was fabricated. Three kinds of carbon materials (carbon fibers, multiwall carbon nanotubes and carbon blacks) were utilized as the supports of the sensing and reference electrodes for the CO sensors. The results revealed that the effective Pt loadings on the electrodes increased in the following order: carbon fibers (CFs) > multiwall carbon nanotubes (MWCNTs) > carbon blacks (CBs), leading to the increasing of the sensitivities toward CO in same order. In other words, the sensor using Pt/CFs as the sensing electrode (SE) showed the highest sensitivity with the value of 0.077 μA/ppm and shortest response time in the range of CO concentration from 1 to 200 ppm at room temperature. Further, a reproducible and stable response against 50 ppm CO was obtained for the sensor with Pt/CFs SE materials. Moreover, a low detection limit of 0.1 ppm for CO was also examined, suggesting that the sensor can be convenient for detecting very low traces of CO.

•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.

•Ag-doped hollow urchin-like spheres In2O3 were synthesized via a one-step hydrothermal method.•Sensor response to NO2 was greatly enhanced by slight Ag doping.•Sensor response varied linearly within a certain NO2 concentration.•Excellent NO2 selectivity and high stability were achieved.Ag-doped hollow urchin-like spheres In2O3 hierarchical nanostructures are developed for NO2 detection. Such unique architectures are synthesized by 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. Gas sensing performances of the sensor devices fabricated from pure and Ag-doped In2O3 were systematically investigated. The results indicate that the sensors based on Ag-doped In2O3 in certain molar ratio of AgNO3 to In2O3 (1:100) exhibit the largest response toward 1 ppm NO2, which is almost 23 times higher than that of the sensor based on pure In2O3 at the optimum operating temperature. It demonstrates that the Ag-doping can significantly improve the response to NO2. The excellent and enhanced NO2 sensing performances of Ag-doped In2O3 can be attributed to its novel hierarchical structure and the catalytic activity of Ag nanoparticles.

•Double-shell PdO nanoparticles decorated Fe2O3 hollow nanospheres can directly be synthesized using a facile template-free hydrothermal route combined with subsequent annealing process.•Notably, the internal structure of Fe2O3 hollow spheres could be controlled by varying the reaction time.•The gas sensing devices fabricated from PdO loading Fe2O3 hollow nanospheres with double shell structure showed good selectivity to acetone.Amorphous double-shell Fe2O3 hollow nanospheres decorated with PdO nanoparticles were synthesized via a template-free hydrothermal method combined with subsequent annealing process. The chemical composition and microstructure evolution were examined using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), and transmission electron microscope (TEM). Interestingly, the internal structure of hollow spheres could be controlled by varying the reaction time. The formation of such structures was attributed to the combined effect of ripening and hydrogen ions etching process. The sensing properties of Fe2O3 hollow nanospheres before and after functionalization with PdO nanoparticles were investigated. The PdO-loaded Fe2O3 sensor exhibited obviously enhanced response to acetone compared with their unloaded counterparts. The improvement may arise from unique structure and catalytic effect of PdO in promoting the dissociation of gas molecules.

•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.

Porous α-Fe2O3 microflowers, which were composed of many nanospindles assembled by large numbers of nanoparticles, were successfully synthesized by calcining the FeSO4(OH) precursor prepared through a simple ethanol-mediated method. Various techniques were employed to obtain the crystalline and morphological properties of the as-prepared products. The formation process of such microstructure was proposed according to the morphology and component of the products obtained at different reaction time. Moreover, the obtained α-Fe2O3 was utilized as sensing materials upon exposure to various test gases. As expected, in virtue of the less-agglomerated configuration and unique porous structure, the hierarchical α-Fe2O3 microflowers exhibited higher response as well as faster response/recovery time to acetone when compared with α-Fe2O3 nanoparticles. Significantly, the response time was measured to be 1 s at the low operating temperature of 210 °C.Download high-res image (114KB)Download full-size image

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

Metal oxides with hierarchical microstructures have attracted tremendous attention with respect to their enhanced gas sensing properties. Herein, we reported the facile synthesis of hierarchical ZnFe2O4 yolk–shell microspheres via a template-free solvothermal strategy and the subsequent annealing and chemical etching process. Electron microscopy images undoubtedly demonstrated that the novel ZnFe2O4 architecture was constructed of a large number of nanosheet subunits with a thickness around 20 nm. As a proof-of-concept demonstration of the function, when evaluated as gas sensing materials, the as-prepared ZnFe2O4 yolk–shell microspheres manifested an extremely high response and a low detection limit to acetone at the operating temperature of 200 °C. Significantly, the response to 20 ppm acetone was retained well even after 200 cycles and continuous measurement for 30 days, indicating superior cyclability and long-term stability.

CeO2 decorated SnO2 hollow spheres were successfully synthesized via a two-step hydrothermal strategy. The morphology and structures of as-obtained CeO2/SnO2 composites were analyzed by various kinds of techniques. The SnO2 hollow spheres with uniform size around 300 nm were self-assembled with SnO2 nanoparticles and were hollow with a diameter of about 100 nm. The CeO2 nanoparticles on the surface of SnO2 hollow spheres could be clearly observed. X-ray photoelectron spectroscopy results confirmed the existence of Ce3+ and the increased amount of both chemisorbed oxygen and oxygen vacancy after the CeO2 decorated. Compared with pure SnO2 hollow spheres, such composites revealed excellent enhanced sensing properties to ethanol. When the ethanol concentration was 100 ppm, the sensitivity of the CeO2/SnO2 composites was 37, which was 2.65-times higher than that of the primary SnO2 hollow spheres. The sensing mechanism of the enhanced gas sensing properties was also discussed.Keywords: CeO2/SnO2 composites; gas sensor; heterostructure; hollow spheres; sensing mechanism;

A well-ordered porous three-phase boundary (TPB) was prepared with a polystyrene sphere as template and examined to improve the sensitivity of yttria-stabilized zirconia (YSZ)-based mixed-potential-type NO2 sensor due to the increase of the electrochemical reaction active sites. The shape of pore array on the YSZ substrate surface can be controlled through changing the concentration of the precursor solution (Zr4+/Y3+ = 23 mol/L/4 mol/L) and treatment conditions. An ordered hemispherical array was obtained when CZr4+ = 0.2 mol/L. The processed YSZ substrates were used to fabricate the sensors, and different sensitivities caused by different morphologies were tested. The sensor with well-ordered porous TPB exhibited the highest sensitivity to NO2 with a response value of 105 mV to 100 ppm of NO2, which is approximately twice as much as the smooth one. In addition, the sensor also showed good stability and speedy response kinetics. All these enhanced sensing properties might be due to the structure and morphology of the enlarged TPB.

•Ag@C@SnO2@TiO2 nanospheres (ACSTS) is synthesized via a facile water bath method.•The ACSTS exhibit improved light absorption ability with sufficient dye loading.•Prolonged electron lifetime (171.1 m) is due to reduced charge recombination.•Several kinds of nanospheres with different structures are also investigated systematically.•Upon using ACSTS, an enhanced efficiency of 8.62% is achieved.The hierarchical Ag@C@SnO2@TiO2 nanospheres (ACSTS) have been successfully synthesized by deposition of SnO2 and TiO2 on the Ag@C templates layer by layer. The size of ACSTS is ca. 360 nm while the Ag@C cores have an average diameter of about 300 nm. The rough and porous shell structure consisting of SnO2 and TiO2 ensures a large specific surface area (115.5 m2 g−1). To demonstrate how such a unique structure might lead to more excellent photovoltaic property, several kinds of dye-sensitized solar cells (DSSCs) are also fabricated using different nanospheres based photoanodes. It is found that the ACSTS based DSSC exhibits an obvious improvement in cell performance. According to various technical characterization, the ACSTS can provide dual-functions of light absorption and charge transfer, hence resulting in an enhanced short-circuit photocurrent density of 18.68 mA cm−2 and a higher FF of 63% compared with other DSSCs. The ACSTS cell finally obtains a PCE of up to 8.62%, increasing by 70.4% and 10.2% than hollow TiO2 nanospheres and Ag@C@TiO2 nanospheres based cells, respectively. The improved photovoltaic properties of ACSTS cell can be mainly ascribed to the unique microstructure and the synergistic effect of the encapsulated Ag@C cores.

3D hierarchical flower-like WO3·0.33H2O nanostructures were synthesized via a facile solvothermal method without using any template or surfactant. After annealed at high temperature, the as-prepared WO3·0.33H2O would partly or fully transform into monoclinic WO3 with the morphology almost unchanged. Gas sensing properties of the sensor based on these flower-like nanostructures with the relationship of annealing temperature were also investigated systematically. The experiment results indicate the sensor shows highest response to NO2 when the annealing temperature is 500 °C. At the same time, the detection limit can be as low as ∼5 ppb level. Thus, the novel flower-like nanostructures might be a promising material for designing NO2 gas sensor with high performance.Novel hierarchical flower-like WO3 nanostructures controlled synthesized by one-step solvothermal method.Sensors based on the WO3 nanostructures showed excellent NO2 gas sensing properties down to ppb levels.

Derived from ZnO hollow spheres (ZHSs) as the underlayer and urchin-like TiO2 spheres (UTSs) as the light scattering overlayer, a new bilayered photoanode (ZHS + UTS) is fabricated for use in dye-sensitized solar cells (DSSCs). The ZHSs which directly grow on FTO are synthesized via a one-pot hydrothermal method. The DSSC based on the ZHSs achieves an overall photoelectric conversion efficiency (PCE) of up to 4.84%, which is mainly ascribed to fast electron transport. On the other hand, three kinds of UTSs of different size and morphology are prepared by simply adjusting the titanium precursor dosage. The hierarchical UTSs composed of needle-like thin nanosheets have large specific surface areas, enabling sufficient dye adsorption. In particular, UTS-2 (synthesized using a TiCl3 volume of 0.6 mL) shows strong light-scattering ability and hence is further applied as a scattering layer in the bilayered photoanode. The ZHS + UTS-2 based DSSC finally achieves the highest photocurrent density (Jsc = 18.13 mA cm−2) and therefore an enhanced PCE of 8.67%. The improved photovoltaic performance is attributed to the synergic effects of the ZHSs and UTSs, i.e. the fast electron transport favored by the highly crystallized ZHSs leading to a reduced interfacial charge recombination, the efficient light scattering effect due to the novel morphology of the UTSs, and the sufficient dye adsorption ensured by the large specific surface areas of the ZHSs and UTSs.

•A TiO2/ZnO composite oxide hierarchical nanosphere was synthesized by second steps facile hydrothermal method.•The composite oxide was comprised of 3D urchin-like TiO2 nanospheres and 1D ZnO nanospindles (UTZ).•The UTZs had a better photocatalytic activity than that of either single pure ZnO or TiO2, and P25 TiO2.•We expect that the UTZs would have applications in other research areas including sensors, optoelectronic devices et al.We report the synthesis, characterization and photocatalytic application of ZnO–TiO2 hydrothermal materials. 3D urchin-like TiO2 nanospheres with 1D ZnO nanospindles (UTZ) on the surface had been prepared by the two-step facile hydrothermal method. Photodegradation test of methyl orange (MO) and nitrophenol were examined individually using ZnO (NPs), TiO2 (P25), urchin-like structure TiO2 and ZnO–TiO2 (UTZ). A comparison of the photocatalytic performance of the materials above showed that the ZnO–TiO2 (UTZ) were superior to the other three samples. We expected that the material would have wide potential applications such as chemosensors, UV–vis blockers, self-cleaning, photonic crystals, organic synthesis and solar cells.

•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.

In this paper, the pure and NiO-decorated ZnO flower-like structures with uniform sizes were synthesized by a simple two-step process. As the gas sensing materials of oxide semiconductors gas sensors, their sensing properties were investigated systematically. The results manifested that the sensor based on 8.0 at% NiO-decorated ZnO microflowers showed improved gas sensing properties compared with those of pure ZnO microflowers. The introduction of NiO is believed to not only change the quantity of chemisorbed oxygen, but also form the p-n junction with ZnO. Thus, the functionalization of ZnO with NiO may be a promising method for designing and fabricating the high performance gas sensor.

In this paper, we present a facile one-step microwave assisted hydrothermal route for the synthesis of the unloaded and Pd-loaded SnO2 nanostructures. The Pd was grown in situ on the SnO2 nanostructure, constructing Pd/SnO2. A gas sensor based on the as-prepared Pd/SnO2 was fabricated and tested for response to carbon monoxide gas. The results indicated that the sensor using 3.0 wt% Pd-loaded SnO2 to 100 ppm carbon monoxide performed a superior sensing properties compared to 0 wt%, 1.5 wt%, and 4.5 wt% Pd-loaded samples at a relatively low temperature (100 °C). Such enhanced gas sensing performances could be attributed to both the contribution of Pd-loaded and the in situ method. In addition, the one-step in situ microwave assisted loading process provides a promising and versatile choice for the preparation of gas sensing materials.

Mixed-potential type sensors utilizing stabilized-zirconia (YSZ) and MNb2O6 (M: Co, Zn and Ni) sensing electrodes (SEs) were fabricated and examined for NO2 detection at high temperature. For mixed potential type NO2 sensor present fabricated, the device attached with CoNb2O6-SE has been found to give the highest response at 750 °C compared with the other two devices and could detect even 0.1 ppm NO2 with an acceptable response value (5 mV). The present sensor showed fast response and recovery times to 100 ppm NO2 at 750 °C, which are 3 s and 15 s, respectively. ΔV of the sensor attached with CoNb2O6–SE exhibited segmentally linear relationship to the logarithm of NO2 concentration in the ranges of 0.1–2 ppm and 2–300 ppm, which the sensitivities were 10 and 52 mV/decade, respectively. Moreover, the present device also displayed good repeatability, good wet resistance, slight drifts in 30 days high- temperature measured period, and excellent selectivity in the presence of various interfering gases at 750 °C. Additionally, the mixed potential sensing mechanism was explained quantitatively and further demonstrated from the measurement of polarization curve.

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.

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.

In this work, In2O3 microspheres with hierarchical nanostructures were successfully synthesized by solvothermal method in the presence of oleic acid and urea. The as-synthesized samples were characterized by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). The results indicate that the synthesized hierarchical In2O3 consists of irregular nanorods. Moreover, the gas sensing properties of as-prepared In2O3 hierarchical microspheres were investigated. It was found that the sensor based on such hierarchical nanostructures exhibited high response and good selectivity to NO2 at 145 °C.

A high performance mixed potential type gas sensor based on stabilized zirconia (YSZ) and NiNb2O6 as sensing electrode was fabricated and used for detection of acetone at 650 °C. NiNb2O6 prepared via a facile sol–gel method and sintered at different temperatures was characterized using X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM). The present study mainly focused on the effect of sintering temperature (800 °C, 1000 °C, 1200 °C) of NiNb2O6-SE materials on acetone sensing characteristics at 650 °C. Results indicated that the sensor using NiNb2O6-SE sintered at 1000 °C exhibited the largest sensitivity to acetone in the concentration range of 5–500 ppm at 650 °C, which the slope was −79 mV/decade. The response for the sensor attached with a NiNb2O6-SE sintered at 1000 °C to 100 ppm of acetone was approximately −113 mV. Moreover, the present sensor could detect even 500 ppb acetone with an acceptable response. The present device also displayed fast response and recovery times, good repeatability, slight sensitive effect to humidity, small drifts in 40 days measured periods, and acceptable selectivity at 650 °C. Additionally, the mixed potential mechanism was further demonstrated by polarization curve.

The In-Sn oxides composites made up of nanoparticles were successfully synthesized by an environment friendly one-step hydrothermal approach. The structures and morphologies of the composites were investigated by different kinds of techniques, including X-ray diffraction, field-emission electron scanning microscopy, transmission electron microscopy, electrochemical workstation and H2-TPR technique. Their gas sensing performances of In-Sn oxides composites toward ethanol were investigated using a static system and possible gas-sensing mechanism were investigated. The response (Ra/Rg) of In-Sn oxides composites to 100 ppm ethanol (C2H5OH) was 59.6, which was 7.9 times higher than that of tin oxides at 200 °C. The enhanced response to ethanol can be attributed to the change of charge carrier concentration of the nanocomposites and the variation of oxygen adsorption due to the formation of abundant heterojunctions.

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.

Mixed-potential type YSZ-based sensor utilizing In2O3 as sensing electrode (SE) was fabricated and examined for detection of NO2 at 700 °C. The hierarchical In2O3 oxide material was synthesized via a facile hydrothermal process, and In2O3 sintered at different temperatures were characterized using X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM). The present study mainly focused on the effect of sintering temperature of SE materials (800 °C, 1000 °C, 1200 °C) on NO2 sensing characteristics. As a result, the sensor attached with In2O3-SE calcined at 1000 °C exhibited the largest sensitivity to NO2, which the response to 100 ppm NO2 was as large as126 mV and excellent long-term stability for 30 days tested at 700 °C. It is noteworthy that the NO2 sensitivities for present device showed a small influence by the change of relative humidity at 700 °C. Additionally, the present devices also displayed good repeatability, the excellent sensitivity and relatively good selectivity in conjunction with various interfering gases before and after 30 days high-temperature-aging of 700 °C. It was speculated that the largest sensing performance for the sensor attached with In2O3-SE sintered at 1000 °C was contributed to the special micro-structure of the SE and the highest electrochemical catalytic activity for cathodic reaction of NO2 at the triple phase boundary (TPB).

The pure and Al-doped NiO nanorod-flowers with uniform sizes and well-defined morphologies were synthesized for the first time by a facile solvothermal reaction. As the gas sensing materials of MOS gas sensors, their sensing properties were investigated systematically. The results indicated that the 2.15 at% Al-doped NiO nanorod-flowers showed improved gas sensing properties compared to those of pure NiO nanorod-flowers. The incorporation of Al ions with NiO nanocrystals adjusts the carrier concentration, and induces the change of the oxygen deficiency and chemisorbed oxygen of NiO nanorod-flowers. Thus, the doping of Al3+ into NiO nanorod-flowers should be a promising method for designing and fabricating the high performance gas sensor.Keywords: Al-doped NiO; ethanol; gas sensor; nanorod-flowers; solvothermal

In this study, double-shell composites consisting of inner ZnO hollow microspheres (ZHS) surrounded by outer ZnFe2O4 nanosheets were successfully synthesized. The growth of the ultrathin ZnFe2O4 nanosheets (∼10 nm) on the ZHS outer surface was carried out at room temperature via solution reactions in order to generate a double-shell configuration that could provide a large surface area. As a proof-of-concept demonstration of the design, a comparative sensing investigation between the sensors based on the as-obtained ZnO/ZnFe2O4 composites and its two individual components (ZnO hollow spheres and ZnFe2O4 nanosheets) was performed. As expected, the response of the ZnFe2O4-decorated ZnO composites to 100 ppm acetone was about 3 times higher than that of initial ZnO microspheres. Moreover, a dramatic reduction of response/recover time has been achieved at different operating temperature. Such favorable sensing performances endow these ZnO/ZnFe2O4 heterostructures with a potential application in gas sensing.Keywords: acetone; double-shell; gas sensor; heterostructure; ZnO/ZnFe2O4

We present the preparation of a hierarchical nanoheterostructure consisting of inner SnO2 hollow spheres (SHS) surrounded by an outer α-Fe2O3 nanosheet (FNS). Deposition of the FNS on the SHS outer surface was achieved by a facile microwave hydrothermal reaction to generate a double-shell SHS@FNS nanostructure. Such a composite with novel heterostructure acted as a sensing material for gas sensors. Significantly, the hierarchical composites exhibit excellent sensing performance toward ethanol, which is superior to the single component (SHS), mainly because of the synergistic effect and heterojunction.Keywords: gas sensor; hollow nanostructure; microwave hydrothermal method; semiconductor; α-Fe2O3/SnO2 composites

Semiconductor oxides with hierarchically hollow architecture can provide significant advantages as sensing materials for gas sensors by facilitating the diffusion of target gases. Herein, we develop a facile template-free solvothermal strategy combined with the subsequent thermal treatment process toward the successful synthesis of novel ZnFe2O4 hollow flower-like microspheres. The images of electron microscopy unambiguously indicated that the ZnFe2O4 nanosheets with thickness of around 20 nm assembled hierarchically to form the unique flower-like architecture. As a proof-of-concept demonstration of the function, the as-prepared product was utilized as sensing material for gas sensor. Significantly, in virtue of the porous shell structure, hollow interior, and large surface area, ZnFe2O4 hierarchical microspheres exhibited high response, excellent cyclability, and long-term stability to acetone at the operating temperature of 215 °C.Keywords: acetone; hierarchical structure; sensor; solvothermal method; ZnFe2O4;

ZnO/α-Fe2O3 composites built from plenty of ZnO nanoparticles decorated on the surfaces of uniform round-edged α-Fe2O3 hexahedrons were successfully prepared via a facile solvothermal method. Various techniques were employed to obtain the crystalline and morphological characterization of the as-prepared samples. In addition, a comparative sensing performance investigation between the two kinds of sensing materials clearly demonstrated that the sensing properties of ZnO/α-Fe2O3 composites were substantially enhanced compared with those of the single α-Fe2O3 component, which manifest the superiority of the ZnO decoration as we expected. For instance, the response of ZnO/α-Fe2O3 composites to 100 ppm acetone is ∼30, which is ∼3.15-fold higher than that of primary α-Fe2O3 hexahedrons. The synergetic effect is believed to be the source of the improvement of gas-sensing properties.Keywords: gas sensor; semiconductor; solvothermal method; ZnO/α-Fe2O3 composites; α-Fe2O3 hexahedron;

Hierarchical ZnO/ZnFe2O4 nanoforests with ZnO backbones and ZnFe2O4 nanosheets were successfully prepared by a facile two-step process. The products were fabricated by immersion of the as-synthesized ZnO nanorod arrays in a 0.2 M aqueous solution of ferrous sulfate and subsequent calcination at 500 °C. Various techniques were employed for the characterization of the structure and morphology of the hybrid nanostructures. The results showed that a high density of ZnFe2O4 nanosheets was planted on the surface of the ZnO nanorods. Interestingly, the morphologies of the ZnO/ZnFe2O4 hierarchical nanostructures can be tailored by changing the concentration of the FeSO4 solution and the immersion time. A possible formation process and growth mechanism was proposed as the ZnO nanorods were partly dissolved during the immersion period. In order to demonstrate the potential application of the nanorods, two sensors based on bare ZnO and ZnO/ZnFe2O4 composites were fabricated and their gas-sensing properties were investigated. The results indicated that the obtained advanced ZnO/ZnFe2O4 nanostructures exhibited enhanced sensing properties to ethanol compared to the primary ZnO nanorods. For example, upon exposure to 100 ppm ethanol, the response of the hierarchical ZnO/ZnFe2O4 composites was about 4 times higher than that of the primary ZnO nanorods at the operating temperature of 275 °C.

In this work, flower-like In2O3 nanostructures were prepared with a solvothermal method in the presence of K3C6H5O7·H2O. The as-synthesized samples were characterized by using X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). The results indicate that the synthesized flower-like In2O3 nanostructures were constructed by porous nanosheets. The gas sensing properties of the as-obtained products were investigated. It was found that the sensor based on such flower-like In2O3 nanostructures exhibited high response and good selectivity to NO2.

In this paper, a one-pot synthesis of WO3 hollow sphere nanostructure has been realized via a one-pot template-free solvothermal method. X-Ray diffraction patterns demonstrated that the products are pure monoclinic WO3. Based on the observation of scanning electronic microscopy (SEM) and transmission electron microscopy (TEM), it was revealed that the as-prepared WO3 nanospheres have a diameter of around 2 μm and are hollow structures with shell thickness of about 300 nm which are constructed by numerous oriented nanocrystals. Sensors based on the synthesized WO3 hollow nanospheres exhibited high selectivity to NO2 at low operating temperature. The detection limit can be as low as ∼40 ppb level.

Hybrid Au@In2O3 microstructures with a distinctive core–shell configuration have been successfully synthesized by employing Au@carbon spheres as sacrificial templates. The In2O3 shell can be easily decorated on the Au core by a facile aging process at room temperature (25 °C) combined with a subsequent calcination. Field emission electron microscopy and transmission electron microscopy images revealed that the Au@In2O3 core–shell structures had an average diameter of about 150 nm and the thickness of the porous In2O3 shell was ca. 50 nm. When tested as a potential sensing material for gas sensing, the resulting hybrid Au@In2O3 core–shell structures exhibited a higher response to formaldehyde compared with the pure In2O3 spheres. The enhanced sensing properties of Au@In2O3 core–shell structures were attributed to their intense electron depletion that arose from the catalytic activity of Au nanoparticles and the formation of metal–semiconductor junction.

●Vertically aligned ZnO nanorod arrays were grown on ITO glass substrates.●We using ECD method in a two-electrode cell without any other supporting electrolyte.●ZnO nanorods show dominant UV emission peak and a weak blue green emission.●ZnO nanorods show single-crystal structure.High-ordered ZnO nano-rod arrays with an average length of about 1 µm and diameter of about 50–100 nm were vertically grown on indium-tin oxide (ITO) glass substrates using electrochemical deposition (ECD) method processing in a two-electrode cell at 85 °C. The clear lattice fringes in the image of high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) showed that a single-crystal structure of ZnO nano-rods with the growth direction along the [0 0 2] direction. The cross-sectional field emission scanning electron microscopy (SEM) images results revealed that the deposition time played a key role in elongating the length of the ZnO nano-rods. A sharp peak in the UV region and a weak broad peak in the visible region between 400 and 500 nm were shown in the room temperature photoluminescence (PL) spectra of nano-rod arrays, suggesting the ZnO nano-rod arrays with high-quality crystals were obtained. This synthesis method provided an extremely low cost and easy route for the preparation of 1D ZnO nanostructures.

•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.

A series of La2O3-doped WO3 nanofibers were synthesized by an electrospinning method in this work. The structure composition and morphology of nanofibers were characterized by XRD (X-ray diffraction), field emission scanning electron microscopy (FESEM), selected area electron diffractive (SAED), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). Sensors based on these nanofibers were fabricated by hot-press on the ceramic plates. The gas sensing properties of the undoped and La2O3-doped WO3 (La3+:W6+ =1 mol%, 3 mol% and 5 mol%) nanofibers were tested to various gases. The sensors based on La2O3-doped WO3 nanofibers, in which the molar ratio of La3+:W6+ is 3:100, exhibit highest response toward 100 ppm acetone, having a response about 12.7, which is almost 2 times higher than that of sensor based on pure WO3 nanofibers. These results indicate that the acetone sensors based on the La2O3-doped WO3 nanofibers exhibit fast response and good repeatability characteristics, which are promising for acetone sensors which used by air-quality and environmental monitoring.

A series of mixed potential type gas sensors using stabilized zirconia and M3V2O8 (M: Zn, Co and Ni) sensing electrode were fabricated and applied for detecting acetone at 600 °C. Among the sensors utilizing these composite oxides (Zn3V2O8, Co3V2O8 and Ni3V2O8)–SEs prepared via a facile sol–gel method, the sensor attached with Zn3V2O8–SE exhibited the highest response to acetone at 600 °C. Therefore, the present study mainly focused on acetone sensing performance for YSZ-based sensor using Zn3V2O8–SE at 600 °C. Results showed that the sensor attached with Zn3V2O8–SE could detect even 1 ppm acetone with an acceptable response. Moreover, ΔV of the sensor attached with Zn3V2O8–SE exhibited segmentally linear relationship to the logarithm of acetone concentration in the ranges of 1–10 ppm and 10–400 ppm, which the sensitivities were −16 and −56 mV/decade, respectively. The present device also displayed good repeatability, small drifts in 30 days measured period, and the excellent sensitivity and relatively good selectivity in the presence of various interfering gases before and after 30 days high-temperature-aging of 600 °C. Additionally, polarization curve was measured to further demonstrate the mixed potential mechanism.

Double-shell SnO2/α-Fe2O3 hollow composites were synthesized by a low-cost and environmentally friendly hydrothermal strategy. Various techniques were employed for the characterization of the structure and morphology of hybrid nanostructures. The results revealed that the α-Fe2O3 nanorods grew epitaxially on the surface of hollow SnO2 spheres, which were composed of primary nano-sized particles. The diameter of the α-Fe2O3 nanorods was about 10 nm, and the thickness of the SnO2 spherical shell was about 100 nm. In order to explore the formation mechanism of the composites, the structure features of the double-shell structural SnO2/α-Fe2O3 hollow composites at different reaction stages were investigated. The ethanol sensing properties of the pure SnO2 and SnO2/α-Fe2O3 composites were tested. It was found that such double-shell composites exhibited enhanced ethanol sensing properties compared with the single-component SnO2 hollow spheres. For example, at an ethanol concentration of 100 ppm, the response of the SnO2/α-Fe2O3 composites was about 16, which was about 2 times higher than that of the primary SnO2 nanostructures. The response time of the sensor to 10 ppm ethanol was about 1 s at the operating temperature of 250 °C.

Dispersed porous ZnO/ZnCo2O4 hollow spheres were successfully prepared by annealing the precursor, which was obtained via a facile one-step solvothermal method without any templates or surfactants. The X-ray powder diffraction (XRD) measurement showed that the crystal phase of the sample was a mixture of ZnO and ZnCo2O4. The field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images revealed that the as-synthesized porous ZnO/ZnCo2O4 hollow spheres had an average diameter of about 850 nm and were constructed from a large number of primary nanoparticles. To demonstrate the potential applications of such porous ZnO/ZnCo2O4 composites, the as-prepared products were used to fabricate a gas sensor that was then investigated for gas-sensing performances. Results of the test showed that this sensor had fast response kinetics to acetone at the operating temperature of 275 °C, and a high response to 100 ppm acetone, one that was about 4 times higher than that of sensors based on ZnO/ZnCo2O4 nanoparticles. The remarkable enhancement in the gas-sensing properties of the porous ZnO/ZnCo2O4 hollow spheres was attributed to their unique structure.

Hierarchical α-Fe2O3/NiO composites with a hollow nanostructure were synthesized by a facile hydrothermal method. The structures and morphologies of the composites were investigated by different kinds of techniques, including X-ray diffraction, field-emission electron scanning microscopy, transmission electron microscopy, and energy dispersive spectroscopy. Hierarchical α-Fe2O3/NiO composites were fabricated by growing the α-Fe2O3 nanorods on the surfaces of porous NiO nanosheets with a thickness of ∼12 nm. The gas sensing properties of hierarchical α-Fe2O3/NiO composites toward toluene were investigated using a static system. The response of α-Fe2O3/NiO composites to 100 ppm toluene was ∼18.68, which was 13.18 times higher than that of pure NiO at 300 °C. The enhanced response can be attributed to heterojunction. Meanwhile, the rapid response and recovery characteristics were observed because of the porous hollow structural characteristics and catalytic actions of α-Fe2O3 and NiO.Keywords: gas sensor; heterojunction; hollow nanostructure; hydrothermal; α-Fe2O3/NiO composites

The Au@ZnO yolk–shell nanospheres with a distinctive core@void@shell configuration have been successfully synthesized by deposition of ZnO on Au@carbon nanospheres. Various techniques were employed for the characterization of the structure and morphology of as-obtained hybrid nanostructures. The results indicated that the Au@ZnO yolk–shell nanospheres have an average diameter of about 280 nm and the average thickness of the ZnO shell is ca. 40 nm. To demonstrate how such a unique structure might bring about more excellent gas sensing property, we carried out a comparison of the sensing performances of ZnO nanospheres with different inner structures. It was found that Au@ZnO yolk–shell nanospheres exhibited an obvious improvement in response to acetone compared with the pure ZnO nanospheres with hollow and solid inner structures. For instance, the response of the Au@ZnO nanospheres to 100 ppm acetone was about 37, which was about 2 (3) times higher than that of ZnO hollow (solid) nanostructures. The enhanced sensing properties were attributed to their unique microstructures (porous shell and internal voids) and the catalytic effect of the encapsulated Au nanoparticles.Keywords: acetone sensor; Au@ZnO nanospheres; carbonaceous template

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;

Hierarchical and hollow cylinder comb-like nanostructures with ZnO backbones and SnO2 branches were successfully prepared by an ultrasonic spray pyrolysis process. The observation of field scanning electron microscopy and transmission electron microscopy showed that a high density of SnO2 nanowires grew epitaxially on as-synthesized ZnO nanorod arrays. The diameter and length of the SnO2 nanowires were about 50 nm and 300 nm. The morphologies of ZnO/SnO2 hierarchical nanostructures could be tailored by changing the growth time of SnO2 nanowires. A possible formation process and growth mechanism are proposed from the viewpoint of inside-out Ostwald ripening. In addition, gas sensors based on the hierarchical ZnO/SnO2 nanostructures were fabricated and exhibited good response to ethanol. At an ethanol concentration of 100 ppm, the response was about 11 and the response time was about 1 s at the operating temperature of 275 °C.

In this work, a simple solvothermal method was used for the synthesis of cuboid WO3 crystals without any surface active agent. The calcined WO3 was characterized using field emission scanning electron microscopy (FESEM), X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The results indicate that the sample is composed of cuboid nanobulks. The gas sensing property of a sensor based on the cuboid WO3 was also investigated. It is found that the sensor has a high response to low concentration NO2 at low operation temperature, so the cuboid nanobulks might have a potential application in the fabrication of highly sensitive and low power consumption NO2 gas sensors.

A ZnO core–shell structure with a movable core inside a hollow shell has been rapidly prepared by an efficient microwave hydrothermal method without any template. The synthesis was performed in an aqueous solution using zinc acetate dihydrate as the precursor. The observations of field emission electron microscopy and transmission electron microscopy showed that these ZnO core–shell structures were composed of numerous primary particles with size of tens of nanometers. A series of time-dependent experiments were carried out in order to have a closer inspection of the evolution processes of such structures. When evaluated as a sensing material for gas sensors, the as-synthesized ZnO core–shell structures displayed a high response towards ethanol and good response-recovery properties.

In this study, a series of undoped and V2O5-doped SnO2 nanofibers were synthesized via the electrospinning technique. The morphological and microstructural properties of these nanofibers have been characterized by XRD (X-ray diffraction), FESEM (field emission scanning electron microscopy), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The gas sensing properties of the undoped and V2O5-doped SnO2 (V2O5/SnO2 = 0.5 mol%, 1 mol%, 2.5 mol% and 5 mol%) samples were tested for various gases. The sensor based on the S2 sample (V2O5/SnO2 = 1 mol%) exhibits the largest response toward 25 ppm benzene among these undoped and doped SnO2 nanofibers, exhibiting a response of about 6.25 to 25 ppm benzene, which is almost 3.2 times higher than that of the sensor based on pure SnO2 nanofibers at the optimum operating temperature. The response and recovery time to achieve 25 ppm benzene were about 3 s and 40 s, respectively. These results indicate that the benzene sensors based on the S2 sample exhibit high response, good repeatability and quick response-recovery kinetics.

A bilayered photoanode for dye-sensitized solar cells (DSSCs) was constructed with the top layer using TiO2 hierarchical hollow spheres (THS) and the second layer based on 3D TiO2 nanorods. The anatase THS with diameters of 400–600 nm and with a specific surface area of 100.3 m2 g−1 were synthesized via a simple hydrothermal method. As a light scattering layer, the THS could provide dual-functions of adsorbing dye molecules and strong light-harvesting efficiency. The 3D TiO2 nanorods consisting of 1D vertically aligned rutile TiO2 nanorods (TNR) and 3D TiO2 nanoflowers (TNF) were synthesized by a one-step hydrothermal method. The unique 3D nanostructure could offer a better light scattering capability, faster electron transport and lower electron recombination. Consequentially, the bilayered cell exhibited a much higher short-circuit photocurrent density of 15.60 mA cm−2 and energy conversion efficiency of 7.50%, which indicated a 108.6% and a 36.8% increment of cell efficiency compared to the TNR-F electrode (7.36 mA cm−2, 3.60%) and THS electrode (13.20 mA cm−2, 5.48%), respectively.

Solid electrochemical sensors based on sodium super ionic conductor (NASICON) and spinel-type oxide CoCr2−x MnxO4 (x = 0, 1, 1.2, 1.4 and 2) sensing electrode were designed for sub-ppm H2S detection. In comparison with other spinel-type oxides, the sensor using CoCr1.2Mn0.8O4 showed the maximum response of 178 mV for 10 ppm H2S at 250 °C. The sensor displayed good stability for H2S during the testing period. Moreover, the sensor exhibited excellent selectivity toward H2S against the other interference gases, such as SO2, NO2, CH4, CO, C2H4, H2 and NH3. A sensing mechanism related to the mixed potential was proposed for the sensor based on NASICON and oxide electrodes. The effect of sintering temperature had also been investigated.

Flower-like ZnO hierarchical architectures consisting of different building blocks have been successfully synthesized by a simple microwave-assisted decomposition route. The scanning electron microscopy and transmission electron microscopy results indicated that the morphologies of ZnO hierarchical architectures could be tailored by changing the category of the additives. Gas sensors based on the resulting products were fabricated and their gas sensing properties were tested for a variety of target gases. The results indicated that the sensor based on ZnO with a needle-assembled structure exhibited excellent selectivity and a higher response to NO2 at 75 °C compared to those using the other two flower-like ZnO. The good performance observed here was likely to be the result of the high donor-related intrinsic defects.

The direct synthesis of tin dioxide (SnO2) nanowire arrays on a glass substrate by using an ultrasonic spray pyrolysis method combined with sintering is demonstrated. The products obtained are characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM) and high-resolution TEM. The results show that the SnO2 nanowire arrays consist of single crystalline nanowires, each with a diameter of 50–70 nm and a length of 5–7 μm. There are two different nanowire growth directions because of the oxygen defect growth. The mechanism of the formation and growth of SnO2 nanowire arrays was investigated. A platform gas sensor based on these arrays was fabricated. The sensor exhibits better sensitivity to and selectivity for NO2 than do SnO2 nanoparticles. The gas sensing mechanism is also discussed.

We present a facile one-step template-free route for the synthesis of homogeneous Au-loaded ZnO hollow spheres composed of nanoparticles. The synthesis was performed at a relatively low temperature (90 °C) with short reaction duration (40 min). Field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectrometry (EDX) analysis revealed that Au nanoparticles were homogeneously embedded into ZnO spherical shell, which was composed of nano-sized primary particles. Furthermore, to investigate the influence of Au nanoparticles on the sensor performance, the gas sensing properties of Au-loaded ZnO were studied. A comparative gas sensing investigation between the Au-loaded ZnO and pure ZnO hollow spheres was performed to display the superior sensing properties of the loaded samples. As expected, the sensor using 1.0 mol% Au-loaded ZnO exhibited a high response and fast response and recovery properties to ethanol, which could be attributed to the catalytic effect of Au.

We report a facile synthesis of monodisperse TiO2 spheres with controllable internal structure and surface areas of 17.9–348.8 m2 g−1, which were assembled from porous nanoparticles with average diameters of about 7–16 nm. The internal structures of the as-prepared spheres could be controlled by adjusting the hydrothermal reaction time, while keeping other parameters constant. Finally, novel mesoporous spheres with core–shell structure were assembled. This unique TiO2 core–shell structure with mesopores has filled a gap in the literature on TiO2 structures. In order to demonstrate their potential application, dye-sensitized solar cells (DSSCs) with a double layered photoanode structure were fabricated using either the as-obtained TiO2 spheres with different internal structures or nanometer-sized TiO2 crystals. The results showed that the DSSCs using the mesoporous spheres with core–shell structure as a top-layer exhibited much higher energy conversion efficiency (9.11%) and short-circuit photocurrent density (18.55 mA cm−2), indicating a 38% increase in the energy conversion efficiency compared to those using as-prepared TiO2 nanocrystals (6.62%). This significant improvement in photoelectric properties was mainly due to the excellent light scattering and dye absorption abilities of the novel structures. The results offer useful guidance in designing photoanode materials.

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.

•NiCr2O4 is used as sensing electrode of NASICON-based acetone sensor.•A three-dimensional three-phase boundary was constructed.•The sensing performance was greatly improved by the three-dimensional TPB.•It shows large sensitivity of − 58 mV/decade at 375 °C.The improvement of the sensing performance for the mixed-potential-type acetone sensor using NASICON and Cr-based spinel-type oxide (ACr2O4, A = Zn, Co, Ni) sensing electrode was examined by increasing the length of three-phase boundary (TPB). Among the spinel-type oxides tested, NiCr2O4 was found to be the best suited for the sensing electrode. The NASICON powder was mixed with NiCr2O4 in different mass fraction (10 wt.%, 20 wt.%, 30 wt.% and 40 wt.%) to improve the length of TPB. By mixing NASICON powder into the NiCr2O4 electrode, the contact between NASICON and the sensing material existed not only at the interface between the NASICON layer and the oxide thick film, but also at the inside of the sensing electrode. As a result, a three-dimensional TPB was constructed, and showed an enhancing effect on the sensitivity of the sensor. The sensitivity of the device using NiCr2O4 electrode mixed with 30 wt.% NASICON to 5 ppm–100 ppm C3H6O vapor was − 58 mV/decade at 375 °C, which was higher than other devices (10 wt.%, 20 wt.% and 40 wt.%). It was also observed that the sensor showed a speedy response and recovery kinetics to acetone vapor.

This work focuses on the H2 sensing performance of the sensor with buried Au sensing electrode and spineltype oxide CoCrMnO4 insensitive reference electrode within sodium super ionic conductor(NASICON) film. The sensor showed the highest response to H2 gas on the insensitive material sintering at 800 °C. Compared with those of the traditional structure device, the sensitivity and selectivity of the sensor using buried sensing electrode were greatly improved, giving a response of −177 mV in 9×10−5 g/L H2, which was about 3.5 times higher than that of sensors with traditional structure. Moreover, the ΔV value of the sensing device exhibited linear relationship to the logarithm of H2 concentration and the sensitivity(slope) was −135 mV/decade. A sensing mechanism related to the mixed potential was proposed for the present sensor.

Hierarchical hollow ZnO microspheres were prepared via a one-pot template-free hydrothermal synthesis. It's worth mentioning that the hollowness of these microspheres could be controlled by adjusting the zinc source concentration. Furthermore, these microspheres were integrated into a sensorial structure which exhibited fast response and recovery times and good selectivity to ethanol.

A novel 3D TiO2 nanomesh with a high specific surface area was successfully synthesized via a simple hydrothermal method using NaOH and P25 as the reactive agents. The as-prepared and calcined 3D TiO2 nanomesh samples were characterized by transmission electron microscopy, scanning electron microscopy and X-ray powder diffraction. The performance of DSSCs employing TiO2 as the photoelectrode shows an extreme dependence on the microstructure of the photoanode. Specifically, the double-layer photoanode with a hybrid top-layer composed of TiO2 nanomesh and Degussa P25 gives a high power conversion efficiency of 8.82% and a short circuit photocurrent density of 17.86 mA cm−2, indicating a 47% increase in the conversion efficiency compared to the DSSC with a P25 photoanode (5.98%, 13.33 mA cm−2).

Monodisperse Sn-doped α-Fe2O3 with novel wheat sheaf-like hierarchical architectures were synthesized by a facile one-step hydrothermal method. Field emission scanning electron microscopy and transmission electron microscopy images revealed that these sheaf-like microstructures were built from filamentary crystals-stacked nanoparticles. The high resolution transmission electron microscopy image from a single “filament” showed that all the nanoparticles were highly oriented. The morphology and size of products could be controlled by simply adjusting the concentration of Sn4+. A possible formation mechanism involving oriented aggregation and Ostwald ripening was proposed on the basis of the results of time-dependent experiments. To demonstrate the usage of such α-Fe2O3 hierarchical architectures, the obtained sample was applied to fabricate a gas sensor which was then tested for response to three kinds of gases (ethanol, methanol, and formaldehyde). Results of that test showed that the sensor had high response, and rapid response kinetics to ethanol at the operating temperature of 250 °C.

•Vertically aligned ZnO nanorod arrays with a length of approximate 10μm were successfully prepared by ultrasonic spray pyrolysis process.•The spray time played an important role in extending the length of the ZnO nanorods.•A possible formation mechanism was proposed on the basis of the results of the morphology evolution with different spray time.•Room-temperature photoluminescence spectra of the nanorods showed the near band-edge emission and the deep-level green light emission.Vertically aligned ZnO (zinc oxide) nanorod arrays with a length of approximate 10 μm were successfully prepared by ultrasonic spray pyrolysis process. The as-prepared samples were characterized by X-ray powder diffraction, field emission scanning electron microscopy, transmission electron microscopy and high-resolution transmission electron microscopy. The clear lattice fringes in the image of high-resolution transmission electron microscopy confirmed a single-crystal structure of ZnO nanorods with the growth direction along the [002][002] direction. A possible formation mechanism was proposed on the basis of the results of time-dependent experiments. In addition, the room temperature photoluminescence (PL) spectrum of the nanorods showed a strong UV emission peak and length-dependent green emission.

A trilaminar layer photoanode for dye-sensitized solar cells (DSSCs) was constructed using urchin-like TiO2 hierarchical microspheres and P25. The top layer made of hierarchical microspheres enhanced light scattering; the middle layer consisting of P25 and as-prepared microspheres is a multifunctional layer for DSSCs, high adsorption ability to the dye, light scattering ability, and slow recombination rates coexistence; the bottom layer used P25. The DSSCs based on the photoanode with tripartite-layers structure exhibited a much higher short-circuit photocurrent density of 18.97 mA cm–2 and energy conversion efficiency of 8.80%, which indicated a 36% increase in the conversion efficiency compared to those of the P25 electrode (14.51 mA cm–2, 6.50%). The great improvements of photocurrent density and energy conversion efficiency for hierarchical TiO2 microspheres were mainly attributed to a considerable surface area, a higher light scattering ability, and slower electron recombination rates for the former.

In this work, urchin-like In2O3 microspheres were fabricated using a solvothermal method in the presence of SDS (sodium dodecyl sulfonate) and urea. The as-synthesized samples were characterized using X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM). The results indicate that the synthesized urchin-like In2O3 microspheres were constructed by nanorods. Moreover, the gas sensing properties of as-prepared products were investigated. It was found that the sensor based on such novel urchin-like In2O3 microspheres exhibited high response and good selectivity to O3.

The mixed-potential type gas sensors combining NASICON with oxide electrodes exhibit high sensing performance to typical pollution gases in the atmospheric environment in the intermediate temperature range, indicating a potential in practical application. This paper describes the state-of-the-art for the sensors based on the NASICON, including the current-type, the equilibrium-potential-type and mixed-potential-type. For improving the performance of sensors based on the NASICON and oxide electrodes, two main approaches have been utilized: the developing of new oxide electrode materials and the design of novel sensor structure for enhancing the sensing performance as well as the sensing mechanism involved in the mixed potential.

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.

Hierarchical α-Fe2O3/SnO2 composites were synthesized by a low-cost and environmentally friendly hydrothermal strategy. The structure and morphology of composites were investigated by X-ray diffraction (XRD), field-emission electron scanning microscopy (FESEM), transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS). The results revealed that the α-Fe2O3 nanorods grew epitaxially on the surface of SnO2 nanosheets. The diameter and length of the α-Fe2O3 nanorods were about 10 and 80 nm, respectively, and the thickness of the SnO2 nanosheets was about 15 nm. The acetone sensing properties of the pure SnO2 and α-Fe2O3/SnO2 composites were tested. The results indicated that such hierarchical α-Fe2O3/SnO2 nanostructures exhibited an enhanced acetone sensing properties compared with the primary SnO2 nanostructures. For example, at an acetone concentration of 100 ppm, the response of the α-Fe2O3/SnO2 composites was about 17, which was about 2.5 times higher than that of the primary SnO2 nanostructures. The response time of the sensor to 60 ppm acetone was shorter than 3 s at the operating temperature of 250 °C.

In this work, a novel flower-like indium hydroxide (In(OH)3) structure was fabricated using a hydrothermal method, without any templates or surfactants. In2O3 with similar morphology was formed by annealing precursors In(OH)3. The as-synthesized samples were characterized using X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). The results indicate that the synthesized flower-like In2O3 were formed by porous nanosheets. Moreover, the gas sensing properties of as-prepared products were investigated. It was found that the sensor based on such novel In2O3 nanostructure exhibited high response and good selectivity to NO2.

Hierarchical tin oxide(SnO2) architectures were synthesized with a facile hydrothermal method. In the hydrothermal synthesis, sodium dodecyl benzene sulfonate(SDBS) surfactant plays an important role as structure-directing reagent. The synthesized samples were characterized by powder X-ray diffraction(XRD), field emission scanning electron microscopy(FESEM), transmission electron microscopy(TEM) and high-resolution transmission electron microscopy(HRTEM). The results clearly reveal that the hierarchical architectures of SnO2 were composed of aggregated nanosheets with a thickness of about 100 nm. A possible mechanism for the formation of the SnO2 hierarchical architectures was proposed. In addition, the gas sensing properties of the as-prepared products were investigated and it was found that the sensor based on the special SnO2 hierarchical architectures exhibited a high response and good selectivity to NO2 at the optimal working temperature of 160 °C.

A mixed potential type sensor based on NASICON (sodium super-ionic conductor) was designed for the detection of hydrogen. Two original ways were combined to promote the sensor's sensitivity. LaCrO3 was applied on Au reference electrode as H2 oxidation layer to minimize the H2 response on reference electrode. Additional NASICON layer was coated on Au sensing electrode serving as sensitive electrode for limiting the O2 diffusion. H2 temperature-programmed reduction (TPR) measurement was conducted to test the oxidizability of LaCrO3. The effect of O2 concentration on sensor's sensitivity was discussed to verify the function of additional NASICON layer. The correlation between the thickness of additional NASICON diffusion layer and sensor's sensitivity was also studied. The research showed that the sensor attached with 0.3 mm thick additional NASICON layer exhibited the largest sensitivity to 100–5000 ppm H2 at 400 °C, the slope was −123 mV/decade. In addition, the sensor exhibited excellent selectivity to H2 against the other interference gases, such as CO, NO2, NH3, C7H8, C2H4, CH2O, C3H6O and CH4.

In this article, the NO2 sensing performance of the mixed-potential-type yttria stabilized zirconia (YSZ)-based sensor was improved by modifying the three-phase boundary (TPB). The double-tape casting and the pore-forming method were simultaneously used to obtain the porous YSZ-substrate. Furthermore, a group of tape-casting slurries with different starch concentrations (0 wt%, 5 wt%, 10 wt% and 15 wt%) were applied to form the different contact areas of the three-phase boundary (TPB). The scanning electron microscope (SEM) images showed that YSZ-substrate prepared by using the slurry with 15 wt% starch had the largest contact area. The sensor based on the YSZ-substrate with the largest contact areas and the MnCr2O4 sensing electrode (SE) showed the largest response compared with the other sensors when they were exposed to 10–500 ppm NO2 at 800 °C. The measured polarization (I–V) curves indicated that the present sensors operated under the mixed-potential mechanism. In addition, the sensor showed a good selectivity and repeatability to NO2.

Monodisperse porous α-Fe2O3 ellipsoids composed of primary nanocrystals were synthesized by a facile one-step template-free hydrothermal method. Field emission scanning electron microscopic and transmission electron microscopic results revealed that these porous ellipsoids were built from a large number of nanoparticles with sizes of about 10–20 nm. The morphology and size of the as-prepared products could be tailored by simply adjusting the amounts of hexamethylenetetramine and reaction temperature. A possible formation mechanism was proposed on the basis of the results of time-dependent experiments. When used as sensing materials in chemical sensors, the as-prepared porous α-Fe2O3 ellipsoids exhibit a high response to ethanol.

Novel Zn-doped SnO2hierarchical architectures were synthesized by a simple hydrothermal route. The observation of field-emission electron microscopy and transmission electron microscopy showed that Zn-doped SnO2 hierarchical architectures were composed of one-dimensional nanocones. Interestingly, these nanocones were almost parallel to each other and knitted by other parallel nanocones. The morphology of the products could be controlled by varying the concentration of Zn2+. A possible formation mechanism was proposed from the viewpoint of nucleation and the crystal growth habit. Evidences of dopant incorporation were demonstrated in the X-ray diffraction and X-ray photoelectron spectroscopy measurement of Zn-doped SnO2nanocones. The UV-vis absorption spectra of samples exhibited a blue shift with a decrease of the size of nanocones. Moreover, gas sensors based on the hierarchical Zn-doped SnO2nanocones displayed higher response to ethanol compared with the pure urchin-like SnO2 nanostructures. Finally, based on first-principles calculations, the enhancement in sensitivity toward ethanol could be explained by the strong coulomb binding between ZnSn and its neighboring O vacancies.

Monodisperse α-Fe2O3 discoid crystals have been prepared through a hexamethylenetetramine (HMT)-assisted hydrothermal process combined with subsequent acid-dissolution. First, uniform α-Fe2O3 round-edged hexahedrons with a size of about 1.2 μm were synthesized. Subsequently, by a controlled acid etching process, the as-obtained α-Fe2O3 uniform hexahedrons could be facilely transformed into monodisperse α-Fe2O3 discoid crystals, without influencing the original crystal phase. Both field emission scanning electron microscope results and transmission electron microscope results revealed that the “discuses” were made of piled up nanoparticles. The selected area electron diffraction pattern from the whole discoid crystal displayed that all the nanoparticles were highly oriented, which resulted in the single-crystal “discus” features. To demonstrate the usage of such α-Fe2O3 discoid crystals, the obtained sample was applied to fabricate a gas sensor which was then tested for sensitivity to three kinds of gases (ethanol, methanol and acetone). The results of the test showed that the sensor had a high level of response and good recovery characteristics towards ethanol at the operating temperature of 238 °C.

Nano-sized nickel oxides have been synthesized in a water-in-oil microemulsion. The as-synthesized samples were characterized by powder X-ray diffraction (XRD), transmission electronic microscopy (TEM) and nitrogen adsorption/desorption. The particle size of nickel oxide can be controlled from 11.5 to 31.5 nm by varying the proportion of water, surfactant and oil in the microemulsion, mixing method, and calcining temperature. Gas sensors based on as-synthesized nickel oxide are fabricated and investigated. They exhibit much higher sensitivities to hydrogen sulfide, ethanol and nitrogen dioxide, compared to those based on the conventional bulk NiO. Furthermore, the response of as-synthesized materials to various kinds of target gases increases with the decreasing of the particle size of gas sensors. It is noted that the NiO sensor with particle size of 11.5 nm displays high degree of selectivity, coupled with high response value, making it particularly interesting for H2S-monitoring applications.Highlights► Nano-sized nickel oxides have been synthesized in a microemulsion system. ► The particle size of nickel oxide can be controlled from 11.5 to 31.5 nm. ► They exhibit higher sensitivities to H2S, C2H5OH and NO2 in contrast to bulk NiO. ► The response to all target gases increases with the decreasing of the particle size. ► The NiO sensor (11.5 nm) displays outstanding sensitivity and selectivity to H2S

The uniform hierarchical ZnO ordered nanoclusters were deposited on quartz substrates via a facile chemical bath deposition method. Field emission scanning electron microscopic and transmission electron microscopic results revealed that these hierarchical nanoclusters were built from one-dimensional single crystal nanorods. The morphology of the prepared products could be tailored by changing the deposited time. On the basis of experimental results, a possible growth progress and mechanism of hierarchical ZnO ordered nanoclusters were proposed. In addition, the planar type gas sensor using as-prepared ZnO ordered film deposited for 60 min showed good response to 1 ppm NO2 at room temperature under UV light illumination.Highlights► Hierarchical ZnO composed by monocrystal nanorods were deposited via CBD method. ► A growth progress and mechanism of hierarchical ZnO nanoclusters were proposed. ► A UV-excited sensor with superior response to NO2 at room temperature was fabricated.

A UV light enhanced gas sensor with light reflector was designed and fabricated. Compared with that without reflector, the sensor with reflector had higher efficiency of UV light absorption and utilization. The sensor with reflector exhibited higher response to ethanol than that without reflector at room temperature. The response of the sensor with reflector to 100 ppm ethanol was about 160 under UV illumination. Moreover, the response and recovery kinetics were obviously improved, and the response and recovery time were 50 s and 150 s, respectively. Such obvious improvement on the sensing performance for the sensor can be ascribed to the increasing of the optical path length in the sensing material layer, due to the introduction of the reflector.

A series of tungstate MWO4 (M = Co, Zn and Ni) has been prepared by the polymeric precursor method. Meanwhile, yttria stabilized-zirconia (YSZ) based sensors using these oxides as sensing electrodes were investigated, and among the oxides tested, CoWO4 was found to be the most suitable for the sensing electrode (SE) of the ammonia sensor. The sensor attached with CoWO4 shows the fast response and recovery characteristics (not more than 5 s respectively) and large sensitivity (− 51 mV/decade) at elevated temperature. The electric potential difference (∆V) of the sensor varies almost linearly with the NH3 concentrations in the examined range of 50–1000 ppm. The SEM observation reveals that the special microstructure of CoWO4-SE, bulky rod-like crystals coated by tiny particles, plays a significant role in sensing performance.Highlights► Rodlike CoWO4 crystal is prepared by polymeric precursor method. ► It is used as sensing electrode of NH3 sensor base YSZ electrolyte. ► It shows fast response and recovery either of which is less than 5s. ► It shows large sensitivity of –51 mV/decade at high temperature.

Porous SnO2 hierarchical architectures were synthesized by calcining the precipitates prepared through a facile one-step hydrothermal synthesis method. Field emission scanning electron microscopic and transmission electron microscopic results revealed that these hierarchical nanostructures were built from two-dimensional porous nanosheets. The morphology of the prepared products could be tailored by changing the precursor concentration and reaction time. On the basis of experimental results, a possible mechanism for the formation of the three-dimensional SnO2 hierarchical nanostructure was speculated. Moreover, gas sensing tests showed that the sensor using porous SnO2 hierarchical nanosheets after annealing at 600 °C for 2 h exhibited better gas sensing properties compared with the sensor based on conventionally prepared SnO2 nanoparticles. The enhancement in gas sensing properties was attributed to their unique structures.

Bundle-like α-Fe2O3 nanostructures were successfully synthesized by a simple calcination of β-FeOOH precursor derived from a hydrothermal method in the presence of poly(vinyl pyrrolidone). The as-prepared products were characterized by X-ray power diffraction, field emission scanning electron microscopy, and transmission electron microscopy. The results indicated that bundle-like nanostructures were composed of well-aligned single crystalline nanorods with the diameters of 20–30 nm and the lengths of 200–300 nm. The gas sensing properties of as-prepared products were investigated. It was found that the sensor based on α-Fe2O3 nanostructure exhibited high response, quick response–recovery, and good repeatability to acetone at 250 °C.

This paper focuses on the gas sensing properties of the mixed-potential-type NO2 sensor based on yttria stabilized zirconia (YSZ) and NiO electrode. The sensing performance of the sensor was improved by modifying the three-phase boundary (TPB). Hydrofluoric acid with different concentrations (10%, 20% and 40%) was used to corrode YSZ substrate to obtain large superficial area of TPB. The scanning electron microscope and atomic force microscopic images showed that the 40% HF could form the largest superficial area at the same corroding time (3 h). The sensitivity of the sensor using the YSZ plate corroded with 40% hydrofluoric acid to 20–500 ppm NO2 was 76 mV/decade at 850 °C, which was the largest among the examined HF concentrations. It was also seen that the sensor showed a good selectivity and speedy response kinetics to NO2. On the basis of the measurements of anodic and cathodic polarization curves, as well as the complex impedance of the device, the sensing mechanism was confirmed to involve a mixed potential at the oxide sensing electrode.

Large-scale novel core–shell structural SnO2/ZnSnO3 microspheres were successfully synthesized by a simple hydrothermal method with the help of the surfactant poly(vinyl pyrrolidone) PVP. The as-synthesized samples were characterized using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). The results indicate that the shell was formed by single crystalline ZnSnO3 nanorods and the core was formed by aggregated SnO2 nanoparticles. The effects of PVP and hydrothermal time on the morphology of SnO2/ZnSnO3 were investigated. A possible formation mechanism of these hierarchical structures was discussed. Moreover, the sensor performance of the prepared core–shell SnO2/ZnSnO3 nanostructures to ethanol was studied. The results indicate that the as-synthesized samples exhibited high response and quick response-recovery to ethanol.

In this article, the NO2 sensing performance of the mixed-potential-type yttria stabilized zirconia (YSZ)-based sensor was improved by modifying the three-phase boundary (TPB). The double-tape casting and the pore-forming method were simultaneously used to obtain the porous YSZ-substrate. Furthermore, a group of tape-casting slurries with different starch concentrations (0 wt%, 5 wt%, 10 wt% and 15 wt%) were applied to form the different contact areas of the three-phase boundary (TPB). The scanning electron microscope (SEM) images showed that YSZ-substrate prepared by using the slurry with 15 wt% starch had the largest contact area. The sensor based on the YSZ-substrate with the largest contact areas and the MnCr2O4 sensing electrode (SE) showed the largest response compared with the other sensors when they were exposed to 10–500 ppm NO2 at 800 °C. The measured polarization (I–V) curves indicated that the present sensors operated under the mixed-potential mechanism. In addition, the sensor showed a good selectivity and repeatability to NO2.

•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.

NiO/ZnO composites were synthesized by decorating numerous NiO nanoparticles on the surfaces of well dispersed ZnO hollow spheres using a facile solvothermal method. Various kinds of characterization methods were utilized to investigate the structures and morphologies of the hybrid materials. The results revealed that the NiO nanoparticles with a size of ∼10 nm were successfully distributed on the surfaces of ZnO hollow spheres in a discrete manner. As expected, the NiO/ZnO composites demonstrated dramatic improvements in sensing performances compared with pure ZnO hollow spheres. For example, the response of NiO/ZnO composites to 100 ppm acetone was ∼29.8, which was nearly 4.6 times higher than that of primary ZnO at 275 °C, and the response/recovery time were 1/20 s, respectively. Meanwhile, the detection limit could extend down to ppb level. The likely reason for the improved gas sensing properties was also proposed.

Dispersed porous ZnO/ZnCo2O4 hollow spheres were successfully prepared by annealing the precursor, which was obtained via a facile one-step solvothermal method without any templates or surfactants. The X-ray powder diffraction (XRD) measurement showed that the crystal phase of the sample was a mixture of ZnO and ZnCo2O4. The field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images revealed that the as-synthesized porous ZnO/ZnCo2O4 hollow spheres had an average diameter of about 850 nm and were constructed from a large number of primary nanoparticles. To demonstrate the potential applications of such porous ZnO/ZnCo2O4 composites, the as-prepared products were used to fabricate a gas sensor that was then investigated for gas-sensing performances. Results of the test showed that this sensor had fast response kinetics to acetone at the operating temperature of 275 °C, and a high response to 100 ppm acetone, one that was about 4 times higher than that of sensors based on ZnO/ZnCo2O4 nanoparticles. The remarkable enhancement in the gas-sensing properties of the porous ZnO/ZnCo2O4 hollow spheres was attributed to their unique structure.

Double-shell SnO2/α-Fe2O3 hollow composites were synthesized by a low-cost and environmentally friendly hydrothermal strategy. Various techniques were employed for the characterization of the structure and morphology of hybrid nanostructures. The results revealed that the α-Fe2O3 nanorods grew epitaxially on the surface of hollow SnO2 spheres, which were composed of primary nano-sized particles. The diameter of the α-Fe2O3 nanorods was about 10 nm, and the thickness of the SnO2 spherical shell was about 100 nm. In order to explore the formation mechanism of the composites, the structure features of the double-shell structural SnO2/α-Fe2O3 hollow composites at different reaction stages were investigated. The ethanol sensing properties of the pure SnO2 and SnO2/α-Fe2O3 composites were tested. It was found that such double-shell composites exhibited enhanced ethanol sensing properties compared with the single-component SnO2 hollow spheres. For example, at an ethanol concentration of 100 ppm, the response of the SnO2/α-Fe2O3 composites was about 16, which was about 2 times higher than that of the primary SnO2 nanostructures. The response time of the sensor to 10 ppm ethanol was about 1 s at the operating temperature of 250 °C.