Biomorphic In2O3–ZnO hollow microtubules were synthesized by using cotton as template and followed by calcination. This environmental-friendly method using biomaterials as templates can be used to synthesize metal oxide semiconductor materials with specific morphologies. The structural and microscopy characterization were carried out with X-ray diffraction, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy. The SEM images display the porous structure of the as-obtained hollow microtubules, which is beneficial to increase gas sensing properties of materials. Gas sensors based on In2O3-doped ZnO hollow microtubules were investigated and the test results demonstrate the excellent acetone sensing properties of such porous materials. The response of In2O3–ZnO hollow microtubule sensors to 100 ppm acetone at the optimum temperature of 300 °C is 62.5, meanwhile the response and recovery times are 3 and 7 s, respectively. Moreover, even at 0.1 ppm of acetone detectable response can be observed, and the value is 2.82. At the same time the sensors have selectivity to acetone.

Flower-like porous ZnO was successfully synthesized by a simple hydrothermal method followed by calcination. The morphologies of the as-prepared materials were characterized by scanning electron microscopy (SEM) and the crystal structures were determined by X-ray diffraction. It can be seen in SEM images that each flower-like ZnO unit is composed of randomly arranged ZnO thin flakes which makes the materials extremely porous. Meanwhile, there are numerous through-holes distributed on the surface of ZnO flakes. The gas-sensing properties of the as-prepared materials were investigated, and the results indicate the ultrahigh sensing properties of flower-like porous ZnO to acetone. The response of flower-like porous ZnO sensors to 50 ppm acetone is about 97.8 at the optimum operating temperature of 280 °C. The response and recovery times to 50 ppm acetone are about 2 and 23 s, respectively. Moreover, even at low concentrations of 0.25, 1 and 10 ppm acetone high responses can be observed with the values of 6.7, 15.8 and 30.1. In addition, the as-synthesized flower-like ZnO shows excellent selectivity to acetone and the response to 50 ppm acetone (97.8) is about 4.43 times larger than ethanol (22.1) at the same concentration, which can successfully distinguish acetone and ethanol.

The porous ZnO microflowers, which looks like a flower, has been successfully prepared by a simple hydrothermal method. The structure and morphology of the samples were characterized by X-ray diffraction and Scanning electron microscopy. Meanwhile, the as-prepared samples were applied to fabricate gas sensor device, the gas sensing properties of the sensors based on porous ZnO microflowers were studied. The results indicate the ultrahigh sensitivity and excellent selectivity of Porous ZnO microflowers sensors to ethanol. The response of Porous ZnO microflowers sensor to 100 ppm ethanol is about 123 at 260 °C, which is 4.1 times larger than that of acetone (the response value is 30). The ZnO sensors can successfully distinguish acetone and ethanol which possess similar properties. The results demonstrate that the ZnO sensors have an excellent selectivity to ethanol. The response and recovery time are 4 and 12 s to 50 ppm ethanol, respectively. Moreover, the concentration of ethanol that we can detect is 0.2 ppm, and the response value is 1.65. Thus this work is confirmed that the porous ZnO microflowers sensors have a fantastic gas sensitive property for ethanol.

The sea urchin-like SnO2/Fe2O3 and pure Fe2O3 microspheres were fabricated by a simple hydrothermal method. The SEM images show that SnO2/Fe2O3 products are sea urchin-like microspheres which consisted of numerous nanorods. These outward extension nanorods greatly increase the effective contact area between the materials and gas molecules. The results of gas sensing tests indicate the superiority of this material and the enhancement of sensitivity after doping with SnO2. The SnO2/Fe2O3 sensor response to 100 ppm ethanol at the optimum working temperature of 260 °C is 41.7, which is about four times higher than that of pure Fe2O3 sensor. Meanwhile the sensors possess ultrafast response and recovery, which are 3 and 4 s respectively.

•Pure and Nd-doped In2O3 porous nanotubes have been fabricated.•Such porous nanotubes have been analyzed by SEM, XRD and EDX.•Gas sensors based on the as-synthesized materials have been prepared and tested for formaldehyde sensing properties.Pure and Nd-doped porous In2O3 nanotubes have been successfully synthesized by single-capillary electrospinning method. The SEM images displays the novel structure of Nd-doped In2O3 which has pores distributed on the surface of nanotubes. The subsequent test results demonstrate that Nd-doped porous In2O3 nanotubes possess excellent gas-sensing properties to formaldehyde. The response of Nd-doped porous In2O3 nanotubes to 100 ppm formaldehyde is 44.6 at the optimum operating temperature of 240 °C, which is 3.6 times larger than that of pure porous In2O3 nanotubes (12.5), and the response and recovery times to 100 ppm formaldehyde are 15 and 50 s, respectively.(A) SEM images of (a, b) pure and (c, d) Nd-doped porous In2O3 nanotubes. (B) Responses curves of pure and Nd-doped porous In2O3 nanotube sensors to 100 ppm of formaldehyde at different operating temperatures.

Hollow and porous α-Fe2O3 nanotubes were successfully synthesized by single nozzle electrospinning method followed by annealing treatment. The crystal structures and morphologies of the as-prepared materials were characterized by X-ray diffraction and scanning electron microscopy, respectively. The as-prepared materials were applied to construct gas sensor devices which gas sensing properties were further investigated. The obtained results revealed that porous α-Fe2O3 nanotube gas sensors exhibit a markedly enhanced gas sensing performance compared with hollow α-Fe2O3 nanotube gas sensors, which was about three times higher to 100 ppm acetone at 240 °C. Interestingly, hollow and porous α-Fe2O3 nanotube gas sensors both showed fast response–recovery time and good selectivity, but the porous ones possessed the shorter recovery time. The improved properties could be attributed to the unique morphology of porous nanotubes. Thus, further improvement of performance in metal-oxide-semiconductors materials could be realized by preparation the unique porous structures of nanotubes. Moreover, it is expected that porous metal-oxide-semiconductors nanotubes could be further design as promising candidates for gas sensing materials.

In this work, pure and Al2O3-doped α-Fe2O3 nanotubes were synthesized by a simple single-capillary electrospinning technology and followed calcination treatment. The morphologies and crystal structures of the as-prepared samples were characterized by scanning electron microscopy and X-ray diffraction, respectively. The research of the samples gas sensing properties demonstrates that the ethanol sensing properties of α-Fe2O3 nanotubes are enhanced by doping Al2O3, remarkably. The response value of Al2O3-doped α-Fe2O3 nanotubes to 50 ppm ethanol is 41.8 at the operating temperature 240 °C, which is much higher than that of pure α-Fe2O3 nanotubes (response value is 4). And the lowest detection limit is 300 ppb, to which the response value is about 2. The response and recovery times are about 20 and 60 s–50 ppm ethanol, respectively. In addition, the Al2O3-doped α-Fe2O3 nanotubes possess good selectivity and long-term stability.

Pure and Er-doped In2O3 nanotubes were systematically fabricated by using a single nozzle eletrospinning method followed by calcination. The as-synthesized nanotubes were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectrometry and X-ray powder diffraction (XRD). Compared with pure In2O3 nanotubes, Er-doped In2O3 nanotubes exhibit improved formaldehyde sensing properties at 260 °C. The response of Er-doped In2O3 nanotubes to 20 ppm formaldehyde is about 12, which is 4 times larger than that of pure In2O3 nanotubes. The response and recovery times of Er-doped In2O3 nanotubes to 20 ppm formaldehyde are about 5 and 38 s, respectively. Furthermore, the response of Er-doped In2O3 nanotubes to 100 ppb formaldehyde is 2.19.

•Porous Nd-doped α-Fe2O3 nanotubes (a novel nanostructure) are synthesized.•Materials show ultrahigh sensitivity and good selectivity to acetone.•The lowest detection limit is 500 ppb.Pure and Nd-doped porous α-Fe2O3 nanotubes were successfully synthesized via a facile single-capillary electrospinning method, followed by calcination treatment. The morphologies of the as-prepared samples were characterized by scanning electron microscopy. The crystal structures and components were determined by X-ray diffraction and energy-dispersive X-ray spectroscopy, respectively. The gas-sensing properties of the as-prepared samples were studied, and the results indicate the ultrahigh sensitivity of Nd-doped porous α-Fe2O3 nanotubes to acetone. The response of Nd-doped porous α-Fe2O3 nanotubes to 50 ppm of acetone is about 44 at an operating temperature of 240 °C, which is 17 times larger than that of pure porous α-Fe2O3 nanotubes. Furthermore, the lowest detection limit of acetone is 500 ppb, with a response of 2.4. The response and recovery times to 50 ppm of acetone are about 19 and 50 s, respectively. In addition, the Nd-doped porous α-Fe2O3 nanotubes possess good selectivity and long-term stability.

Pristine In2O3 and ZnO–In2O3 composited nanotubes were synthesized by the electrospinning technic and followed by calcination. The formation of ZnO–In2O3 n–n heterojunctions was found to be highly sensitive to acetone gas. X-ray powder diffraction (XRD), Scanning electron microscope (SEM) and energy dispersive X-ray spectrometry (EDS) were respectively used to characterize surface crystallinity and morphology. The possible mechanism of nanotubes formation was also investigated. Gas sensors were made based on the as-synthesized materials to investigate their gas sensing properties. Compare with the pristine In2O3 nanotubes, ZnO–In2O3 composite nanotubes show the obviously improved of acetone sensitivity. The sensitivity of ZnO–In2O3 composite nanotubes is about 43.2, which is about triple times larger than that of the pristine In2O3 nanotubes to 60 ppm acetone at 280 °C. The response and recovery times of ZnO–In2O3 nanotubes to 60 ppm acetone are about 5 and 25 s. In addition, ZnO–In2O3 composite nanotubes sensor also possesses a good selectivity to acetone.

Pure In2O3 and mixed Fe2O3–In2O3 nanotubes were prepared by simple electrospinning and subsequent calcination. The as-prepared nanotubes were characterized by scanning electron microscopy, powder X-ray diffraction, and energy-dispersive X-ray spectrometry. Gas sensors were fabricated to investigate the gas-sensing properties of In2O3 and Fe2O3–In2O3 nanotubes. Compared to pure In2O3, Fe2O3–In2O3 nanotubes exhibited better gas-sensing properties for formaldehyde at 250 °C. The response of the Fe2O3–In2O3 nanotube gas sensor to 100 ppm formaldehyde was approximately 33, which is approximately double the response of the pure In2O3 nanotube gas sensor. In both cases the response time was ~5 s and the recovery time was ~25 s.

Multiwalled carbon nanotubes (MWNTs) were synthesized through CVD method, and the gas sensitive materials MWNTs/ZnO were obtained by mixing MWNTs and ZnO. The gas sensing properties of the as-prepared materials were investigated. The results show that the gas sensing properties of ZnO sensor are significantly improved by doping MWNTs. The sensitivity, response time and recovery time to 50 ppm ethanol at 260 °C are 46, 4 and 20 s, respectively. We also examined the selectivity of 0.1 wt% MWNTs-doped ZnO sensors to different gases. The results show that the sensor possesses an excellent selectivity to ethanol.

SnO2–Fe2O3 interconnected nanotubes were obtained by combining the single nozzle electrospinning and thermal treatment methods. The results of scanning electron microscopy revealed the special structure of ruptures and interconnected nanotubes in the as-prepared materials. The toluene sensing test results of SnO2–Fe2O3 interconnected nanotubes show that SnO2–Fe2O3 interconnected nanotubes possess excellent toluene gas-sensing properties. The sensitivity of detecting limit (50 ppb) is 2.0 at the optimum operating temperature of 260 °C. The response and recovery times to 1 ppm toluene are about 5 and 11 s, respectively. Moreover, the SnO2–Fe2O3 interconnected nanotube gas sensors exhibit the remarkable selectivity to toluene, and good long-term stability.Keywords: electrospinning; Fe2O3; gas sensors; nanotubes; SnO2; toluene;

Pure and La-doped (5 wt%, 7 wt% and 10 wt%) α-Fe2O3 nanotubes are synthesized by an electrospinning method and followed by calcination. The as-synthesized nanotubes are characterized by scanning electron microscope (SEM) and X-ray powder diffraction (XRD). Compared with pure α-Fe2O3 nanotubes, La-doped α-Fe2O3 nanotubes exhibit improved acetone sensing properties at 240 °C. The response of 7 wt% La-doped α-Fe2O3 nanotubes to 50 ppm acetone is about 26, which is 10 times larger than that of pure α-Fe2O3 nanotubes. The response and recovery times of 7 wt% La-doped α-Fe2O3 nanotubes to 50 ppm acetone are about 3 and 10 s, respectively. Moreover, 7 wt% La-doped α-Fe2O3 nanotubes show a good selectivity to acetone.

α-Fe2O3 nanotubes was successfully prepared by single nozzle electrospinning method. Scanning electron microscope (SEM) was used to characterize the morphology of α-Fe2O3 nanotubes. The gas sensing properties of α-Fe2O3 nanotubes were investigated in detail. The results exhibit relatively good sensing properties to acetone at 240 °C. The response and recovery times are about 3 and 5 s, respectively. The structure of nanotubes is beneficial to the gas sensing properties, which will enlarge the surface-to-volume ratio of α-Fe2O3 and then be available for the transfer of gas, and thus improved the sensor performance consequentially.

SnO2/multiwalled carbon nanotubes (MWNTs) have been studied as gas sensing materials. Compared with pure SnO2, SnO2/MWNTs exhibit improved ethanol sensing properties such as higher sensitivity and quicker response/recovery at 300°C. The sensitivity is 35, 65, 166 and 243 to 500, 1000, 2000 and 3000 ppm ethanol, respectively. The response time is about 1 s, and the recovery time is about 5 s. The sensing improvement is explained in terms of the appropriate basal resistance and enhanced signal transfer brought by MWNTs.

Pure and Pd-doped In2O3 nanofibers are synthesized via a simple electrospinning method and characterized by scanning electron microscopy and X-ray diffraction. Comparing with pure In2O3 nanofibers, Pd-doped In2O3 nanofibers exhibit much higher sensitivity to ethanol at 200 °C. The sensor fabricated from Pd-doped In2O3 nanofibers can detect ethanol down to 1 ppm (the corresponding sensitivity is 4) with good selectivity, and the response and recovery times are 1 and 10 s, respectively. The sensing mechanism and the effect of Pd doping are discussed. The results indicate that the Pd-doped In2O3 nanofibers can be used to fabricate high performance ethanol sensors.

Pure and Cu-doped SnO2 nanofibers are synthesized via a simple electrospinning method, and characterized by transmission electron microscopy and X-ray diffraction. The sensor fabricated from Cu-doped SnO2 nanofibers exhibits improved sensing properties to ethanol at 300 °C. The sensitivity is up to 3 when this sensor is exposed to 5 ppm ethanol. The response and recovery times are about 1 and 10 s, respectively. The linear dependence of the sensitivity on the ethanol concentration is observed in the range of 5–500 ppm. Good selectivity is also observed in our studies. The results make Cu-doped SnO2 nanofibers good candidates for fabricating high performance ethanol sensors.

ZnO microcrystals are synthesized through a facile solution method and characterized by field-emission scanning electron microscopy, transmission electron microscopy, selected area electron diffraction and X-ray diffraction. The ethanol sensing properties of these microcrystals are investigated by spin-coating them on a silicon substrate with Pt electrodes to fabricate a micro-structure sensor. The sensitivity is up to 8 when the sensor is exposed to 50 ppm ethanol, and the response time and recovery time are 10 s and 20 s, respectively. A contact-controlled model is established to explain the sensing properties of the microcrystals, which provides another approach to realize high-performance gas sensors.