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A highly sensitive xylene sensor based on Cr-doped NiO nanotubes was presented, which was prepared using an electrospinning process. The responses of the sensors were investigated using a static system. We found that the sensors based on the Cr-doped NiO nanotubes exhibited a high response towards 50 ppm xylene, with a response of about 88, which was about 63 times higher than that of pure NiO nanotubes. The results obtained for the sensing properties indicate that the gas sensors based on Cr-doped NiO nanotubes are promising for monitoring air-quality and environmental pollution.

In this work, a low-cost and environmentally friendly solvothermal route to the synthesis of indium oxide nanorod clusters was described in the presence of sodium chlorate and urea. The morphologies and structures of the as-prepared 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 clearly revealed that the as-prepared indium oxide was composed of nanorods with a diameter of about 15 nm. The gas sensing properties of the as-prepared indium oxide samples were tested towards various gases. The indium oxide nanorod cluster based sensor showed high response and good selectivity toward NO2 at an operating temperature of 150 °C, giving a response of about 41 to 500 ppb.

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 mixed-potential type stabilized zirconia (YSZ)-based gas sensor using columbite type composite oxide sensing electrode was developed and fabricated, aiming at sensitive detection of ethanol. Among the different oxide sensing electrodes (SEs) developed, the sensor attached with ZnNb2O6-SE was found to achieve the largest sensitivity to ethanol at 625 °C. Furthermore, the result of the effect of sintering temperature on sensing characteristic showed that the sensor utilizing ZnNb2O6-SE sintered at 1000 °C displayed the highest response of −175 mV to 100 ppm ethanol and a low detection limit of 0.5 ppm at 625 °C. ΔV of the present sensor exhibited a segmentally linear relationship to the logarithm of ethanol concentration in the ranges of 0.5–5 ppm and 5–200 ppm, for which the sensitivities were −29 and −112 mV decade−1, respectively. Moreover, the fabricated device also displayed fast response and recovery times, good repeatability, small fluctuation during 30 days continuous high temperature of 625 °C measured periods, and acceptable selectivity to some other interfering gases. Additionally, the sensing mechanism involving mixed potential was further demonstrated by polarization curves.

Pd-doped SnO2 hollow spheres were synthesized via a facile one-step hydrothermal route. Utilized as the materials in sensors, the 3.0 wt% Pd-doped SnO2 demonstrated more excellent gas-sensing properties towards CO than 1.5 wt% and 4.5 wt% Pd-doped SnO2. Compared with the SnO2 hollow spheres gas sensor, the optimum operating temperature of the Pd-doped SnO2 hollow spheres gas sensor dropped to 200 °C from 250 °C; the response value to 100 ppm CO was raised to 14.7 from 2.5 accordingly. Furthermore, the response and recovery times of the 3.0 wt% Pd-doped SnO2 sensor are 5 s and 92 s, respectively, to 100 ppm CO at 200 °C. It is believed that its enhanced gas-sensing performances are derived from the synergistic interactions between the dispersed Pd and the characteristic configuration of the SnO2 hollow sphere. In addition, theoretical calculations have also been performed with periodic slab models by using density functional theory, which explain well our experimental phenomenon.

The present study aimed to develop a high electron lifetime (τr) and low transit time (τd) photoanode material for dye sensitized solar cells (DSSCs). An innovative material structure, a TiO2/ZnO composite oxide hierarchical nanosphere, was synthesized by a two-step facile hydrothermal method. This composite oxide comprised 3D urchin-like TiO2 nanospheres and 1D ZnO nanospindles (ULTZ). The ZnO nanospindles were assembled onto the surface of the 3D urchin-like TiO2 in the hydrothermal process second step. A series of ULTZs made with different growth times were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The DSSCs based on the ULTZs exhibited much higher photoelectric properties and the energy conversion efficiency was 8.78%, which indicated a 30% increase in the conversion efficiency compared to those of the P25 electrode (6.79%); the great improvements of photoelectric properties and energy conversion efficiency for ULTZs based DSSCs were mainly attributed to the superior electronic transmission characteristics.

In this work, we reported a novel toluene sensor based on α-Fe2O3/SnO2 heterostructure nanowires arrays, which were synthesized via combining an ultrasonic spray pyrolysis (for the SnO2 nanowires arrays) method and the subsequent hydrothermal strategy (for the α-Fe2O3 nanorod branches). Various techniques were employed for the characterization of the structure and morphology of the as-obtained products. The results revealed that α-Fe2O3 nanorod branches grew on SnO2 nanowire arrays with an average length of about 800 nm. As a proof-of-concept demonstration of the function, such novel heterostructure nanowires arrays were used as the sensing material for gas sensors. As expected, the heterostructure composites exhibited good sensing performances for toluene, superior to a single component (SnO2 nanowires arrays). For example, the response of the α-Fe2O3/SnO2 composites was up to 5 times higher than that of the primary SnO2 nanowire arrays at 90 °C.

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.

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.

As an ethanol sensing material, the composites of In2O3–SnO2 were composed of In2O3 microflowers and SnO2 nanoparticles. Both In2O3 microflowers and SnO2 nanoparticles were synthesized by hydrothermal method and then mixed in an ultrasonic environment. The morphology and phase composition of the as-synthesized samples were characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The results on gas sensing properties showed that when the mass ratio of In2O3 and SnO2 was 2 : 1, the sensor based on the as-prepared In2O3–SnO2 composite exhibited high response and good selectivity to ethanol at 250 °C. The response to 100 ppm ethanol gas was 53.2. UV illumination stabilized the responses of the sensors while the relative humidity increased. The gas sensing mechanism proposed was that the addition of SnO2 to In2O3 enhanced the catalytic activity for the ethanol reaction, which changed the electrical resistance of the materials. Besides, the morphology was helpful to the gas reaction on the surface of the sensing materials.

Hollow In2O3 microspheres constructed by primary nanoparticles were successfully prepared by thermal treatment of the precursor, which was synthesized via a facile chemical solution route without any templates or surfactants. The images of field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) indicated that the sample was composed of a large number of hollow In2O3 microspheres with diameters of 1–2 μm. In addition, a gas sensor based on the In2O3 hollow microspheres was fabricated and its gas sensing performances were investigated. It was found that sensors based on the as-prepared sample had a low operating temperature (80 °C), and exhibited high response, low detection limit and excellent selectivity to NO2.

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.

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.

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.

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.

In this work, a simple route for the synthesis of irregular In2O3 nanoplates in the presence of oleic acid and urea is described. The structure and morphology of the as-obtained product 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 In2O3 nanostructures are composed of irregular nanoplates. The gas sensing properties of the as-obtained product were investigated. The sensor based on the In2O3 irregular nanoplates exhibits a remarkably enhanced response and a fast response/recovery time towards NO2.

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.

Zinc–tin composite crystallites (cubes and octahedra) synthesized by conventional methods in earlier literature, were proved to have ZnSn(OH)6 structures rather than perovskite structure ZnSnO3 as reported. Furthermore, according to the tolerance factor t introduced by Goldschmidt, ZnSnO3 can not exist as a perovskite structure.

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.