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Fibre-in-tube TiO2–SnO2 fibres (FIT-TSF) have been successfully fabricated via a general crystalline-phase-induced formation strategy. The as-prepared FIT-TSF exhibited excellent sensing properties with an impedance change of three orders of magnitude, an ultrafast response time of 5 s, a recovery time of 8 s, and good reproducibility.

The design of one-dimensional titania with tunable structural hierarchy holds exciting implications for applications such as optoelectronics, sensing and catalysis. Here, we report a general precursor strategy for one-dimensional titania with surface nanoprotrusion and tunable hierarchical structures realized via a titanate route using electrospun titania fibers as precursors. One-dimensional hierarchical hollow titanate (1D-HHT) was first fabricated under mild alkaline hydrothermal conditions from electrospun amorphous TiO2 precursors. The time-dependent evolution of 1D-HHT has been investigated in detail. The concerted effect of alkaline etching and Ostwald ripening is responsible for the surface nanoprotrusion and modulation of structural hierarchy. Anatase TiO2 with the same hierarchical structures can be obtained using controlled calcination. The as-fabricated one-dimensional titania with varying levels of structural hierarchy exhibits about 1.3–1.5 times higher power conversion efficiency than commercial P25 as a photoanode material for dye-sensitized solar cells (DSSCs). The current work may be extended for more functional TiO2-based materials with tunable structural hierarchy and could widen the application range of electrospinning technology for one-dimensional hierarchical structures.

ZnO nanorod arrays decorated with NiO nanosheets on FTO substrates were prepared via a simple ultrasonic spray pyrolysis process combined with chemical bath method. The synthesized samples were characterized and analyzed by scanning electron microscopy and X-ray diffraction. The hierarchical and porous morphologies of ZnO/NiO core–shell nanoheterojunctions could be controlled by changing the growth time of NiO sheets. The “oriented attachment” and “self-assembly” crystal growth mechanisms were proposed to explain the formation of the ZnO/NiO nanostructures. Sensors based on the ZnO/NiO heterojunction nanostructure were fabricated and investigated for their ethanol-sensing properties. The result indicated that response was about 180% toward 100 ppm ethanol at an operating temperature of 200 °C. The growth approach in this work offers a new technique for the design and synthesis of transition metal oxide hierarchical nanoarrays which are promising for gas sensing applications.

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.

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.

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.

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.

We report a facile template-free microwave-assisted approach to synthesize ZnO hollow microspheres. Field emission scanning electron microscopic and transmission electron microscopic results revealed that the average dimensions of these hollow microspheres were about 4 μm in diameter and 400 nm in thickness of spherical shell. Interestingly, the “hollowness” of these ZnO spheres could be adjusted by varying the microwave irradiation time. Moreover, various distinct ZnO structures could be obtained by simply changing the amounts of trisodium citrate dihydrate. On the basis of these results, a possible formation mechanism was proposed. Gas sensors were fabricated based on as-prepared hollow spheres, and their gas sensing properties were tested for response to ethanol and methanol.

Hollow α-Fe2O3 urchin-like spheres were prepared by annealing the FeOOH precursor, which was synthesized via a template-free hydrothermal method. Interestingly, the size and hollowness of spheres could be tailored by adjusting the concentration of ferric sulphate. The sensing properties of the as-prepared samples were investigated.

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.

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.