Fengmin Liu

Design of Au@ZnO yolk-shell nanospheres with enhanced gas sensing properties.

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.

Nanosheet-assembled ZnFe2O4 hollow microspheres for high-sensitive acetone sensor.

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.

Double-Shell Architectures of ZnFe2O4 Nanosheets on ZnO Hollow Spheres for High-Performance Gas Sensors

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.

Highly Enhanced Sensing Properties for ZnO Nanoparticle-Decorated Round-Edged α-Fe2O3 Hexahedrons

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.

Gas sensing with hollow α-Fe2O3 urchin-like spheres prepared via template-free hydrothermal synthesis

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.

Facile synthesis and gas-sensing properties of monodisperse α-Fe2O3 discoid crystals

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.

Growth of SnO2 nanowire arrays by ultrasonic spray pyrolysis and their gas sensing performance

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.

Hierarchical Assembly of α-Fe2O3 Nanorods on Multiwall Carbon Nanotubes as a High-Performance Sensing Material for Gas Sensors

This paper presents a facile hydrolysis reaction and annealing for preparing a novel hierarchical nanoheterostructure via assembly of α-Fe2O3 nanorods onto multiwall carbon nanotubes (MWCNTs) backbones. The as-synthesized nanocomposites were characterized using XRD (X-ray diffraction), FESEM (Field emission scanning electron microscopy), TEM (Transmission electron microscopy), XPS (X-ray photoelectron spectroscopy) and BET (Surface Area and Porosity System). The observations showed uniform α-Fe2O3 nanorods approximately 100-200 nm in length and 50-100 nm in diameter that were hierarchically assembled onto the surface of the MWCNTs. The formation of the heterostructure was investigated by observing the evolution of the microstructure of the products at different reaction times. The X-ray photoelectron spectra (XPS) showed that the ability of the absorbing oxygen was enhanced by the formation of the heterostructure composites. Moreover, as a proof-of-concept presentation, the novel CNTs@α-Fe2O3 hierarchical heterostructure acted as a gas sensitive material. Significantly, the composites exhibited excellent sensing properties for acetone with high sensitivity, exceptional selectivity and good reproducibility. The response of the CNTs@α-Fe2O3 sensor to 100 ppm acetones at 225 °C was nearly 35, which was superior to the single α-Fe2O3 nanorods with a response of 16, and the detection limit of the sensor was 500 ppb. The enhanced properties were mainly attributed to the unique structure and p-n heterojunction between the CNTs and the α-Fe2O3 nanorods.

Fabrication of Well-Ordered Three-Phase Boundary with Nanostructure Pore Array for Mixed Potential-Type Zirconia-Based NO2 Sensor

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 (Zr(4+)/Y(3+) = 23 mol/L/4 mol/L) and treatment conditions. An ordered hemispherical array was obtained when CZr(4+) = 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.

Bilayered photoanode from rutile TiO2 nanorods and hierarchical anatase TiO2 hollow spheres: a candidate for enhanced efficiency dye sensitized solar cells

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.