Shouli Bai

Low-temperature hydrothermal synthesis of WO3 nanorods and their sensing properties for NO2

Tungsten trioxide (WO3) nanorods with an aspect ratio of ∼50 have been successfully synthesized by hydrothermal reaction at a low temperature of 100 °C. The crystal structure, morphology evolution and thermal stability of the products are characterized in detail by XRD, FESEM, FTIR, and TG/DTA techniques. The diameter evolution and distribution of WO3 nanorods strongly depend on hydrothermal temperature and time. Hydrothermal conditions of 100 °C and 24 h ensure the formation of well-defined WO3 nanorods. The transition of the crystal structure from monoclinic WO3 to hexagonal WO3 occurs after calcination at 400 °C. The appropriate calcination conditions of the WO3 nanorods are defined to be 600 °C and 4 h for gas-sensing applications. Response measurements reveal that the WO3 sensor operating at 200 °C exhibits high sensitivity to ppm-level NO2 and small cross-sensing to CO and CH4, which makes this kind of sensor a competitive candidate for NO2-sensing applications. Moreover, impedance measurements indicate that a conductivity mechanism of the sensor is mainly dependent on the grain boundaries of WO3 nanorods. A possible adsorption and reaction model is proposed to illustrate the gas-sensing mechanism.

Quantum-sized ZnO nanoparticles: Synthesis, characterization and sensing properties for NO2

Quantum-sized ZnO nanoparticles have been synthesized at room temperature by a mild sol–gel process using tetraethylorthosilicate (TEOS) as the capping agent to control the particle growth of ZnO. The crystal structure, particle size and optical properties have been investigated by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), photoluminescence (PL) spectra and Raman spectra, respectively. The results show that the ZnO nanoparticles exhibit hexagonal wurtzite structure and the average crystallite size is 5.7 nm which is a little less than TEM results. It has been testified by room-temperature PL spectra that the TEOS capped the surface of ZnO nanoparticles and obviously reduced grain size, as an emission at 520 nm almost disappeared and a new peak with an anomalous blue shift as great as 9 nm, appeared for the TEOS capped ZnO. The sensing tests indicate that the ZnO based sensors not only show a high response to NO2 but also exhibit high selectivity over CO and CH4 at a low operating temperature of 290 °C. The response increases with NO2 concentration and decreases with calcination temperature, and is in agreement with Raman and XRD results.

Synthesis mechanism and gas-sensing application of nanosheet-assembled tungsten oxide microspheres

Nanosheet-assembled tungsten oxide microspheres have been synthesized using rapid sonochemistry followed by thermal treatment. Transient observation of controllable synthesis reveals that the morphological evolution of the product is highly dependent on the ultrasonication time. An assembly mechanism based on oriented attachment and reconstruction is proposed for the sonochemical formation of the nanosheet-assembled microspheres. The obtained samples possess intrinsic non-stoichiometry and a hierarchically porous nano/microstructure, which is beneficial for their utilization in sensing materials and for fast diffusion of gas molecules. The maximum response of the tungsten oxide hierarchical microspheres is 3 times higher than that of commercial nanoparticles for NO2 gas. The gas adsorption–desorption kinetics during the sensing process were mathematically simulated by a derivative method. The first-principles calculation reveals that the NO2 molecule is most likely adsorbed at the terminal O1c site of tungsten oxide, leading to the introduction of new surface states, which are responsible for the intrinsic NO2-sensing properties.

Mechanism enhancing gas sensing and first-principle calculations of Al-doped ZnO nanostructures

Al-doped flower-like ZnO nanostructures have been synthesized by a facile hydrothermal method at 95 °C for 7 h. The structure and morphology of the product were characterized by XRD, FTIR and SEM analysis. The sensing tests reveal that the response is significantly enhanced by Al doping, and the 0.3 wt% Al-doped sample exhibits the highest response of 464 to 10 ppm CO at an operating temperature of 155 °C. A change of the structural defects in Al-doped ZnO is responsible for the enhancement of the sensing properties, which has been confirmed by the room temperature photoluminescence (PL) spectra and X-ray photoelectron spectroscopy (XPS). The response time is reduced disproportionately with the increase in CO concentration by modeling the transient responses of the sensor using the Langmuir–Hinshelwood reaction mechanism. The band structures and density of states for pure ZnO and Al-doped supercells have been calculated using first principles based on density functional theory (DFT). The calculated results show that the band gap is narrowed and the conductance is increased by Al doping, which coincides with the experimental results of gas sensing.

Surface decoration of WO3 architectures with Fe2O3 nanoparticles for visible-light-driven photocatalysis

Zero-dimensional Fe2O3 nanoparticles were successfully decorated on three-dimensional WO3 architectures to constitute a photocatalyst of Fe2O3@WO3 heterojunction. The obtained samples were characterized in detail by X-ray diffraction, scanning electron microscopy, elemental mapping, X-ray photoelectron spectroscopy and UV-Vis absorption spectra. The results indicate that rhombohedral α-Fe2O3 nanoparticles are homogeneously decorated on the surface of monoclinic WO3 architectures, and the constituted n+–n heterojunction results in “redshift” of the optical absorption. The photocatalyst of 1%Fe2O3@WO3 annealed at 400 °C exhibits the highest photocatalytic activity for degradation of Rhodamine B under visible light irradiation. The degradation obeys first-order reaction kinetics with an apparent rate constant of 0.057 min−1. It is suggested that the potential-energy difference between Fe2O3 and WO3 accelerates the separation of photogenerated electron–hole pairs, dominating the enhanced photocatalytic activity. The results presented herein provide new insight for development of a novel visible-light-driven photocatalyst and its potential application in harmful pollutant degradation.

Gas sensing properties of Cd-doped ZnO nanofibers synthesized by the electrospinning method

We used electrospinning followed by thermal treatment to successfully synthesize Cd-doped ZnO nanofibers using different doping concentrations. The research interest is aimed at achieving a significant enhancement of the sensing performance toward CO, which was achieved due to the doping changing the state of the native defect of the ZnO, and which has been confirmed by PL and XPS spectra. The competitive influence of the specific surface and the crystallinity of the ZnO on sensing response is discussed in detail. The sensing mechanism of Cd-doped ZnO sensors for the detection of CO is also discussed. In addition, the band structures and densities of the states for undoped and Cd-doped ZnO were also calculated using first-principles based on a local density approximation LDA + U scheme. The calculated results show that the band gap is significantly narrowed by doping, which concurs with the results determined from UV-Vis spectra.

SnO2@Co3O4 p–n heterostructures fabricated by electrospinning and mechanism analysis enhanced acetone sensing

SnO2@Co3O4 p–n heterostructure nanotubes have been obtained for the first time via precursor nanofibres and subsequent annealing. The structure and properties of heterostructure nanotubes were characterized using various analysis methods. The responses of composites with different SnO2 content to 10 ppm acetone have been measured, and found that the composites exhibited higher sensitivity, lower operating temperature and faster response than pure Co3O4. The improvement of sensing characteristics is attributed to the formation of a p–n heterojunction between the two types of semiconductor oxides.

Carboxyl-directed hydrothermal synthesis of WO3 nanostructures and their morphology-dependent gas-sensing properties

Three different morphologies of tungsten oxides, nanoparticles, nanosheets and hierarchical microspheres, have been successfully synthesized by a facile carboxyl-directed hydrothermal process. The chelation of carboxylic groups with W(OH)6 nuclei is recognized to be the origin of the morphological change. Gas-sensing measurements reveal that the sensing performance varies with WO3 morphology, and the hierarchical WO3 not only exhibits high sensitivity and fast response but also has low operating temperature to toxic NO2. The response of hierarchical WO3 is nearly 2 times and 10 times higher than those of the nanosheets and nanoparticles, respectively. The maximum response of hierarchical WO3 reaches 319 to 10 ppm NO2 at 200 °C. A relationship between morphology and crystal defect is established based on photoluminescence analysis. It is demonstrated that the change in defect feature in crystalline WO3 is responsible for its morphology-dependent gas-sensing properties.

Mechanism of enhancing the formaldehyde sensing properties of Co3O4 via Ag modification

Hollow Co3O4 hierarchical microspheres assembled from many compact nanowires have been successfully synthesized via a facile hydrothermal method from the precursor Co(CO3)0.5(OH)·0.11H2O followed by annealing treatment. The product has a well-defined morphology, is porous, and has a larger surface area as seen through various analytical characterizations; thus, it can act as a good basis for further modification to improve its gas-sensing properties. The sensing tests indicate that the Ag@Co3O4 composite formed via Ag modification can not only improve the sensing response to formaldehyde by several times that of pure Co3O4, but also reduce the optimum operating temperature of the sensor. Furthermore, the gas-sensing mechanism is also discussed in detail, including the effect of Ag addition on the electronic transfer of the Ag@Co3O4 composite. There are still many challenges in making a formaldehyde sensor with high sensitivity using the cheaper noble metal Ag as modified reagent via a facile synthesis and doping method.

Room temperature triethylamine sensing properties of polyaniline–WO3 nanocomposites with p–n heterojunctions

Polyaniline (PANI)–tungsten oxide (WO3) nanocomposites have been successfully synthesized using different weight percentages of tungsten oxide (10–50%) dispersed in a polyaniline matrix by a facile in situ chemical oxidation polymerization. The sensors based on PANI–WO3 nanocomposites were fabricated on a substrate of polyethylene terephthalate (PET) films for detection of triethylamine (TEA) gas at room temperature. It was observed that the sensors of PANI–WO3 nanocomposites show better sensitivity, selectivity, and reproducibility compared to pure PANI, particularly the sensor based on PANI–30% WO3 operating at room temperature exhibits maximum response of 81 to 100 ppm TEA gas, that is 13 times higher than that of pure PANI. The sensing mechanism of the nanocomposites in the presence of TEA atmosphere was discussed in detail, and is attributed to the increase of percentage of doping protonic acid and the formation of p–n heterojunctions between p-type PANI and n-type WO3.