Huiming Ji

Rapid and selective detection of acetone using hierarchical ZnO gas sensor for hazardous odor markers application

Hierarchical nanostructured ZnO dandelion-like spheres were synthesized via solvothermal reaction at 200°C for 4h. The products were pure hexagonal ZnO with large exposure of (002) polar facet. Side-heating gas sensor based on hierarchical ZnO spheres was prepared to evaluate the acetone gas sensing properties. The detection limit to acetone for the ZnO sensor is 0.25ppm. The response (Ra/Rg) toward 100ppm acetone was 33 operated at 230°C and the response time was as short as 3s. The sensor exhibited remarkable acetone selectivity with negligible response toward other hazardous gases and water vapor. The high proportion of electron depletion region and oxygen vacancies contributed to high gas response sensitivity. The hollow and porous structure of dandelion-like ZnO spheres facilitated the diffusion of gas molecules, leading to a rapid response speed. The largely exposed (002) polar facets could adsorb acetone gas molecules easily and efficiently, resulting in a rapid response speed and good selectivity of hierarchical ZnO spheres gas sensor at low operating temperature.

Nanocasting Synthesis of In2O3 with Appropriate Mesostructured Ordering and Enhanced Gas-Sensing Property

Ordered mesoporous In2O3 gas-sensing materials with controlled mesostructured morphology and high thermal stability have been successfully synthesized via a nanocasting method in conjunction with the container effect. The mesostructured ordering, as well as the particle size, crystallinity and pore size distribution have been proved to vary in a large range by using the XRD, SAXRD, SEM, TEM, and nitrogen physisorption techniques. The control of the mesostructured morphology was carried out by tuning the transportation rate of indium precursor in template channel resulting from the different escape rate of the decomposed byproducts via the varied container opening and shapes. The particular relation between the mesostructured ordering and gas sensing property of mesoporous In2O3 was examined in detail. It was found that the ordered mesoporous In2O3 with appropriate mesostructured morphology exhibited significantly improved ethanol sensitivity, response and selectivity performances in comparison with the other ordered mesoporous In2O3, which benefits from the large surface area with enough sensing active sites, proper pore distribution for sufficient gas diffusion, and appropriate particle size for effective electron depletion. The resulting sensing behaviors lead to a better understanding of designing and using such mesoporous metal oxides for a number of gas-sensing applications.

Exposed facets induced enhanced acetone selective sensing property of nanostructured tungsten oxide

WO3 nanorods with exposed (100) and (002) facets were fabricated via a hydrothermal route using different directing agents. Microspheres that were assembled randomly by WO3 nanorods with exposed (002) facets were porous with a specific surface area of 62.2 m2 g−1. Acetone gas sensing properties of the as-prepared sensors were investigated for the breath diagnosis of diabetes. The acetone detection limit of WO3 microspheres was 0.25 ppm, and the response (Ra/Rg) to 1 ppm acetone was 3.53 operated at 230 °C with a response and recovery time of 9 and 14 s, respectively. WO3 samples with exposed (002) facets exhibited better acetone sensitivity and selectivity than those with (100) facets. The sensing mechanism was discussed in detail. Because of the asymmetric distribution of unsaturated coordinated O atoms in the O-terminated (002) facets, the surface had an extent of polarization. Induced local electric dipole moment contributed to easy adsorption and reaction between the (002) facets and acetone molecules with a large dipole moment at low working temperatures. Room temperature PL spectra clearly displayed that the WO3 samples exposed with (002) facets had numerous oxygen vacancies and defects, resulting in excellent acetone sensing properties.

Super-hydrophobic hexamethyl-disilazane modified ZrO2–SiO2 aerogels with excellent thermal stability

Hexamethyl disilazane modified ZrO2–SiO2 aerogels (HMDS/ZSAs) have been prepared by the addition of HMDS into ZrO2–SiO2 gels during the aging process in order to improve their thermal stability at high temperature. FTIR analysis shows that the methyl siloxy groups ((CH3)3Si–O–) could replace the superficial hydroxy groups (–OH) through the condensation between the (CH3)3Si–OH of HMDS and –OH on the initial ZSA surface. HMDS/ZSAs could maintain their three-dimensional network structure without collapse and exhibit a remarkable thermal stability with retention of most physical characteristics up to a temperature of 1000 °C, such as a high surface area (174.4 m2 g−1), high pore volume (0.7246 cm3 g−1) and high elastic modulus (5.23 MPa). TGA and XRD analyses demonstrate that the phase transition of HMDS/ZSAs from amorphous to crystalline occurs up to 1000 °C, which is 100 °C higher than that of ZSAs. It was demonstrated that the modified inert methyl siloxy groups could form small silicon particles (3 to 5 A in diameter) to restrict the migration of grain boundaries, which was favorable to suppress the ZrO2 crystallization at elevated temperatures. Furthermore, HMDS/ZSAs exhibit super-hydrophobic properties with a contact angle of 154°. The high performance of HMDS/ZSAs paves the way for their applications in the field of thermal insulation, energy-absorbing services, environmental remediation, etc.

Effect of sepiolite fiber on the structure and properties of the sepiolite/silica aerogel composite

Sepiolite fiber-reinforced silica aerogel composites for thermal insulators were prepared by dispersing sepiolite fiber in silica sol, aging, solvent exchanging, and drying in supercritical fluid. The surface treated sepiolite fiber and sepiolite/silica aerogel composite were characterized by scanning electron microscope; transmission electron microscope and Fourier transform infrared spectroscopy. The influence of surface treated sepiolite fiber on the mechanical and thermal properties of the aerogel composite was studied. The results indicate the hydroxyl groups on silica sol particles surface able to condense with the hydroxyls of sepiolite fibers with forming Si–O–Si between sepiolite fibers and aerogel matrix in the sol–gel process, which achieves excellent interfacial interaction in the sepiolite/silica aerogel composite, so the mechanical properties of the aerogel composite have been improved effectively without sacrificing much thermal insulating performance.

A CuO–ZnO nanostructured p–n junction sensor for enhanced N-butanol detection

Herein, a novel CuO–ZnO nanostructured p–n junction composite is prepared via the hydrothermal method. It is composed of a ZnO two dimensional (2-D) porous nanosheet assembly and leaf-like 2-D CuO nanoplates. Then, its gas sensing performance toward n-butanol is studied. The 2-D/2-D CuO–ZnO composite sensor shows 2.7 times higher sensitivity than that of pure ZnO at 220 °C. Moreover, its response to n-butanol is 3.5–84 times higher than those for other target gases. This reveals an excellent selectivity toward n-butanol. Its detection limit for n-butanol is calculated to be 0.4 ppm, indicating a potential advantage in low concentration detection. The significant enhancement of the composites sensing performance can be firstly attributed to the p–n junction, which brings electronic sensitization for the composite sensor. Moreover, the porous structure and the open 2-D/2-D heterostructure also contribute to the sensing performance of the composite. These allow the gas molecules to diffuse rapidly, making chemisorption and surface reactions on the p–n junction more easy.

Atomic structure-dominated enhancement of acetone sensing for a ZnO nanoplate with highly exposed (0001) facet

Multilayer-assembled ZnO nanoplates (NP-ZnO) were synthesized, and the relationship between the exposed facets and the sensing performance was investigated. The XRD results showed that NP-ZnO predominantly exposed polar (0001) facets (I0002/I1010 = 2.9), while the PL and XPS spectra indicated that it contained very few oxygen vacancies. For comparison, ZnO nanosheet assemblies and ZnO commercial powder, both with less exposed (0001) facets but much more oxygen vacancies, were also studied. In the gas sensing test, the NP-ZnO sensor displayed the best sensing performance among the three ZnO microstructures sensor. The XPS analysis further confirmed that NP-ZnO was able to adsorb more oxygen species, despite having very few oxygen vacancies. These results revealed that: i) although more oxygen vacancies are generally considered the main reason for the high sensing activity of the (0001) polar facets, this might not be the dominant factor in NP-ZnO; ii) in NP-ZnO, the (0001) facets with few oxygen vacancies displayed an excellent sensing performance mainly due to its special atomic structure.

Calcination system-induced nanocasting synthesis of uniform Co3O4 nanoparticles with high surface area and enhanced catalytic performance

Co3O4 catalytic materials with varying mesoporous periodicity and crystallinity have been successfully synthesized via a calcination system-induced nanocasting method. N-Co3O4 with uniform nanoscale morphology, high specific surface area, and large pore size distribution was obtained in an open system as a calcination process, while M-Co3O4 with long-range mesoporous periodicity and high crystallinity was synthesized using a closed system as the calcination condition. The control of the mesostructure and morphology was carried out by tuning the diffusion rate of the cobalt precursor in the template channel resulting from the different escape rates of the decomposed byproducts via the varied calcination containers. The CO oxidation testing indicated that N-Co3O4 exhibited better catalytic performance than that of M-Co3O4. The difference in activity could be attributed to the uniform nanoscale structure of N-Co3O4, which mesoporous M-Co3O4 lacked. N-Co3O4 had a better performance for CO oxidation due to the uniform nanoparticle structure, higher specific surface area, larger pore size distribution, abundant active oxygen species and Co3+ cationic species on the surface, which accelerated the adsorption and diffusion of reactant molecules and finally improved the reaction activity of N-Co3O4. The resulting catalytic behaviors lead to a better understanding of designing and using such metal oxides for a number of catalytic applications.

The Effect of Propylene Oxide on Microstructure of Zirconia Monolithic Aerogel

Uncracked zirconia (ZrO2) monolithic aerogels were prepared using a sol–gel process accompanied by high temperature supercritical fluid drying technology. Moreover, the experimental results on microstructure of ZrO2 aerogels modified by PO (propylene oxide) were reported. As more PO added to the starting solution, pH rose and the gel time decreased. SEM and N2 adsorption–desorption curves showed that the particle With the molar ratio of PO to Zr increasing from 0 to 2.5, size and connectivity of network of ZrO2 aerogels increased, while average pore radius increased from 6.6 nm to 15.1 nm then decreased to 8.5nm and pore volume increased from 1.2 cc/g to 1.87 cc/g then decreased to 1.44 cc/g.

Formation Mechanism of Plate-like Bi4Ti3O12 Particles in Molten Salt Fluxes

The plate-like Bi4Ti3O12 particles were prepared by molten salt synthesis method. The influence of sintering temperature and cooling process on the microstructure of Bi4Ti3O12 powders was studied. Much larger particles were formed at higher temperatures. The particles could grow larger in slow cooling process. The formation mechanism of plate-like Bi4Ti3O12 particles in Na2SO4-K2SO4 system could be viewed as four processes: (1) solid reaction and nucleation, (2) plate-like structure formation, (3) diffusion and edge nucleation, (4) diffusion and epitaxial growth.