Pure and palladium-loaded Co3O4 hollow hierarchical nanostructures with giant and ultraselective chemiresistivity to xylene and toluene.

Pure and palladium-loaded Co3O4 hollow hierarchical nanostructures consisting of nanosheets have been prepared by solvothermal self-assembly. The nanostructures exhibited an ultrahigh response and selectivity towards p-xylene and toluene. The responses (resistance ratio) of the palladium-loaded Co3O4 hollow hierarchical nanostructures to 5 ppm of p-xylene and toluene were as high as 361 and 305, respectively, whereas the selectivity values (response ratios) towards p-xylene and toluene over interference from ethanol were 18.1 and 16.1, respectively. We attributed the giant response and unprecedented high selectivity towards methylbenzenes to the abundant adsorption of oxygen by Co3O4, the high chemiresistive variation in the Co3O4 nanosheets (thickness≈11 nm), and the catalytic promotion of the specific gas-sensing reaction. The morphological design of the p-type Co3O4 nanostructures and loading of the palladium catalyst have paved a new way to monitoring the most representative indoor air pollutants in a highly selective, sensitive, and reliable manner.

Extremely sensitive and selective NO probe based on villi-like WO3 nanostructures for application to exhaled breath analyzers.

Self-assembled WO3 thin film nanostructures with 1-dimensional villi-like nanofingers (VLNF) have been synthesized on the SiO2/Si substrate with Pt interdigitated electrodes using glancing angle deposition (GAD). Room-temperature deposition of WO3 by GAD resulted in anisotropic nanostructures with large aspect ratio and porosity having a relative surface area, which is about 32 times larger than that of a plain WO3 film. A WO3 VLNF sensor shows extremely high response to nitric oxide (NO) at 200 °C in 80% of relative humidity atmosphere, while responses of the sensor to ethanol, acetone, ammonia, and carbon monoxide are negligible. Such high sensitivity and selectivity to NO are attributed to the highly efficient modualtion of potential barriers at narrow necks between individual WO3 VLNF and the intrinsically high sensitivity of WO3 to NO. The theoretical detection limit of the sensor for NO is expected to be as low as 88 parts per trillion (ppt). Since NO is an approved biomarker of chronic airway inflammation in asthma, unprecedentedly high response and selectivity, and ppt-level detection limit to NO under highly humid environment demonstrate the great potential of the WO3 VLNF for use in high performance breath analyzers.

Ultraselective and sensitive detection of xylene and toluene for monitoring indoor air pollution using Cr-doped NiO hierarchical nanostructures.

Ultraselective and sensitive detection of xylene and toluene with minimum interferences of other indoor air pollutants such as benzene, ethanol, and formaldehyde is achieved using NiO hierarchical nanostructures doped with Cr. Pure and 1.15-2.56 at% Cr-doped NiO flower-like hierarchical nanostructures assembled from nanosheets are prepared by a simple solvothermal reaction and their gas sensing characteristics toward o-xylene and toluene gases are investigated. The 1.15 at% Cr-doped NiO hierarchical nanostructures show high responses to 5 ppm of o-xylene and toluene (ratio of resistance to gas and air = 11.61 and 7.81, respectively) and negligible cross-responses to 5 ppm of benzene, formaldehyde, ethanol, hydrogen, and carbon monoxide. However, pure NiO nanostructures show low responses to 5 ppm of o-xylene and toluene (ratio of resistance to gas and air = 2.01 and 1.14, respectively) and no selectivity toward any specific gas is observed. Significant enhancement of the response and selectivity to o-xylene and toluene is attributed to the decrease in the hole concentration in NiO and the catalytic oxidation of methyl groups by Cr doping.

Selective detection of NO2 using Cr-doped CuO nanorods.

CuO nanosheets, Cr-doped CuO nanosheets, and Cr-doped CuO nanorods were prepared by heating a slurry containing Cu-hydroxide/Cr-hydroxide. Their responses to 100 ppm NO(2), C(2)H(5)OH, NH(3), trimethylamine, C(3)H(8), and CO were measured. For 2.2 at% Cr-doped CuO nanorods, the response (R(a)/R(g), R(a): resistance in air, R(g): resistance in gas) to 100 ppm NO(2) was 134.2 at 250 °C, which was significantly higher than that of pure CuO nano-sheets (R(a)/R(g) = 7.5) and 0.76 at% Cr-doped CuO nanosheets (R(a)/R(g) = 19.9). In addition, the sensitivity for NO(2) was also markedly enhanced by Cr doping. Highly sensitive and selective detection of NO(2) in 2.2 at% Cr-doped CuO nanorods is explained in relation to Cr-doping induced changes in donor density, morphology, and catalytic effects.

Design of highly sensitive C2H5OH sensors using self-assembled ZnO nanostructures.

Various ZnO nanostructures such as porous nanorods and two hierarchical structures consisting of porous nanosheets or crystalline nanorods were prepared by the reaction of mixtures of oleic-acid-dissolved ethanol solutions and aqueous dissolved Zn-precursor solutions in the presence of NaOH. All three ZnO nanostructures showed sensitive and selective detection of C(2)H(5)OH. In particular, ultra-high responses (R(a)/R(g) = ∼1,200, R(a): resistance in air, R(g): resistance in gas) to 100 ppm C(2)H(5)OH was attained using porous nanorods and hierarchical structures assembled from porous nanosheets, which is one of the highest values reported in the literature. The gas response and linearity of gas sensors were discussed in relation to the size, surface area, and porosity of the nanostructures.

Ultra-fast responding and recovering C2H5OH sensors using SnO2 hollow spheres prepared and activated by Ni templates.

Ultra-fast responding and recovering C(2)H(5)OH sensors were prepared using nanoscale SnO(2) hollow spheres with NiO-functionalized inner walls. The exceptional ultra-fast recovery characteristics were attributed to the catalytic surface reaction assisted by NiO at the inner shell.