No abstract

To exploit the size dependent properties of nanoparticles, it is essential to control the particle size. We show that injection of octanethiol into suspensions of ZnO particles can be used to quench particle growth. X-ray photoelectron spectroscopy studies of the adsorption of octanethiol on ZnO single crystals indicate relatively weak adsorption of thiolate and sulfonate species dependent on the crystal orientation. These results suggest that adsorption of thiolate and sulfonate groups on ZnO particles in suspension prevent dissolution of the solid phase. Furthermore, the octanethiol is adsorbed sufficiently strongly to quench growth but can be easily removed for subsequent surface functionalization.

In this communication, we reported for the first time an ultrasensitive nanostructrued sensor that can detect 50 ppt of NH 3 gas in air. Specifically, nanograins of a p-type conductive polymer, polyaniline (PANI), are enchased on an electrospun n-type semiconductive TiO 2 fiber surface. The resistance of the p-n heterojunctions combining with the bulk resistance of PANI nanograins can function as electric current switches when NH 3 gas is absorbed by PANI nanoparticles. As a result, the sensor sensitivity can be significantly improved. The sensor fabricated in this work is 1000 times more sensitive than the best PANI sensor reported in the literature.* To whom the correspondence should be addressed, yulin.deng@ chbe.gatech.edu (Y. Deng).

In solution phase synthesis of nanoparticles, processes such as coarsening and aggregation can compete with nucleation and growth in modifying the particle size distribution in the system. We report on the synthesis of ZnO nanoparticles from Zn(CH3CO2)2, ZnBr2, and Zn(ClO4)2 in 2-propanol. For synthesis from Zn(CH3CO2)2 and ZnBr2, nucleation and growth are fast and are followed by diffusion-limited coarsening. For synthesis from Zn(ClO4)2, diffusion-limited coarsening is observed at shorter times whereas at longer times the particle size increases more rapidly. The rate constant for coarsening at constant temperature increases in the order Br- < CH3CO2 - < ClO4 - indicating that the rate is dependent on anion adsorption. The temperature dependence of the rate constant for coarsening is due to the temperature dependence of the solvent viscosity and the temperature dependence of the bulk solubility of ZnO.

We report on the synthesis of ZnO particles from Zn(CH(3)CO(2))(2) in 2-propanol as a function of the concentration of water, in the absence of a base such as NaOH. Particles with diameters of 3-5 nm are formed depending on time, temperature, and water concentration. The nucleation and growth are slower than in the presence of NaOH, and at longer times the increase in particle size is dominated by diffusion-limited coarsening. The rate constant for coarsening increases with increasing water concentration up to 150 mM, above which the rate constant is 1.1 x 10(-4) cm(3) s(-1), independent of the water concentration. The width of the particle size distribution decreases with increasing water concentration, and at 250 mM water, the full width at half-maximum of the distribution function is essentially the same as for the synthesis of ZnO using NaOH as a reactant. The temperature dependence of coarsening is determined by the bulk solubility of the ZnO nanoparticles and yields an apparent activation energy of 1.12 eV. This is significantly larger than the activation energy of 0.35 eV for coarsening of ZnO from 1 mM Zn(CH(3)CO(2))(2) in 2-propanol with 1.6 mM NaOH.

In solution phase synthesis of nanoparticles, processes such as coarsening and aggregation can compete with nucleation and growth in modifying the particle size distribution in the system. We show that coarsening of ZnO and TiO2 nanoparticles in solution follows the Lifshitz-Slyozov-Wagner rate law for diffusion controlled coarsening originally derived for colloidal systems with micrometer-sized particles, where the average particle size cubed is proportional to time. The rate constant for growth of ZnO in propanol is in the range 10(-4)-10(-2) nm3 x s(-1) and is dependent on the precursor anion and temperature. The coarsening of TiO2 nanoparticles from aqueous Ti(IV) alkoxide solutions is slower due to the low solubility of TiO2 with the rate constant in the range 10(-5)-10(-3) nm3 x s(-1) for temperatures between 150 degrees C and 220 degrees C. Epitaxial attachment of TiO2 particles becomes significant at higher temperatures and longer times. We show that the dominant parameters controlling the coarsening kinetics are solvent, precursor salt, and temperature.

No abstract

The UV irradiation enhanced ZnO fiber sensors for organic gas detection at room temperature are reported in this paper. It has been well-known that the conductivity of ZnO fibers will increase when they are exposed to UV irradiation. However, it was found in this study that the trend of the conductivity increase under UV irradiation could be reversed if the organic gases adsorbed on the ZnO fiber surface. It was found that a linear relationship exists between the current response and the concentration of the organic gas. The results exhibited simple maneuverability for detecting parts per million concentration of organic gases at room temperature. The possible mechanism of photoconduction sensing for detecting an organic gas under UV light irradiation at room temperature is also discussed.