Concentration-specific hydrogen sensing behavior in monosized Pd nanoparticle layers.

In this study, H2 sensing behavior of monosized and monocrystalline Pd nanoparticles has been studied as a function of H2 concentration and measurement temperature. A unique concentration-specific H2 sensing behavior with a 'pulsed' response at larger H2 concentrations and 'saturated' response at lower concentrations has been observed. The threshold concentration required for transition from 'saturated' to 'pulsed' response is very sensitive to measurement temperature. The characteristic change in the sensing behavior can be used to develop a novel sensor capable of determining H2 concentration level having high sensitivity and fast response. This study demonstrates that electrical and gas sensing properties of the nanoparticle layer depend critically on interparticle gaps.

A dual-deposition setup for fabricating nanoparticle-thin film hybrid structures.

This report describes a dual-deposition setup for fabricating well-defined nanoparticles-thin film structures. The setup consists of a particle synthesis section for the gas phase generation of size-selected nanoparticles and a deposition section for the sequential growth of thin film and nanoparticle layers on substrates using vacuum evaporation and atmospheric pressure electrostatic precipitator techniques, respectively. The setup has been used to deposit Pd nanoparticles-Pr thin film structures. Average sizes and size distributions of Pd nanoparticles measured online during the particle synthesis by means of electrical mobility analysis have been compared with those of nanoparticle samples deposited on Pr thin film and other substrates and measured by high resolution scanning electron microscopy and transmission electron microscopy techniques. The setup is useful for depositing a variety of nanoparticles-thin film structures.

Studying nanoparticles in-flight: applications of size-selected free nanoparticles.

Experimental investigation of nanoparticles in form of free particles in an inert gas is advantageous due to absence of substrate effects. Gas-phase synthesis techniques offer many possibilities for producing and manipulating nanoparticles. The most advanced technique produces well-defined nanoparticles, by means of inert-gas evaporation in a flowing gas, size-selection on the basis of the electrical mobility and subsequent sintering into monocrystalline and quasispherical nanoparticles. The electrical charge obtained by nanoparticles allows further manipulation, such as the generation of well-defined nanoparticle pairs, in which the sizes of both the particles can be selected. The application of these well-defined nanoparticles for studying the sintering of metallic nanoparticles is demonstrated. The ability to deliver size-selected nanoparticle aerosols at larger flowrates, in this study up to 30 l/min, allows to deliver larger quantities of well-defined nanoparticles. The size-selection necessitates the charging of the nanoparticle aerosol. We show that photoionization by means of UV irradiation is able to charge nanoparticles with a high efficiency and that by optimizing the radiation intensity the amount of multiple charges can be brought to a minimum, resulting in a geometric standard deviation of the size distribution of 1.05.

Size-dependent gas sensing properties of indium oxide nanoparticle layers.

In2O3 nanoparticle layers having an average size of 8, 11, 15, 21, and 29 nm have been deposited using a two-step method consisting of chemical capping and dip coating techniques. The gas sensing properties in terms of sensor response and response time of the nanoparticle layers towards ethanol have been studied as a function of ethanol concentration and operating temperature. It has been observed that the sensor response increases and the response time decreases with decreasing size in the size range of 5-15 nm. The increase in sensor response at smaller nanoparticle size has been explained in terms of the increase in surface area and particle size becoming comparable to the electron Debye length.

XPS and AFM studies of monodispersed Pb/PbO core-shell nanostructures.

A probe of the core-shell formation and the interfacial properties of monodispersed 30 nm Pb nanoparticles are presented. A direct correlation between the structures of the particle to its chemical states is done, by a careful Ar+ ion depth profiling followed by X-ray Photoelectron Spectroscopy and Atomic Force Microscopy. The study provides a unique insight into the structure of the nanoparticles. A PbO shell of 4 nm is observed to cover a 15-nm core, with a 5 nm interfacial region. The valence band spectra show the movement of the valence band maxima towards and beyond the Fermi-level during the insulator to metal transition along the particle depth. Evidence is also provided for an enhanced preferential sputtering of the PbO shell and an ion induced penetration of PbO into the Pb core.

First preparation of nanocrystalline zinc silicate by chemical vapor synthesis using an organometallic single-source precursor.

A method is presented to prepare nanocrystalline alpha-Zn(2)SiO(4) with the smallest crystal size reported so far for this system. Our approach combines the advantages of organometallic single-source precursor routes with aerosol processing techniques. The chemical design of the precursor enables the preferential formation of pure zinc silicates. Since gas-phase synthesis reduces intermolecular processes, and keeps the particles small, zinc silicate was synthesized from the volatile organometallic precursor [[MeZnOSiMe(3)](4)], possessing a Zn-methyl- and O-silyl-substituted Zn(4)O(4)-heterocubane framework (cubane), under oxidizing conditions, using the chemical vapor synthesis (CVS) method. The products obtained under different process conditions and their structural evolution after sintering were investigated by using various analytical techniques (powder X-ray diffraction, transmission electron microscopy, EDX analysis, solid-state NMR, IR, Raman, and UV/Vis spectroscopy). The deposited aerosol obtained first (processing temperature 750 degrees C) was amorphous, and contained agglomerates with primary particles of 12 nm in size. These primary particles can be described by a [Zn-O-Si] phase without long-range order. The deposit obtained at 900 degrees C contained particles with embedded nanocrystallites (3-5 nm) of beta-Zn(2)SiO(4), Zn(1.7)SiO(4), and ZnO in an amorphous matrix. On further ageing, the as-deposited particles obtained at 900 degrees C form alpha-Zn(2)SiO(4) imbedded in amorphous SiO(2). The crystallite sizes and primary particle sizes in the formed alpha-Zn(2)SiO(4) were found to be below approximately 50 nm and mainly spherical in morphology. A gas-phase mechanism for the particle formation is proposed. In addition, the solid-state reactions of the same precursor were studied in detail to investigate the fundamental differences between a gas-phase and a solid-state synthesis route.

Higher surface energy of free nanoparticles.

We present an accurate online method for the study of size-dependent evaporation of free nanoparticles allowing us to detect a size change of 0.1 nm. This method is applied to Ag nanoparticles. The linear relation between the onset temperature of evaporation and the inverse of the particle size verifies the Kelvin effect and predicts a surface energy of 7.2 J/m(2) for free Ag nanoparticles. The surface energy of nanoparticles is significantly higher as compared to that of the bulk and is essential for processes such as melting, coalescence, evaporation, growth, etc., of nanoparticles.

Evaporation of free PbS nanoparticles: evidence of the Kelvin effect.

The size-dependent evaporation of free-spherical PbS nanoparticles has been investigated by in-flight sintering of size-classified aerosols. The temperature (T(ev)) at which the particle size decreases due to evaporation is found to be size dependent and decreases with decreasing particle size. A linear relationship between the evaporation temperature and the inverse of the particle size is obtained as is the case with size-dependent melting of nanoparticles. This gives a direct evidence of the Kelvin effect and allows one to estimate the surface energy of nanoparticles. The surface energy of PbS nanoparticles has been found to be 2.45 J m(2).