Scanning mass spectrometer for quantitative reaction studies on catalytically active microstructures.

We describe an apparatus for spatially resolving scanning mass spectrometry which is able to measure the gas composition above catalytically active microstructures or arrays of these microstructures with a lateral resolution of better than 100 mum under reaction conditions and which allows us to quantitatively determine reaction rates on individual microstructures. Measurements of the three-dimensional gas composition at different vertical distances and separations between active structures allow the evaluation of gas phase mass transport effects. The system is based on a piezoelectrically driven positioning substage for controlled lateral and vertical positioning of the sample under a rigidly mounted capillary probe connecting to a mass spectrometer. Measurements can be performed at pressures in the range of <10(-2)-10 mbars and temperatures between room temperature and 450 degrees C. The performance of the setup is demonstrated using the CO oxidation reaction on Pt microstructures on Si with sizes between 100 and 300 mum and distances in the same order of magnitude, evaluating CO(2) formation and CO consumption above the microstructures. The rapidly decaying lateral resolution with increasing distance between sample and probe underlines the effects of (lateral) gas transport in the room between sample and probe. The reaction rates and apparent activation energy obtained from such measurements agree with previous data on extended surfaces, demonstrating the feasibility of determining absolute reaction rates on individual microstructures.

Controlling the interparticle spacing of Au-salt loaded micelles and Au nanoparticles on flat surfaces.

The self-organization of diblock copolymers into micellar structures in an appropriate solvent allows the deposition of well ordered arrays of pure metal and alloy nanoparticles on flat surfaces with narrow distributions in particle size and interparticle spacing. Here we investigated the influence of the materials (substrate and polymer) and deposition parameters (temperature and emersion velocity) on the deposition of metal salt loaded micelles by dip-coating from solution and on the order and inter-particle spacing of the micellar deposits and thus of the metal nanoparticle arrays resulting after plasma removal of the polymer shell. For identical substrate and polymer, variation of the process parameters temperature and emersion velocity enables the controlled modification of the interparticle distance within a certain length regime. Moreover, also the degree of hexagonal order of the final array depends sensitively on these parameters.

Microstructured Au/TiO2 model catalyst systems.

The preparation of microstructured Au/TiO2 model catalysts as a first step toward micrometer-scale parallel studies on model catalysts and toward studies of mesoscopic effects in catalytic reactions was investigated by atomic force microscopy and X-ray photoelectron spectroscopy. The model systems, which consist of micrometer-size active areas covered with Au nanoparticles that are separated by similarly sized inactive areas free of Au particles, are fabricated by combining optical lithography methods for microstructuring and ultrahigh vacuum evaporation for Au nanoparticle deposition and by applying suitable cleaning steps. It is demonstrated that practically perfect microstructures with Au nanoparticles of catalytically relevant sizes (2-3-nm diameter) on a clean TiO2 substrate can be produced this way and that the processing steps do not affect the deposited Au nanoparticles, neither in size nor in lateral distribution.