AVS 47th International Symposium
    Surface Science Tuesday Sessions
       Session SS3+MC-TuM

Paper SS3+MC-TuM8
A Microarray Technique for Measuring Adsorption/Desorption Kinetics

Tuesday, October 3, 2000, 10:40 am, Room 210

Session: Technique Innovations: Experiment, Theory and Simulation
Presenter: M.C. Wheeler, National Institute of Standards and Technology
Authors: M.C. Wheeler, National Institute of Standards and Technology
R.E. Cavicchi, National Institute of Standards and Technology
G.E. Poirier, National Institute of Standards and Technology
S. Semancik, National Institute of Standards and Technology
Correspondent: Click to Email
We have developed a novel method for characterizing the kinetics and thermodynamics of adsorption and desorption that uses a 340-element array of micromachined Si hotplates as the sample platform. The method takes advantage of the microhotplates' rapid heating characteristics (10@super 6@ °C/s), intermediate between traditional temperature programmed desorption methods and pulsed laser desorption. The method is isobaric, particularly useful for studying many systems where the desorption rate is significant at room temperature, and readily automated to efficiently produce large data sets for adsorption systems. Under constant gas exposure, all of the array elements are simultaneously subjected to square temperature pulses as short as 5 ms. An example sequence is a high-T cleaning pulse (as high as 800 °C), a low-T adsorption pulse, an intermediate-T desorption pulse, and a final high-T cleaning pulse. Variation of the pulse amplitude and duration allows determination of the adsorption and desorption rate constants and energies. Signal-to-noise is enhanced by using a large array of microhotplate devices and averaging of repeated pulse sequences. The pressure range is extended to over 10@super -5@ Torr using a differentially pumped mass spectrometer system (even higher effective pressures can be achieved with a directed doser). We illustrate the technique by mapping adsorption isotherms and extracting parameters for methanol on CVD-deposited SnO@sub 2@ polycrystalline films, such as are used in gas sensing; however, wide ranges of materials and adsorbates can be investigated with this technique. In addition to emphasizing the convenience, flexibility, simplicity, and efficiency of this automated technique, we will also discuss challenges that we have had to address - the most significant relating to temperature calibration and uniformity.