Observation of Structure-Sensitive Decomposition of Cp(allyl)Pd on Pd Nanodots Formed by MOCVD

The decomposition kinetics of the organometallic precursor cyclopentadienyl-allyl-Pd [Cp(allyl)Pd] on silica and titania substrates (as well as previously deposited Pd) are investigated by continuous metal-organic (MO) CVD under atmospheric pressure. The substrate particles are freshly generated in an aerosol process with well-defined and known densities of surface OH groups, and then coated immediately in a second reactor. Time-resolved data on size and number density of deposited Pd nanodots on the carrier particles are obtained by electron microscopy and used as a means to infer the decomposition kinetics. It is found that decomposition proceeds very rapidly for a few seconds, both by proton-initiated decomposition on surface OH groups and by autocatalysis, until the Pd nanodots have grown to a size of about 2 nm, where autocatalysis comes to a standstill, regardless of precursor concentration, residence time in the CVD reactor, or decomposition temperature. Proton-initiated decomposition on OH groups continues to depend measurably on these process parameters in terms of the number density of Pd dots, even after autocatalysis has stopped. Our observations indicate that autocatalysis is structure-sensitive.