Mobility analysis of nanocluster formation and growth from titanium tetraisopropoxide in a flow tube reactor

Abstract

In an effort to understand the earliest stages of nanocluster growth in gas phase synthesis systems, we apply a flow tube reactor with a differential mobility analyzer-Faraday cage electrometer system to examine nanocluster formation and growth in the sub-3.0 nm mobility diameter range from the decomposition of titanium tetra-isopropoxide (TTIP) in air. Measured mobility distributions are inverted accounting for the DMA transfer function, tubing losses, and the charging efficiency, estimated from a non-steady state charge model within a bipolar ion source. Measurements reveal two types of species in spectra. First, we detect discrete (narrow) peaks falling in the 0.5 nm–1.5 nm size range which appear at different locations for positive and negative measurement modes. Second, we observe a larger, broadly distributed peak, which is similar in intensity for both positive and negative modes after data inversion, and which increases in intensity (concentration) and peak diameter with increasing precursor flow rate, increasing reactor temperature, and increasing residence time. We conclude that the latter broad peak arises from growing nanoclusters in the reactor. The narrower, sub 1.5 nm peaks, are most likely attributable to specific reactive intermediates from the decomposition of TTIP. We find that increasing TTIP concentration, reactor temperature, or increasing residence time, which furthers the extent of nanocluster formation, leads to the depletion of the sub 1.5 nm ions. Because of the large step size in diameter between neighboring sub 1.5 nm peaks, we also find it is unlikely that these peaks arise from small TiO2 clusters. Therefore, our measurements suggest that nanocluster growth from TTIP is facilitated by surface growth of reactive intermediates from TTIP decomposition, with reactive intermediates detectable via mobility analysis. Furthermore, while nanocluster formation and growth is clearly detected, measurements do not make clear that the nanoclusters are fully oxidized TiO2 nanoclusters for the synthesis temperature range examined.