Mass spectrometric analysis of clusters and nanoparticles during the gas-phase synthesis of tungsten oxide

Abstract

The combustion synthesis of nanoscale tungsten-oxide particles from tungsten hexafluoride is investigated in a low-pressure hydrogen/oxygen flat flame. The reactor is equipped with molecular-beam sampling of post-flame gases at variable height above burner (HAB). Main species of the flame, intermediate tungsten species, and tungsten-oxide clusters are studied with time-of-flight mass spectrometry (TOF-MS) as a function of HAB. Various WOx(OH)y are identified within the flame front. With increasing HAB, (WO3)n clusters with increasing cluster size appear in the burnt gases at the expense of the concentration of W1 species. Clusters with n=3–7 arise at 70mm HAB, followed by larger clusters at even larger heights. Clusters up to (WO3)38 were identified. The subsequent formation of nanoparticles is detected with particle mass spectrometry (PMS) and a quartz crystal microbalance (QCM) from 120mm HAB and the increasing particle size and mass flux have been determined.