In-situ characterization of dispersion stability of WO3 nanoparticles and nanowires

The stability of tungsten trioxide (WO3) suspensions in various common polar solvents such as water, acetone, isopropanol (IPA), ethanol, 1-methoxy-2-propanol (1M-2P) and N,N-dimethylformamide (DMF) was investigated. The morphology of WO3 aggregates formed by irregular nanoparticles (d similar to 40 nm, with 1 mu m nominal diameter compact aggregates) and by nanowires of different types (uneven, single or bundled in diameter) and dimensions (nominal lengths of 2, 4, 6, and 10 mu m) were described by means of the small angle static light scattering and the elliptically polarized light scattering (EPLS) techniques. Aggregation of low aspect ratio (bundled) 2 mu m nanowires monitored through the change in spatial extent of the aggregate was found to be minimal (i.e., radius of gyration, R-g similar to 1.8-2.2 mu m in 1-methoxy-2-propanol), with a minimal change in aggregate structure (i.e., fractal dimension, D-f similar to 1.8-1.9 in 1-methoxy-2-propanol) in a time period of about 1 week. Fractal dimension was found to be the lowest for the low aspect ratio nanowires when suspended in N,N-dimethylformamide (D-f similar to 1.4). Aggregates of very high aspect ratio single nanowires (L/D similar to 250 with 10 mu m nominal length) were also observed to form stable dispersions in a period of about a week. Aggregate structures that would lead to observed fractal dimensions were proposed. Information on how well inorganic nanowires are dispersed in various solvents is based singly on the time consuming and intrusive advanced microscopy analyses (such as SEM and TEM) in the literature, and without any reference to the underlying structures. To our knowledge, this study is the first attempt for in-situ description of the underlying causes, such as aggregate morphologies, aggregation rates and solvent types, of the observed dispersion and sedimentation behaviors of inorganic nanowires that were not subjected to any surface treatment or functionalization.