Structure and Photoluminescence of WO_3- Aggregates Tuned by Surfactants

The optoelectronic properties of transition metal oxide semiconductors depend on their oxygen vacancies, nanostructures and aggregation states. Here, we report the synthesis and photoluminescence (PL) properties of substoichiometric tungsten oxide (WO_3-) aggregates with the nanorods, nanoflakes, submicro-spherical-like, submicro-spherical and micro-spherical structures in the acetic acid solution without and with the special surfactants (butyric or oleic acids). Based on theory on the osmotic potential of polymers, we demonstrate the structural change of the WO_3- aggregates, which is related to the change of steric repulsion caused by the surfactant layers, adsorption and deformation of the surfactant molecules on the WO_3- nanocrystals. The WO_3- aggregates generate multi-color light, including ultraviolet, blue, green, red and near-infrared light caused by the inter-band transition and defect level-specific transition as well as the relaxation of polarons. Compared to the nanorod and nanoflake WO_3- aggregates, the PL quenching of the submicro-spherical-like, submicro-spherical and micro-spherical WO_3- aggregates is associated with the coupling between the WO_3- nanoparticles and the trapping centers arising from the surfactant molecules adsorbed on the WO_3- nanoparticles.