Effect of Wettability on the Transport and Fate of Metal Oxide Nanoparticles - FT-4
Nanoparticles have been widely applied in many applications due to their specific physical or chemical properties that differ from those of the bulk counterparts. Since nanoparticles vary significantly in composition, surface charge, crystallinity and geometry, the comparison of their interaction with other materials is challenging, but it is important to characterize and assort nanoparticles quantitatively. Among the many properties of the nanoparticles, surface energy is especially critical for evaluating their potential affinity to important species in the environment. The different levels of toxicities of particles can be attributed to the affinity of particles to different substances in the environment. Many techniques have been attempted for evaluating the surface energy of pure metal oxide nanoparticles. However, preparatory treatments, such as heating or acid cleaning, have been shown to alter the surface drastically. We have designed a benign technique to quantitatively evaluate the surface energy of as received nanoparticles ZnO, CeO2 and P25 TiO2. Using this technique, the surface energies of a series of nanoparticles were determined and ranked under ambient conditions without heating or acid/base treatments. The surface energy calculated by this method for ZnO, CeO2 and P25 TiO2 are 926, 529 and 561 mJ/m2 respectively. The values reported for ZnO and TiO2 are in good agreement with values reported in the literature proving the validity of the new method. We hypothesize that the surface energy of nanoparticles is closely related to their wettability and therefore, the ranking according to the surface energy can be used to predict their interactions with their environmental neighbors, e.g. proteins and cells and also in understanding their aggregation and sedimentation behavior and hence the fate.
The impact of deposition and aggregation on (bio)chemical properties of semiconducting nanoparticles (NPs) is perhaps among the least studied aspects of aquatic chemistry of solids. In this aspect, our aim is to get a mechanistic insight into the effects of co-aggregation and deposition on the catalytic properties of semiconducting metal oxide NPs. Employing a combination of in situ FTIR and ex situ X-ray photoelectron spectroscopy (XPS) and using the Mn(II) oxygenation on hematite (α-Fe2O3) and anatase (TiO2) NPs as a model catalytic reaction, we discovered that the catalytic and sorption performance of the semiconducting NPs in the dark can be manipulated by depositing them on different supports or mixing them with other NPs. We introduce the electrochemical concept of the catalytic redox activity to explain the findings and to predict the effects of (co)aggregation and deposition on the catalytic and corrosion properties of ferric (hydr)oxides. These results provide a new framework for modeling the fate and transport of semiconducting metal oxide NPs in the environment and living organisms: namely, aggregation and co-deposition of NPs which controls the semiconducting properties of NPs and thereby their (bio-)reactivity should be considered as the key parameters for any accurate estimation of fate and transport. Additionally, these results offer new possibilities for rationally tailoring the technological performance of semiconducting metal oxide NPs and can be helpful in discriminating between weakly and strongly adsorbing species.