The reason why nano iron oxide can present different colors is mainly related to its crystal structure, particle size, and surface state.
The crystal structure of nano iron oxide is hexagonal, and the lattice parameters change with the decrease of particle size. When the particle size is large (usually larger than tens of nanometers), iron oxide exhibits a typical α- The Fe2O3 structure, also known as hematite structure, is red in color. This is because typical α- The Fe2O3 structure has a high reflectivity for visible light, absorbing shorter wavelengths (blue-green) in visible light, leaving only longer red wavelengths observed.
When the particle size decreases to the nanoscale, iron oxide undergoes changes in lattice parameters and surface effects, leading to changes in its color. When the particle size is small to a certain extent (usually less than tens of nanometers), iron oxide exhibits magnetism γ- The Fe2O3 structure, also known as magnetic iron oxide, is black in color. This is because the magnetic iron oxide structure has a lower reflectivity for visible light, absorbs more visible light, and does not produce significant reflection, resulting in a black color.
In addition, the surface state of nano iron oxide can also have an impact on color. The surface of nanomaterials has a large specific surface area and is prone to interact with the surrounding environment. For example, when there is an oxide or organic covering layer on the surface of nano iron oxide, their color may change and other colored powders may appear.
Overall, the color of nano iron oxide depends on its crystal structure, particle size, and surface state. Iron oxide with larger particle sizes presents a typical red color, while iron oxide with smaller particle sizes presents a black color, and the surface state also affects the color.
