Hafnium dioxide is a wide band-gap, high-κ material, and Hafnium based compounds have already been integrated into micro-electronic devices. The pure cubic HfO2 phase is promising as it presents a higher permittivity (κ > 25), but needs to be stabilized by addition of divalent or trivalent dopants, which in turn modify the electronic properties of HfO2. Here, we employ a one-pot synthesis approach to produce undoped cubic and monoclinic HfO2 nanoparticles by choice of solvent alone. The average size of these nanoparticles from transmission electron microscopy studies was estimated to be around 2.6 nm. We present a study of the morphology and microstructure and also demonstrate the presence of a strong visible photoluminescence linked to the nanosize of the particles. Furthermore, the synthesis in equivalent conditions of these two phases of HfO2 provides means for direct comparison of the chemical composition and electronic structures of the two polymorphs. This has therefore allowed us to experimentally elucidate similarities and differences in the valence band, band gap states, and conduction band of these pure phases seconded by first principles calculations within the density functional theory.