Nanostructured Er3+- and Er3+/Yb3+-activated hafnium oxide films and nanoparticles were prepared from a stable colloidal suspension and investigated by several techniques such as transmission electron microscopy, X-ray diffraction, dynamic light scattering, atomic force microscopy, Fourier-transform infrared spectroscopy, and photoluminescence. Low roughness and crack-free films were deposited by dip-coating and spin-coating techniques on vitreous SiO2 and Si substrates. Nanostructured particles were also synthesized. Remarkable structural and spectroscopic differences were observed for hafnium oxide-based materials as a function of the Er3+ and Er3+/Yb3+ concentration. The I-4(13/2) I-4(15/2) emission bandwidth and the lifetime of the I-4(13/2) metastable state of Er3+ was tailored through the rare-earth concentration. The Er3+ emission in HfO2 can be explored for photonic applications.