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Undoped and codoped (Eu/Na) ZnO nanocrystals (NCs) were successfully manufactured through an economical sol-gel method. X-ray diffraction (XRD) analysis demonstrated pure hexagonal wurtzite structure without secondary phases for all the samples. The size of the NCs was found to decrease with codoping by Eu3+/Na+ which is related to the existence of strain and stress in the lattice. The dominance of the E2(high) mode in Raman spectra indicates the good crystallinity of the samples. The study from the X-ray photoelectron spectroscopy (XPS) shows the successful insertion of both Eu3+ and Na+ ions into the ZnO lattice and the generation of the zinc and oxygen vacancies (Vo) defects. The band gap energy was reduced and the Urbach energy increased with Na+ content, proving the distortion of the lattice. From the photoluminescence (PL) study, the activation of the Eu3+ ions by Na+ ones was evidenced. Longer PL lifetimes were obtained from Eu3+ ions when they were sensitized by Na+, which may be beneficial to several applications. A process of excitation transfer from both the ZnO host and Na+ sensitizers to the Eu3+ ions was evidenced and discussed. As an application, we tested the performances of the prepared NCs as photocatalysts for Rhodamine B photodegradation under sunlight irradiation. The ZnO NCs codoped with 1% Eu/4% Na displayed the best photodegradation rate with a good stability and a high kinetic rate constant k of 0.021 min−1. The photocatalytic mechanism is discussed in terms of longer radiative recombination from Eu3+ and the generated oxygen vacancies.

Chromium (Cr)-doped zinc oxide (ZnO) nanorods with wurtzite hexagonal structure were prepared through a thermal decomposition technique. The concentration effect of the Cr doping on the structural, morphological, and optical properties of the ZnO nanorods was established by correlating various measurements: transmission electron microscopy (TEM), photoluminescence (PL), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and several UV-visible studies. The obtained nanorods were investigated as photocatalysts for the photodegradation process of methyl orange (MO), under UV-vis light illumination. Different weights and time intervals were studied. A 99.8% photodegradation of MO was obtained after 100 min in the presence of 1 wt.% Cr III acetate hydroxide and zinc acetate dehydrate “ZnO-Cr1”. The kinetic rate constant of the reaction was found to be equal to 4.451 × 10−2 min−1 via a pseudo-first order rate model. Scavenger radicals demonstrated the domination of OH• radicals by those of O2•− superoxide species during the photodegradation. The interstitial oxygen site Oi is proposed to play a key role in the generation of holes in the valence band under visible irradiation. The ZnO-Cr1 photocatalyst displayed good cycling stability and reusability.