[en] Regular sized nanostructures of indium oxide (In₂O₃) were homogeneously grown using a facile route, i.e. a microwave-hydrothermal method combined with rapid thermal treatment in a microwave oven. The presence of Er³⁺ doping plays an important role in controlling the formation of cubic (bcc) and rhombohedral (rh) In₂O₃ phases. The samples presented broad photoluminescent emission bands in the green-orange region, which were attributed to the recombination of electrons at oxygen vacancies. The photocatalytic activities of pure bcc-In₂O₃ and a bcc-rh-In₂O₃ mixture towards the UVA degradation of methylene blue (MB) were also evaluated. The results showed that Er⁺³ doped In₂O₃ exhibited the highest photocatalytic activity with a photonic efficiency three times higher than the pure oxide. The improved performance was attributed to the higher surface area, the greater concentration of electron traps due the presence of the dopant and the possible formation of heterojunctions between the cubic and rhombohedral phases. - Graphical abstract: Photodegradation curves as a function of irradiation time of the samples obtained upon rapid microwave heating. - Highlights: • Efficient and rapid microwave heating to obtain Er³⁺ doped In₂O₃ nanostructures. • Er³⁺ ions doping is fundamental to stabilizing the crystalline rhombohedral phase. • Symmetry breaking induced by vacancies in the lattice leads to photoluminescence. • Surface area of doped sample was two times higher than the surface of pure oxide. • The presence of defects in the lattice structure favors photocatalytic activity.

[en] Highlights: • ZnO nanoparticles were obtained directly on reduced graphene oxide sheets. • rGO promoted electron transport and the reduction of charge carriers recombination. • Nanocomposites show lower onset potential than the requirement for H2O oxidation. • The resulting nanocomposites exhibited high current density during electrolysis. • The overpotential for water oxidation decreased in alkaline pH. - Abstract: Reduced graphene oxide-zinc oxide (rGO-ZnO) nanocomposites were successfully synthesized using a facile microwave-hydrothermal method under mild conditions, and their electrocatalytic properties towards O₂ evolution were investigated. The microwave radiation played an important role in obtainment of well dispersed ZnO nanoparticles directly on reduced graphene oxide sheets without any additional reducing reagents or passivation agent. X-ray diffraction (XRD), Raman and infrared spectroscopies indicated the reduction of GO as well as the successful synthesis of rGO-ZnO nanocomposites. The chemical states of the samples were shown by XPS analyses. Due to the synergic effect, the resulting nanocomposites exhibited high electronic interaction between ZnO and rGO sheets, which improved the electrocatalytic oxidation of water with low onset potential of 0.48 V (vs. Ag/AgCl) in neutral pH and long-term stability, with high current density during electrolysis. The overpotential for water oxidation decreased in alkaline pH, suggesting useful insight on the catalytic mechanism for O₂ evolution.

[en] We report for the first time a rapid preparation of Zn_1_−_2_xCo_xNi_xO nanoparticles via a versatile and environmentally friendly route, microwave-assisted hydrothermal (MAH) method. The Co, Ni co-doped ZnO nanoparticles present an effect on photoluminescence and electrochemical properties, exhibiting excellent electrocatalytic performance compared to undoped ZnO sample. Photoluminescence spectroscopy measurements indicated the reduction of the green–orange–red visible emission region after adding Co and Ni ions, revealing the formation of alternative pathways for the generated recombination. The presence of these metallic ions into ZnO creates different defects, contributing to a local structural disorder, as revealed by Raman spectra. Electrochemical experiments revealed that the electrocatalytic oxidation of dopamine on ZnO attached to multi-walled carbon nanotubes improved significantly in the Co, Ni co-doped ZnO samples when compared to pure ZnO. - Graphical abstract: Rapid synthesis of Co, Ni co-doped ZnO nanoparticles: optical and electrochemical properties. Co, Ni co-doped ZnO hexagonal nanoparticles with optical and electrocatalytic properties were successfully prepared for the first time using a microwave hydrothermal method at mild conditions. - Highlights: • Co"2"+ and Ni"2"+ into ZnO lattice obtained a mild and environmentally friendly process. • The heating method strongly influences in the growth and shape of the particles. • Short-range defects generated by the ions insertion affects the photoluminescence. • Doped ZnO nanoparticles improve the electrocatalytic properties of pure oxide