**4. Conclusions**

The fast microwave syntheses (15 min) of Yb/Eu doped Y2O3 nanostructures was reported. Microwave syntheses revealed itself to be an efficient way for successfully doping the Y2O3 host matrix after dissociating the primary oxides with different acids. In fact, this study revealed the key role of the acids used regarding the final structure and optical behaviour of the nanostructures produced. SEM and STEM revealed that the acetic acid resulted in nanosheets, while both hydrochloric and nitric formed nanospheres with heterogeneities in size. The study indicated the calcination temperature (700 ◦C) at which the conversion into Y2O3 is complete for such nanostructures. This temperature range is expected to cause an impact on production and energy costs of such nanostructures—in fact the calcination temperature is substantially lower than what is usually reported in literature. The optical properties of the produced materials were discussed, and the photoluminescence experiments showed a red Eu3+ emission when exited to a 976 nm source. Moreover, the sphere-like structures revealed enhanced luminescence, when compared to the nanosheets. The lower emission of the nanosheets was associated to a size effect, since acetic acid originated larger structures. The present study opens up to the possibility of these materials to be used as infrared to visible upconverters, and their potential to be integrated in several opto-electronic devices. For further studies, it is imperative to control the particle size and the size distribution of the developed nanostructures, which can be achieved by altering the catalyst or its concentration, but also with further investigation and adjustments on the synthesis parameters.

**Author Contributions:** D.N. and A.P. were responsible for writing the original manuscript with equal contribution; M.M. and T.F. performed the experiments; optical characterization was performed by A.P.; the DSC-TG and XRD characterization was performed by A.P.; D.N. performed the SEM; P.A.C. performed the STEM measurements; A.A., F.S., P.G. and S.G. participated in the review and editing and are responsible for funding, and finally the paper was under the supervision of R.M. and E.F.

**Funding:** This work was supported by FEDER funds, through the COMPETE 2020 Program, and national funds, through the Fundação para Ciência e Tecnologia (FCT), under the projects POCI-01-0145-FEDER-007688 (Reference UID/CTM/50025). The authors also acknowledge funding from the European Commission through the projects 1D-NEON (H2020-NMP-2015, gran<sup>t</sup> 685758-21D) and BET-EU (H2020-TWINN-2015, gran<sup>t</sup> 692373. The work was also partially funded by the Nanomark collaborative project between INCM (Imprensa Nacional—Casa da Moeda) and CENIMAT/i3N. D. Nunes and A. Pimentel acknowledge funding from FCT through the grants SFRH/BPD/84215/2012 and SFRH/BPD/76992/2011, respectively. P.A. Carvalho acknowledges support from the Research Council of Norway through grants 275752 and 197405/F50.

**Conflicts of Interest:** The authors declare no conflicts of interest.
