Preparation and characterization of NaYF₄-based upconverting nanoparticles for solar energy storage systems

Upconversion nanoparticles (UCNPs) are materials that convert near-infrared photons into visible and/or UV emissions. This phenomenon generates high-energy radiation from the absorption of low-energy photons. The synthesis of UCNPs requires a support matrix composed of oxides (Y₂O₃) or fluorides (NaYF₄), which are essential for their optical properties. These materiales have a wide range of applications in bioimaging, drug delivery and energy storage in solar devices.

For this, UCNPs were synthesized by coprecipitation method. For better control of particle size and to obtain a beta (β) crystalline phase, the particles were subjected to heat treatment at 400 °C for 17 h inside a muffle. Subsequently, nanoparticles were deposited on a tetraethyl orthosilicate (TEOS) film synthesized by sol–gel technique, in a molar ratio of TEOS/H₂O/Ethanol at pH of 2, 3 and 5 in a temperature range of 25 to 80 ºC.

Both materials were characterized by infrared spectroscopy (FT-IR), X-ray diffraction (XRD), rheology, confocal (CM) and scanning electron microscopy (SEM). Through FT-IR, the characteristic bands of functional groups corresponding to TEOS were confirmed. The CM results exhibit that UCNPs emit shorter wavelengths and, therefore, higher energy radiation. SEM images show the synthesized films have an irregular and porous surface. However, this behavior increases as the pH decreases and the temperature rises. From the viscoelastic analysis, the films show a stable structure within linear viscoelastic region. Also, their possible transition temperatures from a gel state to a glassy state are observed at 80 ºC.

In conclusion, the particles show upconversion properties and their morphology and size depend on heating time during synthesis. On the other hand, roughness, porosity and structural arrangement of films changes when the pH and temperature of system varied. The viscoelastic properties show stable structures. Finally, the material properties indicate their possible use in solar concentrators for plastic degradation.