Upconversion Phosphors for Efficient Solar Energy Harvesting

Abstract

The underutilisation of the near-infrared (NIR) region in solar harvesting applications, such as semiconductor photocatalysis, necessitated the incorporation of upconverters, which are capable of absorbing multiple NIR photons absorbed and emit UV-Visible photons. This study investigates the emission properties of NaYF4:Yb0.20, Tm/Er0.005, NaBiF4:Yb0.20, Tm(0.005,0.02) and NaBiF4:Yb0.18 - 0.20, Er0.005 - 0.04 powders for upconversion of 980nm radiation to UV-Visible emissions. NaYF4:Yb, Tm/Er upconverters were prepared via hydrothermal synthesis where the structure directing agent (citric acid (CA) or trisodium citrate (TC)), NaF concentration (0.4-2.4 M), pH (5-12) or reaction time (2-16 h) was varied. Upconverters prepared with CA varied in phase composition, crystal shape and size. Weak emissions were observed at lower NaF concentrations (0.4-0.6 M) due to smaller YF3 and a-NaYF4 particles, whilst, higher concentrations (0.8-2.4 M) formed larger a, b-NaYF4, then bNaYF4, particles with more intense emissions at UV-Visible regions. Increasing the pH (5-7) during preparation resulted in hexagonal b-NaYF4 microparticles with similar emission profiles and further increase (9-12) produced shorter and less crystalline upconverters, compromising emission intensities. In contrast, samples prepared with TC resulted in b-NaYF4 hexagonal nanoparticles (0.6-2.4M NaF), where emission intensities increased with NaF concentration. Increasing the pH of these samples to 10 and 12 resulted in smaller nanoparticles and non-crystalline powders, respectively, showing highly quenched emissions. Decreasing reaction times (<16 h) also showed decreased emission intensities. For comparison, NaBiF4-type upconverters with similar Yb3+:Tm3+/Er3+ doping amounts were prepared via precipitation. Increasing the NH4F content added and decreasing its addition rate, obtained upconverters with improved crystallinity, size and structure control, with increased upconverted emission intensities particularly at higher energies compared to its NaYF4 analogues. Changing the reaction temperatures (25-50 _C) or the Yb3+:Er3+ ratios (20-18%:0.5-4%) had variable effects on the crystallinity and structure of the upconverters and similarly with their emissions. Additionally, composites of the NaYF4:Yb0.20,Tm0.005 upconverters were prepared deposition of Au nanoparticles (0.5-5.0 wt.%), where intensities of the higher energy emissions quenched with higher Au loading, and by SiO2 coating, where intensities were unchanged. These composites would serve the basis for future studies.