Synthesis and Potential Application of Rare Earth Doped Fluoride Based Host Matrices

S.P. Tiwari1*, R.S. Yadav2, S.K. Maurya3, A. Kumar3, Vinod Kumar1 and H.C. Swart1

  • 1 Department of Physics, University of Free State, Bloemfontein - 9300, South Africa
  • 2 Department of Zoology, Institute of Science, Banaras Hindu University,

Varanasi - 221005, India

  • 3 Department of Physics, IIT (ISM) Dhanbad - 826004, India
  • 4 Center for Energy, Indian Institute of Technology Delhi, New Delhi -110016, India


Rare earth doped fluoride nanoparticles are potential host matrices for intense upconversion (UC) emissions (Tan et al. 2009; Yi et al. 2006; Maurya et al. 2019; Kumar et al. 2007; Chen et al. 2015; Xu et al. 2015). Some research has been performed on different hosts such as GF4, YF4, LaF4, NaGdF4, NaYF4, NaLaF4, BaGF5, BaYF5, etc. (Li et al. 2018; Chen et al. 2012; Kumar et al. 2015; Qui et al. 2014; Maurya et al. 2018; Runowski et al. 2014; Cao et al. 2011; Ivanova et al. 2008; Shan et al. 2010). These research outputs confirm the ability of these particles for different innovative applications. The main applications viz., temperature sensing, latent fingermarks detection, solar energy harvesting, bioimaging, and photo-catalytic by using these particles have been reported (Cui et al. 2014; Kumar et al. 2009; Xing et al. 2012; Yang et al. 2013; Sun et al. 2011). Tire choice of fluoride hosts has been taken into account due to their high chemical stability

All the arbitrary applications depend on typical synthesis techniques. Synthesis of rare earth doped fluoride phosphors using different methods

Corresponding authors: This email address is being protected from spam bots, you need Javascript enabled to view it has been reported by many researchers (Lojpur et al. 2013; Yi et al. 2004; Liu et al. 2018; Lemyre et al. 2005; Pandey et al. 2018). Tire combustion, co-precipitation, hydrothermal, thermal decompositions etc. are the most common synthesis methods. These synthesis routes are rather significant for the development of particles like oxide, sulfide, phosphate, fluoride-based host materials. Depending on the temperature condition and the approach of the application of material the reaction time and use of organic solvents are mixed in a schematics ratio. Among these preparation methods, combustion synthesis requires very high temperature while other routes have rather low temperature for synthesis (Tiwari et al. 2015; Zeng et al. 2011; Niu et al. 2011). For the synthesis of fluorides, the temperature conditions should be low as at a higher temperature (>500 °C) the fluorides generally degrade to form oxides. Therefore, the structural and optical properties may change accordingly. The co-precipitation, hydrothermal and thermal decomposition methods are therefore better approaches for the preparation of the rare earth doped fluoride phosphor particles, which have been discussed in different parts of this chapter.

The fluoride phosphor particles may be stable in power as well as in the colloidal dispersed state in different solvents (Niu et al. 2011; Ye et al.

2010). For the colloidal stability, the particle size should be a minimum with a uniform shape (Gainer et al. 2014). There are several ways to check the structural properties of fluoride phosphor particles. The crystal structure and crystalline sizes can be confirmed through X-Ray Diffraction (XRD) characterization. The micro/nanoscopic structure can be observed through Field Emission Scanning/Transmission Electron Microscopy (FESEM/ ТЕМ) and the particle shape and sizes can be optimized. The use of different precursors with synthesis may cause impurities in the envelope fluoride particles. Generally, these impurities are organic and can be estimated through Fourier transform infrared (FTIR) spectroscopy. Further, the reaction of these impurities can be examined by the same characterization. Apart from these the Energy Dispersive X-Ray Analysis (EDS), X-ray Photoelectron Spectroscopy (XPS) etc. analysis are useful for elemental stabilizations/ confirmations. Zeta potentials are useful for colloidal dispersion stability. In optical characterizations, the Uv-Vis analysis is useful for the estimation of the optical b

As it was described earlier fluoride phosphor nanoparticles are very significant in many applications. Meruga et al. (2012) demonstrated the security printing/writing applications of NaYF4:Er3+/Yb3+. Wang et al. (2015) discussed the latent fingermarks detection application of fluoride phosphor particles. Ramasamya et al. (2014) successfully utilized the UCNPs in the achievement of the higher efficiency of dye synthesize solar cell. Chen et al. (2015) have demonstrated the powder YF3:Tm3+/Yb3+/Ca2+ in the application of non-contact type temperature sensors. In the bio-imaging fields, Wang et al. (2010) did significant research with NaYF4 doped with different ions. In this present chapter, we have discussed the synthesis, characterization and UC emission in different fluoride hosts.

Synthesis of Fluoride Nanoparticles

The development of high-quality rare earths-doped phosphors with precise shape and size and crystalline phase is fundamental to tune their optical and chemical properties and investigate their applications in different fields. The biggest challenges for the preparation of phosphors are the reproducibility and mono-dispersion of the phosphors. The nucleation and growth process of luminescent phosphors are dependent on the initial crystallization. Victor LaMer had extensively studied the nucleation-growth mechanism and it is in general referred to as the LaMer mechanism (Pound et al. 1952; LaMer et al. 1950). A recent article has been published to explain the kinetic arid thermodynamic processes in the growth of particles (Kumar et al. 2018). An overview of the different kind of UC luminescent phosphors are shown in Table 13.1. Many groups have reported the preparation of phosphor materials using orgcinic solvents as well. Tire prepared crystalline particles need surface modification for mono-dispersion. For applications in aqueous environments, only colloidally stable phosphors can be used. Tire most common methods for the synthesis of fluoride phosphors are thermal decomposition, hydrothermal and co-precipitation. The other preparation strategies are described in Table 13.1 (DaCosta et al. 2014).

All the reported methods of synthesis of nanoparticles with different routes are described below.

Thermal Decomposition Method

This method is usually used for the preparation of fluoride-based phosphors and is almost solely applied in the creation of alkali rare earth tetrafluoride phosphor such as LiYF4, LiGdF4, NaGdF4, NaYF4, NaLuF4, KGdF4, etc. This method is also used to produce rare earth doped/co-doped nanoparticles based on further host lattices with BaYF5, YF3, CaF2, etc. This technique gives excellent control on particle size and shape with comparatively short reaction times with respect to other methods and produces monodispersed phosphors Flaase et al. 2011). Generally orgcinometallic precursors such as rare earth-trifluoroacetates are used. The rare earth-trifluoroacetates precursors are prepared by reacting trifluoroacetic acid (TFA) with Ln- oxides in the laboratory and are also available commercially. A high boiling solvent 1-octadecene (ODE), Oleic Acid (OA)

Synthetic strategy




Materials prepared

Arrested precipitation

Poorly soluble product precipitated within a template or confined space

Simple and fast reaction, cost effective, does not require high temperatures or pressures

Little control over particle shape and size, aggregation is typical, high-temperature post reaction annealing/ calcination step required resulting in aggregation









Combustion synthesis

Typically done to prepare metal oxides M203; nanoparticle formation occurs following rapid heat pulse from combustion of propellant

Very rapid synthesis, cost-effective, low energy cost

Very little control over particle shape, size, and purity, considerable aggregation of particles, observed






Sol-gel method


implemented in the synthesis of metal oxides; prepared materials resemble those prepared by combustion synthesis

Metal acetate or metal alcoxide precursors are very inexpensive, simple reactions

Little control over particle shape and size, high-temperature post reactioia annealing/ calcination step required resulting in aggregation










Synthetic strategy




Materials prepared



Uses 0.3-300 GHz microwave irradiation to heat reaction mixtures

Increased reaction rates, milder reaction conditions decreased energy consumption, high reproducibility

Requires specialized microwave irradiators, limited solvent choice (must be effectively heated by microwaves)




M,03 (M = Pr, Nd,

Sm, Eu, Gd, Tb, Dy)




Micro-emulsion or reverse

Micelle method

Uses the interior aqueous environments of reverse micelles in organic solvents as nano-scale reactors

Very versatile, reproducible, produces homogenous monodisperse materials, control over size and morphology of produced materials

Organic solvents being used, very limited production capacity since relying on amount of aqueous phase that can be solubilized and precursor concentrations





X02 (X = Ce, Sn, Zr)




XZr03 (X = Sr, Ba, Pb)

Flame synthesis

Gas-phase materials are passed through a flame source using a hydrocarbon fuel and 02 as the oxidant.

Heat energy permits formation of crystalline materials.

High-purity products obtained, narrow size distributions, time and cost effective, facile control over

morphology, scalable for industrial applications

Considerable aggregation of particles is observed, problems synthesizing multicomponent materials






Reproduced with permission from Ref. (DaCosta et al. 2014) @ Copyright 2014 Elsevier.

temperature of the precursors (>300 °C). A required solution of Ln- trifluoroacetates is poured into a hot ligand solution. When the reactants are added, due to thermal decomposition, the particles are quickly released and rapid burst nucleation take place (Kumar et al. 2019). These ligands have polar head groups. During the reaction, they help to grow the particles by coordinating. These long hydrocarbon tails provide the bulk solution of the required solubility (Lin et al. 2012). These surface ligands control the particles growth by blocking the extension of the lattice and also stabilize the particle growth by stopping aggregation over the repulsive interactions iir the solution. These metal precursors are expensive, air sensitive and toxic. That is why inert atmosphere is used for annealing purposes. The precipitated material is centrifuged and washed with organic polar solvents. For monodispersed particles, the organic solvents are extracted from the nanoparticles (NPs) and finally colloidally stable particles are collected for a long span (Boyer et al. 2006). The first thermal decomposition method was reported by Mai and his group for the synthesis of NaREF4 (where RE stands for Pr to Lu, Y) co-doped Er3VYb3+ and Tm3+/Yb3+ nanoparticles in 2006 (Mai et al. 2006). They synthesized NaREF4 UCNPs with a decomposition method with a cubic and hexagonal phase. Figure 13.1 shows the hexagonal- phase particles of synthesized UCNPs. The LaMer mechanism was followed for the growth of hexagonal-NaYF4 and cubic-NaYF4 (Boyer et al. 2006; Mai et al. 2006; Wang et al. 2009). Five run monodispersed UCNPs were obtained by tuning and varying the reaction parameters such as concentrations, temperature and reaction time (Wang et al. 2009; Mai et al. 2007). The Ostwald ripening method can also be applied for obtaining monodisperse hexagonal-NaYF4 using rare earth-doped cubic-NaYF4 UCNPs (Wang et al. 2009). The hexagonal-NaYF4 phase can be achieved by refining the reaction criteria via thermal decomposition. Hexagonal-NaYF4 UCNPs doped with lanthanides with uniform shape and size of around 10 run was obtained by re

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