Rare earth nanofluorides: synthesis using ionic liquids

Miroslava Guricová 1 , Jan Pinc 1 , 2 , Juraj Malinčik 3 , Jakub Rak 4 , 5 , Martin Kuchař 4 , 5 ,  and Vilém Bartůněk 6
  • 1 Department of Inorganic Chemistry, Faculty of Chemical Technology, University of Chemistry and Technology, Prague, Czech Republic
  • 2 Department of Metallic Materials and Corrosion Engineering, Faculty of Chemical Technology, University of Chemistry and Technology, Prague, Czech Republic
  • 3 Department of Organic Chemistry, Faculty of Chemical Technology, University of Chemistry and Technology, Prague, Czech Republic
  • 4 Forensic Laboratory of Biologically Active Substances, Department of Chemistry of Natural compounds, University of Chemistry and Technology, Prague, Czech Republic
  • 5 National Institute of Mental Health, Topolová 748, 250 67 Klecany, Czech Republic
  • 6 Department of Inorganic Chemistry, Faculty of Chemical Technology, University of Chemistry and Technology, Technická 5, 166 28 Prague 6, Czech Republic
Miroslava Guricová
  • Department of Inorganic Chemistry, Faculty of Chemical Technology, University of Chemistry and Technology, Prague, Czech Republic
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, Jan Pinc
  • Department of Inorganic Chemistry, Faculty of Chemical Technology, University of Chemistry and Technology, Prague, Czech Republic
  • Department of Metallic Materials and Corrosion Engineering, Faculty of Chemical Technology, University of Chemistry and Technology, Prague, Czech Republic
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, Juraj Malinčik
  • Department of Organic Chemistry, Faculty of Chemical Technology, University of Chemistry and Technology, Prague, Czech Republic
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, Jakub Rak
  • Forensic Laboratory of Biologically Active Substances, Department of Chemistry of Natural compounds, University of Chemistry and Technology, Prague, Czech Republic
  • National Institute of Mental Health, Topolová 748, 250 67 Klecany, Czech Republic
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, Martin Kuchař
  • Forensic Laboratory of Biologically Active Substances, Department of Chemistry of Natural compounds, University of Chemistry and Technology, Prague, Czech Republic
  • National Institute of Mental Health, Topolová 748, 250 67 Klecany, Czech Republic
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and Vilém Bartůněk
  • Corresponding author
  • Department of Inorganic Chemistry, Faculty of Chemical Technology, University of Chemistry and Technology, Technická 5, 166 28 Prague 6, Czech Republic
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Abstract

This review presents a comprehensive summary of the research progress on the synthesis of rare earth fluoride nanomaterials using the most common methods of synthesis. Special focus is on syntheses utilising ionic liquids, which is a new and promising way of preparing nanomaterials without the use of dangerous organic solvents (toxic, flammable, or combustive). Rare earth fluoride nanoparticles can be obtained with a high yield, purity, and crystallinity, and with different morphologies and luminescent properties depending on the selected method of synthesis.

Introduction

Rare earths (REs) are a group of elements consisting of Sc, Y, and the lanthanide group. REs generally possess similar physical and chemical properties, enabling the formation of mixed RE compounds, where trivalent RE ions are contained in various ratios in a single crystal lattice. Most of the lanthanides have partially occupied f-orbital, thus enabling various photon-emitting electron transitions.

RE fluorides (REFs) are particularly an interesting group of RE compounds. Mixed REFs are of great interest (Fedorov et al. 2011) because of their nonlinear optical properties. Their large-scale utilisation is still in a relatively early stage, but their distinct electrical, optical, and magnetic properties make them promising materials in photonics (e.g. lasers, infrared quantum counters, and optical telecommunication). They offer potential use in biomedical applications such as biochemical probes for bioimaging and medical diagnostics and in advanced therapeutics for cancer treatment (Zhao et al. 2014).

Ionic liquids (ILs) and room temperature ionic liquids (RTILs) are salts occurring in liquid state at temperatures below 100°C and at room temperature, respectively. Typical ILs consist of voluminous and highly asymmetric organic cations and organic or inorganic anions, which strongly decreases their melting point. In liquid state, they are preferably composed of single ions and short-lived ion pairs. Possibility of delocalisation of charges on the surface of voluminous ions decreases coulomb forces (Jacob et al. 2006). ILs have attracted wide interest of researchers because of their adjustable viscosity, melting point, acidity, and other physical and chemical properties, as well as their negligible vapour pressure, low combustibility, and high thermal stability (Ding et al. 2007). Moreover, they are powerful solvents for a range of polar and nonpolar compounds, and many chemical reactions can be performed using ILs as a solvent (Nuria and Manuel 2007). Their distinctive properties make them attractive for potential applications in diverse fields such as organic chemistry, electrochemistry, catalysis, physical chemistry, and engineering (Bühler and Feldmann 2006, Guo et al. 2016).

In this paper, we present an overview of the synthesis of nanosized REFs with a focus on syntheses, where ILs are employed as an alternative to the more commonly used solvents, e.g. oleic acid. Approaches based on ILs can be favourably used in the cases, where the production of large quantities of nanosized REFs is desired, in medium- to large-scale syntheses and in industrial applications.

REFs and their properties

REFs in the form of nanoparticles have interesting properties for various applications. Their insolubility in protic solvents is of practical value in synthesis as it enables preparation by precipitation and facilitates purification of the final product. Moreover, their optical properties play an important role in application in the form of bulk crystalline or glass matter or in the form of nanoparticles as a light source or as detectors. Typical well-known examples of REFs utilisation are lanthanum fluoride electrodes doped with europium difluoride or heavy-metal fluoride glasses for fibre technologies. Selected physical properties of some REFs are summarised in Table 1.

Table 1:

Table of selected physical properties of some REFs.

MaterialDensity (g cm−3)Magnetic susceptibility (10−6 cm3 mol−1)Phase transition temperature (°C), phase 1/phase 2
NaYF4 (hexagonal)4.14t (Roy and Roy 1964)−0.28 (Engelhardt and Figgis 1970)691, hexagonal/cubic (Thoma et al. 1963)
NaLaF4 (hexagonal)4.69t (Keller and Schmutz 1964)

4.68c (Zachariasen 1948)
NaPrF4 (hexagonal)4.93t (Keller and Schmutz 1964)5600 (Bukhalova et al. 1969)
NaEuF4 (hexagonal)5.44t (Keller and Schmutz 1964)

5.45c (Zakaria et al. 1997)
NaGdF4 (hexagonal)5.62t (Keller and Schmutz 1964)

5.612c (Aebischer et al. 2006)
950, hexagonal/cubic (You et al. 2002)
NaHoF4 (hexagonal)6.01t (Keller and Schmutz 1964)760, hexagonal/cubic (Roy and Roy 1964)
NaErF4 (hexagonal)6.01t (Keller and Schmutz 1964)775, hexagonal/cubic (Roy and Roy 1964)
NaYbF4 (hexagonal)6.42t (Keller and Schmutz 1964)930, hexagonal/liquid (Fedorov et al. 1983)
KLaF4 (hexagonal)4.52c (Zachariasen 1948)
KCeF4 (hexagonal)4.542t (Brunton 1969)755, hexagonal/cubic (Brunton 1969)
KPrF4 (rhombic)4.14c (Bukhalova et al. 1969)5580 (Bukhalova et al. 1969)
KNdF4 (hexagonal)4.79t (Zakharova et al. 1974)
KSmF4 (hexagonal)4.51t (Hoppe et al. 1980)
KLuF4 (hexagonal)5.15t (Hoppe et al. 1980)

c, Crystallographic; t, theoretical.

Preparation and solubility of REFs

Pure anhydrous bulk REFs can be prepared by treating RE oxides with fluorinating agents, e.g. HF, NH4F, or NH4HF2 (Batsanova 1971). Hydrated REFs can be prepared by precipitation from soluble RE salts using hydrofluoric acid, as all REFs are poorly soluble in water and in acids, in contrary to other RE halogenides (Remy 1961, Batsanova 1971, Greenwood and Earnshaw 1997). Quantitative information on their solubility was only found in literature for fluorides of the general formula REF3 and CeF4. Experimental determination of PmF3 solubility is complicated by the radioactivity of all promethium isotopes (Mioduski 1988). Solubilities of REF3 compounds from different sources scatter significantly, as they are affected by many factors such as pH, fluoride ion concentration, ionic strength, and temperature. A critical evaluation of the REFs solubility was presented by Mioduski et al. (2014, 2015a,b). Anyway, all the sources agree that the solubility of REF3 compounds in water ranges between approximately 10−3 and 10−7 mol dm−3, and it has an increasing trend with increasing atomic number with exceptions of ScF3, which is the most soluble compound of the series, and YF3 with solubility comparable to TbF3 and DyF3. An example of the described trend measured by Itoh et al. (1984) is displayed in Figure 1.

Figure 1:
Figure 1:

Solubility data for the REF3 compounds at 25°C measured by Itoh et al. (1984).

Citation: Reviews in Inorganic Chemistry 39, 2; 10.1515/revic-2018-0016

Optical properties of REFs

REs are in general photonic materials, and each of them possesses specific energy levels and corresponding absorption and emission wavelengths (Remy 1961, Li et al. 2015). The optical properties of REFs also depend on the crystalline structure, which is usually hexagonal or orthorombic for pure REFs (Remy 1961, Batsanova 1971, Greenwood and Earnshaw 1997) and hexagonal, orthorombic, cubic, or tetragonal for the nanoparticles (Xu et al. 2009, Li et al. 2015). Figure 2 displays few examples of REF structures. The presence of specific RE metal in the crystal lattice can induce several types of luminescence including nonlinear light effects, where the most important forms in the case of REFs are down-conversion and up-conversion.

Figure 2:
Figure 2:

Crystal structures of (A) hexagonal NaYF4, (B) cubical NaYF4, and (C) cubical YF3.

Citation: Reviews in Inorganic Chemistry 39, 2; 10.1515/revic-2018-0016

Up-conversion is a multiphotonic optical process, where electromagnetic radiation of a higher wavelength is transformed to radiation of a shorter wavelength – this means that the number of photons emitted is lower than the number absorbed. A typical example is the conversion of infrared light to visible light. Down-conversion is the exact opposite emitting more photons than absorbed. Down-conversion is used, e.g. for optical amplification in erbium-doped glasses or crystals. A schematic representation of these processes is included in Figure 3. Scintillation of various REFs, e.g. cerium fluoride (Moses et al. 1994), is another interesting optical property worthy of further research.

Figure 3:
Figure 3:

Illustration of the up-conversion and down-conversion of REFs.

Adapted from: Suyver et al. (2005).

Citation: Reviews in Inorganic Chemistry 39, 2; 10.1515/revic-2018-0016

The fluorescence frequency depends solely on the of the used RE metal, while the intensity and time of luminescence can be influenced by the size of the nanoparticle (Weller 1965, Feofilov 2002, Stouwdam et al. 2003, DiMaio et al. 2006, Lezhnina et al. 2006, Zhang et al. 2009b, Fedorov et al. 2011, Haase and Schäfer 2011, Li et al. 2015). As a result, doping of a suitable host material with REs is a frequently used alternative to pure REFs, especially convenient in the case of their high price. The specific energy absorptions and emissions can be influenced by the type of RE metal used as the dopant (Feofilov 2002, Haase and Schäfer 2011, Li et al. 2015), while it is almost independent of the host matrix. Intensity of the emission can be amplified by increasing ratio of the RE metal in the matrix (Zhang et al. 2009b).

Compounds of the general formula AREF4 (A=alkali) are an important type of luminescent material, e.g. NaYF4 doped with RE metals such as Er or Yb is commonly used as up-conversion light source (Mai et al. 2006, Xu et al. 2009, Li and Lin 2010, Yao et al. 2012).

Ionic liquids

ILs are relatively young and fast developing phenomena with several previously published reviews (Welton 1999, Sheldon 2001, Dupont et al. 2002, Wasserscheid and Keim 2000, Marsh et al. 2004, Ma et al. 2010). Although they were first described over 100 years ago by Walden (1914), their practical use was established only around 30 years ago. In general, they can be described as molten salts or liquids consisting only of ions. Melting point below 100°C and a low viscosity were introduced to their definition to distinguish them from inorganic molten salts with high melting points, which can be highly corrosive and viscous. In recent years, interest in RTILs consisting of bulky organic cation and organic/inorganic anion increased because of apparently easier handling (Welton 1999). One of the nowadays most used ILs is 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6] and chloride [bmim][Cl] shown in Figure 4.

Figure 4:
Figure 4:

Structures of selected ionic liquids (A) [bmim][PF6] and (B) [bmim][Cl].

Citation: Reviews in Inorganic Chemistry 39, 2; 10.1515/revic-2018-0016

ILs have several advantages over the commonly used organic solvents; the most important being their thermal stability and negligible vapour pressure. Other important properties are little to no flammability, immiscibility with many organic solvents, and their ability to dissolve both organic and inorganic substances with a wide range of polarity. As a result, ILs became an alternative solvent for a number of commonly used organic and inorganic reactions (Welton 1999, Dupont et al. 2002, Marsh et al. 2004, Ma et al. 2010, Bartunek et al. 2017). They also found their use in catalysis (Welton 1999, Sheldon 2001, Dupont et al. 2002, Wasserscheid and Keim 2000, Zeng et al. 2013) and dual-phase extraction (He et al. 2011b). A number of papers on the use of ionic liquids in nanoparticle synthesis was published in the recent years (Dash and Scott 2011, Xu and Zhu 2012, Schadt et al. 2013, Uematsu et al. 2014, Corrêa et al. 2016, Łuczak et al. 2016) with nanoparticles containing REFs not being an exception (Nuria and Manuel 2007, Zhang et al. 2008, Lorbeer et al. 2010), Bartůněk et al. 2011, 2012, 2013a,b, 2015, He et al. 2011a,b, Li et al. 2011b, Lorbeer et al. 2011a,b, 2014, Cybinska et al. 2012). The ILs most widely used for synthesis of REFs are summarised in Table 2.

Table 2:

ILs most frequently used for nanoparticles synthesis with their melting temperatures (Zhang et al. 2006).

Cation typeNameFormulaMelting point (°C)
Dialkylimidazolium1-Butyl-3-methylimidazolium hexafluorophosphate[bmim][PF6]10
1-Butyl-3-methylimidazolium tetrafluoroborate[bmim][BF4]−81
1-Butyl-3-methylimidazolium chloride[bmim][Cl]68
1-Methyl-3-octylimidazolium hexafluorophosphate[omim][PF6]−70
Alkylpyridinium1-Butylpyridinium tetrafluoroborate[bPy][BF4]7

Methods of RE nanofluoride synthesis

Research groups were striving to improve REFs synthesis and make it more effective for the last decade, especially to enable their usage in medicine for cancer treatment and for tissue labelling. Synthesis of REF nanoparticles in large volumes is particularly needed because of their possible applications. Majority of the applied methods for preparation of REFs can be divided into two groups: solvothermal and hydrothermal synthesis, and thermal decomposition, respectively. In the last few years, methods such as microwave-irradiation assisted synthesis, co-precipitation, and pyrolysis were also used. All these methods focus primarily on the control of particles size and crystal phase to tune the desired optical properties like up-conversion. Some other properties as surface stabilisation and subsequent stability of dispersions of prepared nanoparticles are affected by the course of the reaction.

Hydrothermal and solvothermal synthesis

Hydrothermal synthesis of REF nanoparticles is a very simple and cost-effective method. The common procedure is to mix a fluoride precursor with an RE precursor in an aqueous solution, heat it, and subsequently separate the product (Rahman and Green 2009). This method is generally suitable for the preparation of materials of a controlled size and morphology. Another advantage of the method is easy preparation of large amounts of product in a single batch. Water-based systems are also environmentally friendly and can be used as a replacement to several toxic organic solvents.

Nevertheless, the temperature limit of a water environment, 100°C, is often insufficient for the synthesis of the desired product. Therefore, organic solvents are alternatively used for the preparation of REF nanoparticles (Jia et al. 2014), most typically oleic acid and octadec-1-ene (Liu et al. 2009b, Dong et al. 2015). Except for less common organic solvents like linoleic acid, ethylenediamine, or oleylamine, solvothermal synthesis is often performed in ILs (described in detail in the “Synthesis Utilising Ionic Liquids” section).

The solvothermal process can be adapted for the synthesis of a wide range of REFs by adjustment of the experimental parameters (temperature, pressure, pH, and concentration of the precursors). Especially temperature and concentration are known to affect the resulting phase composition (Zhang et al. 2009b) or size of the prepared nanoparticles (Cao et al. 2011). An example of the most common utilisation of the solvothermal method is the synthesis of up-converting NaREF4 nanoparticles doped by erbium and ytterbium (Xu et al. 2009).

Thermal decomposition

In the thermal decomposition syntheses, the products are prepared by decomposition of thermolabile precursors at elevated temperatures. Thermal decomposition is widely combined with other methods for the preparation of nanoparticles with specific properties (Macák et al. 2005). Solvothermal synthesis and thermal decomposition often overlay in the case of REF nanoparticles as solvents with high thermal stability and boiling point such as oleic acid or octadec-1-ene (Li et al. 2012) are mostly employed. In these cases, classification of the synthesis is only a question of the chosen nomenclature.

Morphology, size, and shape of the nanoparticles are mostly affected by temperature and duration of the thermal decomposition. In specific cases, e.g. in the case of the thermal decomposition of Na(CF3COO) and RE(CF3COO)3, the properties of the product can be affected by the ratio of Na to RE or solvent composition (Mai et al. 2006). Thermal decomposition can be also used for the preparation of thin REF films with unique optical properties (Payrer et al. 2013). REF nanoparticles are frequently prepared from precursors such as trifluoroacetates. High-purity nanoparticle synthesis with lower demands on the reaction time and temperature can be achieved by synthesis at low pressures (Liu et al. 2009a).

Microwave-assisted synthesis

One of the most recently developed methods of REFs preparation is the microwave-assisted synthesis. Microwaves are used in cooperation with solvothermal synthesis or thermal decomposition. The benefits of this method compared with conventional methods include reaction time reduction, shorter crystallisation time, increase in yield (Rao et al. 1999), stabilisation, and formation of pure crystalline phases. Moreover, the size of the prepared nanoparticles is considerably lower (Blackwell 2003) compared with that in conventional processes. The time reduction can generally surpass 90% (Ding et al. 2014), mainly because of uniform heating and rapid heating rate (Bilecka and Niederberger 2010, Dambournet et al. 2010). In our recent paper, the time reduction of the hydrothermal synthesis of NaYxGdyYbzEr(1−xyz)F4 from 3 h to 1 min was achieved using a common domestic microwave with a continual output of 1 kW (Bartůněk et al. 2015).

Nanocrystals of PrF3 with a closed-cage sphere-like morphology and a particle size of approximately 3 nm were prepared by combining hydrothermal synthesis and microwave irradiation in (Ma et al. 2007). In comparison, the conventional hydrothermal synthesis leads to nanoparticles of PrF3 aggregating together and reaching a completely different morphology (Ma et al. 2007).

Co-precipitation method

One of the largely applied methods of nanomaterials preparation is the co-precipitation method. The co-precipitation method was used to prepare barium and yttrium fluorides from aqueous solutions at room temperature (Fedorov et al. 2012). NaYF4 nanocrystals were also synthesised by co-precipitation in the presence of various chelators (Fedorov et al. 2012): amino-carboxyl (H2N-C) chelators covering sodium ethylenediaminetetraacetate (Na-EDTA) and sodium nitrilotriacetate (Na-NTA) and hydroxyl-carboxyl (HO-C) chelators covering sodium citrate (Na-citrate), sodium malate (Na-malate), and potassium sodium tartrate (K, Na-tartrate). As a result, up-conversion fluorescent material, cubic NaYF4 nanocrystals co-doped with ytterbium and erbium with a diameter of approximately 10 nm were successfully prepared at room temperature. Concentrations of the different chelators affected the fluorescent properties of the formed NaYF4:Yb:Er nanocrystals in different ways according to their respective physical and chemical properties (Zhou et al. 2010). It should be emphasised that cubic and not hexagonal systems were prepared, where the up-converting properties are not optimal from the point of view of the overall quantum yield. Precipitation of nanocrystals from an aqueous solution of a fluoride-ion source and an RE nitrate solution together with polyethylenimine (Yasyrkina et al. 2014) proves the effect of pH of the formed solution and the order of reagent addition on the solid phase morphology and composition of the NaYF4:Yb:Er nanoparticles. LaF3:Eu3+ nanoparticles were synthesised by the co-precipitation method using glycerine or Triton X-100 as a surfactant and NH4F as a fluorination agent at 50°C (Grzyb et al. 2016). An interesting result was a partial reduction of Eu3+ to Eu2+ by glycerol, inducing white luminescence under UV excitation.

Synthesis utilising ionic liquids

The synthesis of nanofluorides with the utilisation of ILs usually follows the solvothermal synthesis or its combination, e.g. with microwave-assisted synthesis. The application of ILs provides a large temperature window for the synthesis without any problems related to reaching high vapour pressure of the solvent and higher yields. In addition to being solvents in nanoparticle synthesis, ILs can be also used as fluorination agents, co-solvents, other reactants, and templates (Zhang et al. 2009a). The typical procedure utilising ILs is schematised in Figure 5. It is first necessary to prepare a homogenous mixture of precursors with the IL, which can be facilitated by adding various surfactants for the surface stabilisation or functionalisation. Next, the mixture is transferred into a fluorination-resistant container, typically made of Teflon or stainless steel. The fluorination agent is usually added last to trigger the reaction. Several methods can be used for control of the kinetics of the reaction. The most frequent approach is by temperature setup, but microwave treatment can also be used for rapid synthesis (Bartůněk et al. 2015).

Figure 5:
Figure 5:

Scheme of REF nanoparticles synthesis using the ionic liquid approach.

Citation: Reviews in Inorganic Chemistry 39, 2; 10.1515/revic-2018-0016

[bmim][PF6] is a typical example of IL used in REF nanoparticles synthesis (Bartůněk et al. 2011, 2012, 2013a,b) as it is thermally stable well below room temperature. Products of synthesis are easily purified as [bmim][PF6] is miscible with methanol, while excess inorganic reactants can be extracted to water. The products can be easily collected and purified by various techniques, e.g. centrifugation, dialysis, or simple sedimentation and decantation. The behaviour of ILs in water, ethanol, and in ethanol-water system of different ratios was described in the literature (Najdanovic-Visak et al. 2003). Differences in the behaviour can be used to influence the morphology of REF nanoparticles using either pure water, ethanol, or their mixtures. The morphology of REF nanoparticles also depends on the source of the fluoride anion as observed especially with [bmim][BF4] and [bmim][PF6] ILs (Zhong et al. 2009). Various REF nanoparticles were synthesised under optimised conditions in two-phase system oleic acid (n-octanol)/[bmim][PF6], where the IL acted as a crystal phase manipulator and size and shape regulator. Small hexagonal REF nanoparticles grew in IL phase and were obtained after adding a certain volume of n-octanol. This two-phase system combines thermal decomposition method with IL and provides another method for the synthesis of high-quality REF nanoparticles (He et al. 2011a). Uniform nanoparticles were prepared through a facile ethylene glycol/IL 1-methyl-3-octylimidazolium hexafluorophosphate [omim][PF6] interfacial route. Because of their ability to emit a red or green light, nanoparticles of YF3:Eu3+ and YF3:Tb3+ could be used in displays or biolabels (Li et al. 2011a).

Another widely used, safe, and affordable IL is [bmim][BF4] used, e.g. for the synthesis of LaF3 nanoparticles. BF4 undergoes fast hydrolysis at temperatures above 120°C, and the formed free fluoride anions are capable of reacting with RE ions, forming RE-doped LaF3 nanocrystals. A pure orthorhombic phase of nanoparticles without impurities or additional phases is typically obtained by the procedure using [bmim][BF4] in the synthesis (Zhang et al. 2009a).

Another developed procedure for the synthesis of uniform nanofluorides is using [bmim][BF4] as a fluoride anion source and ethylene glycol as a solvent (Nuria and Manuel 2007). The paper presented the preparation of Eu-doped as well as pure REF nanoparticles of a rhombic shape, but the method can be also extended to other lanthanide-fluoride systems. Temperature and concentration of the RE precursors were found to be critical factors for the formation of the nanoparticles. Optimal temperature for the formation of uniform nanoparticles was found to be 120°C, while increase of the temperature to 200°C led to the formation of strongly agglomerated small nanoparticles (Nuria and Manuel 2007).

By combining [bmim][BF4], which plays the role of a template, a solvent, and a reagent, with microwave irradiation, which shortens the reaction time IL, it is possible to synthesise REFs (RE=La-Ln, Y) with various crystal structures (orthorhombic and hexagonal) and many different morphologies (Li et al. 2011b). Formation of the hexagonal phase can be explained by phase transition from the cubic phase. Height of the energy barrier can be controlled by varying the number of fluoride ions coordinated to the RE cation. More fluoride ions in the coordination sphere of RE lead to lower energy barrier and to more probable occurrence of the phase transition (Liu et al. 2009c).

High quality La-doped NaGdF4 up-converting nanocrystals were synthesised in two phase systems consisting an OA-phase (RE-oleate dissolved in oleic acid) as a top phase and an IL-phase ([bmim][BF4]) as a bottom phase, which is also a fluoride-ion source. The products of synthesis are NaGd:Yb,ErF4 nanocrystals of the oil-dispersible cubic phase with a size of 5 nm or of the water-dispersible hexagonal phase occurring on the interface of the two-phase system depending on the amount of methanol used (He et al. 2011b).

Up-converting RE-doped NaYF4 nanoparticles were synthesised by ethylene glycol/IL [omim][PF6] interfacial synthesis [REF]. In this two-phase system, IL acts as a template, reagent, and structure director in the nanoparticle growth. Single-phase cubic NaYF4 crystals with no detectable impurities were obtained without observing any influence of various dopants and their concentrations. The role of IL in the evolution of nanocubes and growth of crystals was further investigated by time-dependent experiments (Zhang and Chen 2010). The two-phase system controls the dual phase transition of NaGdF4. However, using this combination resulted to a cubic phase or a mixture of cubic and hexagonal phases, and the hexagonal NaYF4 is reported as the most efficient host material for IR-to-visible photon conversion (He et al. 2012).

Characterisation of nano-REFs

REFs, typically in the form of a crystalline material, can be characterised by X-ray powder diffraction. Analysis of the phase composition of the prepared material is a crucial step, as the REFs tend to crystallise in various forms. Figure 6 shows an example of a multiphase system of GdF3 prepared by ionic-liquid synthesis. The X-ray powder diffraction analysis can be also used for estimation of the average size of the synthesised nanocrystals using the Scherrer equation (Bartůněk et al. 2013b) or obtaining more complex size information by the peak profile analysis (Ribarik et al. 2001).

Figure 6:
Figure 6:

The XRD pattern of GdF3.

Adopted from (Bartůněk et al. 2013b).

Citation: Reviews in Inorganic Chemistry 39, 2; 10.1515/revic-2018-0016

Elemental analysis of REF crystals may also be required, especially in the case of a synthesis starting from a mixture of REs. Such a synthesis can as well result in a single-phase product, but the stoichiometric composition of the final product can variably differ from the ratio of concentrations of the precursors. Elemental analysis based on atomic absorption spectroscopy, energy-dispersive X-ray spectroscopy, or inductively coupled plasma is usually sufficient for these products. X-ray photoelectron spectroscopy is also a method of choice with an advantage of detection of different oxidation states of the elements present. However, it is strictly a surface method and is limited to a very thin layer of the material, which can be overcome using, e.g. argon plasma spraying for surface polishing.

Electron microscopy is commonly used as a secondary analytical tool. Most often, transmission electron microscopy (TEM) or high-resolution TEM microscopy is employed because these methods also visualise the surface layers of functionalised nanoparticles and various inhomogeneities; see Figure 7. The option of employing computer image processing to obtain size and shape distributions of the nanoparticles is also important (Reetz et al. 2000).

Figure 7:
Figure 7:

TEM images of nano-objects of YF3 prepared by different methods.

(A) mixed crystal entities in one-pot synthesis, (B) homogenous nanoparticles prepared by ionic liquid-mediated synthesis, (C) nanoparticles prepared by microwave synthesis, and (D) nanoparticles coated by silica. Adopted from (Bartůněk et al. 2013b).

Citation: Reviews in Inorganic Chemistry 39, 2; 10.1515/revic-2018-0016

The use of vibrational spectroscopy can face some problems when being employed for analyses of luminescent REF substances. It has to be noted that only small amounts of lanthanide impurities can result in the occurrence of florescence spectra completely converting any vibrational spectroscopy signal (e.g. Raman or infrared radiation).

Optical characterisation is usually a part of the application of REF nanoparticles, but it can also be used to confirm the product identity presence of specific peaks in the spectrum, which can be attributed to an exact transition to energetic levels of RE ions (fluorescence spectra, etc.).

Zeta potential measurements, such as optical scattering, are another important source of information on the size and size distribution of the nanoparticles. In this case, the knowledge on optical parameters of the material is required. Examples of refractive indexes available from the literature are summarised in Table 3. The values for various NaREF4 materials at various wavelengths are usually fluctuating around 1.5. This information can be considered accurate enough because of the approximations included in the analytical method. The ζ potential measurements usually employ spherical approximation, which has to be considered when processing results obtained by this method, especially in the case of noticeably nonspherical nanoparticles. Surface modifications by polymers or other substances with different optical properties can also dramatically shift the ζ potential.

Table 3:

Table of refractive indexes of selected materials measured at given wavelengths.

MaterialRefractive indexWavelength (nm)Reference
β-NaYF4:Yb,Er,Tm1.4748589(Sokolov et al. 2015)
β-NaYF4:Yb,Er1.550976(Tan et al. 2011)
β-NaGdF41.45615, 555(Cichos and Karbowiak 2012)
Fluorite (CaF2)1.4328633(Lifante et al. 1997)

Solid state NMR can be useful for revealing the inner structure of REF nanoparticles or the effects of their doping by other elements. For example, solid-state 139La and 19F NMR spectroscopy of LaF3 nanoparticles reveals that the inorganic core of the nanoparticle preserves the LaF3 structure, but inhomogeneous broadening of the NMR powder patterns arises from distributions of 139La and 19F NMR interactions, confirming a gradual change in the La and F site environments from the nanoparticle core to the surface (Lo et al. 2007). The same study confirms a negligible effect of lanthanide doping on the structure. Such knowledge may be important for predicting properties of the REF nanoparticles.

Applications of REF nanoparticles

The optical and electrical properties of REF nanoparticles (Yuanfang et al. 2008, Wang and Liu 2009, Mader et al. 2010, Li et al. 2015) enable applications such as light sources and detectors and even bio-applications as emerged in the recent years.

Optical applications

The properties of the REF materials make them suitable for various optical applications. One of their key properties is the absence of autofluorescence background that increase the sensitivity of detection. Moreover, REFs are usually less toxic than quantum dots, photostable, and optically tuneable. The nonlinear optical process in nanocrystals and nanoparticles is the main principle in most applications of these materials as light sources. Among many diverse applications are lasers (Huber et al. 1997), waveguides (Dekker et al. 2004), light-emitting diodes (Chen et al. 2010), and up-converting nanofibers with potential use in LEDs, which have been recently assembled and successfully tested (Liu et al. 2016a),b).

Biological applications

Bio-applications of nanosized REFs have two crucial restrictions: nanoparticles have to be water-dispersible and nontoxic for both in vivo and in vitro applications (Kong et al. 2007, Shen et al. 2008). Low systemic toxicity and cytotoxicity can be achieved by selecting REF nanoparticles that are almost insoluble. Water-dispersion systems of REFs can be stabilised by surface modifications with silica, polymers, or hydrophilic functional groups like thiol, amino, or carboxyl groups (Niemeyer 2001), which also enable interactions with biomolecules, thereby achieving good bio-compatibility (Mader et al. 2010).

Up-converting nanofluorides can be used as bioprobes having several advantages over the commonly used organic dyes: possibility to adjust the intensity and frequency of luminescence, high quantum yields, and relatively long lifespan in excited states. Up-converting REFs can also be used for bio-imaging in combination with IR excitation as it prevents from damaging the imaged cells and offers sufficient penetration depths (Diamente and van Veggel 2005, Feng et al. 2006, Bünzli 2010, Mader et al. 2010, Wang et al. 2011). Further applications can be found in drug delivery (Wang et al. 2011) or as advanced mediums for cancer treatment. Nanoparticles delivered into the cancerous tissue up-convert IR irradiation and destroy the cancer cells by the higher energy emission or influence other substances in the tissue resulting, e.g. in the release of active radical species.

Conclusions

REF nanoparticles have many possibilities of industrial applications: in luminescent panels, light-emitting diodes, and detectors and, in recent years, also of medical applications, such as biolabels or advanced devices for cancer treatment. There are many ways to synthesise RE nanofluorides, of which focused on methods utilising ILs that are applied because of their remarkable properties such as low viscosity, inflammability, ease of manipulation, and temperature stability. ILs recently became an alternative solvent for a number of commonly used reactions. They are also used in catalysis and dual-phase extraction. During the synthesis of REF nanoparticles, ILs can act as reagents, co-solvents, and templates and can be used to obtain nanoparticles with high purity and crystallinity. IL application can also be combined with other commonly used methods of REF synthesis, such as solvothermal methods or microwave-assisted synthesis with the benefit of the high yields and overall amount of the product prepared in one batch.

Acknowledgments

This work was funded by the Ministry of the Interior of the Czech Republic (project VG20172020056).

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REVIC publishes high-quality reviews on developments in inorganic chemistry. Critical scientific reviews provide an overview of the current discussions in inorganic chemistry and a perspective on future research directions. Technical reviews offer detailed synthesis protocols, reviews of methodology and descriptions of apparatus. The journal publishes all aspects of solid-state, molecular and surface chemistry.

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