Fluorolytic Sol–Gel Synthesis of Nanometal Fluorides: Accessing New Materials for Optical Applications

inorganics

Review Fluorolytic Sol–Gel Synthesis of Nanometal Fluorides: Accessing New Materials for Optical Applications

Kerstin Scheurell and Erhard Kemnitz *

Humboldt-Universität zu Berlin, Department of Chemistry, Brook-Taylor-Str. 2, D-12489 Berlin, Germany; [email protected] * Correspondence: [email protected]

Received: 20 September 2018; Accepted: 28 November 2018; Published: 3 December 2018

Abstract: The potential of fluorolytic sol–gel synthesis for a wide variety of applications in the field of optical materials is reviewed. Based on the fluorolytic sol–gel synthesis of nanometal fluorides, sols of complex fluorometalates have become available that exhibit superior optical properties over known classical binary metal fluorides as, for instance, magnesium fluoride, calcium fluoride, or strontium fluoride, respectively. The synthesis of transparent sols of magnesium fluoroaluminates of the general composition MgxAlFy, and fluoroperovskites, [K1−xNax]MgF3, is reported. Antireflective coatings fabricated from MgF2, CaF2, MgxAlFy, and [K1−xNax]MgF3 sols and their relevant properties are comprehensively described. Especially the heavier alkaline earth metal fluorides and the fluorperovskites crystallizing in a cubic crystal structure are excellent hosts for rare earth (RE) metals. Thus, the second chapter reflects the synthesis approach and the properties of luminescent systems based on RE-doped alkaline earth metal fluorides and [K1−xNax]MgF3 phases.

Keywords: nanometal ﬂuorides; sol–gel synthesis; antireﬂective coatings; luminescent materials

1. Introduction Nanomaterials chemistry has become an extremely important area of research over the past 40 years. Although numerous nanomaterials have already accessed applications in several areas, the new scientific and industrial revolution driven by the advances in nanomaterials science is still at the beginning. Over the past decades, many new synthesis techniques have been explored, giving access to the fascinating world of nanomaterials with exciting chemical and physical properties. Many different synthesis routes have been developed, resulting in nanomaterials of distinctly distinguished properties. Among several different synthesis approaches, sol–gel synthesis certainly is one of the most powerful synthesis routes in terms of the variety of technical applications. The most prominent and mostly introduced one is doubtlessly the aqueous sol–gel synthesis route, which was pushed mainly by the development of silica, and still is attracting the interest of thousands of chemists and materials scientists worldwide. In a very simple way, this sol–gel synthesis can be described by two consecutive reaction steps, the first one being the hydrolysis of a metal–alkoxide group, followed in the second step by condensation among OH and/or OR groups. Finally, a highly distorted, three-dimensional network of M–O–M bridges is formed, and if performed under appropriate conditions, nanoscopic metal oxides are accessible. However, new non-aqueous sol–gel approaches like ALD (atomic layer deposition) have recently been explored, which allow either the synthesis of novel materials, or in many cases, give access to applications which are not at all possible via hydrolytic sol–gel routes. One of these new non-hydrolytic sol–gel approaches is the so-called ﬂuorolytic sol–gel synthesis that gives access

Inorganics 2018, 6, 128; doi:10.3390/inorganics6040128 www.mdpi.com/journal/inorganics Inorganics 2018, 6, 128 2 of 19 towards novel nanosized metal fluorides [1]. Due to their diverse functionalities, nanometal fluorides have gained tremendous attraction as materials for many applications over the last few years [2–6]. Since the first review about nanofluorides was published in 2006 [2], several further reviews and book chapters have appeared, reflecting specific aspects of the chemistry and materials science of nanometal fluorides [7–17]. Components of solar batteries [18], precursors for laser and scintillation ceramics [19,20], white light sources [21,22], biomedical materials [23–27], or catalysis [8,28,29] are just a few examples for potential applications of nanometal fluorides. Thus, nanometal fluorides have recently gained increasing interest as materials for many different applications. However, among all these different fields of applications, the optical properties of metal fluorides are of outstanding interest because, due to the strong electronegativity of the fluoride anions, metal fluorides usually exhibit large band gaps. As a result of this, several metal fluorides have a significant lower refractive index than the respective metal oxides. As can be predicted from the data for some selected metal fluorides, even the optical window is significantly wider for metal fluorides compared to the respective metal oxides (cf. Table1). That is, metal fluorides are transparent for hard UV irradiation, but also for longer wave numbers in the IR region. Due to their low refractive indices, several metal fluorides are extremely interesting candidates to be used as antireflective (AR) materials for coating, e.g., glass windows, screens made of PMMA or polycarbonate, eye glasses.

Table 1. Some optical data for selected metal ﬂuorides.

◦ Formula Structure FP ( C) Opt. Range n Solubility (g/L)

MgF2 Rutile 1256 120 nm–8 µm 1.38 0.13 CaF2 Cubic 1423 130 nm–8 µm 1.40 0.016 SrF2 Cubic 1477 130 nm–11 µm 1.44 0.11 BaF2 Cubic 1368 150 nm–12 µm 1.48 1.60 150 nm–4 µm 0.01 Quartz SiO2 Trigonal 1713 1.54 50 µm–1000 µm 0.12 am. SiO2

Note, for this kind of application, it is not only the refractive index that is important, but also solubility, stability against water, melting point (temperature stability), or hardness (scratch resistance). Another highly important area of optical applications is luminescent materials for up- and down-conversion (see Reference [3]). Rare earth (RE) metal compounds usually show luminescent properties; however, in pure form they tend for radiation free relaxation due to quenching effects if they are situated too near to each other. In order to circumvent these problems, suitable host materials have to be used in order to separate the RE metals by doping them into the host lattices. Excellent host systems for this purpose are metal fluorides in a cubic structure, like the heavier alkaline earth metal I II fluorides or fluorperovskites M M F3. In spite of the great importance of metal fluoride based optical materials, the focus of this paper is outlining new synthesis approaches to address new application opportunities, which have for the first time become available, based on homodispersed nanoscopic metal fluoride sols that can be obtained in an excellent way, via fluorolytic sol–gel synthesis.

2. Nanometal Fluoride Particles and Their Application as Optical Materials The widely used techniques for the preparation of nanometal fluorides are flame pyrolysis, microwave-assisted solvothermal synthesis, classical precipitation methods from aqueous solutions, and microemulsion routes [3]. The first sol–gel-type method for the synthesis of nanometal fluorides is the trifluoracetate route, developed mainly by Fushihara’s group in the late nineties [30]. However, it is just an indirect route; as a first step, a reaction between a suitable metal precursor with trifluoracetic acid results in the formation of metal trifluoroacetate sols (Equation (1)).

M(OR)x + x CF3COOH → M(OOCCF3)x + x HOR (1) Inorganics 2018, 6, 128 3 of 19

Such sols can be either further processed for coating or to obtain powders after solvent evaporation. Following calcination of coatings or powders eventually results in the formation of metal ﬂuorides due to thermal decomposition of the metal triﬂuoroacetates (Equation (2)).

M(OOCCF3)x → nano-MFx + 1/x(CF3CO)2O + CO2 + CO (2) The drawbacks of this route are sometimes incomplete thermal decomposition, hazardous gaseous byproducts, and the limitation that not all metals form fluoroacetates, nor decompose towards metal fluorides, instead forming metal oxides. Despite all the mentioned drawbacks, this synthesis route was the first highly potent sol–gel-based synthesis approach towards nanometal fluoride materials. A very efficient alternative and direct synthesis method for the preparation of nanometal fluorides is the so-called fluorolytic sol–gel method developed in our group using anhydrous HF as a fluorinating agent [1,7,31]. For convenient handling and dosing, HF can be dissolved in organic solvents like alcohols or ethers, and such HF solutions can easily be reacted with solutions of suitable metal precursors (alkoxides, acetates, chlorides, etc.) preferentially dissolved in the same solvent as HF. Equation (3) displays the general reaction route: M(OR)x + x HFsolv → nano-MFx + x HOR (3) Depending on the metal precursors and the HF content used, different nanofluorides can be prepared:

• Binary fluorides (MgF2, CaF2), e.g., for antireflective coatings; I II • Complex fluorides (MgxAlFy), fluorperovskites (M M F3) for antireflective coatings and hosts for rare earth metal (relevant for luminescent materials);

• Hosts for up and down conversion (CaF2, SrF2, BaF2, complex lanthanide fluorides); • Other applications—composites, dental materials, wood protection, etc. The main features of this powerful fluorolytic sol–gel synthesis were recently reviewed comprehensively [12,32,33], giving deeper insight into the mechanistic details of the synthesis conditions, role of precursors and solvents, etc. The stepwise replacement of the precursor alkoxide groups by fluoride ions was thoroughly studied based on Solid State Nuclear Magnetic Resonance Spectroscopy (MAS-NMR), X-ray Diffraction (XRD), Wide-Angle X-ray Scattering (WAXS), and Small-Angle X-ray Scattering (SAXS) investigations and important data on topological and rheological parameters of these materials were presented. For deeper insight into all these mechanistic aspects, we refer to the cited review articles. Here, we restrict the focus on several synthesis approaches relevant for obtaining fluoride materials for optical applications.

2.1. Binary Fluorides for Antireﬂective Coatings

2.1.1. Magnesium Fluoride Optical materials with a low refractive index (<1.30) are very attractive for the development of antireflective coatings. The Nikon Corporation published in 2008 a method for the preparation of thin films containing MgF2 which is reinforced by a SiO2 binder [34]. After a sol–gel reaction of magnesium acetate and HF in methanol, the crystallinity of the MgF2 particles was low and the surface of the resulting coating was unstable. By heating the MgF2 solution in an autoclave, the crystallinity increased and the resulting films had a high porosity with nanopores between the particles (n = 1.20–550 nm). Due to the porous structure of the MgF2 films, the film strength had to be improved by a SiO2 binder. The combined MgF2–SiO2 coating is wipe- and environment-resistant, but no information is given about the resistance against water. Because of the high porosity of the MgF2–AR coatings associated with a poor adhesion of the resulting films, the application by dip-coating to cover large areas (windows in buildings) is not an industrial standard so far. It is only possible to sputter thin MgF2 films on small substrates, such as lenses. This is the reason only a few publications exist worldwide about this topic, although increasing research activities can be observed recently. Inorganics 2018, 6, x 4 of 19

lenses. This is the reason only a few publications exist worldwide about this topic, although Inorganicsincreasing2018 research, 6, 128 activities can be observed recently. 4 of 19 In 2017, Karthik et al. reported on high performance broad band antireflective mesoporous MgF2 coatings for solar applications [35]. The MgF2 nanoparticles were synthesized from MgCl2 and HF/water.In 2017, After Karthik the etreaction, al. reported the mixture on high was performance transferred broad to an band autoclave antireflective and heated mesoporous up to 200 MgF °C2 coatingsfor several for solarhours applications up to a few [35 ].days. The MgFThe 2nanoparticlesnanoparticles wereobtained synthesized after a fromduration MgCl of2 and1 h HF/water.were still ◦ Aftermesoporous. the reaction, The theresulting mixture coatings was transferred showed tothe an best autoclave optical and properties heated up(100% to 200 transmittanceC for several and hours 0% upreflectance, to a few days. n = The1.16–600 nanoparticles nm). After obtained weather after and a duration thermal of 1stability h were stilltests, mesoporous. no change The in resultingaverage coatingstransmittance showed was the observed, best optical but properties no wipe tests (100% were transmittance carried out. and Hence, 0%reflectance, the stabilityn of= 1.16–600the coatings nm). Afteron the weather substrates and thermalis doubtful. stability Another tests, nodrawback change in is average that the transmittance high HCl content was observed, in the resulting but no wipe sol testscauses were corrosion carried out. and Hence, environmental the stability concerns, of the coatings thus on making the substrates them isinappropriate doubtful. Another for drawbackindustrial isapplications. that the high All HCl these content approaches in the resulting have solthe causes drawba corrosionck of being and environmentaltime- and cost-intensive concerns, thus due making to the themnecessary inappropriate autoclave for treatment industrial making applications. up-scaling All theseillusory. approaches have the drawback of being time- and cost-intensive due to the necessary autoclave treatment making up-scaling illusory. Already in 2012, the synthesis and properties of MgF2 sols obtained from MgCl2 via the new fluorolyticAlready sol–gel in 2012, synthesis the synthesis were reported and properties and comprehensively of MgF2 sols discussed obtained [36]. from The MgCl advantage2 via the of new this fluorolytic sol–gel synthesis were reported and comprehensively discussed [36]. The advantage of approach is that without any further treatment, water-clear ethanolic MgF2 sols were obtained, thiswhich approach were directly is that withoutused for any the further dip-coating treatment, of glass water-clear plates. For ethanolic the preparation MgF2 sols of were antireflective obtained, whichcoatings were with directly a low used refractive for the dip-coatingindex in the of visi glassble plates. range, ForAR the layers preparation built up of by antireflective respective coatings with a low refractive index in the visible range, AR layers built up by respective nanoparticles nanoparticles which form open pore structures are needed. For that reason, at first, the MgF2 whichparticles form in the open sols pore were structures thoroughly are needed.investigated For thatby dynamic reason, at light first, scattering, the MgF2 19particlesF solid state in the NMR, sols 19 wereand XRD. thoroughly investigated by dynamic light scattering, F solid state NMR, and XRD. As can be seen in Figure1, the particle size distribution of MgF is monodisperse, with a maximum As can be seen in Figure 1, the particle size distribution 2of MgF2 is monodisperse, with a atmaximum 8 nm. By at thermal 8 nm. By treatment, thermal the treatment, particle sizethe particle distribution size distribution increases to 60increases nm, apparently to 60 nm, by apparently Ostwald ripeningby Ostwald or coalescence, ripening or but coalescence, in general, suchbut in sols general, are appropriate such sols for are the appropriate coating of different for the substrates,coating of fordifferent example, substrates, glasses (windows,for example, vitrines, glasses lenses) (windows or polymers, vitrines, (displays lenses) or etc.). polymers (displays etc.).

FigureFigure 1.1. HydrodynamicHydrodynamic diameter of particlesparticles inin as-preparedas-prepared MgFMgF22 sols ((a)) andand inin samplessamples afterafter boilingboiling forfor 2424 h h ( b(b) as) as measured measured by by dynamic dynamic light light scattering. scattering Reproduced. Reproduced from from Reference Reference [36] [36] with with the permissionthe permission of the of Royalthe Royal Chemical Chemical Society. Society.

TheThe long-termlong-term stability stability of of MgF MgF2 2sols sols is is crucial crucial for for the the coating coating process. process. Nanoparticles, Nanoparticles, after after sol–gel sol– synthesis,gel synthesis, tend towardtend toward gelation gelati or agglomeration.on or agglomeration. To control To the stability,control the the changestability, of viscositythe change of the of 2 −1 solsviscosity over aof time the sols of 120 over days a time was of measured 120 days (Figure was measured2). Only a(Figure minimal 2). increaseOnly a minimal (0.01 mm increase·s ) over(0.01 thismm time2·s−1) over was observed,this time was meaning observed, these meaning sols are verythese stable. sols are After very the stable. thermal After treatment, the thermal the treatment, viscosity wasthe viscosity significantly was higher. significantly It seems higher. that the It seems sol system that maythe sol be system altered may by introduction be altered by of thermalintroduction energy. of thermalThe energy. MgF2 films coated on soda lime glass were investigated in terms of film porosity, opticalThe properties, MgF2 films and coated mechanical on soda stability. lime glass Figure were3 shows investigated the adsorption–desorption in terms of film porosity, isotherms optical of severalproperties, films and depending mechanical on thestability. treatment Figure temperature, 3 shows the determined adsorption–desorpt by ellipsometricion isotherms porosimetry of several (EP). Itfilms can bedepending seen that on the the overall treatment porosity temperature, of the MgF 2determinedfilms is approximately by ellipsometric 35% independent porosimetry from (EP). the It ◦ treatmentcan be seen temperature. that the overall Obviously, porosity already of the afterMgF2300 filmsC, isthe approximately organic part 35% of the independent gel film (solvent) from the is completelytreatment temperature. removed and Obviously, the inorganic already part (MgF after2) 300 remains °C, the on organic the substrate. part of However, the gel film the pore(solvent) radius is iscompletely increased dueremoved to the and growth the ofinorganic crystalline part grains (MgF at2) higherremains temperature. on the substrate. However, the pore radius is increased due to the growth of crystalline grains at higher temperature. Inorganics 2018, 6, 128 5 of 19 Inorganics 2018, 6, x 5 of 19 Inorganics 2018, 6, x 5 of 19

Figure 2. Viscosity of MgF2 coating solutions as a function of the aging time. Reproduced from FigureFigure 2.2. Viscosity of MgFMgF2 coating solutionssolutions asas aa functionfunction ofof thethe agingaging time.time. ReproducedReproduced fromfrom Reference [36] with the permission2 of the Royal Chemical Society. ReferenceReference [[36]36] withwith thethe permissionpermission ofof thethe RoyalRoyal ChemicalChemical Society.Society.

With a minimum reflectance of 0.35% at 600 nm, the coated MgF2 films showed excellent WithWith aa minimumminimum reflectancereflectance ofof 0.35%0.35% atat 600600 nm,nm, thethe coatedcoated MgFMgF22 filmsfilms showedshowed excellentexcellent antireflective properties. An increase of the treatment temperature led only to a slight reduction of antireflectiveantireflective properties. properties. An An increase increase of of the the treatm treatmentent temperature temperature led led only only to toa slight a slight reduction reduction of this reflection, which corresponds to the open pore structure determined. ofthis this reflection, reflection, which which corresponds corresponds to tothe the open open pore pore structure structure determined. determined.

Figure 3. Adsorption (filled (filled symbols) and desorp desorptiontion (open symbols) isotherms of MgF 2 thinthin films films Figure 3. Adsorption (filled◦ symbols) and desorption (open symbols) isotherms of MgF2 thin films treatedtreated at at 300, 400, and 500 °C,C, measured by ellipsometric porosimetry (a). Derivative Derivative pore pore radius treated at 300, 400, and 500 °C, measured by ellipsometric porosimetry (a). Derivative pore radius distributions as calculatedcalculated fromfrom the the respective respective desorption desorption branch branch (b ).(b Reproduced). Reproduced from from Reference Reference [36] distributions as calculated from the respective desorption branch (b). Reproduced from Reference [36]with with the permissionthe permission of the of Royalthe Royal Chemical Chemical Society. Society. [36] with the permission of the Royal Chemical Society. The mechanical stability, which is crucial for any practical application, but was not presented for The mechanical stability, which is crucial for any practical application, but was not presented AR layersThe mechanical reported inside stability, the publications which is crucial cited before,for any waspractical tested application, with the Crockmeter but was not test presented using felt for AR layers reported inside the publications cited before, was tested with the Crockmeter test orfor steel AR woollayers stamps reported (force inside 4 N) the (Figure publications4). On borosilicate cited before, glass, was even tested after with 500 abrasionthe Crockmeter cycles with test using felt or steel wool stamps (force 4 N) (Figure 4). On borosilicate glass, even after 500 abrasion feltusing and felt 25 or cycles steel withwool steel stamps wool (force (hardness 4 N) (Figure 0000), no 4). scratches On borosilicate are visible. glass, The even MgF after2 films 500 exhibited abrasion cycles with felt and 25 cycles with steel wool (hardness 0000), no scratches are visible. The MgF2 ancycles appreciable with felt stability and 25 under cycles mechanical with steel stress.wool (hardness As can be seen,0000), the no MgF scratches2 antireflective are visible. coatings The MgF from2 films exhibited an appreciable stability under mechanical stress. As can be seen, the MgF2 thefilms raw exhibited material MgClan appreciable2 possess excellent stability optical under and mechanical mechanical stress. properties. As can Unfortunately, be seen, the however, MgF2 antireflective coatings from the raw material MgCl2 possess excellent optical and mechanical theseantireflective sols are inappropriatecoatings from for the industrial raw material application MgCl due2 possess to the high excellent amount optical of HCl and which mechanical is formed properties. Unfortunately, however, these sols are inappropriate for industrial application due to duringproperties. the solUnfortunately, synthesis. This however, HCl may these cause sols high ar corrosione inappropriate of the industrialfor industrial coating application plants, and due thus to the high amount of HCl which is formed during the sol synthesis. This HCl may cause high isthe rather high of amount fundamental of HCl than which real interest.is formed during the sol synthesis. This HCl may cause high corrosion of the industrial coating plants, and thus is rather of fundamental than real interest. corrosion of the industrial coating plants, and thus is rather of fundamental than real interest. InorganicsInorganics 20182018,, 66,, 128x 6 of 19 Inorganics 2018, 6, x 6 of 19

Figure 4. Photographs of MgF2 thin films on (a) soda-lime and (b) borosilicate glass substrates that

FigurehadFigure undergone 4.4. PhotographsPhotographs the Crockmeter of of MgF MgF2 2thin thintest ﬁlms filmswith on felton (a ) (aand soda-lime) soda-lime steel wool. and and (b The) ( borosilicateb) numbersborosilicate glassat theglass substrates right substrates side that of hadthatthe undergoneimageshad undergone correspond the Crockmeter the to Crockmeter the respective test with test number feltwith and felt steelof and abra wool. steelsion Thecycles.wool. numbers The All samplesnumbers at the have rightat the been side right oftreated theside images atof 500the ◦ correspond°Cimages prior correspond to toanalysis. the respective to Reproduced the respective number from ofnumber abrasion Reference of abra cycles. [36]sion Allwith cycles. samples the All permission havesamples been have of treated the been Royal at treated 500 ChemicalC at prior 500 toSociety.°C analysis. prior to Reproducedanalysis. Reproduced from Reference from [Reference36] with the [36] permission with the ofpermission the Royal Chemicalof the Royal Society. Chemical Society. A few yearsyears later, ScheurellScheurell etet al.al. [[37,38]37,38] reportedreported onon thethe optimizationoptimization ofof thethe ﬂuorolyticfluorolytic sol–gelsol–gel synthesisA few ofof years MgFMgF2 2later,for for a alarge-scaleScheurell large-scale et process, al.process, [37,38] starting starting report fromed from on magnesium themagnesium optimization acetate acetate tetrahydrate. of thetetrahydrate. fluorolytic After Aftersol–gel a mild a dehydration (100 ◦C, under vacuum) of the acetate tetrahydrate to the nearly water-free acetate and mildsynthesis dehydration of MgF 2(100 for °C,a large-scale under vacuum) process, of startingthe acetat frome tetrahydrate magnesium to acetatethe nearly tetrahydrate. water-free Afteracetate a reactionandmild reaction dehydration with with water-free water-free(100 °C, HF, under itHF, was itvacuum) possiblewas possible of to the obtain toacetat obtain cleare tetrahydrate clear MgF 2MgFsols2 (Figuresolsto the (Figure nearly5). 5). water-free acetate and reaction with water-free HF, it was possible to obtain clear MgF2 sols (Figure 5).

ii Figure 5.5. MgF2 solsol stabilized stabilized by by Al(O Al(OPr)Pr)33 afterafter 2 2 days days of of fluorination fluorination prepared prepared in in a a 10 10 L L reactor. reactor. Reproduced from Reference [37] with the permission of the Royal Chemical Society. ReproducedFigure 5. MgF from2 sol Reference stabilized [37] by with Al(O theiPr) pe3 afterrmission 2 days of the of Royalfluorination Chemical prepared Society. in a 10 L reactor. Reproduced from Reference [37] with the permission of the Royal Chemical Society. Directly after the reactionreaction of thethe magnesiummagnesium acetate with HF, thethe formedformed MgFMgF22 sols areare turbidturbid because of the interaction of the very active MgF2 particles. Depending on the concentration becauseDirectly of the after interaction the reaction of the of thevery magn activeesium MgF acetate2 particles. with HF,Depending the formed on theMgF concentration2 sols are turbid or or dimension of the synthesis approach, clear sols are only obtained after several hours, days, dimensionbecause of of the the interactionsynthesis approach, of the very clear active sols are MgF only2 obtainedparticles. afterDepending several hours,on the days, concentration or sometimes or or sometimes even weeks. Figure6 demonstrates the change in particle size distribution depending evendimension weeks. of Figurethe synthesis 6 demonstrates approach, the clear change sols are in onlyparticle obtained size distribution after several depending hours, days, on or the sometimes sol age, on the sol age, followed by Dynamic Light Scattering (DLS). Obviously, the fluorolytic sol–gel followedeven weeks. by FigureDynamic 6 demonstrates Light Scattering the change(DLS). Obviin particleously, sizethe distributionfluorolytic sol–geldepending synthesis on the of sol metal age, synthesis of metal fluoride nanoparticles occurs according to the model described by Bogush and fluoridefollowed nanoparticles by Dynamic occursLight Scatteringaccording to(DLS). the moObvidelously, described the fluorolytic by Bogush sol–gel and Zukovsky synthesis [39].of metal This Zukovsky [39]. This means that initially, larger aggregates/agglomerates of small particles are formed, meansfluoride that nanoparticles initially, larger occurs aggregates/agglomerates according to the model of describedsmall particles by Bogush are formed, and Zukovsky which decompose [39]. This which decompose slowly with time. For a faster deagglomeration of the sol particles, additivesi such slowlymeans thatwith initially,time. For larger a faster aggregates/agglomerates deagglomeration of ofthe small sol particles, particles additivesare formed, such which as Al(O decomposePr)3 or i as Al(O Pr)3 or tetrametyl orthosilicate (TMOS) and trifluoroactic acid (TFA) were applied in order tetrametylslowly with orthosilicate time. For (TMOS)a faster anddeagglomeration trifluoroactic ofacid the (TFA) sol particles,were applied additives in order such to as accelerate Al(OiPr) 3the or tetrametyl orthosilicate (TMOS) and trifluoroactic acid (TFA) were applied in order to accelerate the Inorganics 2018, 6, 128 7 of 19 InorganicsInorganics 2018,, 6,, xx 7 of 19

2 −1 decompositionto accelerate the of decomposition these agglomerates. of these The agglomerates. viscosity of the The resulting viscosity sols of the is low resulting (2 mm sols2·s·s− is1)) andand low nearly(2 mm 2constant·s−1) and over nearly a time constant of 130 over days. a time of 130 days.

Figure 6.6. ParticleParticle size size distribution distribution measured measured by by DLS DLS depending on on thethe solsol age.age.age. Reproduced Reproduced from from Reference [37] [37] with the permission of thethe RoyalRoyal ChemicalChemical Society.Society.

The film film morphology of the MgF2 coating from Mg(OAc)2 asas a precursor after a thermal The film morphology of the MgF22 coating from Mg(OAc)2 as a precursor after a thermal ◦ treatmenttreatment atat at 500°C500°C 500 Cisis isshownshown shown inin inFigureFigure Figure 7.7. 7TheThe. The particleparticle particle sizesize size isis approximatelyapproximately is approximately 3030 nm.nm. 30 nm.InIn contrastcontrast In contrast toto thethe MgFto the2 films MgF fromfilms MgCl from2 (see MgCl above),(see the above), porosity the of porosity the films of from the films Mg(OAc) from2 is Mg(OAc) only 19%.is Obviously, only 19%. MgF2 films 2from MgCl2 (see above),2 the porosity of the films from Mg(OAc)2 is only 19%.2 Obviously, theObviously, microstructure the microstructure of the porous of the porousMgF2 films MgF stronglyfilms strongly depends depends on the on theprecursor precursor used used in in the the microstructure of the porous MgF2 films2 strongly depends on the precursor used in the fluorolyticfluorolyticfluorolytic sol–gel synthesis process.process.

Figure 7. Scanning electron microscopymicroscopy (SEM) imagesimages ofof MgFMgF2 filmsﬁlms prepared from Mg(OAc)2-based Figure 7. Scanning electron microscopy (SEM) images of MgF22 films prepared from Mg(OAc)22-based ◦ precursor solutions. The The respective respective samples samples have have beenbeen been annealedannealed annealed atat 500500 °C.°C.C. Reproduced Reproduced from from Reference [38] with the permission of the Royal Chemical Society. Reference [38] with the permission of the Royal Chemical Society. The optical properties and mechanical stability are very good, but there is still a disadvantage The optical properties and mechanical stability are very good, but there is still a disadvantage when Mg(OAc) is used as a precursor for the synthesis. That is, the acetic acid formed during the when Mg(OAc)2 is used as a precursor for the synthesis. That is, the acetic acid formed during the when Mg(OAc)2 is used as a precursor for the synthesis. That is, the acetic acid formed during the synthesis reacts with the solvent ethanol to form acetic ester and water. As with many other sol–gel synthesis reacts with the solvent ethanol to form acetic ester and water. As with many other sol–gel routes, water makes the sols unstable on a long-time scale, resulting in increasing viscosity and gelation routes, water makes the sols unstable on a long-time scale, resulting in increasing viscosity and after a few months. Thus, for industrial application, this is an obvious drawback. gelation after a few months. Thus, for industrial application, this is an obvious drawback. Of course, similar to the classical sol–gel synthesis to form SiO , for ﬂuorolytic sol–gel synthesis Of course, similar to the classical sol–gel synthesis to form SiO2, 2for fluorolytic sol–gel synthesis to Of course, similar to the classical sol–gel synthesis to form SiO2, for fluorolytic sol–gel synthesis to to MgF , alkoxides are also suitable as precursor materials. In the case of the magnesium methanolate, MgF2, alkoxides2 are also suitable as precursor materials. In the case of the magnesium methanolate, MgF2, alkoxides are also suitable as precursor materials. In the case of the magnesium methanolate, starting with the reaction of magnesium metal in methanol, the synthesis is very easy and after several starting with the reaction of magnesium metal in methanol,methanol, thethe synthesissynthesis isis veryvery easyeasy andand afterafter hours, water-clear sols in methanol can be obtained. Unfortunately, for a large-scale synthesis process several hours, water-clear sols in methanol can be obtained. Unfortunately, for a large-scale and subsequent dip-coating of substrates, the solvent methanol cannot be used because of its extreme synthesis process and subsequent dip-coating of substrates, the solvent methanol cannot be used toxicity. Moreover, the reaction of neat magnesium with methanol forms hydrogen gas, which might because of its extreme toxicity. Moreover, the reaction of neat magnesium with methanol forms raise safety issues. Magnesium ethoxide is insoluble in alcohols and no homogeneous reaction can hydrogen gas, which might raise safetyfety issues.issues. MagnesiumMagnesium ethoxideethoxide isis insolubleinsoluble inin alcoholsalcohols andand nono be performed to form MgF . Under no circumstances were clear sols obtained. A new synthesis was homogeneous reaction can2 be performed to form MgF2. Under no circumstances were clear sols homogeneous reaction can be performed to form MgF2. Under no circumstances were clear sols obtained.developed A [ 40new], starting synthesis from was Mg(OEt) develop2 edand [40], MCl starting2 (M = Mgfrom or Mg(OEt) Ca) in ethanol.2 and MCl2 (M = Mg or Ca) in obtained. A new synthesis was developed [40], starting from Mg(OEt)2 and MCl2 (M = Mg or Ca) in ethanol. Inorganics 2018, 6, 128x 8 of 19

The chloride quickly dissolves in ethanol and reacts with HF, thus forming HCl in addition to MgF2The. The chloride hydrogen quickly chloride dissolves formed in ethanol can then and react reacts with with unreacted HF, thus forming Mg(OEt) HCl2, forming in addition MgCl to MgF2 and2. Theethanol, hydrogen and consequently, chloride formed a catalytic can then cycle react is with esta unreactedblished. Using Mg(OEt) this2 ,way, forming water-clear MgCl2 andsols ethanol,can be obtainedand consequently, with just a a catalytic small amount cycle is established.of HCl that Using is acceptable this way, for water-clear industrial sols applications. can be obtained Figure with 8 representsjust a small a amount schematic of HCl overview that is of acceptable the reactions. for industrial The optical applications. properties Figure (reflectance8 represents below a schematic1%) of the resultingoverview ofMgF the2 reactions.coatings are The excellent optical properties and as good (reflectance as the below thin 1%)films of from the resulting MgCl2 MgFor Mg(OAc)2 coatings2. However,are excellent the andHCl as content good asis much the thin smaller films than from in MgCl the pure2 or Mg(OAc)MgCl2 system,2. However, but the stability the HCl of content the sols is ismuch significantly smaller than better in thethan pure in th MgCle acetate2 system, system—no but the stabilityester formation of the sols is possible. is significantly Usingbetter this reaction than in cycle,the acetate a new system—no reaction path ester way formation developed is possible.to upscale Using this one-pot this reaction reaction cycle, to a abatch new of reaction 5 L. Through path way the additiondeveloped of tochloride upscale additives this one-pot (AlCl reaction3, CaCl2 to, LaCl a batch3, LiCl, of 5 MgCl L. Through2, SrCl2, the SnCl addition2), the corresponding of chloride additives layers (AlClshowed3, CaClenhanced2, LaCl stability3, LiCl, MgCltowards2, SrCl mechanical2, SnCl2 ),abrasi the correspondingon (steel wool layers00, 25 showedcycles, 4 enhancedN) and improved stability durabilitytowards mechanical against water abrasion [41]. (steel wool 00, 25 cycles, 4 N) and improved durability against water [41].

Figure 8. Schematic representation of the reactions occurringoccurring during the one-pot synthesis using the

chloride method (M = Mg, Ca; xx == 0.05–0.30;0.05–0.30; EtEt == CC22H5).). Reproduced Reproduced from from Reference Reference [40] [40] with the permission of the Royal Chemical Society.

A comparison of antireflective coatings of commercial products with the new MgF2 λ/4 coatings A comparison of antireflective coatings of commercial products with the new MgF2 λ/4 coatings is reported in Reference [42]. The commercial AR products are SiO2 films from Centrosolar is reported in Reference [42]. The commercial AR products are SiO2 films from Centrosolar (Fürth, (Fürth, Germany), 3-layer interference-type AR coatings from Prinz Optics (Stromberg, Germany), Germany), 3-layer interference-type AR coatings from Prinz Optics (Stromberg, Germany), and and DSM coating solutions based on polymer templates (DSM, Heerlen, The Netherland). DSM coating solutions based on polymer templates (DSM, Heerlen, The Netherland). As can be seen from Table2 2,, the the optical optical propertiesproperties (transmittance (transmittance 97.5–99.7%)97.5–99.7%) ofof thethe differentdifferent λ antireflectiveantireflective coatings are comparable. The highest transmittance was achieved withwith λ/4/4 films films from Centrosolar, apparentlyapparently duedue to to the the high high open open porosity porosity (50.2%) (50.2%) inside inside the the layers. layers. However, However, amongst amongst all these porous λ/4 systems, only the novel MgF2 films exhibited superior mechanical film stability all these porous λ/4 systems, only the novel MgF2 films exhibited superior mechanical film stability compared to their SiO2 counterparts under investigation (Figure9). In the Crockmeter tests, using felt compared to their SiO2 counterparts under investigation (Figure 9). In the Crockmeter tests, using (500 cycles) and steel wool (max. 25 cycles), the MgF2 films showed only very few scratches. felt (500 cycles) and steel wool (max. 25 cycles), the MgF2 films showed only very few scratches. Table 2. Film thickness, open porosity, maximum of pore radius distribution, and peak transmittance Table 2. Film thickness, open porosity, maximum of pore radius distribution, and peak transmittance of of antireflective coatings under investigation. Reproduced from Reference [42] with the permission of antireflective coatings under investigation. Reproduced from Reference [42] with the permission of the Royal Chemical Society. the Royal Chemical Society. Film Thickness Open Porosity Max. Pore Radius Peak Transmittance at λ Film System Film Thickness Open Porosity Max. Pore Radius Peak Transmittance at λ Film System (nm) (%) (nm) (%) (nm) (nm) (%) (nm) (%) (nm) Prinz interference 297 <6 n.a. 98.6 515 PrinzDSM interferenceλ/4 297 113 <6 33.2 n.a. (19) 97.5 98.6 545 515 CentrosolarDSM λ/4 λ/4113 105 33.2 50.2 (19) 5.9 99.7 97.5 515545 CentrosolarMgF2 λ/4 λ/4 105 115 50.2 27.3 5.9 8.7 98.5 99.7 540 515 MgF2 λ/4 115 27.3 8.7 98.5 540 InorganicsInorganics 20182018, 6, x, 6 , 128 9 of9 19 of 19 Inorganics 2018, 6, x 9 of 19

Figure 9. Photographs of antireflective coatings subjected to Crockmeter testing using felt (500 cycles) and steel wool (1–25 cycles) as abrasive. The size of the samples shown is 10 × 15 cm². Figure 9.9.Photographs Photographs of of antireflective antireflective coatings coatings subjected subjected to Crockmeter to Crockmeter testing testing using using felt (500 felt cycles) (500 Reproduced from Reference [42] with the permission of the Royal Chemical Society. andcycles) steel and wool steel (1–25 wool cycles) (1–25 as cycles) abrasive. as Theabrasive. size of The the samplessize of the shown samples is 10 shown× 15 cm is2 .10 Reproduced × 15 cm². fromReproduced Reference from [42 Reference] with the permission[42] with the of pe thermission Royal Chemicalof the Royal Society. Chemical Society. 2.1.2. Calcium Fluoride 2.1.2. Calcium Fluoride 2.1.2.The Calciumrefractive Fluoride index of crystalline CaF2 is 1.44 [43]. Similar to MgF2, CaF2 with a sufficient porosityThe is also refractive a promising index of candidate crystalline for CaF λ/42 iscoatings 1.44 [43 to]. Similardecrease to the MgF reflection2, CaF2 with of the a sufficient incident porositylight The refractive index of crystalline CaF2 is 1.44 [43]. Similar to MgF2, CaF2 with a sufficient of glass.is also An a promising advantage candidate of CaF2 against for λ/4 MgF coatings2 is the to very decrease low thesolubility reflection in water, of the incidentwhich could light be of an glass. porosity is also a promising candidate for λ/4 coatings to decrease the reflection of the incident light importantAn advantage factor in of view CaF2 ofagainst the chemical MgF2 is resistance the very low of antireflective solubility in water,coatings which towards could environmental be an important of glass. An advantage of CaF2 against MgF2 is the very low solubility in water, which could be an conditionsfactor in view[44]. ofUntil the chemicalnow, CaF resistance2 films were of antireflective deposited coatingsby physical towards or chemical environmental vapor conditionsdeposition [44 ]. important factor in view of the chemical resistance of antireflective coatings towards environmental techniques.Until now, In CaF2007,2 films Pilvi wereet al. deposited[45] reported by physicalon the atomic or chemical layer deposition vapor deposition (ALD) of techniques. CaF2 on SiO In2/Si 2007, conditions [44]. Until now, CaF2 films were deposited by physical or chemical vapor deposition◦ at 300Pilvi °C et with al. [ 45a low] reported impurity on level. the atomic The resulting layer deposition CaF2 films (ALD) exhibited of CaF a refrac2 on SiOtive2 /Siindex at 300of 1.43,C withbut a techniques. In 2007, Pilvi et al. [45] reported on the atomic layer deposition (ALD) of CaF2 on SiO2/Si thelow transmission impurity level.decreased The resultingwith decreasing CaF2 films wavelength exhibited in a refractivethe UV range. index of 1.43, but the transmission at 300 °C with a low impurity level. The resulting CaF2 films exhibited a refractive index of 1.43, but In 2015, Rehmer et al. [46] reported for the first time the application of nanoscopic CaF2 via the decreasedthe transmission with decreasing decreased wavelength with decreasing in the UVwavelength range. in the UV range. fluorolyticIn 2015, sol–gel Rehmer synthesis et al. route [46] reportedfor antireflective for the first coatings. time the The application synthesis ofof nanoscopictransparent CaF CaF2 2via sol the In 2015, Rehmer et al. [46] reported for the first time the application of nanoscopic CaF2 via the startingfluorolytic from calcium sol–gel synthesisalkoxides routewas not for successful antireflective because coatings. of the The insolubility synthesis of of the transparent alkoxides CaFin the2 sol fluorolytic sol–gel synthesis route for antireflective coatings. The synthesis of transparent CaF2 sol alcohols used. Calcium acetate suspended in mixtures of ethanol and acetic acid provides starting from calcium alkoxides was not successful because of the insolubility of the alkoxides in the transparent CaF2 sols, but the stability of these sols is very poor. After a few days, a gel is formed alcohols used.used. Calcium Calcium acetate acetate suspended suspended in mixtures in mixt ofures ethanol of andethanol acetic and acid acetic provides acid transparent provides dueCaF to 2thesols, esterification but the stability of the of acetic these acid sols iswith very ethanol poor. forming After a few water. days, The a gelbest is suitable formed starting due to the transparent CaF2 sols, but the stability of these sols is very poor. After a few days, a gel is formed material to form CaF2 via sol–gel synthesis is CaCl2. First, the resulting sol is not fully transparent, esterificationdue to the esterification of the acetic of acid the withacetic ethanol acid with forming ethanol water. forming The bestwater. suitable The best starting suitable material starting to butform opaque. CaF 2However,via sol–gel after synthesis the addition is CaCl 2of. First,5 mol the % resultingTMOS as sol a stabilization is not fully transparent, agent, a water-clear but opaque. material to form CaF2 via sol–gel synthesis is CaCl2. First, the resulting sol is not fully transparent, transparent CaF2 sol is formed. The particle size distribution of the CaF2 sol is given in Figure 10. However,but opaque. after However, the addition after of the 5 mol addition % TMOS of 5 as mol a stabilization % TMOS as agent, a stabilization a water-clear agent, transparent a water-clear CaF2 Thesol intensity is formed. weighted The particle distribution size distribution shows two of the classe CaFs2 ofsol particles is given inwith Figure a mean 10. The particle intensity size weighted of 20 transparent CaF2 sol is formed. The particle size distribution of the CaF2 sol is given in Figure 10. nm. distributionThe intensity shows weighted two classesdistribution of particles shows with two a classe means particle of particles size ofwith 20 nm.a mean particle size of 20 nm.

Figure 10. Hydrodynamic diameter of particles of CaF2–sol with 5 mol % TMOS measured by dynamic Figure 10. Hydrodynamic diameter of particles of CaF2–sol with 5 mol % TMOS measured by light scattering. Reproduced from Reference [46] with the permission of the Royal Chemical Society. dynamic light scattering. Reproduced from Reference [46] with the permission of the Royal Figure 10. Hydrodynamic diameter of particles of CaF2–sol with 5 mol % TMOS measured by Chemical Society. Anotherdynamic remarkablelight scattering. precursor Reproduced material from for theRefe synthesisrence [46] ofwith CaF 2thenanoparticles permission of is calciumthe Royal lactate pentahydrateChemical Society. [47]. The solubility of the lactate in ethanol is very good and it needs only a short time Another remarkable precursor material for the synthesis of CaF2 nanoparticles is calcium after reaction with HF to obtain clear CaF sols. A comparison of the optical data between CaF films lactate pentahydrate [47]. The solubility of 2the lactate in ethanol is very good and it needs only2 a on floatAnother glass preparedremarkable from precursor CaCl2 and material Ca(OLac) for2 ·the5H2 Osynthesis is shown of in CaF Figure2 nanoparticles 11. After the is annealing calcium short time after◦ reaction with HF to obtain clear CaF2 sols. A comparison of the optical data processlactate pentahydrate (500 C, 15 min), [47]. the The porosity solubility of the of CaF the2 lactatefilms from in ethanol the lactate is very precursor good isand much it needs higher only (48%) a between CaF2 films on float glass prepared from CaCl2 and Ca(OLac)2·5H2O is shown in Figure 11. short time after reaction with HF to obtain clear CaF2 sols. A comparison of the optical data between CaF2 films on float glass prepared from CaCl2 and Ca(OLac)2·5H2O is shown in Figure 11. InorganicsInorganics 20182018,, 66,, 128x 1010 ofof 1919

After the annealing process (500 °C, 15 min), the porosity of the CaF2 films from the lactate dueprecursor to the is high much organic higher amount (48%) ofdue the to precursor. the high Therefore,organic amount the reﬂectance of the precursor. of this coating Therefore, is nearly the zeroreflectance and the of refractive this coating index is nearly around zero 1.26. and the refractive index around 1.26.

Figure 11.11. TransmittanceTransmittance and and reﬂectance reflectance spectra spectra (a )( asa) wellas well as refractiveas refractive index index (b) of(b calcined) of calcined CaF2 CaFthin2 ﬁlmsthin films and for and a glassfor a substrateglass substrat as functione as function of the wavelength.of the wavelength. Reproduced Reproduced from Reference from Reference [47] with [47] the permissionwith the permission of the Royal of th Chemicale Royal Chemical Society. Society.

Unfortunately,Unfortunately, the the mechanical mechanical stability stability of the of coating the coating from the from MgF 2thesol MgF obtained2 sol fromobtained a magnesium from a lactatemagnesium is very lactate poor. Alreadyis very afterpoor. one Already cycle of af feltter treatment, one cycle the of CaFfelt2 filmtreatment, was completely the CaF2 destroyed.film was Bycompletely contrast, thedestroyed. surface ofBy the contrast, CaF2 coating the surface from CaCl of 2thewas CaF only2 coating barely damaged,from CaCl even2 was after only 100 barely cycles withdamaged, steel wool. even Theafter difference 100 cycles between with thesteel two wool precursors. The difference in optical andbetween mechanical the two properties precursors can bein rationalizedoptical and mechanical based on the properties lactic acid can formed be rationalized when starting based with on the the lactate, lactic whichacid formed (i) has when a high starting boiling pointwith andthe lactate, (ii) tends which to polymerize (i) has a evenhigh moreboiling in orderpoint to and maintain (ii) tends organics to polymerize inside the layerseven more that thermally in order decomposeto maintain at organics higher annealing inside the temperatures layers that only,thermally thus creatingdecompose many at pores higher due annealing to the blowing temperatures effect of theonly, decomposition thus creating products many pores (mainly due CO to2 theand blowin water).g Startingeffect of with the inorganicdecomposition precursors, products on the (mainly other hand,CO2 and no volatilewater). products Startingare with formed inorganic at higher precursors annealing, on temperature; the other hand, thus,the no densificationvolatile products process are is notformed counteraffected, at higher annealing resulting intemp mechanicallyerature; thus, evidently the densification stronger films. process is not counteraffected, resulting in mechanically evidently stronger films. 2.2. Complex Fluorides for Antireﬂective Coatings 2.2. Complex Fluorides for Antireflective Coatings Magnesium Fluoro Aluminates—MgxAlFy and [K1−xNax]MgF3

MagnesiumAntireflective Fluoro filmsAluminates—Mg from binaryxAlF fluorides,y and [K such1−xNax as]MgF MgF3 2 and CaF2, exhibit excellent optical properties and are resistant against abrasion and scratches. At this time, coated glasses with such films Antireflective films from binary fluorides, such as MgF2 and CaF2, exhibit excellent optical can be used for lenses, vitrines, and internal windows. A remaining drawback is still water resistance, properties and are resistant against abrasion and scratches. At this time, coated glasses with such meaning stability towards environmental influences for any outdoor application of such coatings. films can be used for lenses, vitrines, and internal windows. A remaining drawback is still water It is still under debate whether this water sensibility arises from dissolution of the MgF2 coatings resistance, meaning stability towards environmental influences for any outdoor application of such or is rather an effect caused by the lowering the MgF2–particle–particle attraction inside the MgF2 coatings. It is still under debate whether this water sensibility arises from dissolution of the MgF2 layer resulting in a kind of delamination. We do believe in the second hypothesis. Several complex coatings or is rather an effect caused by the lowering the MgF2–particle–particle attraction inside the fluorides, especially magnesium fluoro aluminates, exhibit equally or even lower refractive indices, MgF2 layer resulting in a kind of delamination. We do believe in the second hypothesis. Several but fortunately, are insoluble in water [32]. complex fluorides, especially magnesium fluoro aluminates, exhibit equally or even lower refractive To investigate such compounds to be used for optical films, synthesis conditions were discovered indices, but fortunately, are insoluble in water [32]. giving direct access to water-clear sols of these complex metal fluoride sols. Note, so far there are To investigate such compounds to be used for optical films, synthesis conditions were no reports on the synthesis of transparent fluorometalate sols containing homodispersed fluoride discovered giving direct access to water-clear sols of these complex metal fluoride sols. Note, so far nanoparticles via any other synthesis route. Finally, synthesis protocols for different magnesium there are no reports on the synthesis of transparent fluorometalate sols containing homodispersed fluoroaluminates (MgAlF , Mg AlF and MgAl F ) and perovskite type fluoride compounds via the fluoride nanoparticles via5 any2 other7 synthesis2 8route. Finally, synthesis protocols for different fluorolytic sol–gel approach were successful. After synthesis, in all cases, colorless and clear colloidal magnesium fluoroaluminates (MgAlF5, Mg2AlF7 and MgAl2F8) and perovskite type fluoride dispersions were obtained [48]. With 19F MAS NMR, signal shifting by thermal impacts due to the compounds via the fluorolytic sol–gel approach were successful. After synthesis, in all cases, colorless evaporation of oxygen-containing groups (remaining OR groups from the precursors) was observed. and clear colloidal dispersions were obtained [48]. With 19F MAS NMR, signal shifting by thermal Moreover, the thermal impact finally led to the decomposition of the complexes into the pure fluorides impacts due to the evaporation of oxygen-containing groups (remaining OR groups from the MgF and AlF above 500 ◦C. precursors)2 was3 observed. Moreover, the thermal impact finally led to the decomposition of the As can be seen from Figure 12, the antireflective behavior of the films built up by complex complexes into the pure fluorides MgF2 and AlF3 above 500 °C. fluorides is excellent, e.g., reflections below 1% and in the case of Mg2AlF7, even below 0.5%. Inorganics 2018, 6, x 11 of 19

As can be seen from Figure 12, the antireflective behavior of the films built up by complex fluorides InorganicsInorganics2018, 6, 1282018, 6, x 11 11of 19 of 19 is excellent, e.g., reflections below 1% and in the case of Mg2AlF7, even below 0.5%. Unfortunately, the transmissionsAs canof beall seen layers from Figureare not 12, theon antireflectivethe highest behavior level offor the antireflective films built up by coatings complex fluorides(93–97%). Unfortunately,Obviously,is excellent, due the to e.g., transmissionsthe reflections higher crystallinity below of all 1% layers and of in areth thee notcomplexcase on of Mg the fluorides,2AlF highest7, even level thebelow films for 0.5%. antireflective are Unfortunately, not totally coatings theclear transmissions of all layers are not on the highest level for antireflective coatings (93–97%). (93–97%).and absorbance Obviously, and due scattering to the higher can crystallinitytake place. ofExpectedly, the complex the fluorides, stability the of films these are layers not totally towards clear Obviously, due to the higher crystallinity of the complex fluorides, the films are not totally clear andwater absorbance is excitingly and high scattering (see Figure can take 13). place. Expectedly, the stability of these layers towards water is and absorbance and scattering can take place. Expectedly, the stability of these layers towards excitinglywater high is (seeexcitingly Figure high 13). (see Figure 13).

(a) (a) (b)) Figure 12. (a) Reflectance curve of layers 1–3; (b) Transmission curves of layers 1–3; Layer 1: Figure 12.Figure(a) Reﬂectance12. (a) Reflectance curve of curve layers of 1–3;layers (b )1–3; Transmission (b) Transmission curves curves of layers of 1–3;layers Layer 1–3; 1:Layer MgAlF 1: 5; MgAlF5; Layer2: MgAl2F8; Layer3: Mg2AlF7. Reproduced from Reference [48] with the permission of Layer2:MgAlF MgAl25F; 8Layer2:; Layer3: MgAl Mg2F28AlF; Layer3:7. Reproduced Mg2AlF7. Reproduced from Reference from [Reference48] with the[48] permissionwith the permission of the Royal of Chemicalthe Royalthe Society.Chemical Royal Chemical Society. Society.

Figure 13. Layer 1 (from MgAlF5 sol) after 14 days of water bath. Reproduced from Reference [48] with the permission of the Royal Chemical Society. Figure 13.13. Layer 1 (from(from MgAlFMgAlF55 sol) after 14 daysdays ofof waterwater bath.bath. Reproduced from Reference [[48]48] with thethe permissionpermission ofof thethe RoyalRoyal ChemicalChemical Society.Society. After thermal densification at 450 °C for 15 min, the coated glasses were put into a bath with de- ionized water. Fourteen days later, the◦ glasses were removed from the water bath and dried at room After thermal densification densification at at 450 450 °CC for for 15 15 min, min, the the coated coated glasses glasses were were put put into into a bath a bath with with de- temperature. All coatings were scratch-resistant, meaning they could not be rubbed off from the de-ionized water. Fourteen days later, the glasses were removed from the water bath and dried ionized substrateswater. Fourteen with a tissue.days later,Thus, thethe connectivityglasses were be removedtween the fromsubstrate the (glass)water andbath the and films dried (complex at room at room temperature. All coatings were scratch-resistant, meaning they could not be rubbed off temperature.fluorides) All was coatings very stro wereng. Note, scratch-resistant, respective MgF 2meaning layers can theyeasily couldbe rubbed not off be after rubbed 5 to 8 daysoff only.from the from the substrates with a tissue. Thus, the connectivity between the substrate (glass) and the films substrates withAnother a tissue. group Thus, of complex the connectivity fluorides are be thetween perovskite the substrate like complexes (glass) and(MIM theIIF3 ).films Perovskites (complex (complexfluorides)are fluorides)was nearly very insoluble stro wasng. very Note,compounds strong. respective Note,and theirMgF respective refractive2 layers MgFcan indices easily2 layers are be in canrubbed a low easily offrange beafter rubbed(KMgF 5 to 83 : offdays 1.404 after only. and 5 to 8 daysAnother only.NaMgF group3: 1.364). of These complex characteristics fluorides make are them the interestinperovskiteg candidates like complexes as AR layer (M materials,IMIIF3). Perovskites although I II are nearlyAnotherthey insoluble have group not been ofcompounds complex reported fluorides so and far fortheir aresuch refractive the applicatio perovskite ns.indices This like isare complexesobviously in a low due (Mrange toM the F (KMgFfact3). Perovskitesthat3 gas: 1.404 phase and are deposition is complicated, and sols of such materials were not available so far. However, Schütz et al. nearlyNaMgF insoluble3: 1.364). These compounds characteristics and their make refractive them indicesinterestin areg incandidates a low range as AR (KMgF layer3: materials, 1.404 and NaMgFalthough3: [49] very recently reported the synthesis and characterization of perovskite mixed phases 1.364).they have These not characteristics been reported make so far them for such interesting applicatio candidatesns. This is as obviously AR layer materials,due to the although fact that theygas phase have ([K1−xNax]MgF3) as coating material for the production of antireflective films on glass substrates. In the notdeposition been reported is complicated, so far for suchand sols applications. of such materials This is obviously were not due ava toilable the fact so thatfar. However, gas phase depositionSchütz et al. is case of alkali-containing fluoride sols, only one clear sol (KMgF3) was formed. It was synthesized at 0 °C complicated, and sols of such materials were not available so far. However, Schütz et al. [49] very recently [49] verywith recentlyTMOS as areported stabilization th additive.e synthesis Based onand dynami characterizationc light scattering of investigations, perovskite a meanmixed particle phases reported the synthesis and characterization of perovskite mixed phases ([K Na ]MgF ) as coating ([K1−xNasizex]MgF diameter3) as coating of 70 nm material in a monodispersed for the production distribution of antireflective was determined. films Afteron1−x glass dip-coatingx substrates.3 on float In the materialcase of alkali-containingglass for theand production thermal fluoride densifying of antireflective sols, at only 450 one°C, films theclear reflectance on sol glass (KMgF substrates. was3) was with formed. 0.15% In the Itin case wasan ofoptimal synthesized alkali-containing range at for 0 °C ◦ fluoridewith TMOSantireflective sols, as onlya stabilization one materials. clear additive. solIn the (KMgF same Based3) manner was on dynami formed. as thec magnesiumlight It was scattering synthesized fluoroalum investigations, atinates 0 C (see with a mean above), TMOS particle the as a stabilizationsize diametertransmission additive. of 70 isnm Basednot in very a onmonodispersed high dynamic (96.1%). light By distri scattering meansbution of investigations,structural was determined. analysis a mean methods, After particle dip-coating such size as diameterXRD on and float of 70glass nm and in a thermal monodispersed densifying distribution at 450 °C, was the determined. reflectance After was dip-coatingwith 0.15% on in floatan optimal glass and range thermal for ◦ densifyingantireflective at 450materials.C, the In reflectance the same wasmanner with as 0.15% the inmagnesium an optimal fluoroalum range for antireflectiveinates (see above), materials. the Intransmission the same manner is not asvery the high magnesium (96.1%). fluoroaluminates By means of structural (see above), analysis the transmissionmethods, such is not as veryXRD highand (96.1%). By means of structural analysis methods, such as XRD and solid-state NMR spectroscopy, Inorganics 2018, 6, 128 12 of 19 an increase of crystallinity of the perovskite phases after thermal treatment at 450 ◦C could be evidenced. Obviously, this might be the reason for the lower transmittance of the thin films on glass substrates after the densifying. To sum up, the fluorolytic sol–gel synthesis has been proven to be a kind of universal direct synthesis approach to water-clear binary metal fluoride, but even complex metal fluorides as well, which can be directly used for manufacturing antireflective coatings via the dip-coating technique. No intermediate treatment, e.g., in autoclaves, is necessary. Additionally, since this synthesis method can easily be scaled up, large-scale industrial application is generally possible.

2.3. Up and Down Conversion Materials

CaF2, SrF2 and BaF2 as Hosts for Rare Earth Metals An interesting field in the area of optical materials is the field of luminescent nanoparticles. Luminescent nanoscale materials exhibit the potential for innovative applications, like medical and biological labels [50], displays [51], fluorescent ceramics [52], and solar cells [53]. Alkali earth metal fluorides are very suitable hosts for rare earth doping because of their cubic structure, their high transparency in a broad spectral range, and low photon energy. A lot of complex synthesis methods for the preparation of such systems exist: Thermally assisted polyol-mediated precipitation synthesis [54], thermally induced precipitation synthesis in water-free methanol [55], and incorporation of Tb3+ in transparent glass–ceramic containing CaF2 nanocrystals [56]. Numerous publications, review articles, or book chapters have appeared over the recent years, covering this wide field of research [4]. Fortunately, fluorolytic sol–gel synthesis is a simple single-step method that also gives direct access to rare earth doped fluoride containing nanoparticles. It seems that practically all rare earth metal fluorides can be directly incorporated into the alkaline earth metal (Ca, Sr, Ba), thus proving again fluorolytic sol–gel synthesis as a very convenient universal synthesis for nanometal fluorides. Here, we exemplarily describe some selected examples just to show how effective, but simple this synthesis approach is. In 2014, Ritter et al. [57] reported on the incorporation of Eu3+ and Tb3+ into nanoscale CaF2 using the fluorolytic sol–gel method with calcium lactate and europium- or terbium acetate as precursors. Equation (4) shows a short rationalization of the reaction path.

(1 − x) Ca(OR‘) + x Ln(OR”) + (2 + x) HF → Ca Ln F + (2 − 2x) R’OH + 3x·R”OH 2 3 1−x x 2+x (4) OR’ = Lactate; OR” = Acetate, x = 0 . . . 0.1

The clear sols of CaF2:Eu in methanol with different dopant concentrations show a bright red luminescence upon excitation at 393 nm. This leads to several emission bands in the visible spectral 5 7 5 7 range. These emissions fit very well to the transitions D0→ F0 (578 nm), D0→ F1 (590 nm), 5 7 5 7 5 7 D0→ F2 (611 nm), D0→ F3 (647 nm), and D0→ F4 (698 nm). 5 7 With the exception of D0/ F0, which is located in the yellow range, all the other emissions are detectable in the red spectral region. This fact and the low intensity of the yellow band result in an 5 7 5 7 overall reddish appearance. The bands with the highest intensities match to D0→ F1 and D0→ F2 (Figure 14). Excitation of the sols at 366 nm with a handheld UV-lamp reveals an intense fluorescence, yet a different color for all the four cases (Figure 15). In all cases described here, CaF2 was the host for the rare earth metal dopants. By use of other alkali earth metal fluorides as hosts, such as SrF2 or BaF2 and mixtures of them, a broad variation of luminescence properties was obtained [58]. Therefore, high luminescence intensities are achieved without significant quenching effects, even for a 30% rare earth content and more (Figure 16). InorganicsInorganics 20182018,, 66,, x 128 1313 of of 19 19 Inorganics 2018, 6, x 13 of 19 Inorganics 2018, 6, x 13 of 19

Figure 14. Luminescence emission spectra of 0.2 M sols of CaF2:Eux with x = 0.1 (green), 1 (blue), 2 Figure 14. Luminescence emission spectra of 0.2 M sols of CaF2:Eux with x = 0.1 (green), 1 (blue), 2 (orange),Figure 14.14. 4 (red),Luminescence and 10 (black), emission emission excited spectra spectra at 393of of 0.2 0.2nm. M M The sols sols inset of of CaF CaF shows2:Eu2:Eux the xwithwith integr x x= =ated0.1 0.1 (green), emission (green), 1 1(blue),area (blue), in 2 (orange), 4 (red), and 10 (black), excited at 393 nm. The inset shows the integrated emission area in the2(orange), (orange), 560–720 4 4 (red),nm (red), range. and and 10Reproduced (black), excitedexcited from at atReference 393 393 nm. nm. The [57]The inset withinset shows theshows permission the the integrated integr ofated the emission Royalemission areaChemical area in the in the 560–720 nm range. Reproduced from Reference [57] with the permission of the Royal Chemical Society.560–720the 560–720 nm range.nm range. Reproduced Reproduced from Referencefrom Reference [57] with [57] the with permission the permission of the Royal of the Chemical Royal Chemical Society. Society. Society.

3+ 3+ Figure 15. Picture of Eu - and3+ Tb -doped3+ CaF2 excited with 366 nm (from left to right: CaF2:Tb10, Figure 15. Picture of3+ Eu - and3+ Tb -doped2 CaF excited with 366 nm (from left to2 right: Figure 15. Picture of Eu3+- and Tb3+-doped CaF excited2 with 366 nm (from left to right: CaF :Tb10, mixtureFigure 15.of 52%Picture of CaF of Eu2:Tb10,- and and Tb 48%-doped of CaF CaF2:Eu10,2 excited CaF 2with:Eu4.8,Tb5.2, 366 nm (fromand CaF left2:Eu10). to right: Reproduced CaF2:Tb10, CaFmixture2:Tb10, of 52% mixture of CaF of2 52%:Tb10, of and CaF 248%:Tb10, of CaF and2 48%:Eu10, of CaF CaF22:Eu4.8,Tb5.2,:Eu10, CaF2:Eu4.8,Tb5.2, and CaF2:Eu10). and Reproduced CaF2:Eu10). frommixture Reference of 52% [57] of CaF with2:Tb10, the permissi and 48%on of of CaF the 2Royal:Eu10, Chemical CaF2:Eu4.8,Tb5.2, Society. and CaF2:Eu10). Reproduced Reproducedfrom Reference from [57] Reference with the [permissi57] withon the of permission the Royal ofChemical the Royal Society. Chemical Society. from Reference [57] with the permission of the Royal Chemical Society.

T1 T2 Total Intensity [a.u.] MX :Eu10_15Al T1 T2 Total intensity [a.u.] MF :Eu10_5BA (A) T1 T2 Total Intensity [a.u.] 2 (B) T1 T2 Total intensity [a.u.] MF2 :Eu10_5BA MX :Eu10_15Al (B) 2 (A) T1 T2 2 Total Intensity [a.u.] MX2:Eu10_15Al T1 T2 2Total intensity [a.u.] MF :Eu10_5BA (A) 2 2 (B) 2 2 2 2 1 1 BaF 1 2 1 BaF 1 BaF 1 BaF2 0 BaF2 0 2 SrF 2 SrF BaF 0 CaF SrF BaF 2 0 CaF SrF BaF 2 2 2 2 2 SrF 2 2 2 SrF 0 CaF SrF BaF 2 0 CaF SrF BaF 2 2 2 2 CaF SrF 2 2 2 SrF CaF SrF BaF 22 CaF SrF BaF CaF 2 2 2 2 CaF 2 2 2 CaF2 CaF2 CaF2 2 2 Intensity [a.u.] Intensity [a.u.] Intensity [a.u.] Intensity [a.u.] Intensity [a.u.] Intensity [a.u.]

550 575 600 625 650 675 700 725 750 550 575 600 625 650 675 700 725 750 550 575 600 625 650 675 700 725 750 550 575 600 625 650 675 700 725 750 550 575 600Wavelength 625 650 [nm] 675 700 725 750 550 575 600 625 650 675 700 725 750 Wavelength [nm] Wavelength [nm] FigureFigure 16. 16. ComparisonComparison of of the the luminescence luminescence emission emission spectra spectra of of 0.2 0.2 M M sols sols of CaF 22:Eu10,:Eu10, SrF2:Eu10, SrF2:Eu10, Figure 16. Comparison of the luminescence emission spectra of 0.2 M sols of CaF2:Eu10, SrF2:Eu10, andandFigure BaF BaF 16.22:Eu10:Eu10 Comparison with with 15 15 mol molof the % % ofluminescence of aluminium aluminium emissionisopropoxide isopropoxide spectra (A (A) )ofor or 0.2 5 5 mol molM sols % % of ofof benzoic benzoicCaF2:Eu10, acid acid SrF2:Eu10, ( (BB)) as as an an and BaF2:Eu105 with 715 mol %5 of aluminium7 isopropoxide (A) or 5 mol % of benzoic acid (B) as an additive.additive.and BaF 2:Eu10T1: T1: 5D Dwith0→0→7F 151F, 1 T2:,mol T2: 5 D%D0 →of0→7 Faluminium2.F Reproduced2. Reproduced isopropoxide from from Reference Reference (A) or 5[58] mol [58 with] % with of the benzoic the permission permission acid (B of) ofas the the an additive. T1: 5D0→7F1, T2: 5D0→7F2. Reproduced from Reference [58] with the permission of the RoyalRoyaladditive. Chemical Chemical T1: 5D Society. Society.0→7F1, T2: 5D0→7F2. Reproduced from Reference [58] with the permission of the Royal Chemical Society. Royal Chemical Society. Inorganics 2018, 6, 128 14 of 19 Inorganics 2018, 6, x 14 of 19

ThisThis isis aa newnew andand promisingpromising resultresult andand aa bigbig stepstep forwardforward towardstowards aa higherhigher light efficiency.efficiency. LuminescenceLuminescence intensities,intensities, lifetimes,lifetimes, andand quantumquantum yieldsyields ofof thethe rarerare earthearth ionsions steadilysteadily increaseincrease fromfrom CaFCaF22 viavia SrFSrF22 toto BaFBaF22. InIn the the past past years, years, core–shell core–shell structured structured nanoparticles nanoparticles possessing possessing an an inorganic inorganic core core surrounded surrounded by anby inorganican inorganic shell haveshell been have invented been invented to reduce to radiation-free reduce radiation-free relaxations, relaxations, to improve luminescentto improve lightluminescent output, to light increase output, quantum to increase efficiency, quantum and toeffi solveciency, the and problems to solve from the undesired problems energy from undesired transfers inenergy multirare-earth-doped transfers in multirare-earth-doped systems, which result systems, in a divergent which result color impression.in a divergent Using color the impression. fluorolytic sol–gelUsing the route, fluorolytic in 2017, sol–gel core–shell route, structured in 2017, metalcore–shell fluoride structured nanoparticles metal fluoride were prepared nanoparticles for the were first timeprepared [59]. Thisfor the method first enablestime [59]. the This synthesis method of largeenables amounts the synthesis (kilogram) of larg of nanostructurede amounts (kilogram) core–shell of particlesnanostructured in one batch core–shell at room particles temperature. in one Thebatch remarkable at room temperature. behavior of the The prepared remarkable nanoparticles behavior inof thethe case prepared of SrF nanoparticles2:Eu10@n×SrF in2 (then = case 0–3) of is SrF given2:Eu10@ in Figuren×SrF 172 .(n = 0–3) is given in Figure 17.

Figure 17. Core–shell particles SrF2:Eu10@n×SrF2 (n = 0–3). Upper left: Pictures at daylight and under Figure 17. Core–shell particles SrF2:Eu10@n×SrF2 (n = 0–3). Upper left: Pictures at daylight and under UV excitation (366 nm). (A) PL emission spectra (ex. 393 nm); (B) DLS particle diameter (number); UV excitation (366 nm). (A) PL emission spectra (ex. 393 nm); (B) DLS particle diameter (number); (C) (C) Eu3+ luminescence lifetime (em. 590 nm); (D) luminescence intensity (ex. 393 m), particles with Eu3+ luminescence lifetime (em. 590 nm); (D) luminescence intensity (ex. 393 m), particles with different amounts of Eu3+ doping are given for comparison. PLQY: Photoluminescence quantum yield. different amounts of Eu3+ doping are given for comparison. PLQY: Photoluminescence quantum yield. Reproduced from Reference [59] with the permission of the Royal Chemical Society. Reproduced from Reference [59] with the permission of the Royal Chemical Society. As can be seen in Figure 17, the particle diameter increases with the number of shells from 8 to As can be seen in Figure 17, the particle diameter increases with the number of shells from 8 to 14 nm. In the same manner, the luminescence lifetime and total emission intensity increase with every 14 nm. In the same manner, the luminescence lifetime and total emission intensity increase with additional SrF2 shell, whereby the greatest impact of luminescent intensity was found for the ﬁrst shell. every additional SrF2 shell, whereby the greatest impact of luminescent intensity was found for the The improving luminescence properties with every additional shell are a clear evidence of first shell. the successive formation of a core–shell system. TEM images of SrF2:Eu10 core particles and The improving luminescence properties with every additional shell are a clear evidence of the SrF2:Eu10@3×SrF2 core–shell particles directly prove the increase of particle size (Figure 18). successive formation of a core–shell system. TEM images of SrF2:Eu10 core particles and SrF2:Eu10@3×SrF2 core–shell particles directly prove the increase of particle size (Figure 18). As per the description in the part-magnesium fluoro aluminates (see above), fluorolytic sol–gel synthesis has been also successfully applied for the synthesis of nanoscaled, homodispersed complex calcium metal fluoride sols (CaAlF5, Ca2AlF7, LiMgAlF6) using CaCl2, MgCl2, LiOMe, and Al(OiPr)3 as metal precursors, which were reacted with anhydrous HF in ethanol. All complex metal fluoride sols showed long-time stability and transparency, even after months. These complex metal fluorides recently gained enormous interest as hosts for fluorescent materials due to the suitable sizes of the ionic radii of alkaline earth cations. In addition, they exhibit high ionic strength, hardness, and good isolation behavior and are stable over a wide temperature range [60]. Inorganics 2018, 6, 128 15 of 19 Inorganics 2018, 6, x 15 of 19

Inorganics 2018, 6, x 15 of 19

Figure 18.18.(A )(A Transmission) Transmission electron electron microscopy microscopy (TEM) images (TEM) of SrF images2:Eu10 andof (BSrF) SrF2:Eu102:Eu10@3 and× SrF(B2) SrF(selected2:Eu10@3×SrF particles2 (selected are surrounded particles by are black surrounded lines). Reproduced by black lines). from Reference Reproduced [59] from with theReference permission [59] withof the the Royal permission Chemical of Society.the Royal Chemical Society.

2.4. CompositesAs per the description in the part-magnesium fluoro aluminates (see above), fluorolytic sol–gel synthesis has been also successfully applied for the synthesis of nanoscaled, homodispersed complex Inorganic fillers are able to enhance the performance of organic polymers in terms of thermal calcium metal fluoride sols (CaAlF5, Ca2AlF7, LiMgAlF6) using CaCl2, MgCl2, LiOMe, and Al(OiPr)3 oras mechanical metal precursors, behavior which associated were reactedwith electrical with anhydrous or thermal HF conductivity. in ethanol. Furthermore, All complex metalbecause fluoride of the specialsols showed optical long-time properties stability of MgF and2, the transparency, resulting composites even after (polymer months. and These inorganic complex filler metal material) fluorides are totally transparent (Figure 19) [61,62]. Figurerecently 18. (A gained) Transmission enormous electron interest microscopy as hosts for fluorescent(TEM) images materials of SrF due2:Eu10 to theand suitable (B) sizes of the

SrF2:Eu10@3×SrFionic radii2of (selected alkaline particles earth cations.are surrounded In addition, by blac theyk lines). exhibit Reproduced high ionic from strength, Reference hardness, [59] and good with theisolation permission behavior of the and Royal are Chemical stable over Society. a wide temperature range [60].

2.4. Composites2.4. Composites Inorganic Inorganicfillers are ﬁllersable to are enhance able to enhancethe perfor themance performance of organic of organicpolymers polymers in terms in of terms thermal of thermal or or mechanicalmechanical behavior behavior associated associated with electrical with electrical or thermal or conductivity. thermal conductivity. Furthermore, Furthermore, because of because the of the special opticalspecial properties optical propertiesof MgF2, the of MgFresulting2, the composites resulting composites (polymer and (polymer inorganic and inorganicfiller material) ﬁller are material) are totally transparenttotally transparent (Figure 19) (Figure [61,62]. 19 )[61,62].

Figure 19. Photograph of a representative composite sample with MgF2 (10 wt %) in PolyHEMA. Reproduced from Reference [62] with the permission of the Royal Chemical Society.

The synthesized MgF2 sols via the fluorolytic sol–gel method, stabilized by trifluoroacetic acid, were successfully mixed with acrylate monomers and after polymerization, volume composite or thin films with filler contents up to 40 wt % can be formed (Figure 20).

Figure 19. FigurePhotograph 19. Photograph of a representative of a representative composite compositesample with sample MgF2 with(10 wt MgF %)2 in(10 PolyHEMA. wt %) in PolyHEMA. ReproducedReproduced from Reference from [62] Reference with the [62 pe] withrmission the permission of the Royal of Chemical the Royal Society. Chemical Society.

The synthesizedThe synthesized MgF2 sols MgFvia the2 sols fluorolytic via the fluorolytic sol–gel method, sol–gel stabilized method, stabilizedby trifluoroacetic by trifluoroacetic acid, acid, were successfullywere successfully mixed with mixed acrylate with acrylatemonomers monomers and after and polymerization, after polymerization, volume volume composite composite or or thin thin films filmswith withfiller fillercontents contents up to up 40 towt 40 % wt can % be can formed be formed (Figure (Figure 20). 20).

Figure 20. Scheme of composite preparation from MgF2 sols as thin film coatings or volume composites. Reproduced from Reference [62] with the permission of the Royal Chemical Society.

Figure 20. Scheme of composite preparation from MgF2 sols as thin film coatings or volume composites. Reproduced from Reference [62] with the permission of the Royal Chemical Society. Inorganics 2018, 6, x 15 of 19

Figure 18. (A) Transmission electron microscopy (TEM) images of SrF2:Eu10 and (B)

SrF2:Eu10@3×SrF2 (selected particles are surrounded by black lines). Reproduced from Reference [59] with the permission of the Royal Chemical Society.

2.4. Composites Inorganic fillers are able to enhance the performance of organic polymers in terms of thermal or mechanical behavior associated with electrical or thermal conductivity. Furthermore, because of the special optical properties of MgF2, the resulting composites (polymer and inorganic filler material) are totally transparent (Figure 19) [61,62].

Figure 19. Photograph of a representative composite sample with MgF2 (10 wt %) in PolyHEMA. Reproduced from Reference [62] with the permission of the Royal Chemical Society.

The synthesized MgF2 sols via the fluorolytic sol–gel method, stabilized by trifluoroacetic acid, wereInorganics successfully2018, 6, 128 mixed with acrylate monomers and after polymerization, volume composite16 of or 19 thin films with filler contents up to 40 wt % can be formed (Figure 20).

Figure 20.20.Scheme Scheme of of composite composite preparation preparation from from MgF2 MgFsols as2 sols thin ﬁlmas thin coatings film orcoatings volume composites.or volume composites.Reproduced Reproduced from Reference from [62 Reference] with the [62] permission with the of permission the Royal Chemicalof the Royal Society. Chemical Society.

In the case of the polymer poly(1,4-BDDMA), an incorporation of MgF2 nanoparticles from 10 to 40 wt % leads to a signiﬁcantly reduction of the refractive index from 1.47 to 1.43. Thus, for polymers that need a precisely tuned low refractive index, e.g., as in the case of cladding materials for glass ﬁbers, this offers new opportunities for further progress in this regard.

3. Summary and Outlook This article has focused on the potential arising from the recently developed fluorolytic sol–gel synthesis, which gives direct access to homidispersed nanoscopic metal fluoride materials. The big advantage of the fluorolytic sol–gel approach is the direct access of water-clear sols containing homodispersed metal fluoride nanoparticles. The application of anhydrous HF prevents the agglomeration of NPs and gelation. All the other routes that are based on aqueous HF solutions have to be post-treated in autoclaves, followed by several separation steps, thus being extremely time-demanding and costly and far away from industrial-scale application. Fluorolytic sol–gel synthesis gives not only water-clear sols for manufacturing AR coatings, but also allows very simple, up-scalable synthesis of all imaginable rare earth doped alkaline earth metal (Ca, Sr, Ba) fluoride systems that are of high interest as up- and down-converting materials. Moreover, modified in the right manner, these homodispersed metal fluoride NPs can be used to down tune the refractive index of several organic polymers resulting in transparent metal fluoride–polymer composites. However, there are several other applications away from those reflected here, since the manufacturing of metal fluoride sols allows coating for other purposes too. Thus, sodium fluoroaluminate near the eutectic composition of the AlF3–NaF system (Na1.3AlF4.3) is used under the trade name “Nocolock” by Solvay for the welding of an aluminum-based heat exchanger in automobiles. Moreover, KZnF3 has become increasingly important over the past years as an anticorrosion reagent for aluminum passivation in order to replace copper by aluminum in outdoor air conditioners. In both cases, the application of nanoscopic fluorometalate soles instead of commonly used pastes of micrometers big particles in organic binders offers significantly improved application technologies and drastically improved efficiency. It is worth mentioning that sols could even be used to coat pipes etc. from the inner side, which is impossible based on highly viscous pastes. Metal fluoride coatings are also of interest in the case of corrosion protection against aggressive fluoride-based compounds, e.g., hydrogen fluoride or even fluorine gas. Repeated coatings of, e.g., transparent MgF2 layers that are thermally densified after each single coating step can effectively protect the coated layers of, e.g., glass or diverse metal surfaces. An extremely important field of application arises from the low solubility of, e.g., CaF2 or SrF2 for dental applications. Sols of just several nanometers small MF2 particles can be used to easily introduce these MF2–NPs into caries lesions, creating a kind of depot therein this way. Due to the very low solubility of, e.g., CaF2, a controlled release of fluoride and calcium ions over time takes place exactly at that place where these ions are needed for re-mineralization. Thus, an extensive, repeated supply of soluble fluorides which is still state-of-the-art becomes superfluous. Even the introduction of such MF2–NPs into polymeric dental materials is possible and offers further advantages for the development of new dental materials. Inorganics 2018, 6, 128 17 of 19

All these applications of metal ﬂuorides are only possible if homodispersed nanoparticles are accessible, and those can be obtained via the ﬂuorolytic sol–gel approach as presented here.

Funding: The authors thank the research training network GRK 1582/2 “Fluorine as a Key Element” of DFG (Deutsche Forschungsgemeinschaft) for funding. This project was partly also funded by the German Federal Ministry of Economics and Technology (Grant 03ET1235C). Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Kemnitz, E.; Gross, U.; Rüdiger, S.; Shekar, C.S. Amorphous Metal Fluorides with Extraordinary High Surface Areas. Angew. Chem. Int. Ed. 2003, 42, 4251–4254. [CrossRef][PubMed] 2. Kuznetzov, S.V.; Osiko, V.V.; Tkatchenko, E.A.; Fedorov, P.P. Inorganic nanofluorides and related nanocomposites. Russ. Chem. Rev. 2006, 75, 1065–1082. [CrossRef] 3. Fedorov, P.P.; Luginina, A.A.; Kuznetsov, S.V.; Osiko, V.V.Nanofluorides. J. Fluorine Chem. 2011, 132, 1012–1039. [CrossRef] 4. Fedorov, P.P.; Kuznetsov, S.V.; Osiko, V.V. Photonic. In Electronic Properties of Fluoride Materials; Tressaud, A., Poeppelmeier, K., Eds.; Elsevier: Amsterdam, The Netherlands, 2016; pp. 7–31. 5. Feng, W.; Sun, L.-D.; Zhang, Y.W.; Yan, C.-H. Synthesis and assembly of rare earth nanostructures directed by the principle of coordination chemistry in solution-based process. Coord. Chem. Rev. 2010, 254, 1038–1053. [CrossRef] 6. Li, C.; Lin, J. Rare earth fluoride nano-/microcrystals: Synthesis, surface modification and application. Mater. Chem. 2010, 20, 6831–6847. [CrossRef] 7. Rüdiger, S.; Gross, U.; Kemnitz, E. Non-aqueous sol–gel synthesis of nano-structured metal fluorides. J. Fluorine Chem. 2007, 128, 353–368. [CrossRef] 8. Rüdiger, S.; Kemnitz, E. The fluorolytic sol–gel route to metal fluorides—A versatile process opening a variety of application fields. Dalton Trans. 2008, 9, 1117–1127. [CrossRef][PubMed] 9. Kemnitz, E.; Rüdiger, S. Alain Tressaud. In Functionalized Inorganic Fluorides; Wiley: Hoboken, NJ, USA, 2010; pp. 69–97.

10. Kemnitz, E.; Wuttke, S.; Coman, S.M. Tailor-Made MgF2-Based Catalysts by Sol–Gel Synthesis. Eur. J. Inorg. Chem. 2011, 2011, 4773–4794. [CrossRef] 11. Kemnitz, E. Nanoscale metal fluorides: A new class of heterogeneous catalysts. Catal. Sci. Technol. 2015, 5, 786–806. [CrossRef] 12. Kemnitz, E.; Noack, J. The non-aqueous fluorolytic sol–gel synthesis of nanoscaled metal fluorides. Dalton Trans. 2015, 44, 19411. [CrossRef] 13. Kemnitz, E. Handbook of Sol-Gel Science and Technology; Klein, L., Aparicio, M., Jitianu, A., Eds.; Springer: Cham, Switzerland, 2018. 14. Kemnitz, E.; Coman, S. New Materials for Catalytic Applications; Elsevier: Amsterdam, The Netherlands, 2016; pp. 133–187. 15. Krahl, T.; Kemnitz, E. Aluminium fluoride—The strongest solid Lewis acid: Structure and reactivity. Catal. Sci. Technol. 2017, 7, 773–796. [CrossRef] 16. Lucier, B.E.G.; Johnston, K.E.; Arnold, D.C.; Lemyre, J.-L.; Beaupre, A.; Blanchette, M.; Ritcey, A.M.; Schurko, R.W. Comprehensive Solid-State Characterization of Rare Earth Fluoride Nanoparticles. J. Phys. Chem. C 2014, 118, 1213–1228. [CrossRef] 17. Naccache, R.; Yu, Q.; Capobianco, J.A. The Fluoride Host: Nucleation, Growth, and Upconversion of Lanthanide-Doped Nanoparticles. Adv. Opt. Mater. 2015, 3, 482–509. [CrossRef] 18. Huang, X.; Han, S.; Huang, W.; Lin, X. Enhancing solar cell efﬁciency: The search for luminescent materials as spectral converters. Chem. Soc. Rev. 2012, 42, 173–201. [CrossRef][PubMed] 19. Basiev, T.T.; Doroshenko, M.E.; Fedorov, P.P.; Konyushkin, V.A.; Kuznetsov, S.V.; Osiko, V.V.; Akchurin, M.S.

Efﬁcient laser based on CaF2–SrF2–YbF3 nanoceramics. Opt. Lett. 2008, 33, 521–523. [CrossRef] 20. Aballea, P.; Suganuma, A.; Druon, F.; Hostalrich, J.; Georges, P.; Gredin, P.; Mortier, M. Laser performance of

diode-pumped Yb:CaF2 optical ceramics synthesized using an energy-efficient process. Optica 2015, 2, 288–291. [CrossRef] Inorganics 2018, 6, 128 18 of 19

21. Yang, L.W.; Han, H.L.; Zhang, Y.Y.; Zhong, J.X. White emission by frequency up-conversion in 3+ 3+ 3+ Yb -Ho -Tm triply doped hexagonal NaYF4 nanorods. J. Phys. Chem. 2009, 133, 18995–18999. [CrossRef] 22. Méndez-Ramos, J.; Yanes, A.C.; Santana-Alonso, A.; del-Castillo, J. Highly efﬁcient up-conversion and bright

white light in RE co-doped KYF4 nanocrystals in sol–gel silica matrix. Chem. Phys. Lett. 2013, 555, 196–201. [CrossRef] 23. Wang, F.; Deng, R.; Wang, J.; Wang, Q.; Han, Y.; Zhu, H.; Chen, X.; Liu, X. Tuning upconversion through energy migration in core-shell nanoparticles. Nat. Mater. 2011, 10, 968–973. [CrossRef] 24. Soukka, T.; Rantanen, T.; Kuningas, K. Photon upconversion in homogeneous ﬂuorescence-based bioanalytical assays. Ann. N. Y. Acad. Sci. 2008, 1130, 188–200. [CrossRef] 25. Tian, G.; Gu, Z.J.; Zhou, L.J.; Yin, W.Y.; Liu, X.X.; Yan, L.; Jin, S.; Ren, W.L.; Xing, G.M.; Li, S.J. 2+ Mn Dopant-Controlled Synthesis of NaYF4:Yb/Er Upconversion Nanoparticles for in vivo Imaging and Drug Delivery. Adv. Mater. 2012, 24, 1226–1231. [CrossRef][PubMed] 26. Peng, J.; Sun, Y.; Liu, Q.; Yang, Y.; Zhou, J.; Feng, W.; Zhang, X.; Li, F. Upconversion nanoparticles dramatically promote plant growth without toxicity. Nano Res. 2012, 5, 770–782. [CrossRef] 27. Min, Y.; Liu, J.; Padmanabhan, P.; Yeow, E.; Xing, B. Recent advance of biological molecular imaging based on lanthanide-doped upconversion-luminescent nanomaterials. Nanomaterials 2014, 4, 129–154. [CrossRef] [PubMed] 28. Dimitrov, A.; Koch, J.; Troyanov, S.I.; Kemnitz, E. Aluminum Alkoxide Fluorides Involved in the Sol–Gel

Synthesis of Nanoscopic AlF3. Eur. J. Inorg. Chem. 2009, 2009, 5299–5301. [CrossRef] 29. Astruc, A.; Cochon, C.; Dessources, S.; Célérier, S.; Brunet, S. High specific surface area metal fluorides as catalysts for the fluorination of 2-chloropyridine by HF. Appl. Catal. A Gen. 2013, 453, 20–27. [CrossRef]

30. Fujihara, S.; Tada, M.; Kimura, T. Preparation and characterization of MgF2 thin ﬁlm by a triﬂuoroacetic acid method. Thin Solid Films 1997, 304, 252–255. [CrossRef] 31. Kemnitz, E.; Scholz, G.; Rüdiger, S. Sol-Gel-Synthesis of Nano-Scaled Metal Fluorides—Mechanism and Properties. In Functionalized Inorganic Fluorides: Synthesis, Characterization and Properties of Nanostructured Solids; Tressaud, A., Ed.; John Wiley & Sons Inc.: Hoboken, NJ, USA, 2010; pp. 1–35. 32. Weber, M.J. Handbook of Optical Materials; CRC Press: Boca Raton, FL, USA, 2003. 33. Scholz, G.; Kemnitz, E. Sol-Gel Synthesis of Metal Fluorides: Reactivity and Mechanism. In Modern Synthesis Processes and Reactivity of Fluorinated Compounds; Groult, H., Leroux, F., Tressaud, A., Eds.; Elsevier Inc.: Amsterdam, The Netherlands, 2017; pp. 609–650.

34. Ishizawa, H.; Niisaka, S.; Murata, T.; Tanaka, A. Preparation of MgF2-SiO2 thin ﬁlms with a low refractive index by a solgel process. Appl. Opt. 2008, 47, C200-5. [CrossRef] 35. Karthik, D.; Pendse, S.; Sakthivel, S.; Ramasamy,E.; Joshi, S.V.High performance broad band antireflective coatings

using a facile synthesis of ink-bottle mesoporous MgF2 nanoparticles for solar applications. Sol. Energy Mater. Sol. Cells 2017, 159, 204–211. [CrossRef] 36. Noack, J.; Scheurell, K.; Kemnitz, E.; Garcia-Juan, P.; Rau, H.; Lacroix, M.; Eicher, J.; Lintner, B.; Sontheimer, T.;

Hofmann, T.; et al. MgF2 antireflective coatings by sol–gel processing: Film preparation and thermal densification. J. Mater. Chem. 2012, 22, 18535. [CrossRef] 37. Scheurell, K.; Noack, J.; König, R.; Hegmann, J.; Jahn, R.; Hofmann, T.; Löbmann, P.; Lintner, B.; Garcia-Juan, P.; Eicher, J.; Kemnitz, E. Optimisation of a sol–gel synthesis route for the preparation of

MgF2 particles for a large scale coating process. Dalton Trans. 2015, 44, 19501. [CrossRef] 38. Scheurell, K.; Kemnitz, E.; Garcia-Juan, P.; Eicher, J.; Lintner, B.; Hegmann, J.; Jahn, R.; Hofmann, T.; Löbmann, P.

Porous MgF2 antireflective λ/4 films prepared by sol–gel processing: Comparison of synthesis approaches. J. Sol-Gel Sci. Technol. 2015, 76, 82–89. [CrossRef] 39. Bogush, G.H.; Zukovsky, C.F. Uniform silica particle precipitation: An aggregative growth model. J. Colloid Interface Sci. 1992, 142, 19–34. [CrossRef] 40. Krahl, T.; Broßke, D.; Scheurell, K.; Lintner, B.; Kemnitz, E. Novel aspects in the chemistry of the non-aqueous

fluorolytic sol–gel synthesis of nanoscaled homodisperse MgF2 sols for antireflective coatings. J. Mater. Chem. C 2016, 4, 1454–1466. [CrossRef] 41. Schütz, F.; Scheurell, K.; Scholz, G.; Kemnitz, E. Effects of Chloride Additives on Mechanical Stability

and Environmental Durability of Porous MgF2 Thin Films. In Nanostructured Thin Films IX; Lakhtakia, A., Mackay, T.G., Suzuki, M., Eds.; SPIE: Bellingham, WA, USA, 2016; Volume 9929, p. 992914. Inorganics 2018, 6, 128 19 of 19

42. Löbmann, P.Antireflective coatings by sol–gel processing: Commercial products and future perspectives. J. Sol-Gel Sci. Technol. 2017, 83, 291–295. [CrossRef] 43. Malitson, I.H. A redetermination of some optical properties of calcium ﬂuoride. Appl. Opt. 1963, 2, 1103–1107. [CrossRef] 44. Barta, C.; Fendrych, F.; Recker, K.; Triska, A.; Wallrafen, F. Influence of crystallisation conditions on the microtexture

of the directionally solidified eutectic of the MgF2–CaF2 system. Cryst. Res. Technol. 1990, 25, 1287–1298. [CrossRef] 45. Pilvi, T.; Arstila, K.; Leskelä, M.; Ritala, M. Novel ALD Process for Depositing CaF2 Thin Films. Chem. Mater. 2007, 19, 3387–3392. [CrossRef]

46. Rehmer, A.; Scheurell, K.; Kemnitz, E. Formation of nanoscopic CaF2 via a fluorolytic sol–gel process for antireflective coatings. J. Mater. Chem. C 2015, 3, 1716–1723. [CrossRef] 47. Rehmer, A.; Kemnitz, E. Characterization of nanoscopic calcium fluoride films. In Nanostructured Thin Films IX; Lakhtakia, A., Mackay, T.G., Suzuki, M., Eds.; SPIE: Bellingham, WA, USA, 2016; Volume 9929, p. 99290F. 48. Schütz, F.; Bäthge, M.; Scheurell, K.; Scholz, G.; Feist, M.; Kemnitz, E. Development of complex magnesium fluoro aluminates via the fluorolytic sol–gel synthesis. J. Mater. Chem. C 2018, 6, 72–82. [CrossRef] 49. Schütz, F.; Lange, L.; Scheurell, K.; Scholz, G.; Kemnitz, E. Synthesis and Characterization of Perovskite-Type

[K1−xNax]MgF3 Mixed Phases via the Fluorolytic Sol–Gel Synthesis. Crystals 2018, 8, 66. [CrossRef] 50. Rocha, U.; Jacinto, C.; Silva, W.F.; Guedes, I.; Benayas, A.; Maestro, L.M.; Elias, M.A.; Bovero, E.; van Veggel, F.C.J.M.;

Sole, J.A.G.; et al. Sub-Tissue Thermal Sensing Based on Neodymium-Doped LaF3 Nanoparticles. ACS Nano 2013, 7, 1188–1199. [CrossRef][PubMed] 51. Wang, Q.H.; Setlur, A.A.; Lauerhaas, J.M.; Dai, J.Y.; Seelig, E.W.; Chang, R.P.H. A nanotube-based ﬁeld-emission ﬂat panel display. Appl. Phys. Lett. 1998, 72, 2912–2913. [CrossRef] 52. Santana-Alonso, A.; Mendez-Ramos, J.; Yanes, A.C.; del-Castillo, J.; Rodriguez, V.D. Up-conversion in sol–gel 3+ 3+ 3+ derived nano-glass–ceramics comprising NaYF4 nano-crystals doped with Yb , Ho and Tm . Opt. Mater. 2010, 32, 903–908. [CrossRef] 3+ 53. Shalav, A.; Richards, B.S.; Trupke, T.; Kramer, K.W.; Gudel, H.U. Application of NaYF4:Er up-converting phosphors for enhanced near-infrared silicon solar cell response. Appl. Phys. Lett. 2005, 86, 013505. [CrossRef]

54. Feldmann, C.; Roming, M.; Trampert, K. Polyol-Mediated Synthesis of Nanoscale CaF2 and CaF2:Ce,Tb. Small 2006, 2, 1248–1250. [CrossRef] 55. Wang, J.S.; Miao, W.R.; Li, Y.X.; Yao, H.C.; Li, Z.J. Water-soluble Ln3+-doped calcium ﬂuoride nanocrystals: Controlled synthesis and luminescence properties. Mater. Lett. 2009, 63, 1794–1796. [CrossRef] 56. Sun, X.Y.; Gu, M.; Huang, S.M.; Jin, X.J.; Liu, X.L.; Liu, B.; Ni, C. Luminescence behavior of Tb3+ ions in

transparent glass and glass-ceramics containing CaF2 nanocrystals. J. Lumin. 2009, 129, 773–777. [CrossRef] 3+ 3+ 57. Ritter, B.; Krahl, T.; Rurack, K.; Kemnitz, E. Nanoscale CaF2 doped with Eu and Tb through ﬂuorolytic sol–gel synthesis. J. Mater. Chem. C 2014, 2, 8607–8613. [CrossRef] 58. Ritter, B.; Haida, P.; Fink, F.; Krahl, T.; Gawlitza, K.; Rurack, K.; Scholz, G.; Kemnitz, E. Novel and easy

access to highly luminescent Eu and Tb doped ultra-small CaF2, SrF2 and BaF2 nanoparticles—Structure and luminescence. Dalton Trans. 2017, 46, 2925–2936. [CrossRef] 59. Ritter, B.; Haida, P.; Krahl, T.; Scholz, G.; Kemnitz, E. Core–shell metal fluoride nanoparticles via fluorolytic sol–gel synthesis—A fast and efficient construction kit. J. Mater. Chem. C 2017, 5, 5444–5450. [CrossRef] 60. Rehmer, A.; Scheurell, K.; Scholz, G.; Kemnitz, E. Sol–Gel-Synthesis of Nanoscopic Complex Metal Fluorides. Nanomaterials 2017, 7, 362. [CrossRef] 61. Noack, J.; Schmidt, L.; Gläsel, H.-J.; Bauer, M.; Kemnitz, E. Inorganic–organic nanocomposites based on sol–gel derived magnesium Fluoride. Nanoscale 2011, 3, 4774–4779. [CrossRef] 62. Noack, J.; Fritz, C.; Flügel, C.; Hemmann, F.; Gläsel, H.-J.; Kahle, O.; Dreyer, C.; Bauer, M.; Kemnitz, E. Metal fluoride-based transparent nanocomposites with low refractive indices. Dalton Trans. 2013, 42, 5706–5710. [CrossRef]

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