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Page1of16Journal of Materials Chemistry CView Article OnlineDOI:10.1039/C6TC02737GSet of manganese ions activated fluoride phosphors (A,BF6:Mn4+, A=KNa, B=Si, Ge, Ti): Synthesis below O C and efficient room-temperaturephotoluminescenceFei Tang, Zhicheng Su," Honggang Ye," Mingzheng Wang," Xin Lan,' David LeePhillips,Yongge Cao, and Shijie XuaTransition-metal-ions activated solid-state phosphors are of particular interest for thedevelopment of LED-based white light sources. In addition to their relatively low cost,these luminescent materials show exceptionally high luminescence efficiencyespecially at room temperature and above,due to the involvement and promotion ofthermal phonons.Inthisarticle,wepresentacomprehensiveinvestigationonthesetof manganese ions doped fluoride phosphors (A2BF6:Mn*t, A=K, Na, B=Si, Ge, Ti),includingthesynthesisproceduresandvariouscharacterizationswiththeemphasisofoptical spectroscopic characterizations.All the phosphors synthesized at a temperatureof -16 C by chemical co-precipitation method exhibit intense red color emissions atroom temperature under the excitation of light with a wide range of wavelengths from450 nmto325nm.Inmost of the phosphors thephonon-assisted luminescencedominates in the spectra, which is evidenced by Raman scattering measurements.X-raydiffraction data of the samples reveal thatK2SiF6:Mn4+crystallizes in cubicphase, while the remaining crystals have hexagonal structures, but with differentsymmetries for K,TiF6:Mnttand NaSiF6:Mntt, respectively. More interestingly, thewell-resolved spectral splitting was observed on the major phonon-assistedluminescence signatures of both K2SiF6:Mn4+and K,TiF6:Mn*samples, indicatingthe occurrence of complicated but fascinating phonon-assisted transition processes inthe phosphors.IntroductionDriven by the big demand and rapid development of warm white light-emitting diodes(WLED), various Mn-activated alkali-metal hexafluoride red phosphors have beenexplored in recent years, primarily due to their distinct advantages in synthesis andluminescenceaspects.-Comparedwiththetraditional commercial redphosphorsusing Eu2+ ions as active centers, for example, CaAISiNg:Eu?+6. 7 which is usuallyaccompanied by a broad emission band, A2BF6:Mn4+ (i.e., A=K, Na, B=Si, Ge, Ti)phosphors exhibit sharp emission lines at red region with a high PL quantum yield(QY).Without use of expensive rare earth oxides and hard synthetic conditions, suchas high temperature and high pressure,the preparation of A,BF6:Mn4+phosphorsbecomes easier and much cheaper, which makes this kind of phosphors morecompetitive than the present commercial products. Nevertheless, the possibility oflarge varying chemical valance state of Mnelement from +7 to O also brings a greatDepartment of Physics, and Shenzhen Institute of Research and Innovation (SIRI),TheUniversity of Hong Kong, P. R. China.Corresponding author. Email: sjxu@hku.hkbDepartmentof Chemistry,TheUniversityofHongKong,P.R.China°Department of Physics, Renmin University of China, Beijing 100872, P. R. China1
1 Set of manganese ions activated fluoride phosphors (A2BF6:Mn4+, A=K, Na, B=Si, Ge, Ti): Synthesis below 0 oC and efficient room-temperature photoluminescence Fei Tang,a Zhicheng Su,a Honggang Ye,a Mingzheng Wang,a Xin Lan,b David Lee Phillips,b Yongge Cao,c and Shijie Xua* Transition-metal-ions activated solid-state phosphors are of particular interest for the development of LED-based white light sources. In addition to their relatively low cost, these luminescent materials show exceptionally high luminescence efficiency, especially at room temperature and above, due to the involvement and promotion of thermal phonons. In this article, we present a comprehensive investigation on the set of manganese ions doped fluoride phosphors (A2BF6:Mn4+, A=K, Na, B=Si, Ge, Ti), including the synthesis procedures and various characterizations with the emphasis of optical spectroscopic characterizations. All the phosphors synthesized at a temperature of -16 oC by chemical co-precipitation method exhibit intense red color emissions at room temperature under the excitation of light with a wide range of wavelengths from 450 nm to 325 nm. In most of the phosphors the phonon-assisted luminescence dominates in the spectra, which is evidenced by Raman scattering measurements. X-ray diffraction data of the samples reveal that K2SiF6:Mn4+ crystallizes in cubic phase, while the remaining crystals have hexagonal structures, but with different symmetries for K2TiF6:Mn4+and Na2SiF6:Mn4+, respectively. More interestingly, the well-resolved spectral splitting was observed on the major phonon-assisted luminescence signatures of both K2SiF6:Mn4+ and K2TiF6:Mn4+ samples, indicating the occurrence of complicated but fascinating phonon-assisted transition processes in the phosphors. Introduction Driven by the big demand and rapid development of warm white light-emitting diodes (WLED), various Mn4+-activated alkali-metal hexafluoride red phosphors have been explored in recent years, primarily due to their distinct advantages in synthesis and luminescence aspects.1-5 Compared with the traditional commercial red phosphors using Eu2+ ions as active centers, for example, CaAlSiN3:Eu2+ , 6, 7 which is usually accompanied by a broad emission band, A2BF6:Mn4+ (i.e., A=K, Na, B=Si, Ge, Ti) phosphors exhibit sharp emission lines at red region with a high PL quantum yield (QY).1 Without use of expensive rare earth oxides and hard synthetic conditions, such as high temperature and high pressure,8 the preparation of A2BF6:Mn4+ phosphors becomes easier and much cheaper, which makes this kind of phosphors more competitive than the present commercial products. Nevertheless, the possibility of large varying chemical valance state of Mn element from +7 to 0 also brings a great aDepartment of Physics, and Shenzhen Institute of Research and Innovation (SIRI), The University of Hong Kong, P. R. China. * Corresponding author. Email: sjxu@hku.hk bDepartment of Chemistry, The University of Hong Kong, P. R. China cDepartment of Physics, Renmin University of China, Beijing 100872, P. R. China Page 1 of 16 Journal of Materials Chemistry C Journal of Materials Chemistry C Accepted Manuscript Published on 19 September 2016. Downloaded by Cornell University Library on 20/09/2016 06:17:28. View Article Online DOI: 10.1039/C6TC02737G
Journal ofMaterialsChemistryCPage2of16ViewArticleOnlinDOI:10.1039/C6TC02737Gchallenge in the synthetic process, which may result in undesired byproductscontaining Mnt(x+4)ions.Therefore, how to control the Mn valance state of being+4 during the reaction process is crucial to the preparation of A2BF:Mn4+ redphosphors withdesirableluminescence characteristics.Inthisstudy,wereporttherational svnthesis oftheseries of A,BF:Mn4+red phosphorsby usinga lowtraureowchmaltraonmdpftygouIt is known that in A2BF6:Mn4* red phosphors, B4ions are commonly coordinatedwith six Fions to form a regular octahedron,,l The introduction of Mnt ions intothe host lattice usually happens via substituting Btions in the view of their samevalence states and the lower formation energy.2 Thus, to avoid an excess latticedistortion, the choice of B4+ ions having similar ionic radius with Mn4+ is desired forsuch substitution process. In contrast, A element does not engage in the replacementbehavior of Mn+ ions and is generally alkali metal or alkaline earth metal element(A2).3, 14 On the other hand, the flexible selection of A element may offer us a goodprobability to turn the emission wavelength to meet our demand through adjusting theA-F bond strength to some extent which can lead to a large luminescence peakposition displacement.The two kinds of crystalline lattice structures,namelyhexagonal and cubicphases,havebeen verifiedforthisfamily of phosphorcrystals.Amongst them, K2SiF6:Mn+4crystal is found toform in the only cubic structure, butexhibit characteristic luminescence spectroscopic properties analogous to thehexagonal ones.15Thus far, several synthesis approaches have been developed to prepare this kind ofphosphors, including wet chemical etching,31617 cation exchange reactionoxydoreduction reaction's and hydrothermal reaction.1920 Each of them represents afeasiblepreparationrouteandshowssomeinterestingresults.However,basedontheconsideration of time consumption and the usage of strong corrosive HF solution, itstill remains a great challenge to develop a facile synthesis route for the low cost massproduction. Recently, Wei et al. proposed a low-temperature co-precipitation approachto synthesize K2MF6:Mn*t(M-Si and Ge) fluoride phosphors.10 This method candramatically shorten the preparation period.At the same time,low temperatureenvironment canefficientlydepresstheevaporationof HF solutionwhich may causesevere harm to human body.However, the synthesis temperature reported by themwas around or even above the boiling point of HF (19.5 C), which may still result inmassive evaporated HF gas. Inspired by the low-temperature co-precipitation idea, weimprove the route to make the synthesis to be carried out at temperature of-16 c.Byemploying different compounds containing B4+ions,for example,BO2,H2BF6, andK,BF asa reactant,we synthesize set of A,BF6:Mn* phosphors, e.g.K2SiF6:Mn**(KSFM),K,TiF6:Mn4*(KTFM),Na2SiF6:Mn**(NaSFM),andK,GeFg:Mn**(KGFM). And then we do comprehensive characterizations, includingX-ray diffraction, scanning electron microscope, steady-state and time-resolvedphotoluminescence (PL) and Raman scattering, as well as necessary ab initiocalculations, on these samples. These study activities make us obtain an overallstate-of-the-art understanding of this new class of red phosphors.2
2 challenge in the synthetic process, which may result in undesired byproducts containing Mn+x(x≠4) ions. Therefore, how to control the Mn valance state of being +4 during the reaction process is crucial to the preparation of A2BF6:Mn4+ red phosphors with desirable luminescence characteristics. In this study, we report the rational synthesis of the series of A2BF6:Mn4+ red phosphors by using a low temperature slow chemical titration method proposed first by Liu group.9, 10 It is known that in A2BF6:Mn4+ red phosphors, B4+ ions are commonly coordinated with six F- ions to form a regular octahedron.1, 11 The introduction of Mn4+ ions into the host lattice usually happens via substituting B4+ ions in the view of their same valence states and the lower formation energy.12 Thus, to avoid an excess lattice distortion, the choice of B4+ ions having similar ionic radius with Mn4+ is desired for such substitution process. In contrast, A element does not engage in the replacement behavior of Mn4+ ions and is generally alkali metal or alkaline earth metal element (A2).13, 14 On the other hand, the flexible selection of A element may offer us a good probability to turn the emission wavelength to meet our demand through adjusting the A-F bond strength to some extent which can lead to a large luminescence peak position displacement. The two kinds of crystalline lattice structures, namely hexagonal and cubic phases, have been verified for this family of phosphor crystals. Amongst them, K2SiF6:Mn+4 crystal is found to form in the only cubic structure, but exhibit characteristic luminescence spectroscopic properties analogous to the hexagonal ones.15 Thus far, several synthesis approaches have been developed to prepare this kind of phosphors, including wet chemical etching,3,16,17 cation exchange reaction,1 oxydoreduction reaction18 and hydrothermal reaction.19,20 Each of them represents a feasible preparation route and shows some interesting results. However, based on the consideration of time consumption and the usage of strong corrosive HF solution, it still remains a great challenge to develop a facile synthesis route for the low cost mass production. Recently, Wei et al. proposed a low-temperature co-precipitation approach to synthesize K2MF6:Mn4+(M=Si and Ge) fluoride phosphors.10 This method can dramatically shorten the preparation period. At the same time, low temperature environment can efficiently depress the evaporation of HF solution which may cause severe harm to human body. However, the synthesis temperature reported by them was around or even above the boiling point of HF (19.5 oC), which may still result in massive evaporated HF gas. Inspired by the low-temperature co-precipitation idea, we improve the route to make the synthesis to be carried out at temperature of -16 oC. By employing different compounds containing B4+ ions, for example, BO2, H2BF6, and K2BF6 as a reactant, we synthesize set of A2BF6:Mn4+ phosphors, e.g. K2SiF6:Mn4+(KSFM), K2TiF6:Mn4+(KTFM), Na2SiF6:Mn4+(NaSFM), and K2GeF6:Mn4+(KGFM). And then we do comprehensive characterizations, including X-ray diffraction, scanning electron microscope, steady-state and time-resolved photoluminescence (PL) and Raman scattering, as well as necessary ab initio calculations, on these samples. These study activities make us obtain an overall state-of-the-art understanding of this new class of red phosphors. Journal of Materials Chemistry C Page 2 of 16 Journal of Materials Chemistry C Accepted Manuscript Published on 19 September 2016. Downloaded by Cornell University Library on 20/09/2016 06:17:28. View Article Online DOI: 10.1039/C6TC02737G
Page3of16Journal ofMaterialsChemistryCView Article OnlineDOI:10.1039/C6TC02737GExperimentalCommercial available chemicals were used as reactants and precursors tabulated inTable sl of supporting information. All these materials were of analytical gradewithout further modification before usage.Considering the corrosive feature of HFthe plastic containers, instead of glass or metal ones, were required to have beenalwaysusedintheentiresynthesisprocess.Synthesis of MaterialsThe red phosphors of A,BF6:Mn4* were prepared using the two-step chemicalco-precipitation approach. In the entire synthesis process the surrounding temperaturewas kept at -16 c. The synthesis of KMF compound was completed in the first stepThe major procedures were schematically depicted in Fig. 1(a). The synthesized KMFcompound will be used as the Mn* ions precursor in the next step of preparation. Forthe synthesis ofKMF compound,vigorous stirringoperationwasfirstly carried out inmixing HF solution and KHF2 powder to form a colorless solution, followed byaddingKMnO4powdersto form a black uniform mixture solution.Herein,HOsolution was employed as an efficient reducing agent to drop slowly into the abovesolution, usually accompanying with the generation of a large amount of oxygen.BeforefinishingtheadditionofH,O,a slowchangeof solutioncolorfromblacktobrown-yellowshallbeobserved,whichimpliesthereductionofchemical valancestate of Mn element from +7 to +4. It is required to note that the addition rate of H,O2solution shall be carefully controlled to prevent formation of other valance states ofMn ions in the system.The reaction process may be described by,2KMnO,+8HF+3H,0,+2KHE,→2K,MnF.+8H0+30,个(1)After KMF was fully precipitated from the reaction solution, vacuum filtrationoperation was proceeded to filter KMF precipitation, then followed by washingprocesses for three times to eliminate other impurities.The drying process was carriedout at 80C before the achievement of KMF powder.In the next major process the red-phosphor A,BFg:Mn*+ powders were synthesized.As illustratedinFig.l(b),theKMFpowderobtainedinthelast stepwasemployedasa reactant in this preparation process.It was first added into HF solution undervigorous stirring to form a uniform gold-yellow solution.Then the B elementcompounds were subsequently dropped into the solution undera continuous stirringoperation.Herein, it shall benoted that the different compounds ofB elements areselected for the synthesis of different red phosphors. For KTFM, KGFM, KSFM, theirB element precursors were chosen to beK,TiF6, GeO2,and H2SiF6,respectively.InthecaseofNaSFM,H2SiF6wasalsoadoptedasSi+sourcematerialjustlikeKSFMAfter some reaction time, yellow precipitation can be observed at the bottom ofcontainer.Then viatheprocessesof filtration,washing and drying in sequence,thefinal phosphors were obtained.3
3 Experimental Commercial available chemicals were used as reactants and precursors tabulated in Table s1 of supporting information. All these materials were of analytical grade without further modification before usage. Considering the corrosive feature of HF, the plastic containers, instead of glass or metal ones, were required to have been always used in the entire synthesis process. Synthesis of Materials The red phosphors of A2BF6:Mn4+ were prepared using the two-step chemical co-precipitation approach. In the entire synthesis process the surrounding temperature was kept at -16 oC. The synthesis of KMF compound was completed in the first step. The major procedures were schematically depicted in Fig. 1(a). The synthesized KMF compound will be used as the Mn4+ ions precursor in the next step of preparation. For the synthesis of KMF compound, vigorous stirring operation was firstly carried out in mixing HF solution and KHF2 powder to form a colorless solution, followed by adding KMnO4 powders to form a black uniform mixture solution. Herein, H2O2 solution was employed as an efficient reducing agent to drop slowly into the above solution, usually accompanying with the generation of a large amount of oxygen. Before finishing the addition of H2O2, a slow change of solution color from black to brown-yellow shall be observed, which implies the reduction of chemical valance state of Mn element from +7 to +4. It is required to note that the addition rate of H2O2 solution shall be carefully controlled to prevent formation of other valance states of Mn ions in the system. The reaction process may be described by, 4 2 2 2 2 6 2 2 2KMnO +8HF+3H O +2KHF 2K MnF +8H O+3O → ↑ . (1) After KMF was fully precipitated from the reaction solution, vacuum filtration operation was proceeded to filter KMF precipitation, then followed by washing processes for three times to eliminate other impurities. The drying process was carried out at 80 oC before the achievement of KMF powder. In the next major process the red-phosphor A2BF6:Mn4+ powders were synthesized. As illustrated in Fig. 1(b), the KMF powder obtained in the last step was employed as a reactant in this preparation process. It was first added into HF solution under vigorous stirring to form a uniform gold-yellow solution. Then the B element compounds were subsequently dropped into the solution under a continuous stirring operation. Herein, it shall be noted that the different compounds of B elements are selected for the synthesis of different red phosphors. For KTFM, KGFM, KSFM, their B element precursors were chosen to be K2TiF6, GeO2, and H2SiF6, respectively. In the case of NaSFM, H2SiF6 was also adopted as Si4+ source material just like KSFM. After some reaction time, yellow precipitation can be observed at the bottom of container. Then via the processes of filtration, washing and drying in sequence, the final phosphors were obtained. Page 3 of 16 Journal of Materials Chemistry C Journal of Materials Chemistry C Accepted Manuscript Published on 19 September 2016. Downloaded by Cornell University Library on 20/09/2016 06:17:28. View Article Online DOI: 10.1039/C6TC02737G
JournalofMaterialsChemistryCPage4of16ViewArticleOnlirDOI:10.1039/C6TC02737GsStirtK-MnFs(a)(b)(A=KorNa.B=SiorGeorTi)Fig. 1 Schematic diagrams of the synthesis of (a) K,MnF6 powder and (b)A2BF6:Mn++(A=K orNa,B=Ti,Si, or Ge)red phosphors via low-temperatureco-precipitation methods.MaterialscharacterizationX-raydiffraction(XRD)patternsofthefinalproductswererecordedatascanrateof0.02 °swith an X-ray diffractometer (Type D8 Advance ECO, Bruker, UK). In thisdiffractometer a Cu K-alpha line was used as the irradiation source.The acceleratingvoltage and applied current were set at 40 kV and 80 mA, respectively, for the XRDmeasurements. The morphology of the samples studied was analyzed by using aSU-8010 cold-field emission scanning electron microscope (HITACHI UHR, Japan)at an accelerating voltage of 5 kV.High-resolution PL spectra were measured at room temperature on a home-madePL setup. The 476 nm line from an Ar-Kr ion mixed gas laser (Coherent Innova-70)was used as theexcitation sourceforPLmeasurements.The luminescence signal wasdispersedbyamonochromator(Spex75oM)beforetransformingintoanelectricalsignal withaphotomultipliertube(HamamatsuR928).Alock-inamplifier(StanfordResearch SR830)togetherwitha standardoptical chopperwasemployedtogainahigh signal to noise ratio. Finally, a data acquire module was used to convert theelectrical signal into digital data that can be accepted by computer.Room-temperatureRamanspectraofthesamplesweremeasuredonaconfocalmicro-Raman system (WITech-Alpha) by using the 514.5 nm line of an Ar ion laseras the excitation light source.The laser beam was focused on the samples via asingle-modeopticalfiber.Thescattered signal waspassedthroughalongpassfiltertoeliminate the Rayleigh line before light dispersing in a monochromator (ActonA
4 Fig. 1 Schematic diagrams of the synthesis of (a) K2MnF6 powder and (b) A2BF6:Mn4+ (A=K or Na, B=Ti, Si, or Ge) red phosphors via low-temperature co-precipitation methods. Materials characterization X-ray diffraction (XRD) patterns of the final products were recorded at a scan rate of 0.02 o s -1 with an X-ray diffractometer (Type D8 Advance ECO, Bruker, UK). In this diffractometer a Cu K-alpha line was used as the irradiation source. The accelerating voltage and applied current were set at 40 kV and 80 mA, respectively, for the XRD measurements. The morphology of the samples studied was analyzed by using a SU-8010 cold-field emission scanning electron microscope (HITACHI UHR, Japan) at an accelerating voltage of 5 kV. High-resolution PL spectra were measured at room temperature on a home-made PL setup. The 476 nm line from an Ar-Kr ion mixed gas laser (Coherent Innova-70) was used as the excitation source for PL measurements. The luminescence signal was dispersed by a monochromator (Spex 750M) before transforming into an electrical signal with a photomultiplier tube (Hamamatsu R928). A lock-in amplifier (Stanford Research SR830) together with a standard optical chopper was employed to gain a high signal to noise ratio. Finally, a data acquire module was used to convert the electrical signal into digital data that can be accepted by computer. Room-temperature Raman spectra of the samples were measured on a confocal micro-Raman system (WITech-Alpha) by using the 514.5 nm line of an Ar ion laser as the excitation light source. The laser beam was focused on the samples via a single-mode optical fiber. The scattered signal was passed through a longpass filter to eliminate the Rayleigh line before light dispersing in a monochromator (Acton Journal of Materials Chemistry C Page 4 of 16 Journal of Materials Chemistry C Accepted Manuscript Published on 19 September 2016. Downloaded by Cornell University Library on 20/09/2016 06:17:28. View Article Online DOI: 10.1039/C6TC02737G