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Thin Film Materials Research Group, Korea R

(KRICT), 141, Gajeong-ro, Yuseong, Daejeon

kr; changhae@krict.re.kr; Fax: +82-42-861-4

† Electronic supplementary informationprocedure, characterization, and gures.

Cite this: RSC Adv., 2014, 4, 23218

Received 23rd March 2014Accepted 11th April 2014

DOI: 10.1039/c4ra02550d

www.rsc.org/advances

23218 | RSC Adv., 2014, 4, 23218–2322

Synthesis of red-emitting nanocrystalline phosphorCaAlSiN3:Eu

2+ derived from elementaryconstituents†

Junsang Cho,* Bo Keuk Bang, Seok Jong Jeong and Chang Hae Kim*

A highly efficient red-emitting nitride phosphor of CaAlSiN3:Eu2+ is

prepared via one-pot ammonothermal synthetic route starting from

elementary constituents of Ca, Al, Si, and Eu. Respective elements are

able to be dissolved in the supercritical solution of ammonia, trans-

formed into intermediates of metal amides, and consecutively con-

verted to metal nitride of nanocrystalline CaAlSiN3:Eu2+.

In the past few decades, nitridosilicates (or nitrides) as well asoxonitridosilicates (or oxynitrides) have attracted a great deal ofresearch interest because they have excellent mechanical,chemical and thermal stabilities as well as structural diversities.In this regard, much attention has been paid to investigate theEu2+-doped nitride and oxynitride phosphor materials such asa-and b-SiAlON,1,2 M2Si5N8 (M¼ Ca, Sr, Ba),3 MSi2O2N2 (M¼ Ca,Sr, Ba),4 and CaAlSiN3

5 because they are the promising phos-phor candidates used for industrial applications in white light-emitting diodes (LEDs). Among them, CaAlSiN3:Eu

2+ showedsuperb red-emitting luminescent performance due to its highquantum efficiency (�80%), radiation stability, and remarkablethermal quenching behavior.5

In terms of the synthesis of CaAlSiN3:Eu2+ (shortly CAS-

N:Eu2+) phosphors, a wide range of conventional methods havebeen reported: (1) solid state reaction of constituent nitrides athigh temperature (SSR: 1600–1800 �C),6 (2) carbothermalreduction and nitridation of oxide (CRN: 1400–1500 �C),7 and(3) self-propagating high-temperature synthesis from a CaAlSialloy (SHS: 1450–1550 �C).8 Recently, a synthesis from alloy-derived ammonometallates in ammonia solution has beenreported.9,10 However, this method had serious difficultycontrolling the composition of products because alloy materialsof Ca1�xAlSi:Eux with a xed composition rate were pre-synthesized deliberately in order to change the composition

esearch Institute of Chemical Technology

305-600, Korea. E-mail: jscho@krict.re.

151; Tel: +82-42-860-7227

(ESI) available: Details of experimentalSee DOI: 10.1039/c4ra02550d

2

rate of CASN:Eu2+. In addition, it hampered the mechanisticunderstanding of the formation of multinary nitrides since itdid not follow the dissolution–crystallization like sol–gelprocess as the starting materials were from alloy materials, notfrom metal constituents.

To the best of our knowledge, there has been no report thatmultinary nitrides can be synthesized in supercritical ammoniasolution from elementary constituents. Therefore, to improvethe limitations in the previous method, for the rst time, wehave developed and systematically investigated the preparationof Eu2+-doped nitridosilicates of CASN:Eu2+ through the disso-lution–crystallization process starting from elementaryconstituents such as Ca, Al, Si, and Eu at a lower reactiontemperature of 580 �C with an energy-efficient and benignprocess. This novel solution-based one-pot approach providesus with not only a convenient way to control the compositionrate of products, but also a deep insight into how respectivemetals convert to amides, to imides, and ultimately tomultinarynitrides in the ammonia system.

It is well known that oxide materials are synthesized inaqueous media and nitride materials are also able to besynthesized in ammonia solution in the same sol–gel-likeprocess. In the ammonia system, the elementary constituentscan be dissolved to amides, and then condensed to imides andnally to nitrides in a sequence with releasing ammonia.11 Thisis highly dependent on the reaction temperature and pressureof the ammonia system. In the meantime, in order to increasethe solubility of constituents (metals) as well as increase thespeed of crystallization, mineralizers are typically used as theycan play a signicant role in facilitating the synthesis by theformation of the intermediate of metal complex.12 In oursystem, sodium azide is employed as a mineralizer (or ux)because sodium azide is chemically more stable than sodiumamide. Sodium amide is easily contaminated by oxygen due toits high reactivity toward oxygen or moisture and sodium azidecan generate highly puried sodium and nitrogen when ther-mally decomposed between 250–300 �C, generating sodiumamide and hydrogen simultaneously.12

This journal is © The Royal Society of Chemistry 2014

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It has been reported that metal elements of Ca,13,14 Al,15 Si(semimetal),16 and Eu17,18 are allowed to be soluble in sodiumamide–ammonia solution. Many studies have been conductedon the formation of a variety of metal amides, indicating that allelements employed in our synthetic system such as Ca, Al, Si,and Eu are able to react with fused alkali metal amide (e.g.NaNH2) or ammonia solution. It was well known that metalelements in ammonia are able to form metal amides. Zeuneret al. reported that metal amides could be used as reactiveprecursors prepared by dissolving metals in liquid and/orsupercritical ammonia because of the low decompositiontemperature of metal amides.19,20 In addition, in the presence ofsodium ux, Li et al. referred metal amides or its polymerizedproducts as ammonometallates, converted from pure metals.9

Thus, in our synthetic conditions of mixed sodium amide–ammonia solution, respective metals are easily dissolved in thesodium amide–ammonia solution because metals can be con-verted to sodium ammonometallates by forming the adducts ofmetal amide and sodium amide, and these precursors coulddecompose to multinary metal imide and nitride in order.11,12

Aer the complete dissolution of metals in ammonia, thecrystallization process occurred for the synthesis of multinarynitride. The schematic description of the synthesis is presentedin Scheme 1; Eu was excluded for simplicity. Among each metalof Ca (Eu), Al, and Si, the solubility in sodium amide–ammoniasystem is in the order: Si < Al < Ca (Eu). This is mainly attributedto the reactivity of each metal and the number of amide bondsto be made between metal cation and amide ion. Forcalcium, two bonds between the calcium cation and amide ionshould be made in a sodium amide–ammonia system likeCa(NH2)2$Na(NH)2 while aluminum and silicon should haveeach structure like Al(NH2)3$Na(NH)2 and Si(NH2)4$Na(NH)2according to the valence charge of metals. Consequently, siliconis reported as the least reactive metal between them and is onlyslightly dissolved in sodium amide–ammonia solution at 350–400 �C.16

First of all, the preliminary experiments were conducted forthe synthesis of binary Si3N4, starting from an element of Si and

Scheme 1 Schematic diagram for the synthesis of multinary nitride ofCaAlSiN3:Eu

2+, starting from the elements of Ca, Al, Si, and Eu.

This journal is © The Royal Society of Chemistry 2014

NaN3 in the supercritical ammonia solution in an attempt tocheck the feasibility of the synthesis of multinary nitride. It wasinteresting to note that the precursor of sodium ammonosili-cate started to decompose to form NaSi2N3 with sodiumremaining in the host crystal structure.21 However, it was sug-gested that CASN:Eu2+ can be synthesized when Ca and Al areintroduced to the synthesis of NaSi2N3 system. This is becauseNaSi2N3 and CASN:Eu2+ had the same space group, Cmc21, andorthorhombic lattice structure with the close lattice parametersfrom ICDD database: PDF# 01-081-1098 and PDF# 97-016-1796.The XRD patterns showed that well crystallized NaSi2N3 havingthe same crystal structure with CASN was successfully synthe-sized, indicating that the least soluble element of Si was able tobe dissolved and reacted in sodium amide–ammonia solutionat the low temperature of 580 �C (Fig. 1). This stronglyencouraged us that Si can be dissolved and converted to theintermediate of sodium ammonosilicate, and condensed tosodium silicon nitride through the ammonothermal processaer that.

Since the least reactive element of Si could be dissolved inthe melted sodium amide–ammonia system, we rmly believedthat the intermediates of respective sodium ammonometallatesof Ca, Al, Si, and Eu can be formed aer they were completelydissolved in the ammonia system and simultaneous conversionto nitride occurred because the solubility of other materials isbetter than that of silicon.

Second, we investigated the intermediate products of CaAl-SiN3:Eu

2+ obtained at the reaction temperature of 500 �C for 50hours to examine the intermediate structure and the dissolu-tion of pure metals. The XRD patterns of the as-preparedsample showed that any strong peaks from puremetals were notobserved except for the peaks from Na2CaSiO4, Na2SiO3, andAlN. These were suggested to be oxidized from the binary orternary intermediates. It has been reported that binary metalamides adducted with NaNH2 are existed such as KEu(NH2)3,17

NaCa(NH2)3,22 NaAl(NH2)423 and, NaSi(NH2)521 Therefore, metalelements were completely dissolved in ammonia with amineralizer and transformed into the intermediates of amide,

Fig. 1 XRD patterns of NaSi2N3 under the reaction of Si and NaN3 atthe reaction condition of 580 �C for 10 days.

RSC Adv., 2014, 4, 23218–23222 | 23219

Fig. 3 XRD patterns of synthesized CaAlSiN3:Eu2+ phosphors with

various treatment conditions: (a) as-synthesized and (b) after washingtreatment, (c) after acid treatment 1 MHCl for 5min, and (d) for 20min,respectively.

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and then condensed to imide or nitride to some extent.However, aer washing treatment, XRD patterns from CaAlSiN3

and unreacted Si were observed (Fig. 2). This is attributed to nothomogeneously stirring the sample during the reaction. As aresult, a large amount of unreacted Si element staying at thebottom of basket were analyzed. However, more importantly, itwas suggested that the least reactive element, Si was not fullyconverted to silicon amide at that reaction condition, resultingin strong peaks of raw Si in the XRD patterns. Nevertheless,some part of Si was already dissolved and reacted to form CASNbecause the color of the acquired sample changed to gray incomparison with silver and black of starting powders. More-over, collected samples had a size of a few hundred nanometers.As shown in Fig. S1, ESI,† samples before washing showed ananobar-like shape, but aer washing they were supposed to bedissolved in a mixed solvent of water or ethanol, with onlyround-shaped plates remaining. Compared with SEM images ofthe raw starting materials with a relatively large size in fewmircometers or millimeters, depicted in Fig. S2, ESI,† it wasshown that the elementary constituents of metals were de-nitely dissolved in the sodium amide–ammonia solution andthen the crystallization process occurred to generate nano-sizedplates of CASN:Eu2+ phosphor.

Finally, we studied the crystal phase, morphologies,compositions, and optical properties of the CaAlSiN3:Eu

2+

phosphors synthesized under the reaction at 580 �C for 20 dayswith various treatment conditions: (1) as-synthesized, (2) aerwashing treatment with water and ethanol, (3) aer treated with1 M HCl acid for 5 min, and (4) for 20 min, respectively. TheXRD patterns shown in Fig. 3 indicated that CASN wassuccessfully synthesized, consistent with the PDF card# 97-016-1796 of CaAl0.54Si1.38N3. For the sample before washing, theimpurity phases of AlN and CaO were detected, but they wereremoved aer washing treatment. This is due to the facts thatpoorly crystallized powders were hydrolysed in water andwashed out and the heaviest CASN powders were precipitatedmore rapidly than others during centrifugation: (density of

Fig. 2 XRD patterns of intermediate products of CaAlSiN3:Eu2+

phosphors collected at the reaction temperature of 500 �C for 50 h: (a)as-synthesized and (b) after washing treatment.

23220 | RSC Adv., 2014, 4, 23218–23222

CASN: 3.7919 g cm�3).8 When the sample was treated with 1 MHCl acid, the crystallinity of CASN:Eu2+ was slightly improved,but an impurity phase of Al2O3 appeared with increasing acidtreatment time up to 20 min, showing that the residue of Al inthe sample were most likely oxidized to Al2O3. In order tomeasure the atomic concentration of samples, EDS analysis wascarried out. It was shown that both samples before and aerwashing contained a high oxygen content (�40%). This is due tothe fact that unreacted ammonometallates were oxidized toamorphous metal oxide when the reactor was unsealed to air.However, aer treatment with acid, the oxygen content in thesample drastically decreased as these amorphous oxide resi-dues were removed by acid treatment, while nitride compo-nents remained because they were resistant enough to the acidtreatment (Table S1, ESI†).

SEM images of the CASN:Eu2+ phosphors shown in Fig. 4showed that plate-like nanocrystals were synthesized before andaer washing, which was consistent with previous reports.10 Itwas speculated that, in the prolonged reaction time, nano-barsor nano-particle-like crystals seemed to aggregate to form nano-plate-like crystals in comparison with the shape of sampleacquired at the early reaction period at 500 �C for 50 h (i.e.nanorods or nanoparticles). Accordingly, a small amount ofremaining nanorod or nanoparticle crystals are also observed inFig. 2(a and b). However, the research on the detailed mecha-nism for the formation of nano-plates of CASN:Eu2+ has notbeen conducted yet. Despite the acid treatment, the size andmorphology of the samples remain the same.

The room-temperature PL excitation and emission spectra ofCASN:Eu2+ were shown in Fig. 5. The excitation spectra covered theregion fromnear UV to visible with the strongest two peaks locatedat 325 nm and 450 nm due to the excitation of Eu2+ ion transitionfrom 4f7 to 4f65d1. The 5d level was strongly dependant on theouter crystal eld and split by the ligand eld strength of the localsymmetry around the Eu2+ ions. In this case, the 5d orbital of Eu2+

This journal is © The Royal Society of Chemistry 2014

Fig. 4 SEM images of synthesized CaAlSiN3:Eu2+ phosphors with

various treatment conditions: (a) as-synthesized and (b) after washingtreatment, (c) after acid treatment 1 MHCl for 5min, and (d) for 20min,respectively.

Fig. 5 PL excitation and emission spectra of CaAlSiN3:Eu2+ from

bottom to top: as-synthesized, after washing treatment, after acidtreatment 1 M HCl for 5 min, and for 20 min, respectively.

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is split into two levels such as T2g and Eg due to the tetrahedralsymmetry of surrounding rigid network of [AlN4] and [SiN4]. Underexcitation of 450 nm, strong red emission peak centered at 650 nmwas observed. Aer washing treatment with water and acid, theboth PL excitation and emission intensity increased because non-emitting amorphous materials were removed by washing treat-ment. Acid treatment enabled an slight increase in the PL emis-sion intensity. However, they showed a relatively low PL efficiencyof �5% compared to conventionally prepared phosphors (Fig. S3,ESI†) due to an extremely low reaction temperature of 580 �Cinstead of 1600 �C, but had an excellent thermal stability. Thetemperature-dependent PL emission showed that PL intensityremained at 75% of the initial value with heating sample up to180 �C, shown in Fig. S4, ESI.†

This journal is © The Royal Society of Chemistry 2014

Additionally, in order to evaluate the effect of a mineralizerand reaction time, the amount of sodium azide and reactiontime were altered respectively. The more sodium azideemployed in the ammonia system, the more crystallized CAS-N:Eu2+ powders were synthesized, with the improved XRDpatterns shown in Fig. S5, ESI.† This is because sodium azideplayed an important role in the facilitation of the dissolution ofraw materials, nucleation and growth of multinary crystals.When an insufficient amount of mineralizer was used, such asthe molar ratio of Na/(Ca + Eu) ¼ 0.5, a large amount ofunreacted Si was found, indicating that a mineralizer was reallyimportant to dissolve the pure metal solutes, especially Si due tothe least reactive element, in ammonia solution. In a prolongedreaction period of up to 30 days, more converted CASN:Eu2+

powders were prepared with the conversion yield of up to 85%(Fig. S6, ESI†).

Overall, all elements were dissolved in sodium amide–ammonia solution to form ammometallates of Ca, Al, Si andEu at 400 �C, and these precursors were condensed togetherto form multinary intermediates with increasing the reactiontemperature from 400 �C to 580 �C: from the binary amide ofM–(NH2)x (M ¼ Ca(Eu), Al, Si) to the ternary amide imides ofCa–(NH)–Al, Ca–(NH)–Si, and Al–(NH)–Si, and the quaternaryimide nitrides of Ca–N–(Si)Al.11,24–27 An indication of theadduct type with NaNH2 were omitted for the simplicity. Atthe critical temperature, pressure, and concentration ofmultinary nitrides containing Ca, Al, Si, and N, they weresuggested to decompose to a seed of CASN:Eu2+ accompa-nying simultaneous a nucleation and a following crystalliza-tion growth. Unfortunately, at present, it is uncertain whetherthe intermediate of ammonometallates or their condensedforms (i.e. imides, imide nitrides, nitrides) exist at themolecular level or partially polymerized level depending ontheir size. However, it was thought that each ammonome-tallate existing at the molecular level may condense eachother promptly to the certain polymerized products contain-ing quaternary elements, considering the reactivity ofamides, which would be used a seed of CASN:Eu2+ later. It isdifficult to characterize intermediate complex because theammonometallates are so reactive in air that they trans-formed to an amorphous phase with a low crystallinityimmediately aer the reactor was unsealed under atmo-spheric conditions. Thus, we are in the process of systemat-ically investigating the intermediate complexes to fullyunderstand the formation mechanism of multinary nitrides,especially from elementary constitutents.

In conclusion, the novel ammonothermal synthesis of CAS-N:Eu2+, starting from respective metal constituents was inves-tigated. This synthetic method is an easy and convenient way tocontrol the composition ratio of products, and provides insightinto understanding the formation mechanism of the multinarynitride of CASN:Eu2+ under a supercritical ammonia systemwith a sequence of metal, sodium ammonometallates, imide,and multinary nitride at a lower reaction temperature of 580 �C.It allows us to synthesize various nitrides via elementary metals-driven ammonothermal process.

RSC Adv., 2014, 4, 23218–23222 | 23221

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Acknowledgements

This work is nancially supported by Korea Research CouncilIndustrial Science and Technology (KK-1307-B9).

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