HF-Free Hydrothermal Route for Synthesis of Highly Efficient Narrow

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A HF-free hydrothermal route for synthesis of highly efficient narrowband red emitting phosphor K2Si1-xF6:xMn4+ for warm white LEDs Lin Huang, Yiwen Zhu, Xuejie Zhang, Rui Zou, Fengjuan Pan, Jing Wang, and Mingmei Wu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04989 • Publication Date (Web): 31 Jan 2016 Downloaded from http://pubs.acs.org on February 2, 2016

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Chemistry of Materials

A HF-free hydrothermal route for synthesis of highly efficient narrow-band red emitting phosphor K2Si1-xF6:xMn4+ for warm white LEDs Lin Huang, Yiwen Zhu, Xuejie Zhang, Rui Zou, Fengjuan Pan, Jing Wang* and Mingmei Wu Ministry of Education Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, KLGHEI of Environment and Energy Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou, Guangdong 510275, PR China

ABSTRACT: With superior photo-luminescent properties, the recently discovered A2MF6:Mn4+ material holds the potential in replacing the commercial rare-earth-doped (oxy)nitride phosphors for solid state lighting and display. We report here a green synthetic route to synthesize narrow red emitting K2SiF6:Mn4+ without the usage of toxic and volatile HF solution. We show that K2SiF6:Mn4+ are produced in common low-toxic H3PO4/KHF2 liquid instead of high-toxic HF liquid and systematically investigate its morphology and photo-luminescence properties. Moreover, the reaction mechanism is comprehensively discussed in detail. We find that not only does H3PO4/KHF2 play the same key roles as HF in the process of stabilizing Mn4+ and promoting Mn4+ into the host K2SiF6 but also exhibits more excellent ability than HF in controlling the concentration of Mn4+ ion in the host K2SiF6. By demonstrating its application in white LED with tunable chromaticity coordinate and correlated color temperature, we show our hydrothermal strategy based on low-toxic H3PO4/KHF2 solution system will open the opportunity for the narrow red emitting A2MF6:Mn4+ to be synthesized in large scale towards white LED industry adoptions.

1. INTRODUCTION

ing white LEDs suffer many weaknesses, for instance

Luminescent materials or phosphors have played a

lower CRI and high CCT, and fail to be used in general

key role as light convertor in determining the optical

lighting and back lighting source for display.1–3 Thereaf-

properties of white LEDs including color rendering index

ter, many researchers worldwide have been devoted to

(CRI), correlated color temperature (CCT), etc., since

develop red emitting phosphors which may be suitable

white LEDs was firstly invented in 1996 and expected as

for white LEDs, including oxides, sulfides, nitrides, or-

an promising energy-saving and environmentally friendly

ganic compounds, et al.4–14 Nowadays, blue LED excita-

lighting device. Cerium doped yttrium aluminum garnet

ble commercial red emitting luminescent materials are all

(YAG:Ce3+) is well known as such a kind of a commercial

rare-earth

yellow emitting phosphor, which was firstly used to com-

Ca2Si5N8:Eu2+

bine with blue LED chip to generate white light. Unfortu-

strong broadband excitation and intense emission. Nev-

nately, due to the lack of red emission, the correspond-

ertheless, the synthesis conditions are rigorous for ni-

ion

activated

and

ACS Paragon Plus Environment

phosphors,

CaSiAlN3:Eu2+,

for

instance,

etc. All of them exhibit


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trides. High temperature (> 2000 K) and high pressure (~

as the reductant to reduce KMnO4 in HF liquid. Pan's

1 MPa) furnace and oxygen/water free glove box are

group reported a one-step hydrothermal method to pro-

necessary and expensive instruments to ensure the ex-

duce a series of A2MF6:Mn4+ phosphors using ox-

cellent luminescent properties of rare earth nitrides.

ide/KMnO4 as starting materials in HF/H2O environment.

Moreover,

Eu2+

ion shows broadband emission and low

More recently, Chen and Liu's research team21,22 devel-

color purity, which greatly limit its application in back light

oped a new synthesis based on the strategy of "a two-

source for display. Therefore, novel narrow red emitting

step cation exchange". Firstly, a pure K2MnF6 was pro-

phosphors with excellent luminescent properties under

duced by the redox reaction between KMnO4, H2O2 and

facial or mild synthesis conditions are still in great need.

KHF2 in a concentrated HF solution. Then the as-

Recently, series of narrow red emitting fluorides

obtained K2MnF6 reacts K2SiF6 in a relatively dilute HF

A2MF6:Mn4+ activated by Mn4+ ion (A = alkali metal ion, B

solution by a direct cation exchange method to generate

= Si, Ge, Ti, .etc)

have attracted more and more

K2(Si,Mn)F6. In summary, all of these synthetic routes to

attentions. For example, K2TiF6:Mn4+ shows strong

synthesize excellent narrow red emitting A2MF6:Mn4+

broadband excitation in the wavelength ranging from UV

phosphors without exception involved a reduction of

to blue region, matching well with the emission from UV

permanganate and the subsequent crystallization of

or blue LEDs, and exhibits intense red emission at 630

A2MF6 matrix in an indispensable HF-rich solution. Un-

nm with high quantum yield up to 98 %,23 much larger

fortunately, hydrofluoric acid HF shows unwanted hyper

than 75-80 % of nitrides.11 Also it has excellent lumines-

toxicity and may cause fatal damages for human body if

cence properties under high joint temperature of LEDs.

operating improperly. Also, the extra demand of safety

Mn4+ ion in K2TiF6 almost shows no red emission degra-

precautions will inevitably increase phosphor costs. Be-

dation at 500 K while Eu2+ ion in CaSiAlN3 gives red

sides, different kinds of organic reagents are used in the

emission loss of about 17 %,24 suggesting that the for-

following cleaning processes for all the synthesis routes

mer shows promising application in high power white

mentioned above. Therefore, new green synthetic route

LEDs for general lighting. More importantly, it does not

without the usage of toxic, volatile and flammable chemi-

need rigorous synthesis conditions, for instance, high

cal solvent are in great and emerging need for narrow

temperature or high pressure or oxygen/water free glove

red emitting A2MF6:Mn4+ phosphors and will greatly pro-

box like nitrides, to synthesize A2MF6:Mn4+.

motes their application in mass production scale in the

15–22

Until now, there are three kinds of synthesis routes

future.

previously developed, i.e., chemical etching in room

In this paper, we develop a novel green hydrothermal

temperature, hydrothermal method and the so-called

synthetic route for the first time to produce K2SiF6:Mn4+

“cation exchange” or “ionic exchange”. Sadao et al.15-19

in common low-toxic H3PO4/KHF2 liquid instead of high-

synthesized Mn4+-doped alkaline hexa-fluorides by etch-

toxic HF liquid and systematically investigated its mor-

ing corresponding elementary substance (Si, Ge or

phology and photo-luminescence properties. Moreover,

Sn, .etc) in HF/KMnO4/H2O solution where the redox

the reaction mechanism was discussed in detail in order

reaction between elementary substance and KMnO4 in

to deeply understand the key roles of H3PO4/KHF2 in

HF liquid happens and contributes to generate final

synthetic process of K2SiF6:Mn4+, compared to HF. Final-

product directly. Qiu and his co-workers,20 on the other

ly, we demonstrate its potential application in white LED.

hand, utilized SiO2 as inexpensive silicon source to obtain K2SiF6:Mn4+ by similar chemical etching process where H2O2 instead of elementary substance was used

2. EXPERIMENTAL SECTION


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2.1. Preparation of soluble manganese (IV) (MnL2, L =

were obtained by mixing up 0.12 g silicone, 0.005 g

HPO42-)

YAG:Ce3+ and x g KSFM. During which, x was valued 0,

According to the synthesis provided by Jáky et al.,26,27

0.01, 0.02, 0.05, 0.1, and numbering as LED-0 - 4 re-

20 mL dark purple solution which contained 0.1317 g

spectively. After vacuum treatment to remove air bubbles,

KMnO4 (A.R.) was added dropwise into 40 mL 0.5 M

the mixture was coated on the lead frame and solidified

potassium formate CH2O2·K solution in the ultrasonic

under 150 oC for 4 h to produce WLEDs. The WLEDs

vibration environment to produce the active MnO(OH)2.

were operated at 3.0 V with drive current 20 mA.

The precipitate was then collected by centrifugation and washed by distilled water twice. These dark brown solid was dispersed in 200 mL 3 M phosphoric acid followed by ultrasonic vibration for 30 mins and then let stand for 48 hours to generate a uniform solution. After diluting with 3 M phosphoric acid to 250 mL, the resultant 3 mmol/L solution MnL2 was prepared for the further synthetic procedure. This solution was stable at room temperature (RT) for more than 15 days.

2.4. Characterization Absorption spectrum was recorded with a Cary 5000 UV-Vis-NIR spectrophotometer (Varian) equipped with double out-of-plane Littrow monochromator. The solid products were characterized by using powder X-ray diffraction (XRD) studies (Rigaku D/MAX 2200 VPC) in the range of 10

o

< 2 θ < 70

o

at a scanning rate of 5

o

/ min

with CuKa1 radiation (λ = 1.5405 Å) on a silicon wafer. The morphology and elemental composition of the as-

2.2. Hydrothermal synthesis of K2Si1-xF6: xMn4+ (KSFM)

prepared products were measured by scanning electron

The stating materials SiO2 (99.5 %, grain diameter

microscope (SEM) on a FEI Quanta 400 instrument.

was 15 ± 5 nm), KHF2 (A.R.) and as-prepared MnL2 so-

Transmission electron microscope (TEM) and selected

lution were mixed thoroughly and charged into a 40 mL

area electron diffraction (SAED) analyses were per-

Teflon cup. In a typical process, 4 mmol SiO2 and 15 ml

formed on a JEOL 2010 instrument. The photolumines-

MnL2 solution were added into this reaction. Besides,

cence excitation (PLE) and photoluminescence (PL)

KHF2 was added variably from 12 to 120 mmol to inves-

spectra within the temperature range 293 - 500 K as well

tigate the optimum synthesis condition and to study the

as the decay curves were measured using an Edinburgh

reaction mechanism. After ultrasonic vibration for 30

Instruments FSP920 Time Resolved and Steady State

mins, the Teflon vessel was sealed up in a stainless

Fluorescence Spectrometers equipped with a 450 W Xe

steel autoclave and kept at 180 oC for 6 hours. After re-

lamp, a 60 W μF900 microsecond flash lamp, TM300

action, the autoclave was then cooled naturally to RT.

excitation monochromator and double TM300 emission

The resulting products were collected by centrifugation

monochromators

and washed by distilled water twice, and dried under

sensitive PMT. The spectral resolution of the steady

vacuum at 40 oC for 24 hours.

measurements was about 0.05 nm in UV-Vis. The sam-

and

thermo-electric

cooled

red-

ple was mounted in an Oxford OptistatDN2 nitrogen cryostat for PLE and PL measurements above RT. The

2.3. LEDs fabrication WLEDs with different correlated color temperatures (CCTs) were fabricated by combining blue-chip (453 nm, 3V, 30 mA, Sanan optoelectronic Co. Ltd), YAG:Ce3+ yellow phosphor and different weight of the as prepared KSFM red phosphor. Various phosphor-silicone mixtures

quantum yield (QY) of the sample was measured using a barium sulfate coated integrating sphere (150 mm in diameter) attached to the FSP920. Electroluminescence (EL) spectra were recorded at a forward-current and measured using an Ocean Optics Instruments QE65000. Electron Paramagnetic Resonance (EPR) analyses were



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measured on spectrometer (BRÜCKNER A300-10-12) with frequency of 9.8 GHz and attenuator of 30 dB.

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Fig. 2 shows the SEM, TEM and the SAED of the asprepared KSFM. The SEM image (Fig. 2A and 2B) clearly exhibits that the particles have a decanedron shape with the average size of about 10 μm and the size dis-

3. RESULTS AND DISCUSSION Fig. 1a gives the XRD patterns of the as-synthesized product KSFM. It is obviously seen that there is no distinct impurity peaks and all the sharp diffraction peaks can be indexed to the standard patterns of cubic phase K2SiF6 (JCPDS No. 85-1382) with a unit cell a = 8.14 Å (space group = O5h-Fm3m). Mn4+ ion would replace Si4+ site in the center of the [SiF6]2- octahedron for inducing less distortion defects because of close ionic radius and the same valence. Fig. 1b shows typical PL and PLE spectra of KSFM at RT. Monitored at 630 nm, there are two broad excitation band in the UV and blue region between 300 and 500 nm, full width at half maxima (FWHM) are 41 and 43 nm respectively, which are attributed to the 4A2g → 4T1,2g transitions of Mn4+ ion. Under excitation at 450 nm, the PL spectrum has series of sharp peaks around 580 - 650 nm, due to the 2Eg → 4A2g transitions of Mn4+, these results areidentical with the previous work. The inner PL QY under 450 nm excitation is measured to be 28 %. The photo images, taken under natural light and 365-nm portable ultraviolet lamp, show that the cream-colored KSFM powders emit intense red light.

persion is relatively uniform.The TEM image (Fig. 2C) indicates that the KSFM particle is solid. The SAED (Fig. 2D) of the marked area (Fig. 2C) of a single particle suggests that it has a nature of a single crystal, of which the distinct diffraction dots can be indexed to the cubic phase K2SiF6. These results suggests that each particle is an individual single crystal. Fig. 2E shows the structure of [KF12] decanedron viewed in [44-4] direction, of which the schematic outline perfectly matches with the actual shape of the particles, a characteristic decanedron shape of [KF12] cage, as shown in Fig. 2C. In the past years, KSFM had been mainly synthesized by the wet chemical etching process. Commonly, the corresponding particles show a nature of poly-crystalline and have irregular morphology and uneven size dispersion of about several ten microns. Comparatively, in present case, the as-obtained KSFM particles have a nature of single crystal, of which the morphology is more regular and the size dispersion is more uniform. They are rare but desirable characteristics that are not present in the previously identified KSFM, from which pc-white LEDs with more uniform white light will benefit.

Fig. 2 (A, B) SEM and (C) TEM images of the assynthesized KSFM phosphor with a nominal Si:F atom ratio Fig. 1 (a) XRD pattern and (b) RT excitation and emission spectra of the synthesized KSFM phosphor with a nominal Si:F atom ratio of 1:48. Photograph of the KSFM taken under natural light (c) and 365-nm UV-light (d), respectively.

of 1:48. (D) SAED patterns of the selected area of (C) (marked with white circle). (E) Structure of [KF12] cage viewed in [44-4] direction.

Temperature-dependent PL performance is an important parameter for phosphors to be used for high-


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power WLEDs applications where the joint-temperature

spectively. As shown in Fig. 3e,

KSFM shows the

31*10-3

at 500 K, which is

It is obviously seen in

chromaticity shift of about

the PL spectra of the as-synthesized KSFM at the tem-

44*10-3

of LED chip reaches over 150

oC.

much smaller than

of the CSASNE. And it in-

ion

creases gradually with the temperature increasing.

almost exhibits no emission shift as the temperature in-

Moreover, KSFM shows high color purity of ~100 % at

creases. Interestingly, the integrated emission intensity

the whole temperature range of RT-500 K, which is

of Mn4+ ion in the whole wavelength of 590-660 nm ini-

mainly due to the tiny shifting and broadening of sharp

tially increases greatly, then enhances gradually and

red emission peaks of Mn4+. Fig. 3f shows the CIE color

finally decreases significantly at 440 K as the tempera-

coordinates of the as-synthesized KSFM at the tempera-

ture increases, as seen in Fig. 3b. At 340 K (67

420

ture range of RT-500 K. At RT, the color coordinates are

it is about

(0.692,0.308). With the temperature increasing, they are

108 %, 110 %, 110 % and 98 % of the initial intensity at

(0.685,0.315)@420 K (147 oC), (0.684,0.316)@440 K

298 K, respectively. As a reference, the commercial red

(167 oC) and (0.680,0.319)@500 K (227 oC), respective-

LED-used phosphor (Sr)CaAlSiN3:Eu2+ (CSASNE, pur-

ly. Obviously, KSFM shows excellent the color stability.

perature range of RT to 500 K (Fig. 3a) that

K (147

oC),

440 K(167

oC)

and 500 K(227

oC),

Mn4+

oC),

chased from LUMING Technology Group Co. Ltd) can

In summary, KSFM exhibits excellent thermal quench-

only maintain 81 % initial intensity at 500 K. Such an

ing behavior and perfect color stability including tiny

abnormal thermal quenching behavior of

Mn4+

ion is

chromaticity shift and high color purity, suggesting that it

much different from rare earth ions, of which the PL in-

is a potential red emitting phosphor for high quality and

tensity generally decreases with increasing temperature.

high power WLEDs.

The reason why Mn4+ ion shows such an anomalous phenomenon is that its emission is attributed to the vibronic radiative transition around zero phonon line (ZPL) and the intensity of vibronic emission of Mn4+ ion generally increases as the temperature increases.23 It is clearly seen in Fig. 3c and 3d that the emission intensities of both Stokes (Is) and anti-Stokes (Ia) vibronic transitions increase with increasing temperature and comparatively the anti-Stokes vibronic emission increases more greatly. The ratio of Ia to Is shows a liner temperature dependence from 0.34 at RT to 0.55 at 500 K (227 oC). The color stability can be quantifiably described by the chromaticity shift (ΔE) using the following equation (1): 28,29

△E = (u't −u'o )2 + (v't −v'o )2 + (w't −w'o )2

(1)

where u' = 4x/(3-2x+12y), v' = 9y/(3-2x+12y) and w' = 1-

Fig. 3 (a) Emission spectra of the as-synthesized KSFM and

u'-v'. u' and v' are the chromaticity coordinates in u'v'

(b) thermal quenching behaviors of the as-synthesized

uniform color space, x and y are the chromaticity coordi-

KSFM and CSASNE at T = RT-500 K. (c) The temperature-

nates in CIE 1931 color space, and o and t are the

dependent PL intensity of the as-synthesized KSFM in the

chromaticity shift at 298 K and a given temperature, re-

wavelength range of 605-655 nm, and 605-620 nm (d, Ia)



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and 620-655 nm (d, Is). (e) Chromaticity shift of KSFM and

Fig. 4 (a) XRD patterns, (b) the normalized integrated emis-

CSASNE at T = RT to 500 K. (f) CIE color coordinates of

sion intensity and (c, λex = 365 nm) photographs of serial of

the as-synthesized KSFM at T = RT and 500 K.

KSFM samples with various nominal Si:F atom ratio.

As discussed above, the nominal atom ratio of Si to F

Additionally, the elemental composition analysis of se-

is 1:48, which is greatly larger than 1:6, i.e., stoichio-

rial KSFM samples with various nominal Si:F atom ratio

metric atom ratio of Si to F in ionic groups [SiF6]2-. The

were quantitatively carried out using energy dispersive

question what is the role of excess F sources in KSFM

spectrometer (EDS) in order to further prove the purity of

comes naturally. Consequently, the effects of the atom

the as-synthesized KSFM samples. It is clearly seen in

ratio of Si to F on structure and PL properties of KSFM

Fig. 5 that the atom percentage of K, Si, F were about

were systematically investigated. Fig. 4a shows the XRD

19 %, 8 % and 71 %, respectively, all of which almost

patterns of serial of KSFM samples with the nominal

keep unaltered as the nominal atom ratio of Si to F in-

atom ratio of Si to F increasing from 1:6 to 1:60, which

creases from 1:6 to 1:60, which is close to 2:1:6, i.e., the

are labeled as A to H, respectively. Obviously, all the

stoichiometric atom ratio of K2SiF6. Since Mn content

sharp diffraction peaks of the sample A with stoichio-

was below the limit of detection, the atom ratio of Mn

metric atom ratio of Si to F (1:6) in ionic groups [SiF6]2-

was not given. Also, some light elements such as C, O

can be also indexed to the standard patterns of cubic

and P were also detected but not provided. In our previ-

phase K2SiF6 (JCPDS No. 85-1382). As the nominal at-

ous work, we found that up-conversion emission intensi-

om ratio of Si to F increases, there is no any tiny impurity

ty of NaYF4:Yb,Er was greatly enhanced after post-

phase in samples B to H, which suggests that the for-

treatment in an aqueous solution of NH4HF2 and NaF.30

mation of pure matrix K2SiF6 does not benefit from the

Such an enhancement was attributed to the sodium ion

excess F source, i.e., KHF2. Fig. 4b and 4c exhibits the

insertion in NaYF4 containing sodium vacancy after post-

dependence of the emission intensity of Mn4+ ion on the

treatment in an aqueous solution of NH4HF2 and NaF. In

excess F source. The sample A almost does not give

present case, those results above suggest there is no

any red emission. As the nominal atom ratio of Si to F

potassium or fluorine vacancy in the as-synthesized

increases, red emission intensity of

Mn4+

ion in K2SiF6

KSFM samples and extra KHF2 does not play a key role

gradually increases, then reaches the maximum at the

in inserting K+ or F- ion in K2SiF6, which improves the

Si/F ratio of 1:48 (sample G) and finally decreases. The

emission intensity of Mn4+.

red emission intensity of the sample G is about 200 time as intense as the sample A. All these results above suggest that the Mn4+ doping content may change along with the increase of fluoride input.


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Fig. 5 Product yield and quantitative elemental composition data of serial KSFM samples with various nominal Si:F atom ratio.

In general, the emission intensity of luminescent materials can be improved simply by increasing the concentration of activators. In order to confirm whether KSFM samples show similar dependence, KSFM samples with various nominal Si:F atom ratios were further studied by using EPR technique, as shown in Fig. 6. As representative, EPR spectra of two samples with a nominal Si:F atom ratio of 1:48 (Sample G) and 1:6 (Sample A) were given. It is obviously seen in Fig. 6a that Sample A al-

Fig. 6 EPR spectra of (a) the as-synthesized sample A with

most does not show characteristic six hyperfine signals

a nominal Si:F atom ratio of 1:6 and (b) sample G with a

of Mn4+ ion. Comparatively, the EPR profile of Sample G in Fig. 6b shows characteristic six hyperfine components of Mn4+ between 300 and 400 mT, due to the 1/2 ↔ - 1/2

nominal Si:F atom ratio of 1:48 at RT. (c) Differential EPR absorption intensities of KSFM samples with various nominal Si:F atom ratios.

transition of Mn4+ ion with d3 electronic configuration.31,32

A clear depiction of the KSFM host structure is more

Other KSFM samples with nominal atom ratios of Si:F

helpful to understand the formation mechanism of KSFM.

between 1:6 and 1:48 give similar EPR profile, which are

It is reported that K2SiF6 is of cubic phase with a unit cell

not provided here. Fig. 6c shows the differential EPR

of a = 8.14 Å. As shown in Fig. 7a and 7b, every Si4+ ion

absorption values (ΔAbs) of Mn4+ ion in all KSFM samples

is closely coordinated with six F- ions, forming a [SiF6]2-

with various nominal Si:F atom ratios. The differential

octahedron ionic group, and every K+ ion is surrounded

absorption values (ΔAbs) of Mn4+ ion is obtained by sub-

with 4 [SiF6]2- octahedrons by sharing three F ions of

tracting the maximum values, as marked in Fig. 6b. Ob-

each [SiF6]2-, forming a [KF12] decahedral cage. That is

viously, the ΔAbs value of Mn4+ ion initially increases with

to say that the crystalline structure of K2SiF6 is not "rigid"

increasing nominal Si:F atom ratios, then reaches the

enough since [SiF6]2- ion group is isolated each other by

maximum value at nominal Si:F atom ratios of 1:48

large K+ cation. It is therefore strongly expected that if K+

(Sample G) and finally decrease. These results indicate

ions and [SiF6]2- ionic group coexist in solution phase,

that the actual concentration of Mn4+ ions in K2SiF6 crys-

K2SiF6 may precipitate and crystallize as seen in Fig. 7c.

tal is controlled by input amount of KHF2 raw materials

It is also reported that the solubility of K2SiF6 is 0.084

even if the initial amount of MnL2 as Mn4+ ion source is

g/100

fixed in all samples. The dependence of EPR signal of

crystallization process of K2SiF6 can be expressed in

KSFM on input amount of KHF2 (Fig. 6c) is perfectly

equation (2):

consistent with that of PL (Fig. 4b), suggesting that the dependence of red emission intensity of KSFM on input amount of KHF2 is inherently due to the changing concentration of activators, i.e., Mn4+ ion, controlled by KHF2.

mL

H2O.25

Accordingly,

the

dissolution-

K2SiF6 (s) ↔ 2K+ (aq) + [SiF6]2- (aq) (2) As red emitting center, Mn4+ ion is strongly expected to substitute for Si4+ since they have the same ionic valence and approximate radius (0.53 Å for Mn4+ and 0.40 Å for Si4+). However, it is well known that it is more difficult for free Mn4+ ion to prevent being reduced or oxi-



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Page 8 of 13

dized in solution since Mn4+ ion will easily decompose

and potential to properly reduce MnO4- into Mn4+. For

into lower and higher valence Mn ion. Comparatively,

instance, Adachi et al. simply synthesized Mn4+ doped

[MnF6]2- ionic group is more stable in solution. Therefore,

fluorides by wet chemical etching of Si/SiO2 in

how to obtain an active

[MnF6]2-

ionic group becomes the

HF/KMnO4 mix solution or HF/KMnO4 + H2O2 if want to

most key factor in the synthetic process of high efficient

accelerate reaction,15,17,20 while Pan's group tried to use

red emitting KSFM.

hydrochloric acid HCl replacing HF but failed to obtain Mn4+. And Pan found that the redox reaction between KMnO4 and HCl solution which makes KMnO4 totally reduce to Mn2+ at 120 oC hydrothermal system.32 Those results strongly supported the first important role of HF liquor we proposed. The second important role of HF liquor is to provide enough F- ion to complex with Mn4+ to maintain [MnF6]2ionic group before Mn4+ ion is reversibly reduced or ox-

Fig. 7 (a) Coordination environment and (b) crystal structure of K2SiF6 unit cell. (c) Schematic illustration on the formation of K2SiF6 crystal in solution phase.

ided. For instance, Chen and Liu's research team had developed a new synthesis based on the strategy of "cation exchange".23 Firstly, a pure K2MnF6 powder was precipitated after the reaction between KMnO4, H2O2 and

As mentioned in introduction section, there are three

KHF2 in a concentrated HF solution. Then, the as-

main synthetic experiment processes until now, including

obtained K2MnF6 precursor as dopant reacted with

chemical etching route, hydrothermal method and the

K2SiF6 in HF solution. Therein, excess HF plays a key

so-called “cation exchange” strategy. After carefully

role of preventing the decomposition reaction of [SiF6]2-

summarizing all those previous researches, we found

and especially [MnF6]2-. Otherwise, [MnF6]2- can be hy-

that they all involved three sub-procedures as follows:

drolyzed into mixed-valence Mn oxides and hydroxides.

1. The formation process of [MnF6]2- mostly from the

For instance, other researches found that when KSFM is

direct reduction of permanganate in a high concentrated

put into a water solution, it degrades seriously and only

HF solution;

remains about 9 % of the initial luminescence intensity of

2. The crystallization process of fluoride host from the

KSFM.25,34

reaction of the corresponding Si source, for instance,

Based on the above discussions, one can conclude

elementary substance, salt or oxide, in high concentra-

that high-toxic HF liquor is necessary for the reduction of

tion HF solution;

MnO4- ion and the stabilization of Mn4+ ion in liquid. Con-

3. The doping or diffusion process of [MnF6]2- dopant into fluoride host in high concentration HF solution. Obviously, high-toxic HF liquor is necessary for existence of [MnF6]2- ionic group. Hereafter we will fully discuss the key role of high-toxic HF liquor in obtaining [MnF6]2- ionic group in whole synthetic process of high efficient red emitting KSFM. The first important role of HF liquor, we think, is to provide the reasonable acidity

sequently, how to avoid the use of high-toxic HF liquor and obtain [MnF6]2- ionic group at the same time becomes a big challenge or hot issue for synthesis of high efficient red emitting KSFM. In this paper, we propose a novel green synthetic route to produce K2SiF6:Mn4+ though replacing high toxic HF by common non-toxic acid H3PO4 with strong complexity. Hereafter, we will comparatively discuss the roles of H3PO4 in detail and deeply understand the reaction


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mechanism and then the enhancement mechanism of

weak EPR signals (Fig. 6a) and poor typical red emis-

red emission with the increasing amount of KHF2. The

sion (Fig. 4b and 4c) from Mn4+ ions can be detected.

overall schematic processes on how to finally obtain

According to equation (5), the transformation from MnL2

[MnF6]2-

into [MnF6]2- does not happen, due to the scarcity of

are is presented in Fig. 8.

As mentioned, once Mn4+ was obtained by reduction in

enough KHF2, and consequently MnL2 may be decom-

a solution system, it will rapidly suffer hydrolysis before

posed into Mn2+, as decribed by the quation (8) shown

transforming into [MnF6]2- if without the protection of HF.

below:

To get rid of the use of HF, initially a stable manganese (IV) solution

MnL2 (L = HPO42-) was acquired by two

steps strategy provided by Jáky et al.,26,27 as shown in equation (3) and (4) : (s) + 3CO2 (g) +

(8) The above side reaction can be supported by Fig. 2S. When the amount of KHF2, i.e., a nominal Si:F atom ratio,

2MnO4- (aq) + 3CHO2- (aq) + 3H2O (l) → 2MnO(OH)2 5OH-

H2O (l) + MnL2 (aq) → 1/2O2 (g) + Mn2+ (aq) + H2L (aq)

(aq.) (3)

increases above 1:6, more and more MnL2 tranform into [MnF6]2-, according to equation (5), which results in en-

MnO(OH)2 (s) + 2H2L (aq) → MnL2 (aq) + 3H2O (l) (4) The typical absorption spectra of KMnO4 and MnL2 solutions are consistent with those reported by Jáky et al.,21 as seen in Fig. S1, which strongly suggests that soluble manganese Mn(IV)L2 based on H3PO4 is successfully obtained. Thereafter, such an intermediate MnL2 slowly turning into [MnF6]2- and meanwhile SiO2 transfers into [SiF6]2- in a H3PO4 solution when [HF2]was added, as expressed by the equation (5-6) below: MnL2 (aq) + 3[HF2]- (aq) + H+ (aq) ↔ [MnF6]2- (aq) + 2H2L (aq) (5) SiO2 (s) + 3[HF2]- (aq) + H+ (aq) → [SiF6]2- (aq) +

hanced EPR signal and red emission from Mn4+ ion, as shown in Fig. 6 and Fig. 4, respectively. Also we find out that there is no efficient KSFM acquired from using KF as the potassium and fluorine source in exchange, though the Si:F input ratio remains the same (Fig. S3), due to the decrease of acidity is not conductive to the formation of [MnF6]2-. To sum it up, the high-toxic HF liquor can be simply replaced by the combination of H3PO4/KHF2 and the doping concentration of Mn4+ in KSFM can be tuned simply by adjusting the addition of KHF2 through regulating the [MnF6]2- content in the hydrothermal system.

2H2O (l) (6) Finally, K2Si1-xMnxF6 precipitate in solution, promoted by the following equation (7): 2K+ (aq) + (1-x)[SiF6]2- (aq) + x[MnF6]2- (aq) ↔ K2Si1xMnxF6

(s) (7)

In above processes, it is obviously seen that non-toxic H3PO4/KHF2 couple plays a key role as the same complex reagent as commonly used toxic HF in stabilizing

Fig. 8 Transformation process of quadrivalence Mn in solu-

Mn4+ ion and facilitating the formation process of [MnF6]2-.

tion phase.

Such a reaction mechanism can be strongly supported by the effect of the amount of KHF2 on red emission performance of K2Si1-xMnxF6. When a nominal Si:F atom ratio was kept at stiochiometircal 1:6, pure K2SiF6 phase can be obtained (Fig. 4a, curve A) and contrarily much

It is well known that YAG:Ce is a famous yellow emitting phosphor use to fabricate the WLED with blue LED chip. However, the white light quality is not good enough because there is seriously scare of the red emission in



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Page 10 of 13

the electroluminescence spectra of the WLED. In order

0.25, 0.1, 0.05, respectively. And (d) the chromaticity coor-

to demonstrate the potential application of KSFM in solid

dinates of LED-0 - 4. The inset of (d) shows photographs of

state lighting, the WLEDs were comparatively fabricated

LED-0 and LED-4 under a drive current of 20 mA.

with/without the as-prepared KSFM. The electroluminescence (EL) spectra of the fabricated WLEDs with/without the as-prepared KSFM, operated at 3.0 V with drive current 20 mA, were recorded as shown in Fig. 9. It is clearly seen in Fig. 9a that the EL spectrum of blue LED chip perfectly overlaps with the PLE spectrum (λem = 630 nm) of KSFM, suggesting that KSFM can make the full use of the blue light emitted from the LED chip. Fig. 9b shows the EL spectrum of WLED (labeled as LED-0) fabricated by combining blue LED chip and only yellow YAG:Ce. It gives white light with the chromaticity coordinates of (0.325, 0.337), approximately close to the equal energy white light point of (0.333, 0.333), as shown in Fig. 9d. The color render index (CRI), the correlated color temperature (CCT) and the luminous efficacy of the as fabricated WLED-0 are 68.6, 6267 K and 80 lm/W. Fig. 9c shows the EL spectral of the WLED (labeled as LED-1, LED-2, LED-3 and LED-4) fabricated by combining blue LED chip and the mixture of yellow YAG:Ce and KSFM with different weight ratio of 0.5, 0.25, 0.1, 0.05, respectively. It is obviously seen in the EL normalized at 550 nm that as the wt. ratio of YAG:Ce to KSFM decreases,

4. CONCLUSION In summary, we have developed a new environmental friendly hydrothermal strategy for the first time for the narrow-band red emitting phosphor K2SiF6:Mn4+ free of the participation of high toxic HF liquor. The formation mechanism shows that H3PO4/KHF2 plays the same key roles as HF in the process of stabilizing Mn4+ and promoting Mn4+ into the host K2SiF6 and exhibits more excellent ability than HF in controlling the concentration of Mn4+ ion in the host K2SiF6.The as-synthesized K2SiF6:Mn4+ shows good thermal stability, inconspicuous chromaticity shift and better high-temperature resistance than commercial red phosphor (Sr)CaAlSiN3:Eu2+. CRI and CCT tunable WLEDs were successfully fabricated using the as-prepared red emitting K2SiF6:Mn4+. These results indicate that this hydrothermal strategy based on low-toxic H3PO4/KHF2 system may open a new green synthetic route to narrow red emitting A2MF6:Mn4+ and will greatly promotes their application in mass production scale in the future.

the sharp red emission of KSFM increases and broadband blue emission of LED chip decrease. Accordingly,

ASSOCIATED CONTENT

the chromaticity coordinates move toward warm white

Supporting Information. Information Absorption spectra, photographs,

light area and that of LED-4 is (0.394, 0.423). The CRI,

the dependence of integrated emission intensity of KSFM

the CCT and the luminous efficacy of the as fabricated

on different fluorine sources. This material is available free

WLED-4 are 67.6, 3840 K and 40 lm/W.

of

charge

in

PDF

format

via

the

Internet

at

http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], Tel: +86-20-84112112, Fax: +86-20-84111038. Fig. 9 The EL spectra of the (a) blue LED chip, (b) WLED (LED-0) fabricated with YAG:Ce3+, (c) WLED (LED-1 - 4) fabricated with YAG:Ce3+/KSFM of different wt. ratio 0.5,

Author Contributions All authors have given approval to the final version of the manuscript.


Page 11 of 13

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8.

Notes The authors declare no competing financial interest.

Guo, C.; Luan, L.; Chen, C.; Huang, D.; Su, Q., Preparation of Y2O2S:Eu3+ phosphors by a novel decomposition method. Mater. Lett. 2008, 2008 62, 600-602.

9.

ACKNOWLEDGMENT

with Eu2+ and Ce3+ for LEDs. Opt. Mater. 2007, 2007 30, 375-

This work was financially supported by the “973” programs

(2014CB643801),

the

NSFC

(51572302

and

Jia, D.; Wang, X.-j., Alkali earth sulfide phosphors doped 379.

10.

Uheda, K.; Hirosaki, N.; Yamamoto, Y.; Naito, A.; Nakaji-

21271191), the Joint Funds of the National Natural Science

ma, T.; Yamamoto, H., Luminescence Properties of a Red

Foundation of China and Guangdong Province (U1301242),

Phosphor, CaAlSiN3:Eu2+, for White Light-Emitting Diodes.

Electrochem. Solid-State Lett. 2006, 2006 9, H22.

Teamwork Projects of Guangdong Natural Science Foundation (S2013030012842), Guangzhou Science & Technology

11.

Li, Y. Q.; van Steen, J. E. J.; van Krevel, J. W. H.; Botty,

Project (2015B090926011 and 2013Y2-00118), Natural

G.; Delsing, A. C. A.; DiSalvo, F. J.; de With, G.; Hintzen,

Science

H.

Foundation

of

Guangdong

Province

(2014A030313114) and the Fundamental Research Funds for the Central Universities (14lgqt02).

T.,

Luminescence

properties

of

red-emitting

M2Si5N8:Eu2+ (M=Ca, Sr, Ba) LED conversion phosphors.

J. Alloys Compd. 2006, 2006 417, 273-279. 12.

Xie, R.-J.; Hirosaki, N., Silicon-based oxynitride and nitride phosphors for white LEDs—A review. Science and

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