Optical properties of rare earth-activated Ca3(PO4)2:RE (RE = Eu3+

and Dy3+) phosphors prepared by wet chemical synthesis

S K RAMTEKE1, A N YERPUDE1,* , N S KOKODE2 and S J DHOBLE3

1Department of Physics, N.H. College, Bramhapuri, Chandrapur 441206, India2N.H. College, Bramhapuri, Chandrapur 441206, India3Department of Physics, RTM Nagpur University, Nagpur 440033, India

*Author for correspondence (atulyerpude@gmail.com)

MS received 2 December 2020; accepted 24 February 2021

Abstract. Optical properties of rare earth-activated Ca3(PO4)2:RE (RE = Eu3? and Dy3?) phosphors obtained by wet

chemical synthesis method, are reported in this paper. The phase confirmation of synthesized phosphor host is done by

X-ray diffraction study. SEM images of the sample host reveal irregular morphology with collection of particles and

porous nature. The crystal structure is not viewed with naked eyes, although particle size ranges from 2–5 micrometres.

Photoluminescence characterization of Ca3(PO4)2:Eu3? gives characteristic orange–red emission at wavelengths of 594,

613 and 617 nm upon near UV excitation at 397 nm attributed to 5D0 ?7F1,

7F2 transition of rare earth Eu3? ion.

Whereas Ca3(PO4)2:Dy3? gives blue–yellow emissions at wavelengths 483 and 574 nm due to near UV excitation at

wavelength of 351 nm, which are corresponding to 4F9/2 ?6H15/2,

6H13/2 transitions of dopant ion Dy3?. Colour

coordinates from CIE diagram of 483 and 574 nm are obtained at (x = 0.0781, y = 0.1704) and (x = 0.471, y = 0.527) for

the Ca3(PO4)2:Dy3? phosphor. Whereas in the case of Ca3(PO4)2:Eu

3? for an emission wavelength of 613 nm, the colour

coordinates are found to be (x = 0.674, y = 0.325). The presented optical investigation of phosphor displays that the

reported phosphor can be a potential contender for ecofriendly solid-state lighting.

Keywords. Phosphate; rare earth; wet chemical method; solid-state lighting.

1. Introduction

In routine occurrence, luminescence materials have enacted

an inestimable position with its utmost accessible application

that is self-lit signalization. This phenomenon indicates the

ability of a material to accumulate energy from (artificial)

lighting, along with visible and ultraviolet (UV) spectrum

close to it and moderately emit it as luminous in the murk at a

normal temperature [1]. Over the previous decade,

advancement in synthesis and characterization of alkaline

earth phosphate phosphors has resulted in significant atten-

tion in this class of materials because solid-state lighting has

many advantages over fluorescent and incandescent lights,

for example, its durability, efficiency and environmentally

friendly characteristics [2–4]. Now, research area including

the white light-emitting diode (wLED) and phosphor con-

verted light emitting diode (pcLED) has become an area of

enormous interest to the researchers all over the world [1–4].

Generally, for the production of white light, a combination of

a blue LED with yellow luminescence from Y3Al5O12:Ce3?

(YAG:Ce3?) phosphor compound is used [5,6].Nevertheless,

white light emission can be obtained usingRGBphosphors by

exciting with near UV chip, which can improve the lumen

output and also enhances colour rendering index (CRI) [7,8].

Recently, it is found that phosphate-based phosphor has

exquisite thermal and hydrolytic stabilities. In particular,

phosphates havingmolecular formula (ABPO4,A = K?, Li?,

Na? and B = Ca2?, Sr2?, Mg2?) materials are found to be

relevant hosts for white light-emitting phosphors [9–11].

Over recent years, several lanthanides activated phosphate-

based phosphors were reported which are useful in wLED

applications [12–15]. Amongst the rare earth elements

available as activators, Eu and Dy are the most commonly

used activators as they give rise to sharp emissions in the

visible blue–green–red region of the electromagnetic spec-

trum [16–19]. Recently, Nagpure et al [20] reported lumi-

nescence properties of phosphate-based Ca3(PO4)2 phosphor

doped with Eu and Dy by solid-state synthesis. Zhang et al[21] reported steep brightness in Eu3?-activated Ca3(PO4)2red phosphor for near UV light emitting diode appliances.

They also prepared the same phosphor with different mor-

phologies by using surfactants and shown that fluorescence

intensity was highly affected by the use of specific surfactants

[22]. Li et al [23] studied the phase transformation obtained

by the controlled quenching and increasing the Eu2? content

in Ca3(PO4)2:Eu2?. They also found that cooling rate

parameter automatically impacts luminescence properties

with improved thermal emission stability. Furthermore,

Bull. Mater. Sci. (2021) 44:174 � Indian Academy of Scienceshttps://doi.org/10.1007/s12034-021-02476-5Sadhana(0123456789().,-volV)FT3](0123456789().,-volV)

tunable blue–green emission was also reported recently in b-Ca3(PO4)2:Eu

2? and Tb3? phosphor due to energy trans-

mission mechanism between the co-dopants, which was

responsible for high quantum efficiency reaching up to 90%

[24]. The phosphate host, Ca3(PO4)2 phosphor has the prop-

erty of heterovalent substitution of Ca2? by various cations to

form the new phases and its copious crystallographic sites for

the doped activator tunable to photoluminescence. Because

of this property, it caughtmuch attention of researchers. Thus,

these phosphors have much ability and potential in the uti-

lization ofwLEDs for their tunable emission. In this paper,we

report the wet chemical synthesis of Ca3(PO4)2:RE3?

(RE3? = Eu3?, Dy3?) phosphors and their photolumines-

cence characterization.

2. Experimental

Both the rare earth-activated phosphate phosphors, Ca3(PO4)2:Eu

3? and Ca3(PO4)2:Dy3? were prepared by wet

chemical method. The doping concentrations of Eu3? and

Dy3? were 1 mol%. For the preparation of Ca3(PO4)2:Eu3?,

calcium nitrate (Ca(NO3)2�6H2O) (analytical reagent (AR)),

ammonium dihydrogen phosphate (NH4H2PO4) (AR) and

europium oxide (Eu2O3) (AR) were used as reactants. The

calcium nitrate and ammonium dihydrogen phosphate were

taken in separate borosil beakers and dissolved in relevant

ratios of distilled water. In a test tube dopant, Eu2O3 was

taken and it dissolved with dilute HNO3. Then, Eu2O3 is

converted to nitrate form. Afterwards, europium nitrate

solutions were then mixed into a beaker and stirred until the

solution becomes transparent. The beaker is then kept on a

magnetic stirrer at 80�C for 10 h. After complete dehy-

dration of the solution mixture, crystalline powder was

obtained as a product. This crystalline powder was taken in

agate mortar and pestle, and grinded to obtain a fine pow-

dered sample. The powder thus obtained was annealed at

800�C for 3 h in a muffle furnace which was then utilized

for further characterization.

3. Results and discussion

3.1 XRD pattern of Ca3(PO4)2 phosphor

X-ray diffraction (XRD) pattern of as-prepared Ca3(PO4)2host is shown in figure 1. The ICCD standard data file no.

861585 is also given for comparison with the obtained XRD

pattern of the prepared compound. The standard data file is

compatible with the sample XRD pattern indicating that the

desired host lattice was obtained from the wet chemical

synthesis. The XRD peaks showed the position of atoms

within lattice structure and crystal size of prepared sample.

From this observation, Ca3(PO4)2 powder was formed with

no impurity phases and implies the accomplish formation of

the homogeneous compound.

3.2 SEM micrographs of Ca3(PO4)2 phosphor

Figure 2 shows SEM images of Ca3(PO4)2 host samples for

two different magnifications. Although the crystal structure

is not visible, morphology and average particle size of the

sample are evident from the SEM imaging. The sample has

an irregular surface structure with a collection of particles.

The porous nature of the sample is visible which may be

due to the evolution of gases during the chemical reaction.

The size of the particles is in the range of 2–5 lm.

3.3 Photoluminescence properties of Ca3(PO4)2:Dy3?

phosphor

The photoluminescence excitation and emission spectra of

Ca3(PO4)2:Dy3? phosphor are shown in figures 3 and 4,

respectively. Multiple peaks are obtained in the excitation

spectra sighted at 483 nm emission wavelength. The exci-

tation peaks were obtained at 326, 351, 366 and 389 nm in

UV region. One of the most prominent peaks which is at

351 nm wavelength is used for the measurement of emis-

sion spectra. Strong emission at 483 and 574 nm in the

blue–yellow region is obtained upon UV excitation attrib-

uted to 4F9/2 ?6H15/2,

6H13/2 transitions of dopant ion

Dy3? [25–27]. The origins of Dy3? emissions at 483 and

574 nm are due to magnetic and electric dipoles. In most of

the compounds studied that, when Dy3? is situated at a low

symmetry (with an omission of inversion centre), the yellow

emission is effective, while the blue emission is tenacious

when Dy3? is situated at a high symmetry (with an inver-

sion centre). From the depicted figure 4, observed blue

emission is more durable than yellow emission.

3.4 Photoluminescence properties of Ca3(PO4)2:Eu3?

phosphor

Figure 5 delineates the photoluminescence excitation

spectra of Ca3(PO4)2 phosphor activated by Eu3? ion

sighted at an emission wavelength of 613 nm. Three char-

acteristic peaks are found at 364, 385 and 397 nm in the

Figure 1. XRD pattern of Ca3(PO4)2 host.

174 Page 2 of 5 Bull. Mater. Sci. (2021) 44:174

near UV wavelength range. The most prominent peak of the

excitation spectrum which is at 397 nm is used to obtain the

photoluminescence emission spectra as shown in figure 6. It

was observed that powerful emission of wavelengths at 594,

613, 617 nm peaked in the orange–red region of the visible

spectrum. These emission wavelengths are assigned to5D0 ?

7F1,7F2 transitions of rare earth Eu3? ion [28–31].

In the second and prominent emission peak, a doublet is

evident; this may be due to the crystal field splitting effect

during the incorporation of dopant ion in the host lattice.

3.5 Chromaticity coordinates of Ca3(PO4)2:Eu3?

and Ca3(PO4)2:Dy3? phosphors

The CIE chromaticity diagram of Ca3(PO4)2:Eu3? and

Ca3(PO4)2:Dy3? phosphors excited by near UV wavelength

Figure 2. SEM images of Ca3(PO4)2 phosphor.

Figure 3. Photoluminescence excitation spectrum of Ca3(PO4)2:

Dy3?, kem = 483 nm.

Figure 4. Photoluminescence emission spectrum of Ca3(PO4)2:

Dy3?, kex = 351 nm.

Figure 5. Photoluminescence excitation spectrum of Ca3(PO4)2:

Eu3?, kem = 613 nm.

Bull. Mater. Sci. (2021) 44:174 Page 3 of 5 174

is shown in figure 7. The colour coordinates of the Ca3(PO4)2:Dy

3? phosphor of the blue colour (483 nm) is

(x = 0.0781, y = 0.1704) and of the yellow colour (574 nm)

is (x = 0.471, y = 0.527). The colour coordinates of the

Ca3(PO4)2:Eu3? phosphor of orange–red colour (613 nm) is

(x = 0.674, y = 0.325). The obtained CIE colour coordi-

nates for Ca3(PO4)2:Eu3? and Ca3(PO4)2:Dy

3? phosphors

are around the edge of the CIE diagram signify the immense

colour purity of their corresponding emissions [32,33]. Both

the Ca3(PO4)2:Eu3? and Ca3(PO4)2:Dy

3? phosphors can be

efficiently excited by near UV wavelength giving strong

visible emissions. Further, the CIE colour coordinates of

synthesized phosphor suggest that these contenders can be

utilized for solid-state lighting [34].

4. Conclusion

The rare earth Ca3(PO4)2:RE (RE = Eu3? and Dy3?) phos-

phors synthesized by wet chemical synthesis. The synthesized

phosphor was characterized for XRD to confirm the phase;

SEM to study morphology, PL excitation and emission. The

colour chromaticity properties were studied using CIE. Colour

coordinates are calculated from the CIE diagram and found

around the edge of the CIE diagram. Ca3(PO4)2:Eu3? phos-

phor gives characteristic orange–red emission, whereas Ca3(PO4)2:Dy

3? gives blue–yellow emissions due to near UV

excitation. The optical properties of the synthesized phosphor

indicate that the reported phosphor can be a potential con-

tender for environmentally friendly based solid-state lighting.

Acknowledgements

A N Yerpude is thankful to Gondwana University,

Gadchiroli, India (project ref. no./GUG/DIIL/609/2020,

dated 14/02/2020), for financial assistance and Gondwana

University, Gadchiroli, for constant encouragement.

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