# Synthesis and Photoluminescence Study of Bi3+ and Pb2+ Activated Ca3(BO3)2

Authors: A. B. Gawande; R. P. Sonekar; S. K. Omanwar
Journal: International Journal of Optics
 (2014)

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## Abstract

The photoluminescence properties of Pb2+ and Bi3+ doped Ca3(BO3)2 prepared by solution combustion synthesis technique are discussed. The structure of the prepared phosphor is characterized and conformed by XRD and FTIR. SEM images of the prepared materials show irregular grains with agglomerate phenomena. Prepared phosphors achieved the band emissions, respectively, at 365 nm and 335 nm corresponding to the transition P
1
→
S
0
1
3. Optimum concentration, critical transfer distance, and Stokes shift of the synthesized materials were measured. These phosphors may provide an efficient kind of luminescent materials for various applications in medical and industry.


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## Body

## 1. Introduction

Several researchers studied the luminescence properties of Bi3+ doped phosphors [1–6]. Bismuth can exist in materials in different valence states, such as 0, +1, +2, +3, and +5, or even mixed valence states of +1 and +5. In all of these valence states only Bi3+ is normally most stable in most host materials. Usually, the emission peaks of Bi3+ occur in the ultraviolet, blue, green, or even red wavelength regions with variation of host materials. For example, GaBO3 : Bi3+ [7] and YBO3 : Bi3+ [8] show UV emission, LnNbO4 : Bi3+ [9] gives blue emission, CaWO4 : Bi3+ [10] and Ca3Al2O6 : Bi3+ give green emission [11], and Bi4Ge3O12 [12] displays red emission at low temperature. Pb2+ doped phosphors have been the interest of many researchers. Many attempts have been made to synthesize Pb2+ activated phosphors which emit in varying range from 290 to 470 nm when excited by UV light [13–22]. In recent years, great efforts have been taken by many researchers to discover and develop new rare-earth and transition-metal ion-doped material systems as luminescent materials with high absorption in the UV spectral region [23]. Inorganic materials containing metal ions with s2 (Pb2+, Tl+, Sn2+, Sb3+, Bi3+) configuration can be used in X-ray imaging devices, low pressure lamps, and high-energy physics. For example, Pb2+ doped BaSi2O5, which emits a broad band around 350 nm under UV excitation, is one of the earliest known phosphors for photocopying lamps [24].In the present work, two inorganic phosphors Ca3(BO3)2 : Bi3+ and Ca3(BO3)2 : Pb2+, were prepared using solution combustion synthesis technique. The synthesized phosphors were characterized by using the powder X-Ray Diffractometer (XRD), Fourier Transform InfraRed (FTIR), and Scanning Electron Microscope (SEM). The photoluminescence properties of these phosphors at room temperature were studied using a Spectrofluorometer.


## 2. Experimental

The phosphors were prepared by a novel technique which is slight variation of solution combustion synthesis method. Detailed description of the method is given in our previous work [25–30]. Bi3+ and Pb2+ doped Ca3(BO3)3 phosphors were obtained by the combustion of aqueous solution containing stoichiometric amounts (using oxidizer/fuel ratio) of calcium nitrate (AR), lead nitrate (AR), bismuth nitrate (AR), ammonium nitrate (AR), urea (AR), and boric acid (AR) as boron source (Table 1). All the precursors were dissolved in a china dish using little amount of double distilled water. The dish containing the solution was introduced into a muffle furnace maintained at 550
±
10°C. The solution undergoes dehydration followed by decomposition with the evolution of large amount of gases (oxides of nitrogen and ammonia) and ignited to burn with a flame yielding voluminous powder of Ca3(BO3)3 : Bi3+ and Ca3(BO3)3 : Pb2+. These raw powders were sintered for 3 h at 800
±
10°C and quenched to room temperature on aluminum plate and crushed into a fine powder. The prepared powder samples were then subjected to the powder XRD analysis. PL measurements at room temperature were performed on Hitachi F-7000 Spectrofluorometer in the range 200–500 nm.Table 1
Molar ratio of ingredients used for material preparation.
Material
Molar ratio
(
Ca
(
1
-
x
)
Bi
x
)
3
(
BO
3
)
2
Ca(NO3)2 : Bi(NO3)3 : H3BO3 : CO(NH2)2 : NH4NO3
3
×
(
1
-
x
) : 3
×
x : 2 : 6 : 3
(x
=
0.001
,
0.002
,
0.003
,
0.005
,
0.01)
(
Ca
(
1
-
x
)
Pb
x
)
3
(
BO
3
)
2
Ca(NO3)2 : Pb(NO3)2 : H3BO3 : CO(NH2)2 : NH4NO3
3
×
(
1
-
x
) : 3
×
x : 2 : 6 : 3
(x
=
0.001
,
0.003
,
0.005
,
0.01
,
0.02
,
0.03,
0.04)


## 3. Results and Discussion

### 3.1. X-Ray Diffraction Pattern of Synthesized Materials

The powder XRD of synthesized material, analyzed on Rikagu Miniflex II X-ray Diffractometer, is shown in Figure1 and found in good agreement with the ICDD file (01-070-0868). The crystal structure of the prepared material Ca3(BO3)2 can be refined to be rhombohedral, space group R-3c with lattice parameters a
=
b
=
8.638 Å and c
=
11.849 Å.Figure 1
X-ray powder diffraction patterns of Ca3(BO3)2 synthesized by solution combustion synthesis.


### 3.2. FTIR Spectra of Synthesized Materials

The FTIR spectra measured at room temperature are shown in Figure2. Generally, the FTIR analysis of the studied borate material shows four distinct frequency regions, from 1200 to 1600 cm−1 and from 800 to 1200 cm−1, which are assigned to the stretching vibrations of both triangular BO3 and tetrahedral BO4 units, respectively. As usual, frequencies of B–O vibrations depend on the boron coordination. Stretching frequencies of a coordinated BO
n groups decrease as the coordination number n increases. Thus, the experimental results indicate that B–O bond asymmetric stretching of BO3 groups is located in the region from 1150 to 1500 cm−1. Moreover, the condensation of BO3 groups (either chains or rings) leads to two types of B–O bands. One is short B–O bands (oxygen bonded to a single B atom or nonbridging oxygen). The other is longer B–O–B bonds. The responding B–O stretching frequencies are accordingly distributed into two bands or groups of bands, centered near 1450 and 1200 cm−1 [31]. As seen in Figure 2, the strong bands observed above 1200 cm−1 should be assigned to the B–O stretching mode of the triangular [BO3]3− groups, while the bands with maxima at about 700–800 cm−1 should be attributed to the B–O out of plane bending, which confirms the existence of the [BO3]3− groups [32, 33].Figure 2
FTIR spectra of Ca3(BO3)2 synthesized by solution combustion synthesis.


### 3.3. SEM Images of Synthesized Phosphors

Figure3 shows the Scanning Electron Microscope (SEM) images of powders prepared at 800°C. It was observed that the microstructure of the phosphors consisted of irregular grains with agglomerate phenomena. The average size of the powders is about 2–12 μm. The results show that phosphors have a good crystallinity and a relatively low sinter temperature.SEM images of (a) Ca3(BO3)2 : Bi3+ and (b) Ca3(BO3)2 : Pb2+.
(a)
(b)


### 3.4. Photoluminescence of Ca3(BO3)2 : Bi3+

Figure4 displays the excitation and emission spectra of Ca3(BO3)2 : Bi3+. The ground state of Bi3+ ion with 6s2 configuration is the 1S0 spin-orbit singlet state and the excited states of the 6s6p configuration are 3P0, 3P1, 3P2, and 1P1 states in sequence of energy increase. The transition between 1S0 and 1P1 is parity and spin-allowed, while the transition between 1S0 and 3P1 is largely allowed due to spin-orbit coupling. Other transitions are strictly forbidden. Typically at room temperature, emission is observed from the 3P1 → 1S0 transition. As an activator, excitation usually occurs from the 1S0 ground state to the 3P1 or 1P1 excited state because the 1S0 → 3P0 and 1S0 → 3P2 transitions are strongly forbidden. The emission of Bi3+ ions originates from the 3P0 state at low temperatures [34], while at higher temperatures the emission occurs mainly from the 3P1 level, in which transition is allowed by spin-orbit mixing of the 3P1 and 1P1 states. In Figure 4, broad excitation bands are in the region from 260 nm to 310 nm with maximum at 290 nm which can be ascribed to 1S0 → 3P1 transition of Bi3+. The emission band is observed at 365 nm from the 3P1 excited state level to the 1S0 ground state upon excitation with 290 nm. Additionally, we observed no splitting or multiple bands in the emission spectra, which indicate that Bi3+ here is supposed to occupy the position of Ca2+ ions in the lattice. In view of the extensive literature on the luminescence of Bi3+ in inorganic host, it can be seen that Bi3+ in inorganic hosts absorbs light of wavelengths mainly in the 200–310 nm range and emits in the 330–450 nm range.Figure 4
PL and PLE spectra of(
Ca
(
1
-
x
)
Bi
x
)
3(BO3)2 (x = 0.001, 0.002, 0.003, 0.005, 0.01), excitation spectra monitored at 365 nm emission, and emission spectra monitored at 290 nm excitation.Further, the photoluminescence spectra of Ca3(BO3)2 : Bi3+ with different Bi3+ doping concentrations were investigated. The variation of emission intensity of Bi3+ with different Bi3+ doping concentration is shown in Figure 5. It is observed that, with increasing Bi3+ doping concentration, the emission intensity of Ca3(BO3)2 : Bi3+ increases and reaches a maximum at 0.003 mol. After this concentration, emission intensity decreases due to concentration quenching phenomenon. In this case, the energy transfer occurs from one activator to another until an energy sink in the lattice is reached. Finally, the Stokes shift of the synthesized phosphor was calculated to be 7086 cm−1 using the excitation band at 290 nm and the emission band at 365 nm.Figure 5
Variation in emission intensity of(
Ca
(
1
-
x
)
Bi
x
)
3(BO3)2 (x = 0.001, 0.002, 0.003, 0.005, 0.01).


### 3.5. Photoluminescence of Ca3(BO3)2 : Pb2+

The photoluminescence spectrum of Pb2+ doped in Ca3(BO3)2 host material is shown in Figure 6. It can be described by the 1S0 → 3P0,1 transition, which originates from the 6s2 → 6s16p2 interconfigurational transition. Typically at room temperature, emission is observed from the 3P1 → 1S0 transition [35], although at low temperatures the highly forbidden 3P0 → 1S0 emission is also observed [36]. The excitation band for the synthesized phosphor Ca3(BO3)2 : Pb2+ was observed at 270 nm, which is assigned to the 1S0 → 3P1 transition and the emission band was observed at 335 nm which can be ascribed to transition from 3P1 excited state to the 1S0 ground state. No splitting or multiple bands in the emission spectra for prepared phosphor are observed. Hence, we believed that the Pb2+ ions are incorporated at only one site (Ca2+ ion site) in the crystal lattice. In many inorganic hosts, the emission band of Pb2+ ion is in the UV region. It is also known that, in some hosts, Pb2+ ion emits in visible region. This diversity is depending strongly on the site occupied by Pb2+ ions, electronegativity of the ligand, crystal structure of the host lattice, and temperature [14, 37].Figure 6
PL and PLE spectra of(
Ca
(
1
-
x
)
Pb
x
)
3(BO3)2 (x = 0.001, 0.003, 0.005, 0.01, 0.02, 0.03, 0.04), excitation spectra monitored at 335 nm emission, and emission spectra monitored at 270 nm excitation.The luminescence intensities of Pb2+ doped phosphors always depend on the doped Pb2+ ions concentration [38–40]. Therefore, the photoluminescence spectra of Ca3(BO3)2 : Pb2+ with different Pb2+ doping concentrations were investigated. The variation of emission intensity of Pb2+ with different Pb2+ doping concentration is shown in Figure 7. It is observed that the position and shape of the photoluminescence bands have exhibited no obvious changes with Pb2+ concentration. The emission intensity of Ca3(BO3)2 : Pb2+ increases and reaches a maximum at 0.005 mol. The Stokes shift was calculated to be 7186 cm−1 using the excitation peak at 270 nm and the emission peak at 335 nm.Figure 7
Variation in emission intensity of(
Ca
(
1
-
x
)
Pb
x
)
3(BO3)2 (x = 0.001, 0.003, 0.005, 0.01, 0.02, 0.03, 0.04).According to Blasse [41], for the critical concentration the average shortest distance between nearest activator ions is equal to the critical transfer distance R
0. R
0 is, in fact, the critical separation between donor (activator ion) and acceptor (quenching ion), at which the nonradiative rate equals that of the internal single ion relaxation. The R
0 value can be practically calculated using the following equation:
(1)
R
0
=
2
×
(
3
V
4
π
χ
c
N
)
1
/
3
,
where χ
c is the critical concentration, N the number of Ca2+ ions in the Ca3(BO3)2 unit cell, and V the volume of the unit cell. By taking the values of χ
c
=
0.003 (for Bi3+) and 0.005 (for Pb2+), N
=
8 and V
=
255.20 Å3, respectively; the critical transfer distance R
0 for Ca3(BO3)2 : Bi3+ and Ca3(BO3)2 : Pb2+ was measured to be about 27 Å and 23 Å, respectively.


## 3.1. X-Ray Diffraction Pattern of Synthesized Materials

The powder XRD of synthesized material, analyzed on Rikagu Miniflex II X-ray Diffractometer, is shown in Figure1 and found in good agreement with the ICDD file (01-070-0868). The crystal structure of the prepared material Ca3(BO3)2 can be refined to be rhombohedral, space group R-3c with lattice parameters a
=
b
=
8.638 Å and c
=
11.849 Å.Figure 1
X-ray powder diffraction patterns of Ca3(BO3)2 synthesized by solution combustion synthesis.


## 3.2. FTIR Spectra of Synthesized Materials

The FTIR spectra measured at room temperature are shown in Figure2. Generally, the FTIR analysis of the studied borate material shows four distinct frequency regions, from 1200 to 1600 cm−1 and from 800 to 1200 cm−1, which are assigned to the stretching vibrations of both triangular BO3 and tetrahedral BO4 units, respectively. As usual, frequencies of B–O vibrations depend on the boron coordination. Stretching frequencies of a coordinated BO
n groups decrease as the coordination number n increases. Thus, the experimental results indicate that B–O bond asymmetric stretching of BO3 groups is located in the region from 1150 to 1500 cm−1. Moreover, the condensation of BO3 groups (either chains or rings) leads to two types of B–O bands. One is short B–O bands (oxygen bonded to a single B atom or nonbridging oxygen). The other is longer B–O–B bonds. The responding B–O stretching frequencies are accordingly distributed into two bands or groups of bands, centered near 1450 and 1200 cm−1 [31]. As seen in Figure 2, the strong bands observed above 1200 cm−1 should be assigned to the B–O stretching mode of the triangular [BO3]3− groups, while the bands with maxima at about 700–800 cm−1 should be attributed to the B–O out of plane bending, which confirms the existence of the [BO3]3− groups [32, 33].Figure 2
FTIR spectra of Ca3(BO3)2 synthesized by solution combustion synthesis.


## 3.3. SEM Images of Synthesized Phosphors

Figure3 shows the Scanning Electron Microscope (SEM) images of powders prepared at 800°C. It was observed that the microstructure of the phosphors consisted of irregular grains with agglomerate phenomena. The average size of the powders is about 2–12 μm. The results show that phosphors have a good crystallinity and a relatively low sinter temperature.SEM images of (a) Ca3(BO3)2 : Bi3+ and (b) Ca3(BO3)2 : Pb2+.
(a)
(b)


## 3.4. Photoluminescence of Ca3(BO3)2 : Bi3+

Figure4 displays the excitation and emission spectra of Ca3(BO3)2 : Bi3+. The ground state of Bi3+ ion with 6s2 configuration is the 1S0 spin-orbit singlet state and the excited states of the 6s6p configuration are 3P0, 3P1, 3P2, and 1P1 states in sequence of energy increase. The transition between 1S0 and 1P1 is parity and spin-allowed, while the transition between 1S0 and 3P1 is largely allowed due to spin-orbit coupling. Other transitions are strictly forbidden. Typically at room temperature, emission is observed from the 3P1 → 1S0 transition. As an activator, excitation usually occurs from the 1S0 ground state to the 3P1 or 1P1 excited state because the 1S0 → 3P0 and 1S0 → 3P2 transitions are strongly forbidden. The emission of Bi3+ ions originates from the 3P0 state at low temperatures [34], while at higher temperatures the emission occurs mainly from the 3P1 level, in which transition is allowed by spin-orbit mixing of the 3P1 and 1P1 states. In Figure 4, broad excitation bands are in the region from 260 nm to 310 nm with maximum at 290 nm which can be ascribed to 1S0 → 3P1 transition of Bi3+. The emission band is observed at 365 nm from the 3P1 excited state level to the 1S0 ground state upon excitation with 290 nm. Additionally, we observed no splitting or multiple bands in the emission spectra, which indicate that Bi3+ here is supposed to occupy the position of Ca2+ ions in the lattice. In view of the extensive literature on the luminescence of Bi3+ in inorganic host, it can be seen that Bi3+ in inorganic hosts absorbs light of wavelengths mainly in the 200–310 nm range and emits in the 330–450 nm range.Figure 4
PL and PLE spectra of(
Ca
(
1
-
x
)
Bi
x
)
3(BO3)2 (x = 0.001, 0.002, 0.003, 0.005, 0.01), excitation spectra monitored at 365 nm emission, and emission spectra monitored at 290 nm excitation.Further, the photoluminescence spectra of Ca3(BO3)2 : Bi3+ with different Bi3+ doping concentrations were investigated. The variation of emission intensity of Bi3+ with different Bi3+ doping concentration is shown in Figure 5. It is observed that, with increasing Bi3+ doping concentration, the emission intensity of Ca3(BO3)2 : Bi3+ increases and reaches a maximum at 0.003 mol. After this concentration, emission intensity decreases due to concentration quenching phenomenon. In this case, the energy transfer occurs from one activator to another until an energy sink in the lattice is reached. Finally, the Stokes shift of the synthesized phosphor was calculated to be 7086 cm−1 using the excitation band at 290 nm and the emission band at 365 nm.Figure 5
Variation in emission intensity of(
Ca
(
1
-
x
)
Bi
x
)
3(BO3)2 (x = 0.001, 0.002, 0.003, 0.005, 0.01).


## 3.5. Photoluminescence of Ca3(BO3)2 : Pb2+

The photoluminescence spectrum of Pb2+ doped in Ca3(BO3)2 host material is shown in Figure 6. It can be described by the 1S0 → 3P0,1 transition, which originates from the 6s2 → 6s16p2 interconfigurational transition. Typically at room temperature, emission is observed from the 3P1 → 1S0 transition [35], although at low temperatures the highly forbidden 3P0 → 1S0 emission is also observed [36]. The excitation band for the synthesized phosphor Ca3(BO3)2 : Pb2+ was observed at 270 nm, which is assigned to the 1S0 → 3P1 transition and the emission band was observed at 335 nm which can be ascribed to transition from 3P1 excited state to the 1S0 ground state. No splitting or multiple bands in the emission spectra for prepared phosphor are observed. Hence, we believed that the Pb2+ ions are incorporated at only one site (Ca2+ ion site) in the crystal lattice. In many inorganic hosts, the emission band of Pb2+ ion is in the UV region. It is also known that, in some hosts, Pb2+ ion emits in visible region. This diversity is depending strongly on the site occupied by Pb2+ ions, electronegativity of the ligand, crystal structure of the host lattice, and temperature [14, 37].Figure 6
PL and PLE spectra of(
Ca
(
1
-
x
)
Pb
x
)
3(BO3)2 (x = 0.001, 0.003, 0.005, 0.01, 0.02, 0.03, 0.04), excitation spectra monitored at 335 nm emission, and emission spectra monitored at 270 nm excitation.The luminescence intensities of Pb2+ doped phosphors always depend on the doped Pb2+ ions concentration [38–40]. Therefore, the photoluminescence spectra of Ca3(BO3)2 : Pb2+ with different Pb2+ doping concentrations were investigated. The variation of emission intensity of Pb2+ with different Pb2+ doping concentration is shown in Figure 7. It is observed that the position and shape of the photoluminescence bands have exhibited no obvious changes with Pb2+ concentration. The emission intensity of Ca3(BO3)2 : Pb2+ increases and reaches a maximum at 0.005 mol. The Stokes shift was calculated to be 7186 cm−1 using the excitation peak at 270 nm and the emission peak at 335 nm.Figure 7
Variation in emission intensity of(
Ca
(
1
-
x
)
Pb
x
)
3(BO3)2 (x = 0.001, 0.003, 0.005, 0.01, 0.02, 0.03, 0.04).According to Blasse [41], for the critical concentration the average shortest distance between nearest activator ions is equal to the critical transfer distance R
0. R
0 is, in fact, the critical separation between donor (activator ion) and acceptor (quenching ion), at which the nonradiative rate equals that of the internal single ion relaxation. The R
0 value can be practically calculated using the following equation:
(1)
R
0
=
2
×
(
3
V
4
π
χ
c
N
)
1
/
3
,
where χ
c is the critical concentration, N the number of Ca2+ ions in the Ca3(BO3)2 unit cell, and V the volume of the unit cell. By taking the values of χ
c
=
0.003 (for Bi3+) and 0.005 (for Pb2+), N
=
8 and V
=
255.20 Å3, respectively; the critical transfer distance R
0 for Ca3(BO3)2 : Bi3+ and Ca3(BO3)2 : Pb2+ was measured to be about 27 Å and 23 Å, respectively.


## 4. Conclusion

Two inorganic phosphors were prepared by a solution combustion synthesis method followed by heating of the precursor combustion ash at 800°C for 3 h in air. SEM images of the synthesized phosphors show irregular grains with agglomerate phenomena. Synthesized phosphors Ca3(BO3)2 : Bi3+ and Ca3(BO3)2 : Pb2+ achieved the band emissions, respectively, at 365 nm and 335 nm corresponding to the transition 3P1 → 1S0. The optimum concentration of Bi3+ and Pb2+ in Ca3(BO3)2 is measured to be 0.003 and 0.005 mol, respectively. For these values of optimum concentrations the critical transfer distance, R
0, was measured to be about 27 Å and 23 Å, respectively. Finally, the Stokes shifts of Ca3(BO3)2 : Bi3+ and Ca3(BO3)2 : Pb2+ were measured to be 7086 cm−1 and 7186 cm−1, respectively. The emission bands of both the phosphors are in the UV region and the phosphors can be potential candidates for application in UV lamps.



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