[STRUCTURAL STUDIES OF PHOSPHOR DOPED WITH PRASEODYMIUM USING FOURIER TRANSFORM INFRARED (FTIR) SPECTROSCOPY AND FOURIER TRANSFORM RAMAN SPECTROSCOPY

KAJIAN TERHADAP STRUKTUR FOSFOR KALSIUM TITANIA DIDOPKAN DENGAN PRASEODYMIUM MENGGUNKAN SPEKTROSKOPI INFRAMERAH TRANSFORMASI FOURIER

Aishah. S.1,2*, and Hussin R.2

1 LENDT Group, Industrial Technology Division, Malaysian Nuclear Agency, Bangi, 43000 Kajang, Selangor Darul Ehsan, Malaysia.

2 Faculty of Science, Universiti Teknologi Malaysia, 81310 Skudai, Johor

*[email protected]

Abtract

Studies of phosphor material have witness a rapid growth in research and development of luminescence phenomenon due to their diverse application. Calcium titanate (CaTiO3) compound based phosphor doped with praseodymium provides a subject for fundamental structural studies aiming at exploring a new functional material. The effect of calcium concentration on structural properties of CaTiO3 phosphor was investigated by Fourier transform infrared (FTIR) spectroscopy and Fourier transform Raman spectroscopy (FT-Raman). FTIR studies confirmed the presence of the functional groups modes of vibrations. The bands in the region of 416-460 cm-1 was assigned to Ti-O-Ti bending modes. Meanwhile, the band in the region of 515-570 cm-1 was attributed to Ti-O stretching vibration. Only 6 Raman active modes of CaTiO3 are detected in the samples by FT-Raman spectroscopy Keywords: phosphor, calcium titanate, Fourier transform infrared (FTIR) spectroscopy

Abstrak

Kajian terhadap bahan fosfor telah berkembang dengan pesat dalam bidang penyelidikan dan pembangunan didorong oleh fenomena pendar cahaya untuk diaplikasikan terhadap pelbagai aplikasi teknologi. Bahan fosfor berasaskan kalsium titania (CaTiO3) didopkan dengan praseodymium menjadi subjek dalam kajian terhadap struktur asas bahan untuk dijadikan sebagai bahan berfungsi. Kesan penambahan kalsium terhadap struktur asas fosfor (CaTiO3) dikaji dengan menggunakan spektroskopi inframerah transformasi fourier (FTIR) dan spektroskopi Raman transformasi fourier(FT-Raman). FTIR mengesahkan kehadiran mod getaran kumpulan berfungsi titania. Mod getaran pada julat 416-460 cm-1 disebabkan oleh pembengkokan Ti-O-Ti. Manakala, pada julat 515-570 cm-1, mod regangan Ti-O dapat dikenalpasti. Hanya 6 modes aktif bagi CaTiO3 yang dapat dikesan oleh spektroskopi FT- Raman.

Katakunci: fosfor, kalsium titania, spektroskopi inframerah transformasi Fourier

1.0 INTRODUCTION

Luminescence in solid is the phenomenon in which electronic states of solid is excited by an external source and this excitation energy is released as light. When the energy comes from short-wavelength light, usually ultraviolet light, the phenomenon is called photoluminescence [1]. There are two basic kinds of luminescence materials recognized as inorganic (phosphor) and organic (organoluminophosphors) luminescence. Substances contains of solid luminescence materials are called as phosphors. Usually, inorganic phosphor consist of a crystalline “host material” added with small amount of certain impurities that are called the “activators”. -type prasedodymium 3+ doped CaTiO3: Pr is potential red phosphor for various display application such as for safety and emergency marking.

Philippe Boutinaud et. al. [2] reports on luminescence properties of praseodymium doping in titanate host material show high luminescence efficiency and attract much attention. From previous studies, a lot of report have been published focus on luminescent properties compare to local structure of titania based phosphor or another phosphor material. Nowadays, studies on the structure of phosphor material and their relations to luminescence properties are less conducted by researchers. From literature review, Kong Li et. al. [3] was reported that the structure of the sample are related to the luminescence properties. In addition, effect of doping material and percentage of host concentration were effected the structural and luminescence behavior.

3+ In these work, structural studies of xCaO-(100-x)TiO2:1%Pr , where 10≤ x(mol%)≤ 40 phosphor was investigated by solid state reaction method. Optimum percentage of CaO content in TiO2 host lattice and their effect on structural behaviour of vibration was studied in calcium titanate host phosphor. The samples were characterize by Fourier Transform Infrared (FTIR) spectroscopy and Fourier Transform Raman (FT-Raman) spectroscopy.

2.0 EXPERIMENTAL

Calcium (99% CaO: Riedel-de Haen), dioxide (99% TiO2: Fluka Alrich were employed as the raw materials. Praseodymium oxide (99% P203: Acros) was incorporated as a doped material in appropriate amounts for charge compensation purposes.

The calcium titanate phosphor doped with praseodymium are synthesized using solid state reaction method using high temperature. Starting material were mixed and homogenized through grinding process using ball milling for 1hour. Then, the sampled were sintered at 1000°C for 6 hours. Lastly, the furnace shall be set to room temperature to let the mixture to cool down. After the reactions, the samples will be characterized by Fourier Transform infrared spectroscopy (FTIR) and Fourier Transform Raman spectroscopy (FT-Raman).

The samples were prepared using potassium bromide method (KBr) pellet on powder samples by infrared excitation scan. The IR spectra were recorded using 10 scans and resolution at 4cm-1. The IR spectra of samples were recorded in range of 400–2000 cm-1. The Raman spectra were measured by Jobin Yvon HR 800 UV in the spectral range of 100–1200 cm-1. The sample was excited with an argon ion laser (λ = 514.5nm) with power of about 20mW. The digital intensity data were recorded at intervals of 4 cm-1 and the spectral resolution was about 4 cm-1.

3.0 RESULTS AND DISCUSSION

3.1 The effect of calcium concentration on host structure by FT-Infrared spectroscopy

Infrared spectroscopy analysis was carried out to study the effect of different concentration of Ca atom to the properties of CaTiO3 crystal structure. The infrared spectra different concentration of CaO in xCaO-(100-x)TiO2, where 10≤ x(mol%) ≤ 40 doped with 1 mol% Pr3+ with are shown in Figure 1. The infrared spectra of these samples, which were sintered at 1000oC, showed main bands around 710, 570, 527, 460 and 415 cm-1. The broad bands in the -1 -1 region 710 cm and 570 cm band were assigned to Ti-O stretching vibrations of TiO6. The absorption band at 527 cm-1 could be assigned to the Ti-O stretching vibration [4]. While the vibration at 460 and 415 cm-1 attributed to the vibration of Ti-O-Ti bending mode [5]. FTIR band and their assignment of CaTiO3 structure were list on Table 1. From Figure 1, the wavenumber of the transmittance peaks of Ti-O bonds corresponds to vibration at 710 cm-1 shift to smaller value when the concentration of Ca increased. The wavenumber of transmittance peaks in 10Ca-90TiO2 -1 -1 (mol %) was near 710 cm while in 40Ca-60TiO2 (mol %) was shift near to 675 cm . According to the study, when the concentration of Ca in the sample increases, it is shifted to smaller wavenumbers. Shifted to smaller wavenumber showed less energy is needed by titanium (Ti) and oxygen (O) atoms of Ti-O bond in the molecule for all samples to vibrate with their certain wavenumber. Hence, reduction in energy has defined that the bonding of Ti- O bond length becomes increasingly elongated in cell size [6].

The infrared-active mode Ti-O-Ti bending vibrations (δ(Ti-O-Ti)) at 460 cm-1 became clearly determined in infrared spectra and slightly shifted to lower value when x was increased. The addition of higher CaO (x CaO mol% are increase) content will enhance the formation of Ti-O-Ti bonding. The spectrum are shifted to lower wavenumber, hence, higher energy are given to the samples in addition of xCaO mol%. The peaks at this region have higher energy to break the bond and easily determine at x= 30 CaO mol% and x=40 CaO mol%. All the bands (710, 570, -1 527, 460 and 415 cm ) were vibrate in the TiO6 octahedra lattice.

Table 1: FTIR band and their assignment of CaTiO3 structure

Wavenumber (cm-1) Modes of vibration

710 Ti-O stretching vibration of TiO6

570 Ti-O stretching vibration of TiO6 527 Ti-O stretching vibration 460 Ti-O-Ti bending 415 Ti-O-Ti bending

)

) Ti

- ) Ti

- O O - - O) mol % CaO O - - Ti Ti ( ( Ti Ti

( ( δ v

v O) x=40 δ -

Ti

(

v

x=30

x=20

Intensity (a.u) x=10

1200 1100 1000 900 850 800 750 700 650 600 550 500 450 400 -1 Wavenumber (cm )

3+ Figure 1: FTIR spectrum of xCaO-(100-x)TiO2:1%Pr , different concentration of CaO 10≤ x (mol%) ≤40

3.2 The effect of percentage of Ca on host lattice by FT-Raman spectroscopy

3+ 3+ Figure 2 shows the Raman spectra of xCaO-(100-x) TiO2:Pr in series of 0≤ x (mol%)≤ 40 where Pr ions were constant at 1mol %. The Raman spectra of CaTiO3 shown a number of sharp peaks superimposed on a broad feature -1 -1 3+ between 150 cm and 700 cm region. Raman-active modes were detected for the powder of xCaO-(1-x) TiO2:Pr when x was in the range of 10 to 40 mol%. As x increased, several peaks diverged and new Raman-active modes appeared. Therefore, the percentage of Ca2+ ion was able to increase or decrease the structural organization in perovskite structure. As a result, the percentage of Ca in CaTiO3 promoted a different coordination of interaction between the ions, which mainly aroused from the stretching, torsional and bending vibrations of the shorter metal- oxygen bonds, contributing to the observation of the Raman-active modes.

There were a total of 24 Raman-active modes for orthorhombic CaTiO3. Factor group analysis gave the Raman active vibration modes as 7Ag+7 B1g+5B2g+5B3g. Although 24 Raman active modes were expected, several of these modes could not be detected because of their low polarizabilities [7]. Hence, only six Raman active modes were commonly observed.

3+ The Raman spectra of xCaO-(100-x)TiO2: Pr where 10≤ x(mol%) ≤ 40 had only six Raman actives mode which were observed at ~150, ~260, ~415, ~450, ~517 and ~630 cm-1. It was found that the Raman active modes for these samples were in good agreement with Sanjay et al. [8]. According to Marques et. al., [9] band at ~150cm-1 was -1 related to the vibration of Ca bonded to a TiO3 group (Ca–TiO3) lattice mode. The band observed at ~260cm could be assigned to the O-Ti-O bending vibration mode or caused by the tilting phenomenon between the [TiO6]-[TiO6] -1 -1 -1 clusters. The bands at ~415cm , ~450cm and ~517cm were assigned to Ti-O3 torsional (bending or internal -1 vibration of oxygen cage) modes. Particularly, the ~620cm mode of CaTiO3 was assigned to Ti-O symmetric stretching vibration mode [10,11]. The Raman bands and their assignment are listed in Table 2.

Table 2: Raman band and their assignment of CaTiO3 structure

Raman Bands (cm-1) Assignment 150 (Ca–TiO3) 260 δ(O-Ti-O)

415 δ(Ti-O3) 450 δ(Ti-O3) 517 δ(Ti-O3) 620 υs(Ti-O)

) 3 3

O - TiO O) -

- (Ti δ Ca O) (Ti -

δ

(Ti s v x mol%

x=40

x=30

x=20

x=10

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 Raman Shift / cm-1

3+ Figure 2: Raman spectra of xCaO-(100-x) TiO2 doped with 1 mol% Pr

An analysis of Figure 2 reports that no drastic spectral changes and only slight shifts were observed on the characteristic position of Raman actives modes when x increased from 10 to 40 mol%. Even though Raman spectra shown in Figure 2 was similar in pattern, but their vibration (peak position and width) were different in 10≤ x mol % ≤ 40 in CaTiO3 crystals, which were related to the effect of different percentage on the calcium titanate network. As x increased, the band at 434cm-1 diminished and slightly shifted due to a reduction in the degree of Ti-site order. The intensity 251 cm-1 bands decreased with an increasing x. Thus, it is suggested that these band was related to the rotation of the oxygen octahedral cage associated with the orthorhombic distortion. At lower percentage of x (x=10 mol %) stronger interactions between Ti-O bonds occurred, suggesting an increase of [TiO6]-[TiO6] clusters vibration at around 250 cm-1 in perovskite structure. When x=30, a narrow peaks at 510 cm-1 started to appear. Therefore, the intensity of this peak increased with increasing x, implying a stronger distortion in high CaTiO3 compounds as reported by Zheng et al., (12). When the Ca2+ and Ti4+ varied at a fixed Pr quantity, the bands at ~600 cm-1 diminished presumably due to the Ca-rich. As shown in the figure, the cystallinity was poor in excess Ca. Since the Ca-rich (x=40 mol %), the Ti-O vibration due to this band could not be detected because of low polarizabilities of Ti4+ ions. As a result, this band would diminish due to a reduction in the degree of Ti-site order (13).

4.0 CONCLUSION

Structural characteristic of calcium titanate, CaTiO3 phosphor doped with Pr-ion synthesize via solid state reaction have been investigated. Phase crystallinity of CaTiO3 can be obtained at 1000˚C for 6 hours by conventional solid state reaction method via using CaO and TiO2 as raw materials. As low as CaO concentration (x=10 mol %), the phase of CaTiO3 are obtained in the samples. Therefore, in order to compose a CaTiO3 phase, minimum nominal composition could be used by this method.

-1 Infrared bands have been analyzed for identifying the functional groups of CaTiO3. The bands at around 570 cm -1 -1 and 710 cm are due to the vibration of Ti-O stretching vibration of TiO6 cluster. The band at 527 cm is due to the Ti-O stretching vibrations. Furthermore, the bands near 460 cm-1 and 415 cm-1 is attributed to the Ti-O-Ti bending mode.

There are 24 Raman active modes for the orthorhombic structure of CaTiO3 and can be ascribed by 7Ag + 5B1g + -1 7B2g + 5B3g. However, only 6 Raman active modes are detected in the samples. The bands at ~150 cm are -1 attributed to the Ca-TiO3 lattice vibration mode. Near 260 cm are ascribes to O-Ti-O bending mode and the bands -1 -1 -1 at ~415 cm , ~435 cm , and ~520 cm are assigned to the Ti-O3 torsional mode. In addition, Ti-O symmetric stretching is detected at around 620 cm-1. When the sample are doped with praseodymium, the slight shift observed on the characteristic position of Raman modes which can be related to the degree of crystallization, interaction force between ions, structural defects and influence of doping.

ACKNOWLEDGEMENT

This research was support financially by Ministry of Science Technology and Inovation (MOSTI) under Science Fund research grant Project Number: 03-01-06-SF0053.

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