ОПТИКА И СПЕКТРОСКОПИЯ, 2014, том 116, № 1, с. 106-109
СПЕКТРОСКОПИЯ ^^^^^^^^^^
КОНДЕНСИРОВАННОГО СОСТОЯНИЯ
УДК 535.37.549.75
EFFECT OF CATIONIC AND ANIONIC SURFACTANTS ON MORPHOLOGY AND LUMINESCENT PROPERTIES OF YVO4: Bi, Eu3+ RED PHOSPHORS
© 2014 г. Xiaochun Zhou* and Xiaojun Wang**
*School of Chemistry and Chemical Engineering, Jinggangshan University, Ji'an, Jiangxi, 343009, China **Mechanical and Electronic Department of Guangdong Polytechnic Normal University; Guangzhou 510635, China
E-mail: ncdxzxc@163.com Received March 6, 2013
Using cationic surfactant cetyltrimethylammonium bromide (CTAB) and anionic surfactant sodium dode-cylbenzene sulfonate (SDBS), respectively, YVO4: Bi, Eu3+ red phosphors were prepared by a facile reaction chemistry method and their morphology, structure and luminescent properties were investigated by scanning electron microscope (SEM), X-ray diffraction (XRD) and fluorescence spectrometer. The results showed that the introduction of cationic and anionic surfactants not only greatly influenced their morphology, but induced a remarkable change XRD patterns. The reason that causes the variation of these properties for YVO4: Bi, Eu3+ red phosphors was concluded to be related to different surfactants.
DOI: 10.7868/S0030403414010255
Yttrium orthovanadate (YVO4) is an important optical material in materials science. Owing to its high luminescence efficiency upon electron beam excitation, europium-doped YVO4 powders have been widely used as a red phosphor in color television and cathode ray tubes (CRTs) [1, 2]. More importantly, the luminescence of Eu3+ ions can be well sensitized by the isolated VO 4 group inside YVO4 matrix. These advantages have impelled more and more researchers to study experimental synthesis process and characterization of rare earth-doped YVO4 phosphor. At the same time, Bi3+ acts as a sensitizer of Eu3+ through energy transfer from Bi3+ to Eu3+ in the systems such as YVO4: Bi3+, Eu3+ [3, 4], Y2O3: Bi3+, Eu3+ [5-7]. Compared with phosphors prepared by conventional solid state reaction, the phosphors prepared by a facile reaction chemistry method have better morphology and stronger luminescent intensity [8, 9]. Surfactants are amphiphilic materials that exhibit a double affinity. They consist of a hydrophilic "head" and a hydrophobic "tail." Forming "reverse micelles" surfactants reduce the crystallite size by controlling the distance between them and retain the homogeneity of the final product. Adding the surfactant also reduces the particles surface tension and facilitates the formation of the new phase [10, 11]. Chen et al. [12] reported that the variety and amount of surfactants play a key role in controlling of morphology. Yan et al. [13] reported different morphology of phosphors such as sheet, nano-rods and microspheres can be obtained by hydrothermal process with different surfactants. In addition, the surfactants not only have the role of template to control the surface morphology of phosphors, but also effectively enhance the luminescent efficiency of 5D0-7F2
[14]. So far, the effect of cationic and anionic surfactants on morphology and luminescent properties of YVO4: Bi, Eu3+ red phosphors were not yet reported.
In this work, using cationic surfactant cetyltrime-thylammonium bromide (CTAB) and anionic surfactant sodium dodecylbenzene sulfonate (SDBS) respectively, YVO4: Bi, Eu3+ red phosphors are prepared by a facile reaction chemistry method and the effect of surfactants on morphology ofphosphors is also reported.
1. EXPERIMENTAL
1.1. Synthesis of YVO4: Bi, Eu3+
YVO4: Bi, Eu3+ red phosphors were prepared by a facile reaction chemistry method. All starting materials were of analytical purity. Firstly, the 0.1 mmol Eu2O3 and 0.01 x 1/2 mol Y2O3 were dissolved in concentrated nitric acid to form Eu(NO3)3 and Y(NO3)3 mixture solution. Secondly, 2.5 mmol cationic surfactant cetyltrimethylammonium bromide, 0.01 mol NH4VO3, and 0.2 mmol Bi(NO3)3 • 5H2O were put into the solution of Eu(NO3)3 and Y(NO3)3 at a certain speed. Then the obtained deposition was aged in 40°C water for 1 h. The precursor was gained through the process of drying at 90°C. In the last step, precursor was calcined at 740°C for 40 min in air to obtain the phosphor YVO4: Bi, Eu3+. The doping concentration of Eu3+ is 2 mol% relative to YVO4 host lattice. The as-prepared product was denoted as sample P1. In addition, sample P2 was prepared with anionic surfactant sodium dodecylbenzensulfonate and other conditions are the same as those for synthesizing sample P1.
(a)
(b)
Fig. 1. Low- (left) and high- (right) magnication SEM images of YVO4: Bi, Eu3+ phosphor preparation under different surfactants: (a) CTAB; (b) SDBS.
1.2. X-ray, Luminescence and SEM Measurements
X-ray diffraction (XRD) patterns were measured using a BRUKERD8 FOCUS with CuKa radiation (k = 0.15418 nm), at a scanning rate of 4.0° min—1. PL emission/excitation spectra were measured using an F — 4500 fluorescence spectrometer (Hitachi). The sizes and morphologies of the as-synthesized samples were studied by JSM-6380LA (JEOL) scanning electron microscope (SEM). All the measurements were carried out at room temperature.
2. RESULTS AND DISCUSSION
2.1. Effect of Cationic and Anionic Surfactants on Morphology of Phosphors
Figs. 1a, b shows low- and high-magnication SEM images of YVO4: Bi, Eu3+ prepared with cationic and anionic surfactants by a facile reaction chemistry method. It can be seen that the morphology of phosphors changes dramatically depending on the surfactants. The phosphors with CTAB shown in Fig. 1a exhibit spherical-like particles, and the phosphors with SDBS shown in Fig. 1b exhibit rod-like particles. The result shows that the effect of cationic and anionic surfactants addition on morphology of final product is different.
2.2. X-ray Diffraction (XRD) Patterns
Although comparison of cationic and anionic surfactant proved that the anionic surfactant did not take part in the reaction and its hydrocarbon tail expedited the formation of YVO4 phase, completion of the phase required higher temperatures [11, 15].The X-ray diffraction (XRD) patterns of the YVO4: Bi, Eu3+ red phosphors are shown in Figs. 2a, b. The XRD patterns of samples P1 with cationic surfactant show that the products contained high amounts ofY (YVO4) as main phase and low amounts of B (BiVO4), the XRD patterns of samples P2 with anionic surfactant show that the products contained Y (YVO4) as main phase and higher amounts of B (BiVO4), according to the previous report literature data [2, 16].
2.3. Photoluminescence Properties
The excitation spectra of YVO4: Bi, Eu3+ were shown in Fig. 3 (^em = 621 nm). The broad luminescent band ranged from 200 to 350 nm with a maximum at 278 nm in the spectra. This band is ascribed to a charge transfer from the oxygen ligands to the central
vanadium atom inside the VO4 ion, suggesting the
emission center receives energy transfer from VO4 . From the viewpoint of molecular orbital theory, it can be assigned to transitions from 1A2 (1T1) ground state to
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1A1 (E) and 1E (lT2) excited states of the VO4 ion [17]. The emission spectra of YVO4: Bi, Eu3+ were shown in Figs. 4a, b (^ex = 278 nm). The characteristic emissions of Eu3+ such as 5D1—F1 (539 nm), 5D0—7F1 (596 nm), 5D0-7F2 (617 and 621 nm) and 5D0-7F4
Relative intensity 1000
800 600 400 200 0
200 300
400 500 600 Wavelength, nm
Fig. 3. Excitation spectra YVO4: Bi, Eu phosphor preparation under different surfactants: (1) CTAB; (2) SDBS.
Relative intensity
1000
800 - 617v ^—621
600 " J S1
400 200 596 v " 539 \ x n . / 709 702 \
0 X Ii
1 1 1
500
600 700 Wavelength, nm
Fig. 4. Emission spectra YVO4: Bi, Eu3+ phosphor preparation under different surfactants: (1) CTAB; (2) SDBS.
(702 and 709 nm) are observed, and the relative intensity of 5D0-7F2 is stronger than that of other peaks from Fig. 4. According to the Judd—Ofelt parity law the magnetic dipole transition is permitted while the electric dipole transition is forbidden. The latter is allowed only on condition that the Eu3+ ions occupy a site without an inversion center, which resulted in the electric dipole transition of 5D0-7F2 and emission of red light [14, 18]. The luminescent properties of phosphors strongly depend on the particle size and surface morphology [9]. The crystal field splitting of 5D0-7F2,4 transitions of Eu3+ in YVO4 host (Fig. 4) indicates that the YVO4: Bi, Eu3+ nanocrystallites are well crystallized, and further indicates that YVO4 host are main phase. The emission peaks at 617, 621, 702 and 709 nm are assigned to the 5D0 (At)-7F2 (B2), 5D0 (AO-F2 (E), 5D0 (A1)-1Fa (B2) and D (A1)-1Fa (Ex) transition, respectively [19, 20]. Although the sample P1, P2 the excitation and emission spectra are similar in shape, the bands differ in their intensities. The PL emission intensities of YVO4: Bi, Eu3+ with different morphologies are different under identical measurement conditions. Namely, the spherical-like particles (sample P1) have the highest relative emission intensity, while (sample P2) show the fairly low intensity. Above results indicate that CTAB is more advantageous as a surfactant for effect of luminescent properties than SDBS.
3. CONCLUSIONS
YVO4: Bi, Eu3+ red phosphors were prepared by a facile reaction chemistry method with CTAB and SDBS as cationic and anionic surfactants. The introduction of surfactants does not only change the crystal structure of YVO4: Bi, Eu3+ phosphors, but also effectively improve the luminescent properties of YVO4: Bi, Eu3+ phosphors, especially, CTAB is more advantage as a surfactant for effect of luminescent properties than SDBS. The phosphors prepared with cationic
OnTHKA H CnEKTPOCKOnH^ tom 116 № 1 2014
surfactant have high amounts of Y (YVO4) as main phase and low amounts of B (BiVO4), and the phosphors prepared with anionic surfactant c
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