Composites: Part B 43 (2012) 1192–1195

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Composites: Part B

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Multi wall carbon nanotubes assisted synthesis of YVO4:Eu3+ nanocompositesfor display device applications

Bhaskar Kumar Grandhe a, Vengala Rao Bandi a, Kiwan Jang a,⇑, S. Ramaprabhu b, Ho-Sueb Lee a,Dong-Soo Shin c, Soung-Soo Yi d, Jung-Hyun Jeong e

a Department of Physics, Changwon National University, Changwon 641773, Republic of Koreab Department of Physics, Indian Institute of Technology Madras, Chennai, Indiac Department of Chemistry, Changwon National University, Changwon, Republic of Koread Department of Photonics, Silla University, Busan, Republic of Koreae Department of Physics, Pukyong National University, Busan, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 14 June 2011Received in revised form 5 August 2011Accepted 18 August 2011Available online 28 August 2011

Keywords:A. Ceramic-matrix compositesB. Optical propertiesE. Powder processing

1359-8368/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.compositesb.2011.08.011

⇑ Corresponding author. Tel.: +82 55 213 3425; faxE-mail address: kwjang@changwon.ac.kr (K. Jang).

YVO4:Eu3+ nanocomposites have been synthesized by means of a modified co-precipitation method (CP-CNT). Multi walled carbon nanotubes (MWCNT’s) have been employed in the synthesis of the YVO4:Eu3+

nanocomposites to enhance its photoluminescence efficiency. The prepared nanocomposites were thor-oughly characterized using the characterization techniques namely XRD, SEM, FTIR and Raman scatter-ing. To evaluate the potentiality of the prepared nanocomposites, the same phosphor has also beenprepared by using co-precipitation (CP) method without employing multi walled carbon nanotubesand also by means of conventional solid state reaction method (SSR). The photoluminescence spectraof YVO4:Eu3+ nanocomposites have shown stronger red emission at 619 nm (5D0 ?

7F2) for both the exci-tation wavelengths (254 and 393 nm) than the other two prepared samples. The effect of MWCNT’s onphotoluminescent properties of the YVO4:Eu3+ nanocomposites is also explained.

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1. Introduction

Yttrium vanadate (YVO4) is one of the best candidates with highluminescence efficiency for doping the rare earth ions. It provides awide band gap and suitable Y3+ sites where trivalent rare-earthions can be easily substituted without additional charge compen-sation. Due to its unique electronic structure and well-definedtransition modes, rare-earth ions entrapped in the YVO4 matrixconstitute an essential domain of the lanthanide-based nanostruc-ture families. Europium doped YVO4 phosphors have been used asa red phosphor in color televisions, cathode ray tubes (CRTs), fluo-rescent lamps and in many display device applications [1–3].

Owing to its special structure, extraordinary mechanical and un-ique electronic properties and potential applications, carbon nano-tubes, have attracted considerable attention since they werediscovered [4–6]. To exploit these advantages, it is necessary to at-tach some functional groups or other nanostructures on their sur-face and study the consequences. The combination of carbonnanotubes and other nanostructures are expected to be useful fordifferent applications. Upon scanning the literature, we noticed afew innovative and motivating reports [5–8] that have

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demonstrated the effect of using carbon nanotubes during the syn-thesis of some inorganic compound nanostructures. The results ob-tained by them are very much encouraging. Hence in the presentmanuscript we have made an attempt to study the photolumines-cence properties of multi walled carbon nanotubes assisted synthe-sis of YVO4:Eu3+ nanocomposites. The particle size, microstructureand morphology of any material will primarily depend upon theirpreparation method. We have coated the YVO4:Eu3+ precursor onthe MWCNT’s to enhance its surface to volume ratio. During the sin-tering process, the MWCNT’s will decompose resulting in the for-mation of YVO4:Eu3+ nanocomposites. To evaluate itsphotolumines cence efficiency, we have synthesized the sameYVO4:Eu3+ phosphor by co-precipitation method without employ-ing MWCNT’s and also by conventional solid state reaction method.

2. Experimental

YVO4:Eu3+ nanocomposites were synthesized by means of amodified co-precipitation method. At first, calculated amounts ofyttrium oxide and europium oxide were dissolved in a minimumamount of nitric acid and evaporated to dryness to obtain their cor-responding nitrate salts. Later, those salts were dissolved in dis-tilled water to form yttrium nitrate and europium nitratesolutions. Similarly stiochiometric amount of ammonium vanadate

B.K. Grandhe et al. / Composites: Part B 43 (2012) 1192–1195 1193

was dissolved in distilled water separately. With the help of a mag-netic stirrer, both these solutions were mixed for an hour. Sec-ondly, certain quantity of multi-walled carbon nanotubes wasultrasonically dispersed in the above mixture. Subsequently, suit-able amount of polyvinyl pyrrolidone (PVP) was added as disper-sant and surfactant. Black turbid precipitates were formed afterstirring them at 90 �C for 5 h. The precipitates, thus obtained werefiltered and washed with deionized water for several times andthen dried at 100 �C for 12 h, to get the YVO4:Eu3+ precursor. Final-ly, the YVO4:Eu3+ nanocomposites were obtained by sintering theprecursor powders at 900 �C for 4 h. For an evaluation purpose,YVO4:Eu3+ phosphor was prepared once again by repeating theabove procedure without using MWCNT’s. Apart from this,YVO4:Eu3+ phosphor was also prepared by means of solid-statereaction method. Highly pure and reagent grade chemicals, suchas Y2O3, V2O5 and Eu2O3, were used as starting materials. Requiredchemicals were weighed based on the calculated composition.They were then collected into an agate mortar and grinded withacetone in order to obtain a homogeneous chemical mixture. Itwas then transferred to an alumina crucible and heated in an elec-trical furnace from room temperature to 1100 �C and sintered atthis high temperature for about 5 h with an intermediate grinding.

The prepared phosphors were characterized by means of XRD3003 Seifert diffractometer with Cu Ka line of k = 1.5406 Å overthe 2h range from 20� to 80�. Their morphological features wereexamined on a JEOL JSEM 840A Scanning Electron Microscope. FTIRspectrum of the sample was recorded on a Thermo Nicolet-5700spectrophotometer using the KBr pellet technique from 4000cm�1 to 400 cm�1. Raman spectrum was recorded by using HoribaJobin HR800UV system attached with a He–Ne laser (633 nm) asthe excitation source having an output power of 15 mW with a laserbeam spot size of 100 um using appropriate lens system. Jobin YvonFluorolog-3 fluorimeter equipped with Datamax software was usedto record the photoluminescence spectra of the prepared phos-phors. Xenon lamp (450 W) was used as an excitation source. Allthe spectral profiles were recorded at room temperature.

3. Results and discussion

Fig. 1 shows the XRD patterns of (a) YVO4:Eu3+ phosphor precur-sor prepared by CP-CNT method, (b) YVO4 phosphor prepared byCP-CNT method and sintered at 900 �C, and (c) YVO4:Eu3+ nano-composites prepared by CP-CNT method and sintered at 900 �C.(d) YVO4:Eu3+ phosphor prepared by CP method and sintered at900 �C, and (e) YVO4:Eu3+ phosphor prepared by SSR method and

Fig. 1. XRD patterns of (a) YVO4:Eu3+ precursor prepared by CP-CNT method, (b)YVO4 prepared by CP-CNT method, and (c) YVO4:Eu3+ prepared by CP-CNT method.(d) YVO4:Eu3+ prepared by CP method, and (e) YVO4:Eu3+ prepared by SSR method.

sintered at 1100 �C. We found that the XRD profiles of all the sam-ples are well indexed with tetragonal structure of YVO4 and theyare in good agreement with the standard JCPDS card No. 17-0341.Diffraction peaks related to the MWCNT’s were not found in the sin-tered samples, because it decomposes over 600 �C. Fig. 2 shows theSEM images of YVO4:Eu3+ phosphors prepared by SSR, CP and CP-CNT methods respectively. From the figure we can notice that theYVO4:Eu3+ nanocomposites prepared by CP-CNT method are welldispersed and the average diameter of the grain size was found tobe in the range of 20–30 nm. On the contrary, we can also noticefrom the SEM images of the SSR and CP method samples that thoseparticles are agglomerated and also larger in size when comparedwith the CP-CNT method sample.

Fig. 3a shows the FTIR spectrum of YVO4:Eu3+ nanocompositesprepared by CP-CNT method at 900 �C. The intense transmittancepeak noticed near 833 cm�1 is the characteristic IR band of VO3�

4

which normally arises in the range of 780–920 cm�1. The isolatedpeaks, which contribute to the VO4 tetrahedron, prove that thecrystal structures of the products coincide with its tetragonalphase. We have also observed some insignificant IR bands near1040, 1385, 1635 and 3450 cm�1 which are probably due to OHgroups or atmospheric carbon dioxide which might be adsorbedon the sample surface. During the sample preparation for FTIRspectrum or at the time of spectral measurement, the absorptionof H2O, CO2 from the ambient atmosphere might be possible. Allthe assignments made above are based on the literature reports[9–13]. Fig. 3b shows the Raman spectrum of YVO4:Eu3+ nanocom-posites prepared by CP-CNT method. Normally, vibrations of com-plex VO3�

4 and Y3+ ions in YVO4 unit cells will produce the lowestvibrational frequencies near 160 cm�1 as external modes of Ramanspectra. Here also we noticed a weak band at 157 cm�1 relating toit. Raman bands observed near 183, 221, 259, 376, 487, 813, 838and 890 cm�1 are also in well agreement with the literature re-ported for YVO4:Eu3+ and their assignments are as follows: TheA1g lines are observed at 890 (m1) and 376 cm�1, the B1g modesare observed at 813 (m3), 487 (m4) and 259 (T0) cm�1 and the Eg

mode is observed at 838 (m3) cm�1 [10,14].254 and 393 nm excitable red phosphors are useful in many

applications like Hg-free fluorescent lamps, field emission displays,cathode ray tubes and near UV excitable white LED’s [15–18].Hence, in the present study, we have chosen those two excitationwavelengths to analyze the photoluminescent properties of theprepared phosphors. Fig. 4a and b shows photoluminescence (PL)spectra of phosphors prepared by SSR, CP and CP-CNT methodsand measured with 254 nm and 393 nm as the excitation wave-lengths respectively. For both the excitation wavelengths, theemission spectra did not show any significant changes except forthe variations in their emission intensities. All the spectral profilesconsist of emission bands at around 586, 595, 619, 651 and 700 nmthat were assigned to the electronic transitions of europium ionfrom the 5D0 level to the lower lying levels of 7F0,

7F1, 7F2, 7F3 and7F4 respectively. The strong luminescence of Eu3+ results from anefficient energy transfer from the VO3�

4 group to Eu3+ in YVO4:Eu3+

as reported previously [1,10]. It is yet again confirmed, as we didnot found any emission transitions from its higher excited stateslike 5D1, 5D2 [18]. Emission spectra of all the samples contain thecharacteristic transitions of Eu3+ from its lowest excited state(5D0) only. The most intense transition (5D0 ? 7F2) located at619 nm, corresponds to the red emission which is extensively usedin the lighting and display device applications.

Among the observed 5D0 to 7FJ (J = 0,1, 2, 3 and 4) transitions,the 5D0 ? 7F2 transition observed at 619 nm is an electric dipoletransition and it is sensitive to chemical bonds in the vicinity ofEu3+ ion. On the other hand, the 5D0 ? 7F1 transition (595 nm) isa magnetic dipole one and hardly varies with the crystal fieldstrength around the Eu3+ ion. A strong red emission which is

Fig. 2. SEM image of YVO4:Eu3+ phosphors prepared by (a) SSR method, (b) CP method, and (c) CP-CNT method.

Fig. 3. FTIR and Raman spectrum of YVO4:Eu3+ nanocomposites prepared by CP-CNT method.

Fig. 4. (a) Photoluminescence (PL) spectra of YVO4:Eu3+ phosphors prepared by SSR,CP and CP-CNT methods with (a) 254 nm excitation and (b) 393 nm excitation.

1194 B.K. Grandhe et al. / Composites: Part B 43 (2012) 1192–1195

dominated by an electric dipole transition becomes possible onlywhen Eu3+ ions occupy a site without inversion symmetry [1,10].

Furthermore, in the emission spectra, we have noticed a weak5D0 ? 7F0 forbidden transition at about 586 nm and as well as a5D0 ? 7F2 electric dipole transition at 619 nm of very high inten-sity. It allows us to arrive at a conclusion that in the preparedYVO4:Eu3+ nanocomposites, the Eu3+ ions exist in an environmentwith low symmetry and lack of inversion symmetry [19,20].

From Fig. 4, it can be understood that the nanocomposites pre-pared by means of CP-CNT method are more efficient among allthe prepared samples. It is evident from their SEM images (Fig. 2)that YVO4:Eu3+ phosphor prepared by CP-CNT method possessessmaller grain size and better surface-to-volume ratio. Obviously,there will be more surface Eu3+ ions in the YVO4:Eu3+ nanocompos-ites (CP-CNT) than that in the YVO4:Eu3+ phosphors prepared by CPand SSR methods. Therefore, the higher surface-to-volume ratio inYVO4:Eu3+ nanocomposites, leads to an increase in charge-transfertransitions contributed by surface Eu3+ ions and a decrease of thosecontributed by internal Eu3+ ions than the other phosphors. Smallamounts of residual carbon might be doped into the YVO4:Eu3+

nanocomposites when the as prepared precursors are sintered at900 �C. Carbon doped into YVO4:Eu3+ nanocomposites may perhapshelp to raise the valence band of the YVO4:Eu3+ nanocomposites.

B.K. Grandhe et al. / Composites: Part B 43 (2012) 1192–1195 1195

Therefore, the electronic transition from the valence band to theconduction band could be easier, which is beneficial for lumines-cence [21]. Ajayan et al. [22] reported that when the CNT’s usedas templates are removed by sintering, some oxygen vacancies formin their nanostructures. These oxygen vacancies are known to bethe most common defects in oxides and usually it will act as radia-tive centers in luminescence processes. Consequently, the surfaceeffects, residual carbon and oxygen vacancies altogether result instronger emission intensities of YVO4:Eu3+ nanocomposites thanthe phosphors prepared by means of other methods.

4. Conclusions

XRD and FTIR analysis has confirmed the tetragonal phase ofYVO4:Eu3+ nanocomposites prepared by CP-CNT method. Grainsize of the YVO4:Eu3+ nanocomposites are found to be in the range20–30 nm. It is clear from the obtained results that YVO4:Eu3+

nanocomposites are exhibiting improved photoluminescenceproperties than the other phosphors prepared by co-precipitationand solid state reaction methods. Superior red emission displayfrom the YVO4:Eu3+ nanocomposites compared to the other phos-phors, became possible due to its smaller grain size, presence offew residual carbons and oxygen vacancies. The results revealedby the YVO4:Eu3+ nanocomposites make’s it a possible candidatefor different fluorescent lamp and display device applications.

Acknowledgements

This work was supported by the Priority Research Centers Pro-gram through the National Research Foundation of Korea fundedby the Ministry of Education, Science and Technology (NRF-2010-0029634) and also this work was partially supported by the Na-tional Research Foundation of Korea funded by the Korean govern-ment (NRF-2010-0023034).

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