Journal of Luminescence 145 (2014) 144–147

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Journal of Luminescence

0022-23http://d

n CorrE-m

journal homepage: www.elsevier.com/locate/jlumin

Synthesis and luminescence properties of hexagonal CaTiO3:Eu3+ nanosheets

Wei Yang, Juncheng Hu n

Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, Hubei Province, South-Central Universityfor Nationalities, Wuhan 430074, People’s Republic of China

a r t i c l e i n f o

Article history:Received 28 August 2012Received in revised form19 May 2013Accepted 10 July 2013Available online 19 July 2013

Keywords:NanosheetsGrowth mechanismLuminescence

13/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.jlumin.2013.07.041

esponding author. Tel./fax:+86 27 678 41 302ail address: hujuncheng@mail.scuec.edu.cn (J.

a b s t r a c t

Hexagonal CaTiO3:Eu3+ nanosheets were prepared by a supercritical fluid method. The samples were

characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolutiontransmission electron microscopy (HRTEM) and luminescence spectrometer (LS). Under 466 nm lightexcitation, the CaTiO3:Eu

3+ nanosheets showed a strong red (616 nm) emission corresponding to 5D0-

7F2 transition of Eu3+. The effect of Eu3+ concentration was investigated, among these samples, 0.2 mol%Eu3+doped CaTiO3 nanosheets showed the strongest luminescent intensity. Compared with nanoparti-cles, the nanosheets showed better luminescence property. Growth mechanism of hexagonal CaTiO3

nanosheets was also discussed.& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Phosphors are an important part of white-light LEDs, whichrequire a long life, high rendering index, high luminosity efficiencyand environmental friendly characteristics [1–3]. Eu3+ doped materi-als have high fluorescent efficiency and hypersensitive transition of5D0-

7F2 in the red part of spectrum, and they have attractedconsiderable interest in the fields of high-performance luminescencedevices, catalysts, time resolved fluorescence labels [4–6]. Eu3+

concentrations of these doped materials have great influence onthe luminescence intensity, and the optimum Eu3+ concentrationshave been reported from 1mol% to 26 mol% [7–12].

Perovskite CaTiO3 is widely used for electronic devices, micro-wave technology, varistors immobilization of nuclear wastes andcatalysis [13]. CaTiO3 nanoparticles can be prepared by solid-statereaction, sol–gel, co-precipitation, combustion and hydrothermalmicrowave (HTMW). Flower-like, hollow and core–shell CaTiO3

were obtained using soft or hard templates methods [14–19].Nanosheet materials have high degree of crystallinity and aniso-tropy. These specialties show distinctive physicochemical proper-ties in comparison with conventional nanocrystallites [20]. Rareearth doped CaTiO3 phosphors have better chemical stability andenvironmental friendliness than metallic sulfides and oxides asluminescent materials [21–23].

In this work, we synthesized the Eu3+ doped CaTiO3 hexagonalnanosheets with the enhanced luminescent properties through a

ll rights reserved.

.Hu).

supercritical fluid method. This work presented for the first time thefabrication of Eu3+ doped CaTiO3 hexagonal nanosheets and demon-strated their potential applications as a promising red phosphor forwhite-light LEDs.

2. Experimental

2.1. Materials

Calcium nitrate tetrahydrate (Ca(NO3)2 �4H2O), europium (III)nitrate hexahydrate (Eu(NO3)3 �6H2O), tetrabutyl titanate (Ti(OC4H9)4),ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd,China. All the chemicals were of analytical grade and used withoutfurther purification.

2.2. Synthesis of CaTiO3:Eu3+ nanosheets

In a typical synthesis procedure: 10 mmol Ca(NO3)2 �4H2O wasdissolved in 50 ml ethanol, in which 0.1, 0.2, 0.3 and 0.4 mmol of Eu(NO3)3 �6H2O were added stoichiometrically, respectively, then stirredfor 30 min at room temperature. 10 ml of Ti(OC4H9)4 (about 3.2 mmol)was pipetted out, then suddenly injected into 40 ml ethanol in 100 mlflask and sealed, stirring for 30 min at ambient temperature. The twoprecursor solutions were mixed and then 100 ml ethanol was added,under constant stirring. This mixture was transferred into a 500 mlautoclave and the reaction mixture was purged with 1 MPa N2 forthree times, and then 1MPa N2 was imposed before initiating heating.The mixture was heated to 250 1C for 4 h, then to 265 1C, andmaintained at that temperature for 1.5 h. Finally, the vapor inside

W. Yang, J. Hu / Journal of Luminescence 145 (2014) 144–147 145

was vented. The CaTiO3:Eu3+ samples were collected, dried andcalcined in air at 800 1C for 4 h.

2.3. Characterization

The CaTiO3:Eu3+ samples were characterized by X-ray powderdiffraction (XRD) in a Bruker D8, using a Cu Ka radiation in orderto determine the crystalline structure. Transmission electronmicroscopy (TEM) images were obtained using a Tecnai G20 (FEICo., Holland) transmission electron microscope with an accelerat-ing voltage of 200 keV. The emission spectrum was performed byusing a PerkinElmer LS-55 (PerkinElmer Corporation, USA) lumi-nescence spectrophotometer equipped with a Xenon dischargelamp as an excitation source. For comparison of different samples,the emission spectra were measured at the same instrumentparameters (10 nm for the excitation slit, 10 nm for the emissionslit) at room temperature.

3. Results and discussion

3.1. Structure and morphology

Fig. 1 showed the XRD patterns of 0.2 mol% Eu3+ doped CaTiO3

nanosheets, all of XRD patterns showed the diffraction peaks at 2θ of23.241, 33.141, 47.541, 59.301 and 69.451, they were respectivelyindexed to the (1 0 1), (1 2 1), (0 4 0), (0 4 2) and (2 4 2) planes oforthorhombic with Pnma group space CaTiO3 structure in accordancewith the JCPDS Card no. 22-0153 (unit cell a¼5.440, b¼7.644,c¼5.381 Å). Some weak peaks in the 25–301 range may be theexistence of TiO2. These peaks of TiO2 were accorded with the JCPDSCard no. 75-1748. TiO2 were obtained by the hydrolysis of Ti(OC4H9)4.No other peaks could be found, The chemical stoichiometry of thesample was investigated with EDAX spectrometry, which revealedthat the product consisted of Ca, Ti, O and Eu was close to thatemployed in the synthesis, the doped Eu3+ concentration was about

20 30 40 50 60 70

CaTiO3: Eu3+nanosheets

JCPDS#22-0153

Inte

nsity

(a.u

.)

2 (degree)

0.0 0.9 1.8 2.7 3.6 4.5 5.4 6.3 7.20

500

1000

1500

2000

2500

Eu

Ca TiO

Inte

nsity

(a.u

.)

Envergy (ev)

C

Fig. 1. XRD and EDAX of the 0.2 mol% CaTiO3:Eu3+ sample.

0.2 mol%, this result demonstrated that the Eu3+ could be effectivelydoped into host lattice. The ionic radius of Eu3+ was close to that ofCa2+ and larger than that of Ti4+. The charge compensation may occurby the formation of intrinsic defects such as negatively charged Cavacancies, positively charged oxygen vacancies and/or by reduction ofTi4+ to Ti3+.Thus, the substitution of Eu3+ ion was usually thought tooccupy Ca2+ ion sites of the cube-octahedral (CaO12) instead of Ti4+

sites of the octahedral (TiO6) [24].To study the growth mechanism of CaTiO3 nanosheets, Fig. 2

shows these powders obtained at different reaction times of 1, 2 and4 h, respectively. The average size of nanoparticles was 25 nm whenthe reaction time was 1 h in Fig. 2a. When prolonging the reactiontime to 2 h, the morphology of CaTiO3 was close to hexagonalnanosheets, these nanosheets were surrounded by some smallernanoparticles in Fig. 2b. The formation of nanosheets was governedin the front of solid/supercritical fluid interface, under the highpressure and heat flow [25]. An Ostwald ripening process then ledto the decrease of these nanoparticles around the nanosheets. Morenanosheets were observed, the nanoparticles became smaller andsmaller after 4 h, as shown in Fig. 2c. Fig. 2d shows the HRTEM imageof CaTiO3:Eu3+ nanosheets, the lattice spacing of the nanosheets were0.385 nm, close to the (1 0 1) crystal plane distances of CaTiO3 (JCPDSno. 22-0153). These results further confirmed the presence of highlycrystalline CaTiO3. The growth mechanism was illustrated that thesupercritical environment provided the interface pressure and heatflux needed for growth CaTiO3 nanosheets, the intrinsic crystal growthrate was very high, the growth of the amorphous nanoparticles andtheir aggregation occurred simultaneously under this environment,nanosheets were more energetically favored than smaller nanoparti-cles, these nanoparticles became smaller, and nanosheets becamebigger [26,27].

Fig. 3 shows the excitation and emission spectra of CaTiO3:Eu3+

nanosheets. Excitation spectra (λem¼616 nm, 5D0-7F2 transition of

Eu3+) consisted of one broad band at 400 nm in the ultraviolet (UVA)and one sharp band located at 466 nm in the blue range. Both ofpeaks were due to the crystal field splitting of the Eu3+ f orbital, thepeak at 400 nm was attributed to 7F0-5L6 and the peak at 466 nmwas attributed to 7F0-5D2 [27,28]. Two of the major absorptive peakswere at 400 and 466 nm, which were in good agreement with the UVor blue output wavelengths of GaN-based LED chips, respectively [28].The sample of 0.2 mol% CaTiO3:Eu3+ showed the strongest intensity ofemission. Obviously, the emission of samples strengthen monotoni-cally with increasing of Eu3+ concentration in the range from 0.1 to0.2 mol%, it indicated that the probability of energy transfer increasedbetween Eu3+ and their neighbor Eu3+ ions in cell of Eu3+ dopedCaTiO3. When the Eu3+ concentration was above 0.2 mol%, the non-irradiative interaction between Eu3+ ions became stronger than theenergy transfer, which caused that the excitation energy utilizationrate reduced [12]. The concentration quenching effect gave anexample how the deficiencies affected the luminescence of CaTiO3:Eu3+. In principle, if two Eu3+ ions occupied two Ca2+ sites, it willgenerate one Ca2+ vacancy according to charge compensation. Themore Eu3+ ions were doped into the CaTiO3 cell, the more Ca2+

vacancies would be offered. The intensity of photoluminescencedecreased significantly by the energy transfer process by cross-relaxation between the Eu3+ neighbor ions and as well energytransfer with Eu3+ and Ca2+ vacancies [29]. Jia et al. and Mazzoet al. reported that the phenomenon of concentration quenching inCaTiO3 host were observed at above 28 mol% and above 1 mol%,respectively. We also observed the same effect in the sample with0.2 mol% Eu3+ concentrations.

Morphology of the host materials played an important role incontrolling the chemical, physical, optical, and electronic propertiesfor application [30,31]. Fig. 4 shows the emission of the 0.2 mol%CaTiO3:Eu3+ samples at different reaction times. The nanosheets at4 h showed the strongest emission intensity. It was mainly due to the

Fig. 2. TEM images of 0.2 mol% CaTiO3:Eu3+ sample obtained at 250 1C for different reaction times: (a) 1 h, (b) 2 h and (c) 4 h; (d) HRTEM images of the nanosheets.

400 440 480 580 600 620 640

40

60

80

100

120cd

b

397

466

ex=466 nm

Inte

nsity

(a. u

.)

Wavelength (nm)

em=616 nm

616

EmissionExcitation

a

Fig. 3. Photoluminescence excitation of 0.2 mol% CaTiO3:Eu3+ sample and emissionspectra of CaTiO3:Eu3+ nanosheets for different concentrations: (a) 0.1 mol%,(b) 0.2 mol%, (c) 0.3 mol% and (d) 0.4 mol%.

580 600 620 640 660 68040

60

80

100

120

Inte

nsity

(a. u

.)

Wavelength (nm)

1 h

2 h

4 h

Fig. 4. Photoluminescence emission spectra of 0.2 mol% CaTiO3:Eu3+ sample atdifferent reaction times.

W. Yang, J. Hu / Journal of Luminescence 145 (2014) 144–147146

scattering of the incident light in the surface of the samples becausethe surface morphology affected the scattering of light, by means ofreflectance [32,33]. The nanosheets had unique physical propertiescompared to the irregular nanoparticles.

4. Conclusion

In summary, hexagonal CaTiO3:Eu3+ nanosheets are successfullysynthesized by a supercritical fluid method, the results of XRD and

TEM, hexagonal CaTiO3:Eu3+ nanosheets show well-defined crystal-line. These nanosheets have unique luminescent properties andexhibit a strong red emission under the 466 nm excitation, theoptimal concentration is 0.2 mol%, which is lower than other datain literatures. Furthermore, the novel supercritical fluid method iseasily scaled-up for the preparation of large quantities of thenanoparticles.

Acknowledgments

The Project was sponsored by the Scientific Research Foundationfor the Returned Overseas Chinese Scholars, State Education Ministry.

W. Yang, J. Hu / Journal of Luminescence 145 (2014) 144–147 147

This work was also supported by National Natural Science Foundationof China (20803096 and 21073238), the NSF of Hubei Province(Distinguished Young Investigator Grant 2010CDA082), and NationalBasic Research Program of China (Grant No: 2011CB211704).

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