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Luminescence properties of Tb3+-doped CaMoO4 nanoparticles: annealingeffect, polar medium dispersible, polymer film and core–shell formation†

A. K. Parchur,a A. I. Prasad,b A. A. Ansari,c S. B. Raia and R. S. Ningthoujam*b

Received 17th January 2012, Accepted 25th June 2012DOI: 10.1039/c2dt31257c

Tb3+-doped CaMoO4 (Tb3+ = 1, 3, 5, 7, 10, 15 and 20 atom%) core and core–shell nanoparticles have

been prepared by urea hydrolysis in ethylene glycol (EG) as capping agent as well as reaction medium atlow temperature ∼150 °C. As-prepared samples were annealed at 500 and 900 °C for 4 h to eliminateunwanted hydrocarbons and/or H2O present in the sample and to improve crystallinity. The synthesisednanophosphors show tetragonal phase structure. The crystallite size of as-prepared sample is found to be∼18 nm. The luminescence intensity of the 5D4 →

7F5 transition at 547 nm of Tb3+ is much higher thanthat of the 5D4 →

7F6 transition at 492 nm. 900 °C annealed samples show the highest luminescenceintensity. The intensity ratio R (I[5D4 →

7F6]/I[5D4 →

7F5]) lies between 0.3–0.6 for as-prepared, 500 and900 °C annealed samples. The luminescence decay of 5D4 level under 355 nm excitation showsbiexponential behaviour indicating availability of Tb3+ ions on surface and core regions of particle;whereas, contribution of Mo–O charge transfer to lifetime is obtained under 250 nm excitation. The CIEcoordinates of as-prepared, 500 and 900 °C annealed 5 atom% Tb3+-doped CaMoO4 samples under250 nm excitation are (0.28, 0.32), (0.22, 0.28) and (0.25, 0.52), respectively. The dispersed particles inpolar medium and its polymer film show green light emission. The luminescence intensity is improvedsignificantly after core–shell formation due to extent of decrease of non-radiative rates arising fromsurface dangling bonds and capping agent. Quantum yields of as-prepared samples of 1, 5 and 7 atom%Tb3+-doped CaMoO4 samples are found to be 10, 3 and 2, respectively.

1. Introduction

Study of optical spectra of scheelite type (AXO4, A = Ca, Sr andBa; X = Mo and W) nanomaterials has been an active area inresearch due to their good thermal, chemical and luminescenceproperties. Due to their low vibrational frequency (∼850 cm−1)these nanomaterials have promising applications in photolumine-scence, solid-state lasers, optical fibers and scintillators.1–4

CaMoO4 has tetragonal structure having space group I41/a(C4h

6) with two formula units per unit cell. Both Ca and Mohave S4 point symmetry and the crystal structure has an inversion

centre.5 The possibilities of different electronic transitions inMoO4

2− tetrahedron from O(2p) to Mo(4d) of 5–13 eV(∼250–95 nm) were reported.5–9

CaMoO4 has high absorption cross section (∼103 cm−1) dueto its broad band gap (valence band to conduction band) with abroad emission starting from 330 to 550 nm. Since the emissionband is very broad, it is difficult to tune the color, particularlyfor lighting and display applications. On the other hand, lantha-nide ions have poor absorption cross-section (∼0.01–1 cm−1)due to the forbidden nature of f–f transitions and show poorluminescence emission.10 To overcome this problem, materialswith high absorption cross-section can be manipulated bydoping ions with low absorption cross-section (e.g. lanthanideions, Ln3+ = Sm3+, Eu3+, Tb3+, Dy3+ and Tm3+). The forbiddennature of Ln3+ ion can be relaxed by the crystal field (surround-ings) when Ln3+ ions occupy crystallographic sites of the hostlattice (e.g. CaMoO4). This also gives a possibility of energytransfer from host/sensitizer to the excited state of the activatorby a non-radiative energy transfer mechanism.11 This may resulta high population of photons in the excited level/levels of Ln3+

and thereby enhanced emission of Ln3+ may be expected.Though several papers have been published in this field,however, there is little work on CaMoO4:Tb

3+.To enhance the energy transfer from sensitizer to Ln3+ ion,

one way is to choose CaMoO4 as sensitizer in which Ln3+ ions

†Electronic supplementary information (ESI) available: The parametersobtained after mono-exponential equation fitting to decay data of 900 °Cheated samples given in table. XRD patterns of Tb3+-doped CaMoO4(Tb3+ = 1, 3, 5, 7, 10, 15 and 20 atom%) for as-prepared, 500 and900 °C samples. The emission spectra under excitation wavelength 230,240 and 322 nm excitations, variation in intensity, change in peak posi-tion and FWHM for as-prepared, 500 and 900 °C samples. The typicalfitting to emission peaks. See DOI: 10.1039/c2dt31257c

aDepartment of Physics, Banaras Hindu University, Varanasi-221005,IndiabChemistry Division, Bhabha Atomic Research Centre, Mumbai-400085,India. E-mail: rsn@barc.gov.in; Fax: +91-22-25505151;Tel: +91-22-2559321cKing Abdullah Institute for Nanotechnology, King Saud University,Riyadh-11451, Saudi Arabia

11032 | Dalton Trans., 2012, 41, 11032–11045 This journal is © The Royal Society of Chemistry 2012

occupy the lattice sites of Ca2+ randomly because of similar ionicsizes. Since Ca2+ ion has no f-electrons, the cross-relaxationamong Ln3+–Ln3+ will be low. Recently, CaMoO4 doped withTb3+ ion has been studied.12 It is found that Tb3+ gives a largenumber of transitions in the visible range, but the green emissionband at ∼545 nm is the highest intensity among other transitions.However, the mechanism of energy transfer from the host to acti-vator ion has not been studied in depth. Many other interestingpoints such as excitation wavelengths, effect of concentrationand crystallite size on luminescence of Tb3+-doped CaMoO4

have not been considered much. Very recently, we have reportedthe structural characterization and luminescence properties ofEu3+-doped CaMoO4 nanoparticles.

13a,14

Many preparation routes have been employed to synthesiseCaMoO4 and/or CaMoO4 doped with Ln3+ by different groups,such as the Czochralski technique, conventional solid-state reac-tions, sol–gel and hydrothermal method.15–19 However, the par-ticles prepared by wet chemical routes are not purely crystallineat low temperature preparation.20 Here, we focus on the pre-paration of CaMoO4 doped with Tb3+ ion (Tb3+ = 1, 3, 5, 7, 10,15 and 20 atom%) nanoparticles using the polyol method underurea hydrolysis. Urea hydrolysis helps in the crystallizationprocess at low temperature of 150 °C. The effects of crystallitesize, heat treatment, concentration of Tb3+ and core–shell modifi-cation on luminescence intensity and lifetime are discussed here.The energy-transfer mechanism amongst acceptors (dipole–dipole, dipole–quadrupole and quadrupole–quadrupole) hasbeen analyzed by assuming negligible diffusion amongst donors.Quantum yield measurement has been carried out.

2 Experimental

The nanoparticles of CaMoO4:Tb3+ (Tb3+ = 1, 3, 5, 7, 10, 15

and 20 atom%) were prepared at low temperature (150 °C for3 h) using urea hydrolysis in ethylene glycol (EG). The detailedpreparation method is mentioned elsewhere.13a In brief, calciumcarbonate (CaCO3, 99.99%, Sigma Aldrich), molybdenum chlo-ride (Mo2Cl10, 98%, Acros Organics) and terbium oxide (Tb2O3,99.9%, Sigma Aldrich) were used as starting materials. For atypical preparation of 3 atom% Tb3+-doped CaMoO4, 0.36 g ofCaCO3 and 0.02 g of Tb2O3 were dissolved together in concen-trated nitric acid (HNO3). The mixture was heated at 80 °C toremove the excess acid and the process of removal of excess acidwas repeated five times after addition of double distilled water(5 ml). Subsequently 50 ml of EG was added and the mixturewas ultrasonicated for 30 min. 1 g of Mo2Cl10 (dimer) dissolvedin 50 ml of methanol was then added and the solution wasstirred for 1 h. The solution was heated again at 80 °C. 50 ml ofEG and 2.0 g of urea were then added and placed under ultra-sonication for 1 h for uniform mixing. The reaction mixture washeated up to 150 °C for 3 h under refluxing condition until pre-cipitation was complete. The white precipitate so obtained wascollected by centrifugation and washed five times in methanol toremove excess of EG. Finally, the as-prepared sample wasdivided into three parts. One part of the sample was annealed at500 °C and the other at 900 °C (both in ambient atmosphere at aheating rate of 2 °C min−1 for 4 h) and the third part leftuntreated.

For the preparation of CaMoO4:Tb3+ (3 atom%) coated with

CaMoO4 (CaMoO4:Tb3+@CaMoO4, core–shell) nanoparticles, a

similar polyol process was used as discussed above. The whiteprecipitate of CaMoO4:Tb

3+ (3 atom%) nanoparticles obtainedafter heating the reaction medium for 3 h at 150 °C was allowedto cool to room temperature. To this 2 g of urea was added andthe mixture stirred for 1 h. In the next step, 0.37 g of CaCO3 wasdissolved in nitric acid and the excess amount of acid was evapo-rated by adding double distilled water in another beaker. 1 g ofMo2Cl10 dissolved in 50 ml of methanol was then added andsolution stirred for 1 h. This solution was added to CaMoO4:Tb3+ nanoparticles dropwise and was then stirred for 1 h fol-lowed by heating at 150 °C for 3 h under refluxing conditionuntil precipitation was complete. The precipitate was centrifugedand washed five times in methanol to remove the excess of EG.

Here, urea is a source of ammonia, which acts as precipitatingagent. Its decomposition temperature in water medium (120 °C)is lower than that in the gas/solid phase (190 °C) and it producesammonia and CO2 or CNOH.

13b Ammonia molecules react withCa2+ and Mo5+ ions to form their oxides or/and hydroxides.During the reaction, ammonia also oxidizes Mo5+ to Mo6+ andnucleation of CaMoO4 takes place.13a When the nucleationstarts, surrounding EG molecules cap smaller particles and thus,particle growth is hindered. Also, the final dielectric mediumdoes not allow further particle growth.13c

The crystal structure of the phosphors was identified by aPW 1071 Philips powder X-ray diffractometer (XRD) withNi-filtered Cu-Kα (1.5405 Å) radiation at 30 kV and 20 mA. Allpatterns were recorded over the range 10 ≤ 2θ/° ≤ 70 with a stepsize of Δ2θ = 0.02°. The Scherrer relation was used to calculatethe average crystallite size from XRD spectrum. The relation isexpressed as follows (eqn (1)):

D ¼ 0:89λ

βhklcosθð1Þ

where λ is the wavelength of the X-rays and βhkl the full width athalf maximum (FWHM) of the peak in the XRD pattern.21

Infrared (IR) spectra were measured with a FTIR spectrometer(Bomem MB 102) with a resolution of 1 cm−1. The sample wasmixed with KBr (Sigma Aldrich, 99.99%) in 1 : 5 ratio andpellets were prepared to record the spectra.

The photoluminescence (PL) spectra of these powder phos-phors were recorded using a Hitachi F-4500 spectrometerwith a 150 W Xe lamp as a source at a spectral resolution of5 nm slit width. All the measurements were carried out at roomtemperature. 100 mg of powder sample is pasted over a quartzslide of ∼1 cm in diameter. Here methanol is used as adhesivebetween the powder and quartz slide which was then kept at50 °C for 1 h to remove methanol. This quartz slide containingsample was put on the holder of PL instrument. The anglebetween the incident ray and detector is 90° and sample is at45° to both incident ray and detector. Excitation and emissionslit widths are kept constant at 5 nm. PL decay was recordedwith an Edinburgh instrument F920 equipped with 100 W μsflash xenon lamp as the excitation source with repetition rate of50 Hz and pulse width of 5 ns. The excitation and emissionspectra were corrected from the optical/detector response ofinstrument.

This journal is © The Royal Society of Chemistry 2012 Dalton Trans., 2012, 41, 11032–11045 | 11033

3. Results and discussion

3.1 Structural and morphology studies

Fig. 1 shows the XRD patterns of as-prepared, 500 and 900 °Cannealed samples of 3 atom% Tb3+-doped CaMoO4. It is clearfrom the figure that even the as-prepared sample is highly crys-talline with tetragonal structure. All diffraction peaks match wellwith JCPDS card no 29:0351 (a = 5.226, c = 11.43 Å and V =312.17 Å3). However, some extra peaks are observed in case ofhigh Tb3+ ion doping concentration (above 3 atom%). It is foundthat Tb3+ ions occupy Ca2+ sites up to 3 atom% Tb3+ ion con-centration; above 3 atom% extra XRD peaks (marked with #)can be seen at 2θ = 14.08, 22.98 and 36.65° for 5, 7 and 20atom% Tb3+-doping (Fig. S1(a–c) (see ESI†). At 2θ = 14° thepeak intensity decreases from the as-prepared sample to 900 °C.An extra peak occurs at 22.98° in the as-prepared sample whilethere are several extra peaks for the 500 and 900 °C heatedsamples in the same region. A peak is found for as-prepared and500 °C heated samples at 36.65°. Notably, such peaks at 14,22.98 and 36.65° do not match with JCPDS card no 29:0351.These peaks may be related to MoOn·mH2O, other Tb–Mo–Ocompounds and/or Tb3+ oxides, but could not be distinguishedin this study. The presence of extra phases would be related tothe solubility limit of Tb3+ ion in CaMoO4 and we found similartype of patterns in our earlier studies of Eu3+-doped CaMoO4

above 3 atom% Eu3+.13a It is to be noted that upper limit of sub-stitution up to 3 atom% Tb3+ is due to charge difference withCa2+, and oxygen vacancies as well as phase segregationexpected at high doping concentration of Tb3+. The lattice para-meters of as-prepared 3 atom% Tb3+-doped CaMoO4 are a =5.241, c = 11.402 Å, V = 313.10 Å3 and 900 °C annealedsamples are a = 5.237, c = 11.356 Å, V = 311.51 Å3. The unitcell volume calculated for the as-prepared sample is little high ascompared to the standard JCPDS card 29:0351. This is due tothe broadness of the peak in the case of as-prepared samples.Thus assignment of its exact position is subject to a slight error.

The peak position can be ascertained more accurately in the caseof 500 and 900 °C annealed samples because the peaks aresharp. For the 1 atom% Tb3+-doped CaMoO4, 900 °C annealedsample, a = 5.233, c = 11.382 Å and V = 311.80 Å3. For5 atom% Tb3+ or above, there is no further change in unit cellvolume (within error). The diffraction peaks of Tb3+-dopedCaMoO4 are slightly shifted to higher angles with respect toJCPDS card no 29:0351 which suggest the substitution of Ca2+

(1.07 Å) sites by Tb3+ (0.923 Å) ions up to 3 atom% of Tb3+.The 500 and 900 °C annealed samples show relatively highercrystallinity than the as-prepared samples. The average crystallitesizes determined from the highest intense peak (112) using theScherrer formula for the as-prepared, 500 and 900 °C annealedsamples are found to be ∼18, 25 and 71 nm, respectively.

The theoretical density (Dtd) is calculated using eqn (2):

Dtd ¼ zM=NV ð2Þ

where z is the number of chemical formula units per unit cell,M is molecular weight of the compound, V is the unit cellvolume and N is Avogadro’s number. The Dtd values are foundto be 4.09, 4.13 and 4.18 g cm−3 for 0 (pure), 1 and 3 atom%Tb3+-doped CaMoO4, respectively. This indicates a slightenhancement of density with Tb3+-doping in CaMoO4. To findthe density of pure CaMoO4 we use the unit cell volume calcu-lated in our earlier studies.13a The experimental density of pureCaMoO4 (determined by Archimedes method) varies with temp-erature and a maximum value of 3.85 g cm−3 is observed.22

The FTIR spectra of the as-prepared, 500 and 900 °Cannealed nanoparticles of 3 atom% Tb3+-doped CaMoO4 in thewavenumber range of 400–4000 cm−1 are shown in Fig. 2.It can be seen that all samples show approximately similarvibrations. Group theoretical calculations show 26 modes ofvibrations for scheelite type structures, ΓTd = 3Au + 5Au + 5Bg +3Bu + 5Eg + 5Eu, where g modes are Raman active and four(Au and Eu) modes from five (Au and Eu) are IR active.19,23 Thebands at 1652 and 3419 cm−1 correspond to H–O–H bendingand O–H stretching vibrations of water molecules present on the

Fig. 1 XRD patterns of as-prepared, 500 and 900 °C annealed 3 atom%Tb3+-doped CaMoO4 along with JCPDS card nos. 29-0351 (tetragonalstructure).

Fig. 2 FTIR spectra of as-prepared, 500 and 900 °C annealed 3 atom%Tb3+-doped CaMoO4 samples.

11034 | Dalton Trans., 2012, 41, 11032–11045 This journal is © The Royal Society of Chemistry 2012

surface of nanoparticles, respectively.24,25 The absorption inten-sities of these peaks decrease on annealing the samples at 500and 900 °C. The band at 805 cm−1 is due to the O–Mo–O asym-metric stretching vibration of the MoO4

2− tetrahedron and443 cm−1 corresponds to the stretching vibration of Mo–O(Au mode).26–28 It is found that both the O–Mo–O stretchingvibration and Mo–O vibration are shifted slightly to lower wave-number by ∼3–5 cm−1 (i.e., red shift) on annealing the samplesat 500 and 900 °C. The red shift of Au is responsible for latticeexpansion of CaMoO4. Similar behaviour was observed bySu et al.29 in the case of CaWO4 nanoparticles. The FWHMvalue at 805 cm−1 decreases with heat treatment. The 1383 cm−1

vibration is due to the N–O band from HNO3 used in the samplepreparation. In the as-prepared sample peaks are observed at2853 and 2924 cm−1 indicating C–H stretching vibration fromEG molecules on the surface of CaMoO4:Tb

3+ nanoparticles.In our earlier study, high-resolution transmission electron

microscopy (HRTEM) measurements show that as-preparedCaMoO4 nanoparticles were spherical in shape having diameterin the range of ∼25–30 nm with uniform distribution.13a The900 °C annealed samples have average particle size of∼70–80 nm.

3.2 Photoluminescence study

Fig. 3a shows the excitation spectra of as-prepared Tb3+-dopedCaMoO4 samples (Tb3+ = 1, 3, 5, 7, 10, 15 and 20 atom%),

monitored at 547 nm emission. The f–f transitions of Tb3+

around ∼322 (7F6 → 5D1), 345 (7F6 → 5L8), 355 (7F6 → 5G5),363 (7F6 → 5L10), 373 (7F6 → 5G6) and 382 (7F6 → 5D3) nmalong with Mo–O charge-transfer band (CTB) at 250–290 nmare clearly observed (charge transfer from O2− ligand toMo6− ion in MoO4

2−).30,31a,32 We find that the small hump ofTb–O charge transfer band near ∼250 nm is merged with anMo–O charge-transfer band. The exact positions of Tb–Ocharge-transfer band and 4f–5d transitions of Tb3+ is not clearbecause the latter also occurs at 250–310 nm.31b The intensity ofthe band at 250 nm decreases with addition of Tb3+ and this maybe related to phase segregation of Tb3+ ions. Another possibilityis that there is a decrease of CaMoO4 amount per a specificweight of sample after addition of Tb3+. On increasing Tb3+ con-centration, the intensity related to Mo–O CTB will decrease.In this view, the peak at 250 nm will be related to Mo–O CTB.All excitation peaks show similar behaviour with a high relativeintensity band at ∼250 nm. The peak intensities correspondingto Tb3+ are relatively low in the case of as-prepared samples andan expanded form of these peaks from the 900 °C annealedsample (3 atom% Tb3+) in the 320–390 nm region is shown asthe inset of Fig. 3a. The spectrum shows many sharp peaks forf–f transitions. The position of Mo–O charge-transfer band(CTB) is shifted to higher wavelength by ∼10–20 nm due toannealing of the samples at 500 and 900 °C than in as-preparedsamples. This can be explained as follows: the Mo–O CTB isdue to electronic transition when an electron from oxygen is

Fig. 3 (a) Excitation spectra (monitoring emission at 547 nm) of Tb3+-doped CaMoO4 (Tb3+ = 1, 3, 5, 7, 10, 15 and 20 atom%) of as-preparedsamples. Inset: expansion of excitation spectrum of 3 atom% Tb3+-doped CaMoO4 sample annealed at 900 °C in the range 320–390 nm. Lumines-cence spectra of as-prepared CaMoO4:Tb

3+ nanoparticles at (b) 250 and (c) 355 nm excitation. (d) Integrated area of the 5D4 → 7F5 transition at547 nm (Tb3+) as a function of Tb3+ ion concentration at different excitation wavelengths. Inset shows a photograph of 5 atom% Tb3+-doped CaMoO4

under 266 nm laser excitation (Nd-YAG).

This journal is © The Royal Society of Chemistry 2012 Dalton Trans., 2012, 41, 11032–11045 | 11035

transferred to Mo. In as-prepared samples, there are a relativelylarge number of dangling bonds over the particle surface and thelattice is less ordered as compared to that of heated samples. It isalso expected that there is a higher degree of ionic bondingbetween Mo and O for the as-prepared samples compared to thatfor heated samples, and this results in lower energy absorptionfor heated samples. Consequently, the position of the Mo–Ocharge-transfer band is shifted to higher wavelength by∼10–20 nm upon annealing of the samples at 500 and 900 °Ccompared with the as-prepared samples; similar observationshave been reported in V–O CTB.33a–c A significant improvementin peak intensity is observed in going from as-prepared toannealed samples. This is due to the decrease of non-radiativeprocess arising from –OH vibrations on the surface of the nano-particles and also lowered surface/volume ratio.

Fig. 3b and c show the emission spectra of Tb3+-dopedCaMoO4 (Tb3+ = 1, 3, 5, 7, 10, 15 and 20 atom%) as-preparedsamples by excitation at 250 (Mo–O CTB) and 355 nm (directTb3+). We find four characteristic emission peaks at 492,547, 586 and 621 nm corresponding to 5D4 → 7F6,

5D4 → 7F5,5D4 →

7F4 and5D4 →

7F3 transitions. The emission intensity ofthe 5D4 → 7F5 transition at 547 nm is highest among the othertransitions. The emission spectra of Tb3+-doped CaMoO4 under230, 240 and 322 excitation show similar behaviour (Fig. S2,see ESI†). The broad peaks at 360–480 nm due to contributionsfrom MoO4

2− and 5D3 → 7FJ transitions of Tb3+ are relativelylow in intensity but both fall in the same region. On annealingthe sample at higher temperature, the intensity of broad band at360–480 nm under 322 nm excitation decreases due to decreaseof lattice defects and thus energy transfer from host/MoO4

2− toTb3+ increases and Tb3+ emission intensity is enhanced.

The change in luminescence intensity of Tb3+-doped CaMoO4

nanoparticles with Tb3+ ion concentration has been studiedby fitting the area under emission peaks of 5D4 →

7FJ transitions(J = 5 and 6) with a Gaussian distribution function, eqn (3):

I ¼ I0 þX2i¼1

Ai

wi

ffiffiffiffiffiffiffiffiπ=2

p e2ðλ�λciÞ2=w2i ð3Þ

where I is the observed intensity, I0 is the background intensity,wi the FWHM of the curve, Ai the area under the curve, λ thewavelength and λci the mean value corresponding to the tran-sition. The 5D4 → 7FJ transitions (J = 6 and 5) were fittedbetween 478 and 570 nm. All fittings were carried out afterremoving the background (the background intensity (I0) mayarise from emission intensity of the host CaMoO4 and scatteringof particles after excitation falls on particles. The typical fittingfor 3 atom% Tb3+-doped CaMoO4 annealed at 900 °C under355 nm excitation is shown in Fig. S3 (see ESI†).

The emission intensities of 5D4 → 7FJ transitions (J = 6 and5) (492 and 547 nm) for as-prepared samples are maximumunder 250 nm excitation and decrease in following order 240,230, 322 and 355 nm excitation. The change in emission inten-sity of the 5D4 →

7F5 transition (547 nm) with Tb3+ ion concen-tration (Tb3+ = 1, 3, 5, 7, 10, 15 and 20 atom%) at differentexcitation wavelengths is shown in Fig. 3d and that of the 5D4 →7F6 transition (492 nm) is also shown in Fig. S4 (see ESI†). Theintensity decreases with Tb3+ ion concentration. The inset ofFig. 3d shows the digital photograph of 5 atom% Tb3+-doped

CaMoO4 under 266 nm laser excitation (Nd-YAG); an intensegreen light is observed.

Fig. 4a and b show the emission spectra of Tb3+-dopedCaMoO4 annealed at 500 °C under excitation wavelengths at250 and 355 nm. The emission spectra at 230, 240 and 322 nmexcitation are shown in Fig. S5a–c (see ESI†). The 5D4 → 7F5transition at 547 nm (green emission) shows high luminescenceintensity. The integrated areas under 5D4 →

7FJ transitions (J = 6and 5) decrease with increase of Tb3+ ion concentrations (Tb3+ =1, 3, 5, 7, 10, 15 and 20 atom%), which are shown in Fig. 4cand Fig. S5d (see ESI†), respectively. The luminescence inten-sity is enhanced significantly (∼15 times) by annealing at500 °C, which is related to increase in crystallite size and alsodecrease in non-radiative processes. There is a trend of decreaseof intensity with increase of Tb3+ ion concentration which is thesame as for as-prepared samples.

The emission spectra of Tb3+-doped CaMoO4 (Tb3+ = 1, 3, 5,

7, 10, 15 and 20 atom%) annealed at 900 °C samples are shownin Fig. 5a and b for 250 and 355 nm excitation. The emissionspectra at 230, 240 and 322 nm excitation are shown inFig. S6a–c (see ESI†). It is observed that the luminescence inten-sity increases with Tb3+ ion concentration up to 3 atom% andthen decreases with further increase in Tb3+ ion concentration(Fig. 5c). The luminescence intensity of the 900 °C annealedsample is twice that of the 500 °C annealed sample and 30 timesthat of the as-prepared sample. As compared to the as-preparedand 500 °C annealed samples, 900 °C annealed samples with1–10 atom% of Tb3+ concentration have high luminescenceintensity. Improvement of luminescence is related to the extentof decrease of non-radiative rate arising from O–H vibrationspresent on the surface of the particles and decreased surface/volume ratio after annealing at higher temperatures. The emis-sion peaks corresponding to 5D3 → 7FJ between 360 and480 nm could be observed in case of as-prepared, 500 and900 °C annealed samples under 250 nm excitation. This meansthat the gap between 5D3 and

7FJ levels could not be bridged bymultiphonon relaxation such as O–H and Mo–O–Mo. Interest-ingly, a broad emission peak due to MoO4

2− at 360–480 nmcould be observed in the same region. However, there isimproved luminescence intensity with annealing at highertemperature.

Amongst transitions 5D3,4 → 7FJ, two most prominent tran-sitions are 5D4→

7F6 and 5D4 → 7F5. The variation in intensityratio (R, eqn (4)) of these two transitions will be useful in under-standing the role of quenching from surrounding medium (O–H,dangling bonds) and concentration quenching.

R ¼Ð 508478 I2 dλÐ 570532 I1 dλ

ð4Þ

The intensities of 3–7 atom% Tb3+-doped 900 °C annealedsamples are saturated in both emission and excitation spectrawith 5 nm slit width (not shown here). By assuming the R andFWHM values do not change too much, we have calculated theiremission spectra at 2.5 nm slit width of spectrometer. For theremaining samples emission spectra are recorded with 5 nm slitwidth. The average R values of 3 atom% Tb3+-doped CaMoO4

are found to be ∼0.4, 0.37 and 0.52 for as-prepared, 500 and900 °C samples, respectively, for 250 nm excitation. The

11036 | Dalton Trans., 2012, 41, 11032–11045 This journal is © The Royal Society of Chemistry 2012

variations of R values of these samples under 230, 240, 250, 322and 355 nm excitation with different Tb3+ ion concentrations areshown in Fig. 6a–c. Values increase slightly with Tb3+ ion con-centration in the case of as-prepared and 500 °C annealedsamples, whereas it increases up to 3 atom% and then decreaseswith further increase of Tb3+ ion concentration for 900 °Cannealed samples. The R values are found to be in the range of

∼0.3–0.6 for as-prepared, 500 and 900 °C annealed Tb3+-dopedCaMoO4 samples. Overall, the R value increases on annealingbecause of decrease of non-radiative rate. To the best of ourknowledge this has not been discussed in earlier studies.Reported R values for Tb3+ (0.5–5 wt%) doped Y2O3 werefound to be in the range of 0.15–0.5.33d

Fig. 5 Luminescence spectra of 900 °C annealed CaMoO4:Tb3+

(Tb3+ = 1, 3, 5, 7, 10, 15 and 20 atom%) nanoparticles at (a) 250 and(b) 355 nm excitation. (c) Integrated area under 5D4 →

7F5 transition asa function of Tb3+ concentration at 230, 240, 250, 322 and 355 nm exci-tation. It is to be noted that for the samples of 3–7 atom% Tb3+-dopedCaMoO4 annealed at 900 °C the emission intensities are saturated at5 nm slit width of excitation and emission windows.

Fig. 4 Luminescence spectra of 500 °C annealed CaMoO4:Tb3+

(Tb3+ = 1, 3, 5, 7, 10, 15 and 20 atom%) nanoparticles at (a) 250 and(b) 355 nm excitation. (c) Integrated area under the 5D4 →

7F5 transitionas a function of Tb3+ ion concentration at 230, 240, 250, 322 and355 nm excitation.

This journal is © The Royal Society of Chemistry 2012 Dalton Trans., 2012, 41, 11032–11045 | 11037

It is observed that the peak positions of 5D4 →7FJ transitions

(J = 6 and 5) are slightly shifted to higher wavelength with Tb3+

ion concentration in the case of as-prepared and 500 °C annealedsamples. However, the peak positions are almost unaffected byTb3+ ion concentrations for 900 °C annealed samples (Fig. S7,see ESI†). The FWHM of 5D4 → 7F5 transition (547 nm) ishigher than that of 5D4 → 7F5 transition (492 nm) slightly.However, the slight increase in FWHM does not affect the Rvalue. The change in FWHM of the 5D4 → 7F5 transition withdifferent Tb3+ ion concentrations under 230, 240, 250, 322 and355 nm excitation is shown in Fig. 6d–f and of the 5D4 → 7F6transition is shown in Fig. S8 (see ESI†). The FWHM of 5D4 →7F5 transition at 547 nm varies between ∼8–10 nm for as-pre-pared, 500 °C annealed and 900 °C annealed samples. For atypical case of 1 atom% Tb3+-doped CaMoO4, the FWHMvalues of the 5D4 → 7F5 transition at 547 nm are found to be∼8.4, 8.3 and 8.1 nm for as-prepared, 500 and 900 °C samples,respectively. It is also observed that the FWHM is larger for 250,230 and 240 nm excitation than that for 355 nm excitation at

higher concentration (15 and 20 atom%) of Tb3+ in as-preparedand 500 °C annealed samples, but their FWHM values are verysimilar in the case of 900 °C annealed samples. The first to thirdexcitation wavelengths correspond to the excitation throughMo–O charge-transfer. Excitation at 355 nm corresponds to thedirect excitation of Tb3+.

The distance (RTb–Tb) between Tb–Tb ions for different Tb3+

concentrations can be calculated by using eqn (5):34

RTb�Tb ¼ 23V

4π xN

� �1=3ð5Þ

where V is the unit cell volume, N is the number of dopant sitesavailable in the unit cell and x is the doping concentration(atom%). RTb–Tb values are found to be ∼5.3 and 5.29 Åfor 1 and 3 atom% Tb3+-doped CaMoO4, respectively. TheRDy–Dy calculated by Li et al.35a in Dy3+-doped CaMoO4 was7.2 Å.

Fig. 6 R values for (a) as-prepared (b) 500 and (c) 900 °C annealed samples at 230, 240, 250, 322 and 355 nm excitation with correspondingFWHM at 547 nm shown in (d), (e) and (f ), respectively, for CaMoO4:Tb

3+ (Tb3+ = 1, 3, 5, 7, 10, 15 and 20 atom%).

11038 | Dalton Trans., 2012, 41, 11032–11045 This journal is © The Royal Society of Chemistry 2012

3.3 Core–shell formation study

Fig. 7 shows the luminescence properties of as-prepared core(CaMoO4:Tb

3+) and core–shell (CaMoO4:Tb3+@CaMoO4)

nanoparticles under excitation at 275 nm. It is found that theluminescence intensity of core–shell nanoparticles is enhancedby ∼5.4 times as compared to that of core nanoparticles.Improved luminescence is due to the extent of decrease of non-radiative rate caused by the surface dangling bonds and cappingagent.

3.4 Lifetime study

Fig. 8–10 show the luminescence decay curves for 5D4 level ofTb3+ for as-prepared, 500 and 900 °C annealed Tb3+ ion dopedCaMoO4 (Tb

3+ = 1, 3, 5, 10 and 20 atom%) nanoparticles under250 and 355 nm excitation. The emission wavelength was fixedat 547 nm. All decay curves are fitted by using mono- (eqn (6))and biexponential decay curves (eqn (7)):

I ¼ I0e�t=τ ðmonoexponential decayÞ ð6Þ

I ¼ I1e�t=τ1 þ I2e

�t=τ2 ðbiexponential decayÞ ð7Þwhere I0 is the intensity at time t = 0 and I1 and I2 are the intensi-ties at different time intervals, and their corresponding lifetimesare τ1 and τ2. Since the particles are spherical,13a the sphere canbe divided into two equal volumes (i.e., inner core covered withshell). Based on this, the average lifetime τav can be calculatedusing eqn (8):35b,c

τav ¼ I1τ21 þ I2τ22I1τ1 þ I2τ2

ð8Þ

It is found that the intensity of decay counts decreases withTb3+ ion concentration. The fitting parameters including good-ness of fit (χ2) were calculated for as-prepared, 500 and 900 °Cannealed samples using mono- exponential and bi-exponentialdecay equations. A typical plot of fitting for I vs. time for500 °C annealed 3 atom% Tb3+-doped CaMoO4 sample using

monoexponential decay is shown in Fig. 11a and its inset showsthe ln(I) vs. t plot. The χ2 value for monoexponential fitting wasfound to be ∼12 which is clearly too high. In fact this valuevaries between ∼1.26–27.4 for as-prepared and 500 and 900 °Cannealed samples using a monoexponential equation (Table S1,see ESI†). Therefore, the decay data were fitted using a biexpo-nential equation (Fig. 11b).

Biexponential behaviour is related to an inhomogeneous dis-tribution of activators (Tb3+) in the host (CaMoO4) under theassumption of no other extra phases. Values of τ1 (%) and τ2 (%)vary with the paths of decay or excitation energy. The fittingparameters (I1(%), τ1, I2(%), τ2, τav, χ2) calculated using thebiexponential equation are summarized in Table 1. For a typicalbiexponential fitting to the decay curve of 5 atom% Tb3+-dopedCaMoO4 annealed at 500 °C under 250 nm excitation, lifetimevalues of 152 μs (τ1) and 485 μs (τ2) are obtained with anaverage lifetime of 333 μs (τ) with goodness of fitting value1.637 (χ2). The reported lifetime value of 5 atom% Tb3+-dopedCaMoO4 was 459 μs.36 The τ1 from 250 nm excitation has a68% contribution in average lifetime value while τ1 from355 nm excitation has maximum contribution (88%) in averagelifetime value in as-prepared 5 atom% Tb3+-doped CaMoO4. Wealso find similar behaviour in 500 °C annealed samples. For a900 °C annealed sample, τ1 from 250 nm excitation has a 99%contribution in average lifetime value while τ1 from 355 nm

Fig. 7 Emission spectra of core and core–shell as-prepared samples ofCaMoO4:Tb

3+ (3 atom%). This experiment is carried out using an Edin-burgh instrument F920 with slit width of 3 nm using 275 nm excitation.

Fig. 8 (a, b) Luminescence decay curve (λem = 547 nm) of as-preparedTb3+-doped CaMoO4 samples (Tb3+ = 1, 3, 5, 10 and 20 atom%) nano-particles at (a) 250 and (b) 355 nm excitation.

This journal is © The Royal Society of Chemistry 2012 Dalton Trans., 2012, 41, 11032–11045 | 11039

excitation has a 57% contribution in average lifetime value in5 atom% Tb3+-doped CaMoO4. This implies that τ1 has highcontribution from 250 nm excitation which arises from a moresignificant energy transfer (ET) process from Mo–O and/or Tb–OCT band to Tb3+ in 900 °C annealed samples than as-preparedand 500 °C annealed samples. Shorter τ1 from 355 nm excitationarises from either surface Tb3+ and/or due to inhomogeneitywhereas longer lifetime τ1 from 250 nm excitation arises fromcombination of ET from Mo–O CT to Tb3+ and surface Tb3+ ofparticles in cases of as-prepared and 500 °C annealed samples.

Overall, τ1 and τ2 values increase with change of excitationenergy from 355 to 250 nm. On excitation at 355 nm, excitedphotons at 5G5 come to the 5D4 level non-radiatively throughphonon relaxation. From the 5D4 level, decay starts radiatively aswell as non-radiatively. Up to 1–3 atom% Tb3+ (no extraphases), decays of the 5D4 level of Tb

3+ at the core and surfaceof the particle are different and thus biexponential decay behav-iour occurs. On the contrary, excitation at 250 nm generates ahigh number of excited photons because of the high absorptioncross-section at Mo–O/Tb–O CTB. De-excitation occurs and arelatively large number of photons come to 5D4 level non-radia-tively as compared to that of 355 nm excitation. Since there is alarge energy gap between that at 250 nm excitation and the 5D4

level, energy transfer process is of non-radiative type through

multiphonon relaxation. A relatively large number of excitedphotons at 5D4 level decay to the ground 7FJ, but their paths ofdecay vary at the core and surface of the particle.

The lifetime (τ2) does not change its value significantly foreither 250 and 355 nm excitation so indicating contribution fromcore Tb3+. The average lifetime (τav) values for the

5D4 level ofas-prepared Tb3+-doped CaMoO4 are ∼896, 726, 597, 539 and411 μs for 1, 3, 5, 10 and 20 atom% Tb3+ ion concentrations,respectively under 250 nm excitation. However, the average life-time is more for 250 nm excitation than 355 nm excitation in thecase of as-prepared and 500 °C annealed samples because of ETfrom the Mo–O charge-transfer band to Tb3+ at 250 nm exci-tation. This is also reflected in excitation and emission studies(Fig. 3 and 4). In 900 °C annealed samples, the lifetime (τ1)does not change significantly at both 250 and 355 nm excitation,but their values are higher than that of as-prepared or 500 °Cannealed samples. This is due to extent of reduction of non-radiative rate with annealing. It is observed that lifetime valuedecreases with increasing Tb3+ ion concentration. This is due toa concentration quenching effect. It is to be noted that 900 °Cannealed samples show a better fit with mono- rather thanbiexponential behavior. This is related to a decrease of thesurface effect for 900 °C annealed samples and thus, the corecontribution is more favourable (Table S1, see ESI†).

Fig. 9 (a, b) Luminescence decay curve (λem = 547 nm) of 500 °Cannealed Tb3+-doped CaMoO4 samples (Tb3+ = 1, 3, 5, 10 and 20atom%) nanoparticles at 250 and 355 nm excitation.

Fig. 10 (a, b) Luminescence decay curve (λem = 547 nm) of 900 °Cannealed Tb3+-doped CaMoO4 samples (Tb3+ = 1, 3, 5, 10 and 20atom%) nanoparticles at (a) 250 and (b) 355 nm excitation.

11040 | Dalton Trans., 2012, 41, 11032–11045 This journal is © The Royal Society of Chemistry 2012

The concentration quenching is due to cross-relaxation amongTb3+ ions. This can be expressed as:

5D3;7 F6 !5 D4;

7 F0

With increasing Tb3+ concentration, the emission peak inten-sity from the 5D3 level decreases due to cross relaxation. Theaverage lifetime increases significantly with heat treatment,which is due to decrease of non-radiative processes arising fromwater molecules on the surface of nanoparticles and surface dan-gling bonds. Also, contribution of non-radiative relaxation fromO–H (3410 cm−1) is reduced with increasing heat-treatmenttemperature.

According to Inokuti and Hirayama37 the luminescence decayfunction follows the following behaviour (eqn (9) and (10)):

I ¼ I0 exp�t

τ0� α

t

τ0

� �3=S" #

ð9Þ

α ¼ Γ 1� 3

S

� �c

c0ð10Þ

where c is the acceptor concentration, c0 is the critical transferconcentration and S = 6, 8 and 10 correspond to dipole–dipole,dipole–quadrupole, and quadrupole–quadrupole interaction.

It is to be noted that the lowest concentration of acceptor willshow monoexponential decay. When cross-relaxation amongacceptor ions is present, non-exponential decay behaviour isseen at the initial stage and after that it will be followed bymonoexponential decay for direct excitation of Tb3+ (i.e.355 nm). Also, c0 can be related to donor–accepter energy trans-fer rate (R0) as follows (eqn (11)):37

R0 ¼ 7:346c0�1=3 ð11Þ

Eqn (9) is modified and a graph plotted between [ln(I/I0) +t/τ0] vs. (t)

3/S for S = 6, 8 and 10. Fig. 12 shows [ln(I/I0) + t/τ0]vs. (t)3/S for luminescence decay curve data of 3 atom% Tb3+-doped CaMoO4 of the 900 °C annealed sample. This equationfits well the curve for with S = 6 (χ2 = 0.9821, in the range 3–21on [ln(I/I0) + (t/τ0)] scale). This implies that the fitting follows adipole–dipole interaction in which the transfer rate is pro-portional to the inverse sixth power of donor–acceptor distance.In the initial stage of decay it does not follow straight line behav-iour due to cross relaxation between donor and acceptor. Thecritical transfer distance value calculated from eqn (10) and (11)is found to be ∼6.8 Å for 3 atom% Tb3+-doped CaMoO4

annealed at 900 °C, which is slightly higher than the value cal-culated from XRD analysis (i.e., 5.29 Å). This difference may bedue to the non-inclusion of the donor–donor diffusion parameterduring the calculation.

Notably, the decay follows non-exponential behavior in thecase of the energy transfer process from sensitizer to activator.On excitation at 250 nm, emission occurs at 545 nm after energytransfer from Mo–O CTB to the excited state of Tb3+. Diffusionprocess amongst excited states occurs. After this consideration,the decay of emission intensity at 250 nm excitation can be fittedusing eqn (12):

I ¼ I0eð�t=τ�Dt0:5Þ ð12Þ

where D is related to diffusion and energy transfer. Here, wehave taken one example. After to fitting to data of as-preparedsample (1 atom% Tb3+), I0, τ and D are found to be 794 024counts, 1.546 ms and 1.92 s−0.5, respectively. The goodness ofparameter is found to be 0.9994, which is slightly less than thatfrom a biexponential fit (eqn (7)) to same data (0.9997). Here,the origin 6.1 program was used to fit the equation. In ouropinion, it is difficult to say the fit is better after biexponentialfitting because there are four parameters for the biexponential fitand three parameters for the non-exponential fit.

There is a possible contribution in lifetime value of Tb3+ fromhost emission. In order to minimize this, decay is recorded at586 nm emission and at 250 nm excitation. The host contributionis much reduced in 900 °C heated samples. Fig. 13 shows thedecay curves for 900 °C annealed samples (1, 5 and 7 atom%Tb3+) under excitation at 250 nm and emission at 586 nm.However, the intensity is weaker than that for emission at547 nm (Fig. 10) and thus data were normalized in order tosee the behaviour of the decay; τ1 (%) and τ2 (%) values for1 atom% Tb3+-doped CaMoO4 are 653 (50%) and 1196 (50%),respectively; and 455 (17%) and 644 (83%), respectively for

Fig. 11 (a) Mono- and (b) bi-exponential fittings to luminescencedecay curve (547 nm) of 500 °C annealed 3 atom% Tb3+-dopedCaMoO4 nanoparticles (λex = 250 nm). Fitting parameters are shown inthe figures. Inset of (a) shows the ln(I) vs. t plot.

This journal is © The Royal Society of Chemistry 2012 Dalton Trans., 2012, 41, 11032–11045 | 11041

5 atom% Tb3+-doped CaMoO4. These values are different fromthat monitoring emission wavelength at 547 nm (Table 1). Giventhat the intensity at 586 nm is very much lower than that at

547 nm, the decay process will be different when monitoringemission wavelength changes from 547 to 586 nm. The averagelifetime values for 1, 5 and 7 atom% Tb3+-doped CaMoO4

Table 1 Parameters obtained in the biexponential equation fitting to decay data of as-prepared, 500 and 900 °C annealed samples at 250 and 355 nmexcitation

Sample λexc/nm Tb3+ (atom%) I1 (%) τ1/μs I2 (%) τ2/μs τava/μs χ2 b

As-prepared 250 1 54 511 46 1102 896 2.5423 72 517 28 1000 726 1.6615 68 320 32 821 597 1.979

10 58 198 42 679 539 2.41120 90 39 10 625 411 2.106

355 1 98 8 2 793 546 0.7363 89 9 11 732 667 1.135 88 10 12 522 455 0.962

10 83 26 17 326 239 1.01420 99 25 1 310 60 0.521

500 °C 250 1 61 428 39 1207 932 2.4673 70 219 30 762 547 2.1875 73 152 27 485 333 1.637

10 72 144 28 509 357 1.94220 74 65 26 277 192 1.714

355 1 96 9 4 961 763 1.363 84 30 16 589 471 1.4235 84 17 16 351 281 1.046

10 87 15 13 344 270 1.21720 94 13 6 204 108 0.572

900 °C 250 1 93 14 7 861 701 1.2073 98 607 2 1252 631 1.1845 99 585 1 1290 606 1.118

10 98 568 2 1117 590 1.11220 85 149 15 432 243 0.9

355 1 96 10 4 1119 906 0.7913 88 634 12 1055 710 0.9825 57 554 43 830 699 0.893

10 70 574 30 880 694 0.89120 93 159 7 378 194 0.459

a τav = (I1τ12 + I2τ2

2)/(I1τ1 + I2τ2).b χ2 = ∑kwk

2[Xk − Fk]2/n, χ2 is goodness of fitting, wk is weighting factor for data points (wk ¼ 1=

ffiffiffiffiffiFk

p), Xk is the

calculated lifetime and Fk is the measured lifetime data.

Fig. 12 Plot of [ln(I/I0) + t/τ0] vs. (t)3/S for decay curve (547 nm) of 3

atom% Tb3+-doped CaMoO4 of 900 °C annealed samples for differentvalues of S = 6, 8 and 10 corresponding to dipole–dipole, dipole–quad-rupole and quadrupole–quadrupole interaction (λex = 355 nm).

Fig. 13 Lifetime decay curve (586 nm emission) of 900 °C annealedTb3+-doped CaMoO4 samples (Tb3+ = 1, 5 and 7 atom%) nanoparticlesat 250 nm excitation.

11042 | Dalton Trans., 2012, 41, 11032–11045 This journal is © The Royal Society of Chemistry 2012

samples are 1004, 621 and 623, respectively. For excitation at355 nm, the intensity is very weak and these are not shown here.

3.5 CIE chromaticity study

The CIE coordinates of as-prepared, 500 and 900 °C annealed5 atom% Tb3+-doped CaMoO4 samples under 250 nm excitationare (0.28, 0.32), (0.22, 0.28) and (0.25, 0.52), respectively.These belong to the bluish to green regions. Some reported CIEcoordinate values of 5 mol% Tb3+-doped CaMoO4 on excitationat 254 nm and 5 mol% Tb3+-doped Y2O3 on excitation at264 nm were found to be (0.25, 0.59) and (0.34, 0.47), respec-tively.7,38 The CIE co-ordinates for CaMoO4:Tb

3+@CaMoO4

core–shell nanoparticles are found to be (0.25, 0.46) at 275 nmexcitation.

3.6 Dispersion and polymer film studies

The presence of EG molecules capped on the surface of as-prepared CaMoO4 samples is useful to easy dispersion ofnanoparticles in polar solvents such methanol. For a typical dis-persion, 10 mg of as-prepared 3 atom% Tb3+-doped CaMoO4

was dispersed in 5 ml of methanol followed by ultrasonication.

Well dispersed nanoparticles in methanol could be verified up to∼60 days without settlement. This could be due to hydrogenbonding between MoO4

2− units and/or EG and –OH of metha-nol. Fig. 14a shows emission spectra of re-dispersed as-prepared3 atom% Tb3+-doped CaMoO4 at 250 nm excitation. The 5D4 →7F5 transition shows the highest intensity. It is found that peakpositions of 5D4 →

7FJ transitions (J = 6 and 5) are unaffected inthis process. However, the R value increases (0.34 to 0.38 for3 atom% Tb3+-doped CaMoO4) when the powder sample is dis-persed in methanol. Under exposure of laser source at 266 nm,dark green light is observed (Fig. 15). Further, to make a thinfilm of 3 atom% Tb3+-doped CaMoO4 re-dispersed in poly(vinylalcohol) (PVA), 10 mg of as-prepared sample was mixed with1 g of PVA dissolved in 2.5 ml of distilled water and 2.5 ml ofmethanol was added. The solution was then ultrasonicated for120 s to make a uniform dispersion. This solution was placedover a glass slide using a micropipette and then kept for 4 daysat room temperature for drying, and polymer films of 3 atom%Tb3+-doped CaMoO4 having thickness ∼0.2–0.3 mm were thusprepared. It is also found that addition of borax during the pre-paration of film increases the polymerization. The film showsvery bright uniform luminescence under 266 nm excitation.Fig. 14b shows the emission spectra of CaMoO4:Tb film in PVAafter excitation at 250 nm. It is found that peak positions of5D4 → 7FJ transitions (J = 6 and 5) are unaffected due to dis-persion of nanoparticles in PVA polymer matrix. This film canbe used for the development of optoelectronic devices.

3.7 Quantitative measurement

Variation in emission intensity (Fig. 3–5) is qualitative. In orderto perform quantitative measurement, absolute measurement(based on integrated sphere) was carried out and a detail

Fig. 14 Luminescence spectra of as-prepared 3 atom% dopedCaMoO4 nanoparticles: (a) dispersed particles in methanol and (b) PVAthin film after incorporation of re-dispersed particles at 250 nmexcitation.

Fig. 15 Digital photographs of pellet, PVA film and dispersed particlesin cuvette (3 atom% Tb3+-doped CaMoO4). Green light is observedunder 266 nm excitation using a Nd-YAG laser.

This journal is © The Royal Society of Chemistry 2012 Dalton Trans., 2012, 41, 11032–11045 | 11043

experimental procedure is described in our previous work.33b

The dispersibility of 500 or 900 °C heated samples is poor ascompared to the as-prepared samples and thus these are not per-formed. About 1 mg of as-prepared sample was dispersed in1 ml of absolute ethanol and we observed no decantation of dis-persed particles for a week. Excitation and emission wavelengthswere fixed at 250 and 545 nm, respectively, with slit widths forexcitation and emission fixed at 3 and 0.7 nm, respectively (irisis kept at 90%, step size is 0.1 nm, dwell time is fixed at 0.3 s).Standard Rhodamine 6G in ethanol was used as calibrant.Fig. 16 shows the emission spectra of as-prepared samples of1 and 5 atom% Tb3+-doped CaMoO4 samples. The intensity wasfound to decrease with increase of Tb3+ concentration and thebehaviour was similar to Fig. 3. Quantum yields of as-preparedsamples of 1, 5 and 7 atom% Tb3+-doped CaMoO4 samples arefound to be 10, 3 and 2, respectively.

4. Conclusions

Well-crystalline nanoparticles of Tb3+-doped CaMoO4 have beenprepared at 150 °C using urea hydrolysis in ethylene glycol.As-prepared samples were annealed at 500 and 900 °C to gethigher crystallinity. The synthesised nanophosphors showtetragonal phase structure. Tb3+ can substitute Ca2+ sites up to3 atom% and above this concentration, extra phases are found.The luminescence intensity increases with Tb3+ up to 3 atom%and decreases with further increase of Tb3+. Decrease of lumi-nescence with increasing Tb3+ concentrations is related to a con-centration quenching effect. Luminescence intensity improveswith heat-treatment due to reduction in non-radiative processesfrom surface and O–H groups. From FTIR study, a vibration at805 cm−1 is observed confirming the presence of O–Mo–Oasymmetric stretching vibration of MoO4

2− tetrahedra and443 cm−1 corresponding to the stretching vibration of Mo–O.The luminescence decay studies on direct excitation at 355 nmand indirect excitation at 250 nm can distinguish the energytransfer from Mo–O CTB to Tb3+ and surface contribution to

lifetime. Both luminescence intensity and lifetime value areimproved after annealing the as-prepared samples at 500 and900 °C. The intensity ratio R ([I[5D4 → 7F6]/I[

5D4 → 7F5]) isfound to be in the range of 0.3–0.6. The CIE coordinates ofas-prepared, 500 and 900 °C annealed 5 atom% Tb3+-dopedCaMoO4 samples under 250 nm excitation are (0.28, 0.32),(0.22, 0.28) and (0.25, 0.52), respectively. The as-prepared nano-particles are easily dispersed in polar solvents (methanol) anddispersed particles are incorporated in PVA for formation ofpolymer based films. Both dispersed particles and PVA filmshow green emission. Luminescence emission intensity isimproved significantly by core–shell formation.

Acknowledgements

A. K. Parchur acknowledges the financial support provided byUniversity Grants Commission, Govt. of India through theD. S. Kothari Post Doctoral Fellowship. Authors thankDr T. Mukherjee, Chemistry Group, Dr D. Das and DrR. K. Vatsa, Chemistry Division, BARC for their support andencouragement during this work.

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