Structural, optical and photoluminescence properties of Eu3 + doped ZnO nanoparticles

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 182 (2017) 42–49

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Structural, optical and photoluminescence properties of Eu3 + doped ZnO nanoparticles Odireleng Martin Ntwaeaborwa d,⁎, Sefako J. Mofokeng a, Vinod Kumar b, Robin E. Kroon c a

Department of Physics, CSET, University of South Africa, Johannesburg ZA1710, South Africa Photovoltaic Laboratory, Centre for Energy Studies, Indian Institute of Technology Delhi, New Delhi 110016, India Department of Physics, University of the Free State, Bloemfontein ZA9300, South Africa d School of Physics, University of the Witwatersrand, Private Bag 3, Johannesburg ZA2050, South Africa b c

a r t i c l e

i n f o

Article history: Received 23 September 2016 Received in revised form 16 March 2017 Accepted 30 March 2017 Available online 01 April 2017 Keywords: ZnO Nanoparticles (NPs) Co-precipitation Defects Concentration quenching

a b s t r a c t The structure, particle morphology and luminescent properties of europium (Eu3+) doped ZnO nanoparticles (NPs) prepared by co-precipitation method are discussed. When excited using a 325 nm He-Cd laser, undoped ZnO NPs exhibited weakly the well-known ultraviolet excitonic recombination emission (at ~ 384 nm) and strongly broad band visible emissions associated with defects (at ~600 nm). In addition, the ZnO NPs exhibited green emission at ~600 nm associated with defects when excited using a monochromatized xenon lamp. Upon Eu3+ doping line emissions attributed to 5D0 → 7F1,2,3,4 transitions of Eu3+ ions were observed when the materials were excited using a monochromatized xenon lamp. The exchange interaction mechanism is identified as the cause for concentration quenching of the luminescence of Eu3+ doped ZnO NPs in this study. © 2017 Published by Elsevier B.V.

1. Introduction Synthesis of luminescent semiconducting metal oxide materials has received much attention in the field of research for different technological and optoelectronics applications [1]. In particular, ZnO is a fascinating semiconductor material because it has been extensively explored for light-emitting devices such as light emitting diodes (LEDs), improvement of power conversion efficiency of solar energy, gas sensors and many more applications [2–4] due to various properties that includes high transparency, high electron mobility and an intense room temperature luminescence [5,6]. It is well-known that ZnO have a wide band gap of about 3.37 eV and it is preferred over other semiconductors. However, its fast recombination of generated electron-hole pairs limits its usefulness. As a strategy to minimize this limitation, rare-earths ion doping has been adopted in order to increase the lifetime of charge carriers and subsequently reduce the recombination probability [7]. Since ZnO semiconductor can be easily nanostructured, its optical properties can also be tuned at nanosize scale because of high quantum confinement effects compared to bulk ZnO [3]. The relevant optical and chemical properties of ZnO such as electronic, luminescence and magnetic properties can be easily improved by incorporating relevant impurities into ZnO interstitially through doping and co-doping for a wider range of possible applications [8]. Rare earth (RE) ion doped ZnO ⁎ Corresponding author at: School of Physics, University of the Witwatersrand, Private Bag 3, Johannesburg ZA2050, South Africa. E-mail address: [email protected] (O.M. Ntwaeaborwa).

http://dx.doi.org/10.1016/j.saa.2017.03.067 1386-1425/© 2017 Published by Elsevier B.V.

semiconductors have generated a great interest compared to 3d transitions metal and non-metal transitions doping because of their f → f or f → d internal orbital transitions that give very intense emission lines in the ultraviolet (UV), visible and infrared regions [9,10]. Theoretical investigations revealed that europium (Eu3+) showed tremendous potential as a dopant in ZnO because it exhibits sharp emission in both the UV and visible range [11]. Luminescent properties of Eu3+ doped ZnO have gained interest due to their tremendous potentials in fiber lasers, plasma displays, solar cells and bio-imaging and in field emission display devices in general [12,13]. A complete emission colour display from ZnO:Eu3 + could be achieved because Eu3 + provide both stable chemical and physical properties in ZnO as dopant due to the free or bound excitonic state emissions of ZnO or the energy transfer taking place with the environment to the defects states in ZnO [14]. Many synthesis methods have been used to produce nanostructured ZnO such as nanoneedles, nanonails, nanobelts, nanorods and nanobowls. Examples of these methods are co-precipitation, sol-gel, combustion, solvothermal, microwave etc. [11,15]. Co-precipitaion synthesis is one of the preferred methods due to its advantages including ease of controlling parameters such as temperature, mixing rate, dopant concentrations, degree of alkalinity and acidity or pH of the reaction. By controlling these preparation parameters this method can produce materials with high purity and fine particle size at relatively moderate cost [15,16]. In this study, ZnO NPs doped with different molar concentrations of Eu3+ were synthesized using co-precipitation method. Their structure, morphology and optical properties were investigated by different

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Fig. 1. (a) XRD pattern for pure ZnO and Eu3+-doped ZnO nanoparticles with different concentration of Eu3+, (b) 1.0 mol% Eu3+-doped ZnO nanoparticle with standard files of Zn(OH)2 and Eu2O3 and (c) magnified view of (101) diffraction peak.

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characterization techniques. The influence of Eu3+ doping on the structure, particle morphology and optical properties of ZnO NPs were studied in detail. 2. Experimental Details 2.1. Preparation The ZnO:Eu3+ nanoparticulate phosphors were prepared by co-precipitation method using the analytical reagent, zinc acetate dehydrate Zn(CH3COO)2 H2O, sodium hydroxide pellets (NaOH) and europium nitrate pentahydrate Eu(NO3)3 5H2O. Deionized water was used as a solvent. All the analytical reagents utilized were purchased from Sigma Aldrich and they were used as-purchased without further purification. In a typical preparation, zinc acetate dehydrate and europium nitrate pentahydrate were dissolved in deionized water followed by continuous magnetic stirring for 30 min at room temperature. At the same time, sodium hydroxide pellets were dissolved in deionized water separately under magnetic stirring for 15 min. The solution containing NaOH was added drop-wise to the Zn(CH3COO)2 H2O solution maintaining the pH of the solution at 12. The resulting solution was further stirred for 1 h and the white precipitate was formed. The precipitate was centrifugally separated (at the speed of 5000 rpm for 15 min) and washed several times with deionized water to remove unwanted CHOO– and Na+ ions. The white precipitate was dried in an oven at 60 °C overnight. The precipitate was ground gently using a pestle and mortar. Nanopowders of ZnO and ZnO:Eu3+ powders were prepared. The Eu3+ concentration was varied from 0.2 to 1.0 mol%. 2.2. Characterization The crystal structure of the samples was examined using X-ray diffraction (Bruker D8 Advanced X-ray diffractometer) with monochromatic CuKα (λ = 0.15405 nm) radiation source in the 2θ scan range of 20–80°. The stretching frequency modes were studied with the help of Nicolet Fourier transformed infrared (FTIR) spectrometer. Jeol JSM7800F field emission scanning electron microscope (FE-SEM) coupled with energy dispersive spectrometer (EDS) was used to study particle morphology and chemical elemental composition of ZnO:Eu3 + NPs. The photoluminescence (PL) properties were measured at room temperature using either 325 nm He-Cd laser and monochromatized xenon lamp as the excitation source. The UV–vis reflectance studies were carried out using a Lambda 950 UV–vis spectrometer.

Fig. 2. Full width at half maximum (FWHM) and crystallite size for (101) diffraction peak for pure ZnO and Eu3+-doped ZnO nanoparticles with different molar concentration of Eu3+.

were introduced into Zn2+ interstitial sites are also playing a major role in the shifting of the diffraction peak positions [19,20]. The decrease in intensity of ZnO diffraction peaks indicate that the lattice distortion is induced at higher Eu3+ molar concentration and deteriorates the purity phase of crystallization of ZnO [21]. Narrowing of the diffraction peaks with an increase of Eu3+ concentration suggest that crystallinity was improving. The average crystallite size (D) of ZnO nanoparticles were estimated from prominent (101) diffraction peak using the Scherrer's equation [22]: D¼

0:89λ βcosθ

where D is the average crystallite size, λ the X-ray wavelength (0.15406 nm) used, θ the Bragg diffraction angle and β is a full width at half maximum (FWHM) of the diffraction peak. The crystallite size values of both undoped and Eu3+ doped ZnO nanoparticles are shown in Fig. 2. The crystallite size of undoped ZnO NPs is ~ 14 nm, which was shown to increase to ~35 nm with an increase in molar concentration of Eu3+. The lattice constants of ZnO nanoparticles were calculated using the (100) and (002) diffraction peaks by.

3. Results and Discussion 2

3.1. Structural and Morphological Properties Fig. 1(a) shows the XRD patterns of pure ZnO and ZnO:Eu3+ (with different molar concentration of Eu3+) nanoparticulate phosphors. All diffraction peaks of pure ZnO NPs occur at angles that agree well with the Joint Committee on Powder Diffraction Standard (JCPDS) card no. 36–1451 indicating the formation of hexagonal wurtzite structure of ZnO. Undoped ZnO patterns indicates no trace of any incidental impurities, implying the high purity phase of crystallized ZnO [17]. However, there were secondary phases associated with Eu2O3 (*) and Zn(OH)2 (+) as shown in Fig. 1(b). With an increase in Eu3+ molar concentration, three changes took place on the ZnO crystal structure (i) the intensity of diffraction peaks of Eu2O3 and Zn(OH)2 increases while that one of ZnO are decreasing (ii) the shifting on the peak position and (iii) the narrowing of peak width. An increase in intensity of diffraction peaks of secondary phases might be due to incomplete reaction on further addition of Eu3+ ions whereby most of Eu3+ ions segregate on the surface and small number of Eu3+ ions that were introduced into Zn2+ interstitial sites [18] because of it comparatively bigger ionic radius (Eu3+ ionic radius = 0.95 Å and Zn2+ ionic radius = 0.74 Å). The small amount that

ð1Þ

Sin θ ¼

 
a2  λ2 4
2 2 2 l h þ hk þ k þ 2 c 4a 3

ð2Þ

where θ is the Bragg diffraction angle, hkl are the Miller indices and a, b and c are lattice parameters and the values (a = b = 0.3254 nm and c = 0.5209 nm) are consistent with theoretical lattice constant values [23]. 2

4 h þ hk þ k ¼ 2 3 a2 d 1

2

!

2

þ

l c2

ð3Þ

Eq. (3) was derived from Eq. (2) by substituting sin2θ with Bragg's equation in order to calculate the d-spacing of ZnO nanoparticles and the measured d-spacings are quite reasonable when compared to dspacing from JCPDS no. 36-1451 (Table 1). Fig. 3(a–c) shows the FE-SEM micrographs of undoped and 1.0 mol% Eu3 + doped ZnO NPs. The morphology of undoped ZnO consists of agglomeration of flake-like nanostructures, which was shown to change to oval shape when the Eu3 + ions were incorporated as shown in Fig. 2(a–b). The morphological changes of ZnO due to incorporation of Eu3+ ions have been reported and attributed to the complete change of the surface aspect to oval-like shapes and tensile stress occurring

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Fig. 3. FE-SEM micrographs (a) un-doped ZnO, (b) low and (c) high magnification of 1.0 mol% Eu3+ doped ZnO and (d) EDS analysis of 1.0 mol% Eu3+ doped ZnO nanoparticles.

inherently in Eu3+ doped ZnO [24,25]. Fig. 2(c) shows a high magnification micrograph confirming the oval-like nanostructures. The elemental analysis on the ZnO:Eu3+ nanoparticles using EDS is shown in Fig. 2(d). The inset in the spectrum shows the chosen position on the FE-SEM micrograph where the EDS analysis was performed. The EDS spectrum confirms the presence of zinc (Zn), europium (Eu), oxygen (O) and carbon (C) in the ZnO:Eu3+ nanoparticles. The carbon might be due to carbon tape used to mount the samples. 3.2. Fourier Transform Infrared (FTIR) Analysis The FT-IR spectra of both undoped and Eu3+ doped ZnO NPs recorded at room tempearture in the range of 400 and 4000 cm−1 are shown in Fig. 4. The undoped ZnO spectrum shows two absorption bands around 441 and 565 cm−1 which correspond to the stretching mode frequencies of Zn-O. The band around 867 cm−1 is due to the formation of tetrahedral coordinated Zn. The bands around 1577 and 1412 cm−1 may be related to the symmetric and asymmetric stretching vibrations of carboxylic group originating from the reaction intermediates or small residues of zinc acetate used for preparation of the ZnO nanoparticles. The broad peak and the small one around 3428 and 2364 cm−1 are due to O-H group and CO2 stretching mode [26,27]. Furthermore, signals observed from the 1.0 mol% Eu doped ZnO spectrum around 714, 1050, 1080 and 3609 cm−1 are assigned to CO2– 3 vibration mode [28], C-H stretching mode [29] and O-H group respectively [27]. The broad absorption band around 451 cm−1 in doped ZnO mighty be assigned to Eu-O stretching mode confirming that some Eu3 + ion were introduced in the Zn2+ sites.

the UV region. Similarly, the samples exhibit a sharp decrease in reflectance at approximately 375 nm. The decrease in reflectance is called absorption edge and it is attributed to absorption of the electrons of level 1Sh (fundamental state) to 1Se (excitation state) excitonic transition in ZnO nanoparticles. The characteristic 4f → 4f absorption peaks of Eu3+ are assigned to 7F0 → 5D2, 7F0 → 5D1, 7F1 → 5D1, and 7F1 → 5D0 in the wavelength regime of 465, 525, 535 and 592 nm respectively [24,30]. The inset is the enlarged portion of the spectra (450–650 nm) showing the 4f → 4f transition of Eu3+ ions. A close inspection of this spectrum reveals further extra absorption peak (marked with a rectangular block) around 496 nm for ZnO:Eu3+ (0.2 and 1.0 mol%) samples. This absorption cannot be assigned to any transition of ZnO:Eu3+. It is therefore, assigned to Ho3 + incidental impurities in the sample materials used for the synthesis or characterization. Ho3 + has absorption

3.3. Reflectance and Band-gap Analysis (UV–vis Spectroscopy) The UV–vis reflectance spectra for ZnO and ZnO:Eu3+ nanoparticles are presented in Fig. Fig. 5(a). The spectra of ZnO nanoparticle shows high reflectance in the visible and high absorption in the ultraviolet (UV) region while ZnO:Eu3+ nanoparticles shows f → f absorption in the visible region in addition to ultraviolet (UV) absorption of ZnO in

Fig. 4. FTIR spectra of undoped and Eu3+ doped ZnO nanoparticles.

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Fig. 5. (a) Reflectance spectra and (b) Tauc's plot of un-doped and Eu3+ doped ZnO nanoparticles.

transition at around 495 nm which is assigned to 5I8 → 5F3 transition of Ho3+ [31]. The band-gap energies (Eg) for undoped and Eu3+-doped ZnO nanoparticle were estimated from the extrapolation of the linear portion of the (αhν) 2 vs hν plots (Fig. Fig. 5(b)) from Tauc's plot method [24].   2 ðαhvÞ ¼ A hv−Eg

ð4Þ

where, A is constant, Eg is the optical band gap, h is Plank's constant and α is the absorption coefficient. The optical band-gap of the ZnO (host) is estimated to be 3.27 eV. When Eu3 + ions was incorporated in ZnO, first the bandgap energy decreases from 3.27 to 3.21 eV before increasing linearly with Eu3+ concentration to 3.25 eV as shown inset of Fig. Fig. 5(b). The tailoring of the bandgap of semiconductors due to incorporation of dopant and ions is a well-known phenomenon and this has been attributed to that introduction of new energy levels in the band gap near conduction edge such as shallow level donor impurities. Dopant ions introduce energy levels near the valence band [32] thereby tailoring the bandgap. 3.4. Photoluminescent Analysis Fig. 6 depicts the photoluminescence emission spectra of ZnO NPs. The spectra were measured when exciting the ZnO NPs powder at 325 nm using the He-Cd laser and also a monochromatized xenon

Fig. 6. A deconvolution of the light emission of ZnO nanoparticles excited using a 325 nm (a) He-Cd and (b) monochroatized xenon lamp. The inset of the figures show the ultraviolet emission spectra of ZnO nanoparticles.

lamp. Under 325 nm excitation using the He-Cd laser, ZnO NPs powder exhibit two luminescence bands: the minor ultraviolet emission at 384 nm is related to excitonic recombination (see inset) and the major emission in the visible region at 600 nm is related to defects [32,33]. The less intense ultraviolet emission in this case may be due to the fact that the prepared ZnO NPs have high visible emission intensity because of the high concentration of defects [34]. When exciting using the monochromatized xenon lamp at the wavelength of 325 nm, only defects related visible emission at ~ 600 nm from the ZnO NPs was observed. The broad visible emission spectrum of ZnO NPs were de-convoluted using Gaussian fit in order to identify the present superposition of multiple emission positions. From the de-convoluted spectra, three defect-related visible emission bands were identified and were assigned accordingly to the presence of defects as shown in Fig. 6. It is well known that the visible emission of ZnO is due to the radiative recombination of a photogenerated hole with an electron occupying the oxygen vacancy. The well-known defects in ZnO are oxygen vacancies (Vo), oxygen interstitials (Oi), oxygen antisites (ZnO), zinc vacancies (Vzn), zinc interstitials (Zni) and zinc antisites (OZn). Visible light emission is a result of electron-hole recombination taking place at defect states. The green emission centered at ~530 nm is related to singly ionized oxygen vacancy (V+ o ) and the yellow emission centered at ). The ~ 590 nm may be due to doubly ionized oxygen vacancy (V++ o emission band located at ~ 680 nm is usually originated from oxygen

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Fig. 9. PL spectra for determination of quantum yield of ZnO:0.4 mol% Eu3+. Fig. 7. PL excitaion and emission spectra of ZnO:Eu3+ with different concentration of Eu3+.

interstitials (Oi) [35–39]. Due to the large specific surface area of the nanostructured ZnO, it is most likely that defect states exist in the band gap of the dendritic ZnO nanoparticles demonstrating that the existing oxygen vacancy (deep level trap) attracts interstitial Zn (shallow donor level) [40]. Fig. 7 shows the excitation and emission spectra of Eu3+ doped ZnO recorded at room temperature using monochromatized xenon lamp as excitation source. The excitation spectra were measured by monitoring the emission wavelength at 618 nm and the spectra contains sharp lines at 398, 416, 466 and 538 nm that correspond to the intrinsic 4f → 4f transitions of Eu3+ in the ZnO host. The resulting intrinsic 4f → 4f transitions of Eu3+ were assigned to 7F0 → 5L6, 5D3, 5D2 and 5D1. The emission spectrum shows weak and intense emissions at 593 nm, 618 nm, 646 m and 682–696 nm which are attributed to Eu3 + transitions: 5 D0 → 7FJ (J = 1, 2, 3 and 4), respectively [41,42]. It has been reported that the 5D0 → 7F1 emission transition which is assigned to magnetic dipole becomes intense if Eu3 + ions are located at site with inversion symmetry, whereas 5D0 → 7F2 emission transition that is assigned to electric dipole become intense without inversion symmetry in the ZnO host lattice. In addition, the ratio of integrated maximum emission intensities between 5D0 → 7F2 and 5D0 → 7F1 transitions is called

“asymmetry ratio” which gives a measure of distortion from the inversion symmetry of the Eu3+ ion local environment in host matrix. The intense 5D0 → 7F2 transition or higher (5D0 → 7F2)/(5D0 → 7F1) value indicate the occupancy of Eu3+ ions without inversion symmetry [43, 44]. The calculated asymmetric ratio is about 3.5.and the emission intensity of transition 5D0 → 7F2 is intense than that of 5D0 → 7F1. From these observations, it can be concluded that Eu3 + ions occupies site without inversion symmetry in the host lattice [10]. An increase in Eu3+ concentration enhanced the luminescence intensity and the maximum PL intensity was observed at 0.4 mol% Eu3+ beyond which the luminescence intensity decreases due to concentration quenching effects as demonstrated in Fig. 8(a). Yao et al. reported that there are mechanisms of concentration quenching of rare earths which involves energy transfer between rare earths ions caused by exchange or multipole-multipole interactions [45]. Furthermore, the behaviour of concentration quenching can be explained using different energy transfer models, the reduction in the average distance (Rc) between luminescent centres to favour non radiative energy transfer [46], pairing of activator ions and then shifting to their quenching centres or the excitation energy that reach the quenching centres (impurities) because the resonance between the activator ions enhances the excitation migration due to increase of activator molar concentration [47]. To obtain more detailed information about the concentration quenching of the studied Eu3+ doped ZnO NPs, the separation between

Fig. 8. PL intensity as a function of Eu3+ concentartion.

Fig. 10. Luminescence decay curve of the ZnO:0.4 mol% Eu3+ sample.

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Table 1 The crystallographic planes corresponding to various Bragg angles and the calculated and theoretical d-spacing for ZnO nanoparticle. 2θ (deg.)

Planes

Calculated d-spacing (nm)

Standard d-spacing (nm)

31.72 34.39 36.20 47.48 56.54 62.81 67.90 69.01

(100) (002) (101) (102) (110) (103) (200) (112)

0.2818 0.2604 0.2478 0.1912 0.1627 0.1478 0.1409 0.1379

0.2814 0.2603 0.2475 0.1913 0.1624 0.1477 0.1407 0.1378

3.6. Lifetime Analysis The lifetime of the ZnO:0.4 mol% Eu3+ sample was measured using a monochromatized xenon flash lamp with repitition rate of 50 Hz for excitation at the wavelength of 464 nm. The decay curve is presented in Fig. 10. The curve was well fitted to a biexponential decay using the following equation [51]:

Eu3+ ions which provide information about concentration quenching were calculated using the following equation [48]:  r¼

M ρC NA

13

ð5Þ

where r is the critical distance, M is the molecular weight of zinc oxide (ZnO), ρ the density of zinc oxide, NA is Avogadro constant and C the concentration of Eu3+ ions corresponding to maximum emission intensity. Multipole-multipole interaction mechanisms of energy transfer become effective only when critical distance between rare earth ions is larger than 0.5 nm while the exchange interaction mechanism is effective when the critical distance is below 0.5 nm [31,49]. In the present case the critical distance was calculated as 0.38 nm, suggesting that the exchange interaction is responsible for concentration quenching in this study. 3.5. Quantum Yield Analysis The absolute flourescence quantum yield (QY) or ƞ of the sample with the highest PL intensity (ZnO:0.4 mol% Eu3 +) was measured using the integrating sphere of the FLS980 spectromenter (Edenburgh Instruments Limited). Generally, QY (ƞ) is defined as the ratio of the number of photon emitted (Nem) to the number of photons absorbed (Nabs) [50]. η¼

Nem

ð6Þ

Nabs

There are two methods by which the fluorescence QY can be measured, namely Direct Excitation (DExc) commonly used for liquid samples, and Direct and Indirect Excitation (DI Exc) often used for powders, bulk and thin film samples. The fluorescence QY of our powder sample was measured when the sample was excited at the wavelength of 464 nm using Direct and Indirect excitation (DI Exc) method. The PL spectra used for measuring the the quantum yield are presented in Fig. 9. Spectrum (1) (red online) represents reflection as well as luminescence from the sample, while spectrum (2) (black online) was measured when the excitation was incident on the spectralon (reference plug) but indirectly excites the sample inside the integrating sphere. Spectrum (2) (blue online) represents the reflection of excitation light from the spectralon. The ƞDI Exc value was calculated from the spectral and emission scans (Fig. 9) of the sample and the reference (spectralon) plug using the quantum yield wizard (Eq. (7)) that is implemented in the F980 software and the QY value of 7.8% was obtained. ηDI Exc ¼

SB ðEC −EA Þ−SC ðEB −EA Þ ðSB −SC ÞSA

the absorbed and emitting photons were low for this sample, they were challenging to measure, suggesting that the error margin (that is not easy to quantify) was considerably high for this sample.

ð7Þ

where A and C represents reference samples and B represents the test sample. SA, SB, SC, EA, EB and EC refer to the integrals of the scans. Despite obtaining a decent ƞDI Exc value of 8.7%, both the emission and excitation scans suggested that the number of photons absorbed and those emitted were low as can be inferred from Fig. 9. Since both

  t t þ A2 exp − Iðt Þ ¼ I o þ A1 exp − τ1 τ2

ð8Þ

where, I(t) and Io are the luminescent intensities at certain time t and 0, A1 and A2 are the fitting parameters, τ1 and τ2 are the slow and fast decay components (long and short lifetimes in millisecond). However using these parameters, the average lifetime (τavg) of Eu3 + ions can be calculated by the expression given by. τ¼

Aτ21 þ A2 τ22 A1 τ1 þ A2 τ 2

ð9Þ

The average lifetime (τavg) value for the ZnO:0.4 mol% Eu3+ was determined to be 1.306 ms. 4. Conclusion The wurzite hexagonal ZnO:Eu3 + NPs with different molar concetrations of Eu3+ ions were successfully synthesized via the co-precipitation method. The incorporation of Eu3+ has a great influence on both structure and luminescent intensity of ZnO:Eu3+ NPs. The broad emission spectra observed in the visible region for undoped ZnO is due to different kind of defects. ZnO:Eu3+ NPs shows yellow and red emission and are attributed to Eu3+ transitions. The red emission intensity was measured as a function of Eu3+ doping concentration and the emission of Eu3+ was reduced considerably beyond the concentration of 0.4 mol%. A correlation between experimental results and the theoretical calculations shows that exchange interaction energy transfer is the major mechanism that is responsible for concentration quenching of Eu3+ in ZnO NPs. Acknowledgements This work was financially supported by the South African National Nanoscience Postgraduate Teaching and Training Platform (NNPTTP) of the Department of Science and Technology (DST),Competitive Programme for Rated Researchers (CPRR) (Grant no. CPR20110724000021870) of the National Research Foundation (NRF)-South Africa. The financial support from the rental pool programme of National Laser Center (NLC) (Grant no. NLC-LREGM) is highly recognized. References [1] R. Kumara, A. Umarb, G. Kumara, M.S. Akhtard, Y. Wange, S.H. Kimb, Ce-doped ZnO nanoparticles for efficient photocatalytic degradation of direct red-23 dye, Ceram. Int. 41 (2015) 7773–7782. [2] A. Umar, R. Kumar, G. Kumar, H. Algarni, S.H. Kim, Effect of annealing temperature on the properties and photocatalytic efficiencies of ZnO nanoparticles, J. Alloys Compd. 648 (2015) 46–52. [3] K.P. Raj, K. Sadayandi, Effect of temperature on structural, optical and photoluminescence studies on ZnO nanoparticles synthesized by the standard coprecipitation method, Physica B 487 (2016) 1–7. [4] K.Q. Le, H.P.T. Nguyen, Q.M. Ngo, A. Canimoglu, N. Can, Experimental and numerical optical characterization of plasmonic copper nanoparticles embedded in ZnO fabricated by ion implantation and annealing, J. Alloys Compd. 669 (2016) 246–253. [5] F. Lu, W. Cai, Y. Zhang, ZnO hierarchical micro/nanoarchitectures: solvothermal synthesis and structurally enhanced photocatalytic performance, Adv. Funct. Mater. 18 (2008) 1047–1056.

O.M. Ntwaeaborwa et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 182 (2017) 42–49 [6] S. Sabir, M. Arshad, S.K. Chaudhari, Zinc oxide nanoparticles for revolutionizing agriculture: synthesis and applications, Sci. World J. 2014 (2014) 1–8. [7] A. Khataee, A. Karimi, M. Zarei, S.W. Joo, Eu-doped ZnO nanoparticles: sonochemical synthesis, characterization, and sonocatalytic application, Ultrason. Sonochem. (2015) http://dx.doi.org/10.1016/j.ultsonch.2015.03.016. [8] N. Naseri, M. Yousefi, O. Moradlou, A.Z. Moshfegh, The first on enhanced photoresponsivity of ZnO-TiO2 nanocomposite thin films by anodic polarization, Phys. Chem. Chem. Phys. 13 (2011) 4239–4242. [9] Vinod Kumar, S. Som, Vijay Kumar, Vinay Kumar, O.M. Ntwaeaborwa, E. Coetsee, H.C. Swart, Tunable and white emission from ZnO:Tb3+ nanophosphors for solid state lighting applications, Chem. Eng. J. 255 (2014) 541–552. [10] R. Zamiria, A.F. Lemosa, A. Rebloa, H.A. Ahangar, J.M.F. Ferreiraa, Effects of rare-earth (Er, La and Yb) doping on morphology and structure properties of ZnO nanostructures prepared by wet chemical method, Ceram. Int. 40 (2014) 523–529. [11] M. Chndrasekhar, H. Nagabhushana, Y. SVidya, K.S. Anantharaju, S.C. Sharma, H.B. Premkumar, S.C. Prashantha, B.D. Prashantha, B.D. Prasad, C. Shivakumara, R. Saraf, H.P. Nagaswapura, Synthesis of Eu3+-activated ZnO superstructures: photoluminescence, Judd–Ofelt analysis and sunlight photocatalytic properties, J. Mol. Catal. A 409 (2015) 26–41. [12] N. Yao, J. Huang, K. Fu, X. Deng, M. Ding, M. Shao, X. Xu, Enhanced light harvesting of dye-sensitized solar cells with up/down conversion materials, Electrochim. Acta 154 (2015) 273–277. [13] L.R. Singh, Photoluminescence studies of ZnO, ZnO:Eu and ZnO:Eu nanoparticles covered with Y2O3 matrix, Mater. Sci. Appl. 6 (2015) 269–278. [14] V. Mangalam, K. Pita, C. Couteau, Study of energy transfer mechanism from ZnO nanocrystals to Eu3+ ions, Nanoscale Res. Lett. 11 (2016) 73. [15] G. Vijayaprasath, R. Murugan, T. Mahalingam, Y. Hayakawac, G. Ravi, Enhancement of ferromagnetic property in rare earth neodymium doped ZnO nanoparticles, Ceram. Int. 41 (2015) 10607–10615. [16] A. Pacthak, P. Pramanik, Nano-particles of oxides through chemical method, PINSA 67 (2001) 47–70. [17] K. Shingange, G.H. Mhlongo, D.E. Motaunga, O.M. Ntwaeaborwa, Tailoring the sensing properties of microwave-assisted grown ZnO nanorods: effect of irradiation time on luminescence and magnetic behavior, J. Alloys Compd. 657 (2016) 917–926. [18] T.B. Iveti'c, M.R. Dimitrievska, I.O. G'uth, L.R. Da, S.R. Canin, Luki'c, Petrovi'c, J. Res. Phys. 36 (1) (2012) 43–51. [19] A. Franco Jr., H.V.S. Pessoni, M.P. Soares, Room temperature ferromagnetism in Eudoped ZnO nanoparticulate powders prepared by combustion reaction method, J. Magn. Magn. Mater. 355 (2014) 325–330. [20] M. Najafi, H. Haratizadeh, The effect of growth conditions and morphology on photoluminescence properties of Eu-doped ZnO nanostructures, Solid State Sci. 41 (2015) 48–51. [21] N.A.S. Omar, Y.W. Fena, K.A. Matori, Photoluminescence properties of Eu3+-doped low cost zinc silicate based glass ceramics, Optik 127 (2016) 3727–3729. [22] R.E. Kroon, Nanoscience and the Scherrer equation versus the ‘Scherrer–Gottingen equation’, S Afr J Sci. 109 (5/6) (2013) http://dx.doi.org/10.1590/sajs.2013/a0019 (Art. #a0019, 2 pages). [23] W. Massa, Crystal Structure Determination, Springer-Verlag, Berlin, Heidel-berg, New York, 2000 25. [24] L.F. Koao, F.B. Dejene, R.E. Kroon, H.C. Swart, Effect of Eu3+ on the structure, morphology and optical properties of flower-like ZnO synthesized using chemical bath deposition, J. Lumin. 147 (2014) 85–89. [25] C. Shinho, Synthesis and luminescent properties of Eu3+-doped ZnO phosphors, J. Korean Phys. Soc. 66 (10) (2015) 1559–1563. [26] S. Udayakumar, V. Renuka, K. Kavitha, Structural, optical and thermal studies of cobalt doped hexagonal ZnO by simple chemical precipitation method, J. Chem. Pharm. Res. 4 (2) (2012) 1271–1280. [27] K. Ravichandrika, P. Kiranmayi, R.V.S.S.N. Ravikumar, Synthesis, characterization and antibacterial activity of ZnO nanoparticles, Int. J. Pharm. Pharm. Sci. 4 (4) (2012) 336–338. [28] X. Liu, J. Zhang, L. Wang, T. Yang, X. Guo, S. Wu, S. Wang, 3D hierarchically porous ZnO structures and their functionalization by au nanoparticles for gas sensors, J. Mater. Chem. 21 (2011) 349–356. [29] S. Husain, F. Rahman, N. Ali, P.A. Alvi, Nickel sub-lattice effects on the optical properties of ZnO nanocrystals, J. Optoelectron. Eng. 1 (1) (2013) 28–32.

49

[30] C. Shinho, Synthesis and luminescent properties of Eu3+-doped ZnO phosphors, J. Korean Phys. Soc. 66 (10) (2015) 1559–1563. [31] G. Li, C. Li, Z. Xu, Z. Cheng, J. Lin, Facile synthesis, growth mechanism and luminescence properties of uniform La(OH)3: Ho3+/Yb3+ and La2O3: Ho3+/Yb3+ nanorods, R. Soc. Chem. 12 (2010) 4208–4216. [32] Vinod Kumar, S. Som, Vijay Kumar, Vinay Kumar, O.M. Ntwaeaborwa, E. Coetsee, H.C. Swart, Tunable and white emission from ZnO:Tb3+ nanophosphors for solid state lighting applications, Chem. Eng. J. 255 (2014) 541–552. [33] M. Zhong, G. Shan, Y. Li, G. Wang, Y. Liu, Synthesis and luminescence properties of Eu3+-doped ZnO nanocrystals by a hydrothermal process, Mater. Chem. Phys. 106 (2007) 305–309. [34] T.K. Kundu, N. Karak, P. Barik, S. Saha, Optical properties of Zno nanoparticles prepared by chemical method using poly (VinylAlcohol) (PVA) as capping agent, Int. J. Soft Comput. Eng. 1 (2011) 19–24. [35] M. Willander, O. Nur, J.R. Sadaf, M.I. Qadir, S. Zaman, A. Zainelabdin, N. Bano, I. Hussain, Luminescence from zinc oxide nanostructures and polymers and their hybrid devices, Materials 3 (2010) 2643–2667. [36] S. Talam, S.R. Karumuri, N. Gunnam, Synthesis, Characterization, and Spectroscopic Properties of ZnO Nanoparticles, 2012, International Scholarly Research Network, 2012 1–7. [37] A. Makhal, S. Sarkar, T. Bora, S. Baruah, J. Dutta, A.K. Raychaudhuri, S.K. Pal, Dynamics of light harvesting in ZnO nanoparticles, Nanotechnology 21 (2010) 265703. [38] V. Kumar, H.C. Swart, M. Gohain, V. Kumar, S. Som, B.C.B. Bezuindenhoudt, O.M. Ntwaeaborwa, Influence of ultrasonication times on the tunable colour emission of ZnO nanophosphors for lighting applications, Ultrason. Sonochem. 21 (2014) 1549–1556. [39] K. Shingange, G.H. Mhlongo, D.E. Motaung, O.M. Ntwaeaborwa, Tailoring the sensing properties of microwave-assisted grown ZnO nanorods: effect of irradiation time on luminescence and magnetic behavior, J. Alloys Compd. 657 (2016) 917–926. [40] Z. Zhang, D. Chen, W. Chen, Y. Chen, X. Song, R. Zhan, S. Deng, N. Xu, J. Chen, Thermo-enhanced field emission from ZnO nanowires: role of defects and application in a diode flat panel X-ray source, Appl. Surf. Sci. 399 (2017) 337–345. [41] X. Zeng, J. Yuan, L. Zhang, Synthesis and Photoluminescent properties of rare earth doped ZnO hierarchical microspheres, J. Phys. Chem. C 112 (2008) 3503–3508. [42] B. Han, P. Li, J. Zhang, J. Zhang, Y. Xue, X. Suo, Q. Huang, Y. Feng, H. Shi, First observation of the emission from 5DJ (J = 1, 2, 3) energy levels of Eu3+ in Bi4O3(BO3)(PO4):Eu3+ phosphor, Mater. Lett. 158 (2015) 208–210. [43] P.V. Ramakrishna, D.B.R.K. Murthy, D.L. Sastry, White-light emitting Eu3+ co-doped ZnO/Zn2SiO4:Mn2+ composite microphosphor, Mol. Biomol. Spectrosc. 125 (2014) 234–238. [44] T.T. Van Tran, T.M. Dung Cao, Q.V. Lam, V.H. Le, Emission of Eu3+ in SiO2-ZnO glass and SiO2-SnO2 glass-ceramic: correlation between structure and optical properties of Eu3+ ions, J. Non-Cryst. Solids 459 (2017) 57–62. [45] S. Yao, L. Xue, Y. Yan, Properties of Eu3+ luminescence in the monoclinic Ba2MgSi2O7, Ceramics 55 (3) (2011) 251–255. [46] V. Dordevic, Z. Antic, M.G. Nikolic, M.D. Dramicanin, The concentration quenching of photoluminescence in Eu3+-doped La2O3, J. Res. Phys. 37 (1) (2013) 47–54. [47] G.S.R. Raju, E. Pavitra, S.K. Hussain, D. Balajib, J.S. Yu, Eu3+ ion concentration induced 3D luminescence properties of novel red-emitting Ba4La6(SiO4)O:Eu3+ oxyapatite phosphors for versatile applications, J. Mater. Chem. 4 (2016) 1039. [48] M. Shi, D. Zhang, C. Chang, Tunable emission and concentration quenching of Tb3+ in magnesium phosphate lithium, J. Alloys Compd. 627 (2015) 25–30. [49] R.G.A. Kumar, S. Hata, K. Ikeda, K.G. Gopchandran, Luminescence dynamics nd concentration quenching in Gd2-xEuxO3 nanophosphor, Ceram. Int. 41 (2015) 6037–6050. [50] Edinburgh Instruments, Integrating sphere for measurements of fluorescence quantum yields and spectral reflectance. [online]Available: http://www.edinst.com//wpcontent/uploads/2016/02/FLS980-Series-Reference-Guide-Intergrating-Sphere-pdf (Accessed March 2017). [51] K.N. Kumar, R. Padma, L. Vijayalakshmi, J.S.M. Nithya, M. Kang, Promising red emission from functionalized multi walled carbon nanotubes embedded co-doped Bi3+ + Eu3+: PVA polymer nanocomposites for photonic applications, J. Lumin. 182 (2017) 208–219.