Nd-doped ZnO as a multifunctional nanomaterial

JOURNAL OF RARE EARTHS, Vol. 30, No. 8, Aug. 2012, P. 761

Nd-doped ZnO as a multifunctional nanomaterial Surender Kumar, P.D. Sahare (Department of Physics & Astrophysics, University of Delhi, Delhi – 110 007, India) Received 8 December 2011; revised 3 March 2012

Abstract: Chemically synthesised ZnO and Nd-doped ZnO nanoparticles were investigated for structural, optical, magnetic properties along with photocatalytic activity. Transmission electron microscopy measurement was performed on the undoped and doped ZnO nanoparticles. Compared to the undoped ZnO, Nd-doped ZnO nanoparticles showed enhanced photoluminescent and ferromagnetic properties. The Nddoped ZnO nanoparticles also showed improved photocatalytic properties compared with the undoped ZnO nanoparticles. Furthermore, the effect of UV light irradiation was studied with thermoluminescence (TL) and photoluminescence (PL) measurement techniques. It was found that in case of Nd-doped ZnO nanoparticles TL intensity increased while the green emission in PL spectra decreased with UV-light irradiation. This was attributed to the production of more surface defects on UV irradiation on Nd-doping. Keywords: ZnO nanoparticles; Nd-doping; optical properties; magnetic properties; photocatalytic properties; rare earths

Nanometer-scale materials and semiconductors in particular seem to be important and promising in the development of next-generation electronic and optoelectronic devices[1–3]. In recent years, semiconductor hybrid materials (SHMs) become the centre of attention due to their enhanced optical, photocatalytic and magnetic properties. Especially, synthesis and properties of semiconductor-metal heterostructures such as TiO2, SnO2, ZnO, etc. (doped with Au, Ag, Nd and Gd) have been investigated in recent years due to their potential and important applications in catalysis, cellular imaging, immunoassay, luminescence tagging, spintronics and drug delivery[4–9]. For example, if made magnetic on doping, SHMs will become a kind of multifunctional material with semiconducting, optical, photocatalytic and magnetic properties. ZnO, one of the most popular n-type semiconductor metal oxides, is a versatile material with a wide band gap (3.37 eV) and large exciton binding energy (60 meV)[10]. It has variety of applications such as UV absorption, spintronics, photocatalysts, sensing and UV light-emitting devices. But these properties of ZnO strongly depend on the impurities and defects[11–13]. It is well known that the existence of defects in a semiconductor would lead to corresponding defects energy levels in the band gap[14]. Therefore, we believe that the different types of oxygen defects such as oxygen vacancies and interstitial oxygen in ZnO nanocrystals result in changes in their photoluminescence (PL) and photocatalytic properties. In addition, magnetic properties are also affected by oxygen vacancies[15]. Recent experimental results have shown that the introduction of rare-earth metals (REMs) such as Gd ion in wide band-gap semiconductor, GaN, results in ferromagnetic property[16,17]. This has motivated the researchers

working in this field on REM ion(s) doping in ZnO for spintronics application. Among the wide variety of fabrication approach, the wet chemical approach is a versatile method for low-temperature, large-scale production of various ZnO structure at low cost. However, such chemical approaches provide ZnO with absorbed water and surface hydroxyl groups, which can significantly affect the luminescence properties of ZnO[18,19]. In recent years, different types of SHMs have been studied, such as ZnO-Ag, ZnO-Au, ZnO-Gd, ZnO-Nd, etc.[20–23]. Doping method has been extensively used for the modification the electronic structure of ZnO nanoparticles to achieve new or improved optical, magnetic, catalytic properties. Dopants can segregate on the ZnO nanostructure surfaces or they can incorporate into the lattice or both[24]. Doping in ZnO with a suitable dopant can make it more or less efficient in photodegradation of organic and toxic pollutants. When the Nd3+ ions occupy the substitutional position of Zn2+ ions then cationic vacancies are created locally due to their different ionic states (ionic radii of Zn2+ and Nd3+ are 0.083 and 0.108 nm, respectively). Thus induced cationic vacancies created by Nd3+ doping in the ZnO new kinds of defects[25,26]. Additionally, for device applications, high efciency UV light emitting devices is required, it is important to suppress the visible emission. Among the rare earth metals, Nd is one of the most widely used elements for high power laser applications and recently these lasers have shown their usefulness in inertia confined fusion experiments[27]. Furthermore, Nd3+ doping reduces the band gap energy and enhances the possibility of the photodegradation of dyes under visible light[28], also shown by us in the present study under UV light. Previous reports on Nd-

Corresponding author: Surender Kumar (E-mail: [email protected]; Tel.: +91-11-27667793) DOI: 10.1016/S1002-0721(12)60126-4

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doped ZnO focused on the luminescence, magnetic, electrical and photocatalytic properties in ZnO or TiO2 based materials[29–31]. However, there is a dearth of data on the systematic studies of Nd-doped ZnO nanoparticles which inspired us for the synthesis of such material by wet chemical method and for the investigation of their structural, optical, magnetic and photocatalytic properties. In this report, emphasis on the variation of optical and magnetic properties with the Nd3+ doping is presented. The effect of UV light irradiation is verified with PL and TL measurement techniques. The green emission decreased with UV-light irradiation. Additionally, the photocatalytic properties of Nd-doped ZnO are evaluated in terms of aqueous Rh B dye degradation and realised to be highly efficient catalyst.

1 Experimental 1.1 Synthesis ZnO nanoparticles doped with Nd (0.01, 0.02, 0.03, 0.04 and 0.05 mol.%) were synthesized via wet chemical method using neodymium chloride (NdCl3·6H2O), zinc acetate (Zn(CH3COO)2·2H2O), sodium hydroxide (NaOH) as a starting materials without any further purification. Stoichiometric composition was chosen for zinc acetate and sodium hydroxide. This technique was based on the hydrolysis of the precursor used to prepare ZnO nanoparticles. 0.1 mol/L solution of zinc acetate was prepared in ethanol and refluxed at 80 ºC for 6 h which results in Zn2+ ions. Separately, 0.2 mol/L solution of sodium hydroxide was prepared in ethanol and added dropwise to Zn2+ ions solution under constant magnetic stirring at 40 ºC and kept the reaction for 5 h under continuous stirring. The obtained white precipitates are separated out using centrifugation at 6000 r/min and washed several times with ethanol and dried at 100 ºC for 12 h. For doping, an appropriate amount of NdCl3·6H2O was added to zinc acetate solution keeping all parameters the same and used for further characterization. However, the concentrations of the impurity stated here refer to the amount added to the solution during this process and not the actual amount that would have incorporated into the host of ZnO matrix. But it could be presumed that the concentrations of Nd inside the matrix remain the same. 1.2 Sample characterization X-ray powder diffraction (XRD) patterns of ZnO were recorded from 30° to 80°, using Cu K radiation (=0.154056 nm) on a “Bruker D8” X-ray diffractometer for structural analysis. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) pattern were taken using Tecnai (300 kV), FEI, Holland. The PL of the ZnO nanoparticles was examined with a Cary Eclipse fluorescence spectrophotometer (Varian make). Thermoluminescence glow curves of the samples were recorded on

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Harshaw TL Reader Model-3500HT which is capable of recording the TL from RT to 600 ºC. The samples were irradiated with UV-radiation from the high-pressure mercury vapor lamp of 125 W (Philips, India) was placed inside the quartz tube with a 350 nm filter. Water circulation was carried out through the quartz tube so as to avoid any thermal effects and to serve as well as an IR filter. All experiments were conducted at room temperature. About 5 mg of sample in powder form was taken each time, irradiated for different time intervals and used for TL measurements. Raman spectroscopic measurements were performed on the prepared samples by a Renishaw InVia Reflex Micro Raman spectrometer with air cooled argon laser of wavelength ~514.5 nm. Room temperature M-H loops were taken with maximum field ±2.2 Tesla using Micro Scene EV9 VSM, USA. 1.3 Photocatalytic experiments Photocatalytic studies under UV (<400 nm) light were carried out in an immersion-type, in-house fabricated photochemical reactor. A double-lined quartz tube (with dimensions of 3.5 cm inner diameter, 4.5 cm outer diameter, and 20 cm length) was placed in an outer Pyrex glass reactor of 14 cm inner diameter and 20 cm length. A high-pressure mercury vapor lamp of 125 W (Philips, India) was placed inside the quartz tube. Here also water circulation was carried out to avoid any thermal effects. The appropriate amount of dye solution, to be decomposed, was taken along with the required amount of catalyst in the outer Pyrex container and was constantly stirred to maintain a homogeneous suspension. The dye was dissolved in doubly distilled water. A typical experiment of degradation was carried out as follows: 0.03 g of the catalyst was added to 100 ml of aqueous solution of Rh B with an initial concentration of 5×10–6 mol/L for irradiation experiments. Prior to irradiation, the suspension of the catalyst and dye solution was stirred in the dark for 1 h to reach the equilibrium adsorption. Five millilitre aliquots were pipetted out periodically from the reaction mixture. The solutions were centrifuged, and the concentration of the solutions was determined by monitoring the intensity of 552 nm absorption peak.

2 Results and discussion 2.1 Structural and morphological study The XRD patterns of undoped and Nd-doped ZnO with different Nd-doping contents are shown in Fig. 1. The undoped ZnO powders are identified as a wurtzite structure ZnO. (ICSD card No. 06-2151, space group: P63mc) with lattice constants a=0.32568 nm, c=0.52125 nm. For all the Nd-doped ZnO materials, the diffraction peaks are almost similar to that of pure ZnO. It is possible for Nd3+ ions cooperate with the matrix of ZnO particles to form Nd-Zn-O solid solutions since the radius of Nd3+ is bigger than that of Zn2+. Increasing dopant concentration in a host matrix by different ions may change the lattice parameters because of the ionic

Surender Kumar et al., Nd-doped ZnO as a multifunctional nanomaterial

Fig. 1 XRD patterns of undoped and Nd-doped ZnO nanoparticles at different doping levels: (1) 0.00, (2) 0.01 mol.%, (3) 0.02 mol.%, (4) 0.03 mol.%, (5) 0.04 mol.% and (6) 0.05 mol.% of Nd

radius difference between the dopant and host atoms[32]. It may also generate stress due to this mismatch. XRD data shows that with the doping concentration of Nd3+ ion increasing, however, the intensity of the diffraction peaks decreased and full width at half maximum (FWHM) has gradually increased (Table 1). From Table 1, the obtained lattice parameters for the undoped ZnO nanoparticles are ‘a=0.3284 and c=0.5260 nm’, which are almost identical to the standard values, also the change in lattice parameters (a and c) with Nd-doping given in the Table 1. a and c are the deviation from the a and c (lattice parameters for undoped ZnO), respectively to see the variation trend with the composition of Nd. The value of ‘a’ is decreased with increase of Nd3+ doping while the value of ‘c’ parameter increases with increase of Nd3+ doping or vice versa. This suggests that Nd3+ ions replace the Zn2+ lattice sites or interstitial sites in the sample. Similarly, type of behaviour of ZnO was also observed by Dakhel et al.[33] with Gd3+ doping, however, the changes observed by them in lattice parameter Table 1 Structural properties of pure ZnO and Nd-doped ZnO nanoparticles S. No.

Samples

FWHM a/nm

a/nm c/nm

c/nm

1

Pure ZnO

0.801

0.3284

0.0000 0.5260

0.0000

2

0.01% of Nd

0.905

0.3278

0.0006 0.5261

–0.0001

3

0.02% of Nd

1.051

0.3277

0.0007 0.5261

–0.0001

4

0.03% of Nd

1.121

0.3274

0.0010 0.5267

–0.0007

5

0.04% of Nd

1.143

0.3278

0.0006 0.5265

–0.0005

6

0.05% of Nd

1.164

0.3279

0.0005 0.5261

–0.0001

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are lesser than observed by us with Nd3+. This, further, confirms the doping of ZnO with Nd3+. The increase of doping concentration results the decrease of crystallite size and increase of disorder effect which resulted in the broadening and decrease of intensity of the XRD peaks. TEM images of the ZnO nanoparticles are shown in Fig. 2. Nearly circular shapes for the dark spots in the images indicate that the ZnO nanoparticles are almost spherical. The estimated average particle size corresponding to (a), (b) and (c) are 16, 12 and 9 nm. When the concentration of Nd is in creased the particle size decreased and this is clearly seen in the TEM images. These results are also consistent with other rare earth metal ions doped like Gd-doped ZnO and also in Nd-doped ZnO[34,35]. 2.2 Optical study 2.2.1 Raman According to the group theory, Wurtzite ZnO belongs to the space group C46v with two formula units per primitive cell, a primitive ZnO cell has four atoms (two formula units), each of which occupies C3v sites, leading to 12 phonon branches (nine optical and three acoustic). The optical phonon irreducible representation is given by opt=A1+ 2B1+E1+2E2. The A1 and E1 modes are polar and can split into transverse-optical (TO) and longitudinal-optical (LO) phonons, while the B1 modes are Raman inactive[36]. The A1 phonon vibration is polarized parallel to the c-axis; the E1 phonon is polarized perpendicular to the c-axis. Every mode corresponds to a band in the Raman spectrum. The intensities of these bands depend on the scattering cross-section of these modes. Nonpolar E2 modes are Raman active, and have two wavenumbers, namely E2low and E2high, associated with the motion of oxygen (O) atom and zinc (Zn) sublattice respectively. Strong E2high mode is characteristic of the Wurtzite lattice and indicates good crystallinity. The E1low mode is associated with presence of oxygen vacancies, interstitial Zn or their complexes. A1 (LO) phonon can appear only when the c axis of Wurtzite ZnO is parallel to the sample surface. When perpendicular to the sample surface, E1 (LO) phonon is observed[37]. The presence of Nd3+ in the ZnO lattice deforms the structure and can be detected by Raman analysis. Fig. 3 shows the measured Raman spectra for the Nd-doped ZnO samples with different concentrations of Nd3+. The peaks at 102 cm–1 (E2low) and 436 cm–1 (E2high) are attributed to the nonpolar E2 vibrational modes due to the vibration of Zn and O

Fig. 2 TEM images of undoped (a), 0.03 mol.% doped (b) and 0.05 mol.% doped (c) ZnO nanoparticles

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lattice in Wurtzite ZnO. The intensity of both the E2low and E2high modes decreased gradually with Nd3+ concentration, which can be attributed to the distortion in the crystal. The peak at 1080 cm–1 are attributed to the TO plus LO mode. The peak at 661, 931, 1345, 1417 and 2934 cm–1 are associated with the OCO symmetric bending, C–C vibration, CH3 symmetric bending, CO symmetric stretching and CH3 symmetric stretching present in the radicals (CH3COO–) of the zinc acetate dehydrate respectively[38–43]. The distortion in the crystal is also found out from the XRD results. Obtained ZnO precipitates are already washed several times with ethanol to remove the by-products. The prepared ZnO nanoparticles have residual intermediate compound on the surface in the form of an acetate group, which acts as defect centers for the emission of green luminescence[44]. The observed acetate radicals are loosely bound on the surface of ZnO by dangling bond[45]. 2.2.2 Photoluminescence Fig. 4 shows the emission spectra of undoped ZnO and Nd-doped ZnO nanoparticles. ZnO nanoparticles exhibited a strong visible emission centered at about 565 nm excited by 280 nm and a sharp near band edge emission at around 384 nm. Peak at 384 nm is associated

Fig. 3 Raman spectra of ZnO samples prepared with different Nd amounts: (1) 0.00, (2) 0.01 mol.%, (3) 0.02 mol.%, (4) 0.03 mol.%, (5) 0.04 mol.% and (6) 0.05 mol.%

Fig. 4 PL spectra of the undoped and Nd-doped ZnO with 0.00 (1), 0.01 mol.% (2), 0.02 mol.% (3), 0.03 mol.% (4), 0.04 mol.% (5) and 0.05 mol.% (6) of Nd, measured at room temperature with excitation wavelength of 280 nm

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with the near band edge transition. Broad band is due to the defects at the surface of ZnO such as oxygen vacancies and hydroxyl group[46–49]. PL properties of Nd3+- doped ZnO were studied with doping concentration varied from 0 to 0.05 mol.%. As the concentration of Nd3+ is increased, visible emission enhanced greatly (about 10 times for 0.04 mol.%). However, excessive Nd3+ ions consumed by the ZnO nanoparticles, decreasing the PL intensity, reversely. According to previous report, the reduction of particle’s size usually induced the increase in the content of oxygen vacancies[50]. In present study, Nd-doping decreased the size of ZnO, which might increase the content of oxygen vacancies. However, excessive Nd3+ ions consumed by the ZnO nanoparticles, decrease the fluorescence intensity, reversely. To complete the valence site of one Zn2+, one O2– will be attached to form a ZnO compound while in case of Nd3+, two Nd3+ will be attached to the three O2– to form a Nd2O3 compound. In this process two Nd3+ ions replace the three Zn2+ ions and oxygen concentration remains constant. Thus, the reason on of the change in oxygen vacancies (defect sites such as oxygen vacancies) is not the replacement Zn2+ by Nd3+ but the increase of surface to volume with decrease in size and is well reported in literatures[50–52]. 2.2.3 PL after UV irradiation Fig. 5 shows the PL spectral change obtained in Nd-doped ZnO (0.04 mol.%) under UV irradiation in air at room temperature. The UV irradiation led to a decrease of the green PL band gradually. After 90 min of UV irradiation we achieved almost saturated sate (further UV irradiation did not lead to further evolution of the PL spectrum). The UV irradiation induced quenching of the visible PL band of nanocrystalline ZnO in air is formerly observed and attributed to the photoinduced adsorption of oxygen molecules. UV irradiation in air corresponds to the photooxidation. This effect is attributed to the scavenging of electron by oxygen molecules from the conduction band of ZnO. Under UV illumination, concentrations of oxygen vacancies reduced and will decrease the concentration of recombination centers, and thus the intensity of visible band decreased[53].

Fig. 5 Changes in the PL emission spectra of the Nd-doped ZnO (0.04 mol.%) sample under the UV irradiation (the arrow pointing downwards indicates the decrease in the ‘visible’ emission band with time)

Surender Kumar et al., Nd-doped ZnO as a multifunctional nanomaterial

2.2.4 Thermoluminescence The thermoluminescence glow curves of the undoped and Nd-doped ZnO (0.04 mol.%) nanoparticles irradiated at room temperature in air with increasing exposure of the UV-irradiation with rate of 5 K/s are shown in Fig. 6. It could also be seen from the figure that the peak intensity goes on increasing with the exposure time of the UV-irradiation. There is not much change in the glow curves of the pure ZnO samples except the intensity is very low as compared to the Nd-doped samples. This shows that the number of traps developed in case of the pure samples are much less than the doped samples. The exact mechanism of the recombination of traps in ZnO is not yet completely understood[54,55]. However, as suggested by Secu and Sima[56] the TL, in this case, is mainly due to the recombination of charge carriers released from the surface states associated to the singly occupied oxygen vacancy centres. TL in ZnO associated with the interaction of the vacancies with close neighbour defect sites[57]. This is possibly due to the local charge mismatch in valency and size on trivalent Nd3+ doping. This could also be correlated with the PL visible emission band which increases with the Nd3+ doping but decreases steadily with the exposure of the UV irradiation. This could be understood easily by considering the increasing number of defects on Nd3+ doping due to the local charge imbalance and also due to the stress (because mismatch in ionic size), the different kinds of defects are generated. A single broad peak could be observed at around 640 K. The glow peak is also theoretically fitted (Fig. 7) using the Glow Fit computerized glow curve deconvolution program CGCD software code suggested by Puchalska and Bilski[58]. The trapping parameters for Nd-doped ZnO found from the curve fitting are, peak temperature (Tm) 640 K, order of kinetics (b) 1,

Fig. 6 Thermoluminescence (TL) glow curves for the Nd-doped ZnO (0.04 mol.%) material for different doses (irradiated for different time periods of UV): (1) 4 h, (2) 2 h, (3) 1 h and (7) 0 h (unirradiated sample). The same for the undoped ZnO material are also shown as: (4) 4 h, (5) 2 h, (6) 1 h, (8) 0 h (unirradiated sample). The black-body radiation (the emission from the heating planchet) is also shown as curve (9). The TL intensity (in curves 1–6) is corrected for the plate emission (emission from the heating planchet). The variation of TL intensity with UV-irradiation time (dose) is as shown in the inset

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Fig. 7 Fitted glow curve for 2 h irradiated Nd (0.04 mol.%) doped ZnO nanoparticles

frequency factor (s) 2.21×108 and activation energy (Ea) as 1.2 eV. These values are found to be the same for all the glow curves. It could be seen from the TL response that the ZnO:Nd phosphor could be used for the UV-radiation dosimetry. It could also be seen that the Nd-doped phosphor is around 3 times more sensitive than the undoped ZnO. It also has added advantage that the fading would be low (around 1% in one month) as the TL peaks appear at temperature (~640 K). 2.3 Magnetic study Fig. 8 shows the room temperature magnetization effect for the undoped and Nd-doped ZnO nanostructures. It could be clearly seen from the figure that saturation magnetization increases as Nd concentration increases from 0.01 mol.% to 0.04 mol.%. The saturation magnetization is found to be 0.013, 0.018, 0.025, 0.028, 0.033 and 0.025 emu/g for the samples 0.00, 0.01 mol.%, 0.02 mol.%, 0.03 mol.%, 0.04 mol.% and 0.05 mol.% of Nd, respectively. It is observed that the saturation magnetization decreases as Nd concentration increases beyond 0.04 mol.%. The concentration of oxygen vacancies played an important role in mediating the ferromagnetism exchange between Nd3+ ions. For low con-

Fig. 8 Room-temperature magnetic hysteresis loop of undoped and Nd-doped ZnO: (1) 0.00, (2) 0.01 mol.%, (3) 0.02 mol.%, (4) 0.03 mol.%, (5) 0.04 mol.% and (6) 0.05 mol.% (The inset shows the enlarged view of hysteresis loops)

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centration (0.01 mol.% doped Nd sample), an appreciable ferromagnetic component is developed despite the fact that this sample contains less Nd3+ ions as compared to other samples having more Nd3+ ions. For the samples containing more Nd3+ ions have more oxygen vacancies might have been generated which are responsible for the observed longrange ferromagnetism order. The corecivity is also increased with Nd3+ concentration and maximum for 0.04 mol.%. From these results, the observed ferromagnetic behaviour in the samples may be attributed to defects like oxygen vacancies, which is consistent with the bound magnetic polarons (BMP) model[15]. According to the BMP model, bound electrons in defects, like oxygen vacancies, can couple the Nd3+ ions and cause the ferromagnetic regions to overlap giving rise to long-range ferromagnetic order in the sample. In accordance with the BMP model, the magnetization of the system is assumed to originate from regions of correlated and isolated spins. Similar results also have been observed in Co-doped ZnO[59]. 2.4 Photocatalytic studies The photocatalytic activity of ZnO:Nd (0.04 mol.%) is investigated by photocatalytic decomposition of aqueous solution of the dye Rh B under UV light irradiation as shown in Fig. 9 along with the catalysis of Rh B in the dark without any irradiation. The maximum absorbance for the aqueous RhB dye is observed at around 552 nm. In the presence of ZnO:Nd as the catalyst, the absorbance decreased initially indicating adsorption of the dye Rh B (in the dark). Further, a substantial decrease in the absorbance of Rh B is observed after conducting the reaction under UV light irradiation with time increasing. The solution turned colorless within 60 min of irradiation. Similar experiments are carried out for the pure ZnO nanocrystals as well for the compare study. From these experiments, the variation in the concentrations of the Rh B solutions is plotted against the time (Fig. 9). These results clearly demonstrated that ZnO:Nd decolorizes Rh B faster than synthesized ZnO under similar experimental con-

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ditions. This is attributed to higher oxygen vacancies in ZnO: Nd3+, induced by the Nd3+ ion doping. The presence of a higher concentration of oxygen vacancies is further supported by the enhancement in the PL intensity under excitation with =280 nm (Fig. 4). Though one could not directly derive any correlation between the intensity of the emission in the PL spectrum and the photocatalytic activity in excitonic oxide semiconductors, the suggestion of Jing et al.[60] in which a possible intense PL emission could possibly result in higher photocatalytic activity due to the higher concentration of oxygen vacancies oěered a satisfying explanation for our observations. The pseudo-first order rate constant of ZnO:Nd3+ is 8.2×10–3.

3 Conclusions The synthesis of ZnO phosphor was successfully carried out by wet chemical method. Morphology and particle size were determined via TEM technique. PL intensity increased upto 10 times with the Nd-doping (0.04 mol.%) which made it promising candidate for solid state lighting application. TL studies showed a possibility of considering this material as TLD material. The ZnO:Nd3+ hybrid nanoparticles exhibited room temperature ferromagnetic properties supporting the BMP model. Visible emission band was quenched gradually under the UV-irradiation which was attributed to the photoinduced adsorption of oxygen molecules. Results on photocatalysis also demonstrated that ZnO:Nd decolorized Rh B organic dye faster than the undoped ZnO under UV light irradiation making it a strong photocatalyst. Thus the material is a multifunctional advanced material. Acknowledgments: We are thankful to the University of Delhi for providing research facilities at USIC and partial financial assistance. One of us (Surender Kumar) is thankful to the Council of Scientific and Industrial Research (CSIR), New Delhi for a research fellowship.

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Fig. 9 Photodegradation of Rh B solution with time: (1) adsorption of Rh B in the presence of undoped ZnO, (2) adsorption of Rh B in the presence of Nd-doped ZnO, (3) Rh B in the presence of undoped ZnO under UV irradiation and (4) Rh B in the presence of Nd-doped ZnO under UV irradiation

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