Hydrothermal synthesis and photoluminescence ...

This article was downloaded by: [The Maharaja Sayajirao University of Baroda] On: 07 April 2014, At: 00:33 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Journal of Experimental Nanoscience Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tjen20

Hydrothermal synthesis and photoluminescence properties of cerium-doped cadmium tungstate nanophosphor a

a

a

b

a

Dhaval Modi , M. Srinivas , D. Tawde , K.V.R. Murthy , V. Verma & Nimesh Patel

a

a

Physics Department, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara-390002, Gujarat, India b

Applied Physics Department, Faculty of Technology and Engineering, The Maharaja Sayajirao University of Baroda, Vadodara-390001, Gujarat, India Published online: 03 Apr 2014.

To cite this article: Dhaval Modi, M. Srinivas, D. Tawde, K.V.R. Murthy, V. Verma & Nimesh Patel (2014): Hydrothermal synthesis and photoluminescence properties of ceriumdoped cadmium tungstate nanophosphor, Journal of Experimental Nanoscience, DOI: 10.1080/17458080.2014.899714 To link to this article: http://dx.doi.org/10.1080/17458080.2014.899714

PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &

Downloaded by [The Maharaja Sayajirao University of Baroda] at 00:33 07 April 2014

Conditions of access and use can be found at http://www.tandfonline.com/page/termsand-conditions

Journal of Experimental Nanoscience, 2014 http://dx.doi.org/10.1080/17458080.2014.899714

Hydrothermal synthesis and photoluminescence properties of cerium-doped cadmium tungstate nanophosphor


Dhaval Modia*, M. Srinivasa, D. Tawdea, K.V.R. Murthyb, V. Vermaa and Nimesh Patela a Physics Department, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara390002, Gujarat, India; bApplied Physics Department, Faculty of Technology and Engineering, The Maharaja Sayajirao University of Baroda, Vadodara-390001, Gujarat, India

(Received 26 November 2012; final version received 23 February 2014) The present paper reports synthesis and photoluminescence studies of cadmium tungstate (CdWO4) and cerium-doped cadmium tungstate. The samples were synthesised by low cost and low temperature hydrothermal method and characterised by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM) and photoluminescence analysis. XRD pattern reveals that the CdWO4 has monoclinic wolframite structure. The FTIR spectrum of cerium-doped CdWO4 exhibits broadband below 700 cm1 which is due to the d (Ce–O–C) mode. The TEM images show that size of particle is approximately 60–120 nm in both samples. A broad intense peak was observed at 474 nm when the samples were excited with 263 nm. A broad intense peak was observed at 475 nm when the samples were excited with 600 nm. The intensity of the 474 nm peak decreases with increase in cerium doping concentration. The observation of 475 nm peak when excited with 600 nm is upconversion luminescence. This upconversion emission is due to energy transfer upconversion process involving Cd2þ ions and [WO6]6 ions. Ce3þ ion is responsible for the peak shift of 6 nm. Keywords: CdWO4; doping; hydrothermal method; photoluminescence

1. Introduction In view of the literature survey, it is well known that nanosized inorganic low-dimensional systems exhibit a wide range of optical properties. Luminescent materials in the form of nanobelt, nanoparticles, nanorods, nanowires, and nanotubes are of interest not only for basic research, but also for fascinating applications.[1,2] Divalent metal ion tungstates are of interest for their luminescent properties [3] and metal tungstates have good application prospects in scintillators, optical fibres, microwave applications, humidity sensors, photoluminescence materials, and catalysts, etc.[4–7] Most of the tungstates have scheelite structure or wolframite mainly depending on their cationic radii. Small radii which are in favour of forming wolframite structure and large radii which are in favour scheelite structure.[8,9] In wolframite-structured tungstates the intrinsic luminescence is occurred by the annihilation of self-trapped excitation, and thereby forms excited [WO6]6 complex that can be either excited in the absorption band or in the recombination process.[10] Cadmium tungstate (CdWO4) with a monoclinic wolframite structure is a highly functional material *Corresponding author. Email: [email protected] Ó 2014 Taylor & Francis


2

D. Modi et al.

due to its low radiation damage, low afterglow to luminescence, high average refractive index, high density (7.9 g/cm3), and high X-ray absorption coefficient.[11] Recently, because of its potential use as an advanced medical X-ray detector in computerised tomography [12] has attracted interest. CdWO4 is a monoclinic crystal with intermediate divalent cations and has been described as an ABX4 structure. The tungstate doped with rare-earth nanophosphors has attracted special attention because the corresponding bulk materials have large practical applications in solid state lighting and displays. Therefore, rare-earth ions Ce3þ act as common activators such that it will detect local environments due to their super-sensitive f–f transitions in some hosts. [13] Photoluminescence study of Ce3þ-doped host materials have been studied widely, strong emission of Ce3þ is first observed in LaF3. In particular, a host Ce3þ showed a very intense emission and in tungstate emission was quenched by Ce3þ doping.[13] Currently there are very few reports available on synthesis by various methods, i.e. hydrothermal method in particular and photoluminescent properties of rare-earth-doped cadmium tungstate nanophosphors. An extensive spectral study is required to completely understand the luminescent properties, including energy transfer and morphology of nanocrystalline tungstates. In our earlier work, we have reported the intrinsic PL emissions observed at RT for Ce and Er doped, and Ce þ Er double-doped CdWO4 nanopowder samples.[14] Maximum PL is observed in undoped sample. In this study, we report successful synthesis of ceriumdoped CdWO4 by same low cost hydrothermal method at low temperature and the upconversion intrinsic luminescence is observed for the first time in undoped and Ceþ3-doped nano-CdWO4 powder samples excited by 600 nm xenon lamp. The effects of cerium on the crystallisation, optical properties, and size were investigated.

2. Experimental details 2.1. Sample preparation Cadmium chloride (CdCl2H2O), sodium tungstate (Na2WO42H2O), and cerium oxide (CeO2) of analytical grade were purchased from Alfa Aesar and used as received without any further purification. Distilled water was used as a solvent to prepare all required solutions. Initially 30 ml solution of 0.1 M concentration of CdCl2H2O was prepared by continuous stirring and 30 ml solution of 0.1 M concentration of Na2WO42H2O was added into it dropwise. Thus prepared solution was denoted as sample a. To prepare sample b, the same procedure was adopted as that of sample a and added 30 ml solution of 0.01 M concentration of CeO2. These precursor solutions (sample a, sample b) were transferred to Teflon-lined stainless-steel autoclaves having 90 ml capacity filled with reaction media up to 80% one by one. The autoclave was maintained at a temperature of 95 C for 12 h and air cooled to room temperature. The prepared samples were washed several times with distilled water and finally with absolute ethanol. Finally, a white powder was obtained after drying in vacuum at 80 C for 2 h. Figure 1 shows the flow chart of undoped and ceriumdoped CdWO4 using the hydrothermal method.

2.2. Characterisation The XRD measurements were carried out with a Japan Rigaku D/max X-ray diffractometer, using Ni-filtered Cu Ka radiation. A scan rate of 0.05 /s was applied to record the


Journal of Experimental Nanoscience

3

Figure 1. Flow chart of the synthesis process of CdWO4 using the hydrothermal method.

patterns in the 2u range 10 –70 . FTIR spectra recorded on a Jasco FTIR- 4100 spectrophotometer (Japan) by mixing with KBr in mortar and pestle in the ratio of 1:10. The nanostructure and surface morphology of the CdWO4 were observed by transmission electron microscopy (TEM, Tecnai 20 G2 FEI made). The PL of the samples was investigated on a Shimadzu spectrofluorophotometer at room temperature with a xenon lamp as excitation source.

3. Results and discussion 3.1. XRD XRD patterns revealed that the CdWO4 can be indexed to a pure monoclinic phase of CdWO4 with a wolframite structure with space group P2/c (13); in agreement with JCPDS (Joint Committee on Powder Diffraction Standards) card No. 01-080-0139 and 01-0800137. It is seen from Figure 2 that XRD peaks of undoped CdWO4 sample are highly sharp and intense compared to cerium-doped sample. It implies that the crystallinity decreases with cerium doping. The summary of lattice parameters and average crystallite size calculated using the Scherrer is given in Table 1. D ¼ kλ=bcosu where D is the average crystallite size, k is the constant equal to 0.89, λ is the wavelength of the X-rays equal to 0.1542 nm, and b is full width at half maximum (FWHM).


4

D. Modi et al.

Figure 2. XRD patterns of undoped and cerium-doped samples (samples a and b) of CdWO4.

3.2. FTIR Figure 3 shows FTIR spectra at the wave-number range of 400–4000 cm1 of the CdWO4 particles for both sample a and b. FTIR measurements were done using KBr method at room temperature. The bending and stretching vibrations of Cd–O (542 cm1), W–O (681 cm1) and Cd– O–W (823 cm1) were observed in the undoped as well as doped CdWO4. The FTIR spectrum of sample b exhibits broadband below 700 cm1 which is due to the d (Ce–O–C) mode.[15] The absorption peaks exhibiting bands around 1626 and 3448 cm1 may be attributed to O–H stretching mode of water. Residual water and hydroxy groups are usually detected in as synthesised samples regardless of method used [16] and further heat treatment is necessary for their elimination.

3.3. TEM Figure 4(a) and 4(b) displays TEM photographs (at different magnifications) of undoped and cerium-doped samples (samples a and b) of the CdWO4 prepared at 95 C for 12 h

Table 1. Lattice parameters and average crystallite size. Lattice parameters (nm) Sample

JCPDS card no.

A B

01-080-0139 01-080-0137

a

b

c

Avg crystallite size (nm)

5.0110 5.0280

5.8040 5.8620

5.0500 5.0670

15.31 14.70



5

Figure 3. Room temperature FTIR spectra of samples a and b.

hydrothermal conditions. These images show that the rod formation is better, the boundaries are distinct and the rod surface appears quite smooth. It is clear that these CdWO4 nanorods self-assembled into bundle-like structures. The length of the nanorod is approximately 70–150 nm and the width is about 50 nm as can be seen from Figure 4(a). From Figure 4(b), it is concluded that doping of cerium does not affect morphology of cadmium tungstate in terms of its shape and size.

3.4. Photoluminescence studies The excitation spectra of undoped and Ce-doped CdWO4 phosphors recorded at room temperature is shown in Figure 5(a). There is a broad absorption band between 220 and 325 nm, a small peak of very low intensity around 365 nm, and a broad peak centred around 600 nm. The absorption intensity of the broadband centred around 600 nm of undoped is greater than that of doped sample. Figure 5(b) and 5(c) exhibits PL spectra of undoped and cerium-doped CdWO4 recorded at room temperature using 263 and 600 nm excitation wavelengths, respectively. From the figures, the photoluminescence spectra show broad intense peak at 474 and 475 nm wavelength in violet–green region peak in blue region, respectively. Interestingly, the luminescence intensity decreases when the cerium concentration increases in CdWO4. This has been observed in Figure 5(b) and 5(c). The luminescence observed in CdWO4 is due to the intrinsic defects sites which are responsible and also due to self-trapped holes and radiative recombinations. As it is reported in the literature, the PL emission with less intensity mostly in blue region is due to WO42. The presence of cadmium with tungstate (WO4) enhances 475 nm peak intensity and the emission spreads from violet to green region. The energy at around 2.5 eV probably originating from the WO42 group is mainly responsible for PL emission. It is stated in the literature that the blue emission originates from the tetragonal WO42 groups, while the origin of the green emission remains controversial.[17]


6

D. Modi et al.

Figure 4(a). TEM images of undoped CdWO4 (sample a). (b) TEM images of cerium-doped CdWO4 (sample b).

The intensity of blue–green emission band is more in Figure 5(b) when compared to Figure 5(c). It is due to higher stimulation energy of 263 nm excitation wavelength on comparison with 600 nm excitation wavelength. The cerium-doped sample shows weaker luminescence intensity than that of undoped sample in both the figures. The suppression of intensity is due to the non-radiative 5d–4f transition of the excited Ce3þ. Therefore, Ce3þ ions could serve as non-radiative traps in CdWO4 crystal lattice.[18] The broad emission from 400–600 nm may be obtained with the inclusions of the raspite structure formed due to the thermal stress appearing in the process of crystal growth.[19–21] It is well known that the luminescence spectrum of XWO4 is composed of two main bands; the ‘blue’ one peaking around 420 nm and the ‘green’ one peaking between 490 and 540 nm. There are two defects: (1) regular lattice (WO4)2 defect and (2) defect based on WO3 centre, which are responsible for the complex character of CdWO4 emission. The blue luminescence is an intrinsic feature of CdWO4 and is generally ascribed to the radiative decay of a selftrapped excitation that locates on the regular (WO4)2 group.[22] The green luminescence is of extrinsic origin and it was ascribed to a defect-based WO3 centre [23] possibly with F



7

Figure 5(a). Excitation spectra of a. CdWO4 b. CdWO4:Ce. (b) PL spectra of a. CdWO4 b. CdWO4: Ce recorded with 263 nm excitation wavelength. (c) Room temperature PL spectra of samples a and b recorded with 600 nm excitation wavelength.

centre.[24] PL spectra composed of several sub-bands which are almost distributed throughout the entire 350–600 nm region. Such type of structural shape invokes the presence of four to five Gaussian components. The presence of four to five Gaussian components indicates the excited states of emission centre are relaxed and degenerated under the influence of perturbation. From Figure 5(c) the broad emission from 400–600 nm when excited with 600 nm may be due to upconversion process occurred with reduced intensity at 475 nm of CdWO4 and CdWO4:Ce. The reason for reduction in intensity may be due to the increase of non-radiative transitions and or less excitation energy or with the increase in excitation wavelength. The intensity of 475 nm peak when excited with 600 nm is 60% of the PL emission intensity of 475 nm peak when excited with 263 nm. The similarity of PL spectra and possible observed upconversion spectra for the nano-CdWO4 and CdWO4:Ce phosphors suggests that emission centres may be same in both the cases. The upconverted intrinsic luminescence emission exhibited by CdWO4 and CdWO4:Ce nanophosphors can be explained on the basis of anti-Stokes multi-photon theory involving the capture of electrons in the intermediate levels due to d–d transition of Cd2þ ions. Upconversion process generally occurs

8

D. Modi et al.

Table 2. Peak positions with respect to excitation wavelength. Excitation wavelength (nm)


263 600

Peak position (nm) CdWO4

Peak intensity CdWO4

Peak position (nm) CdWO4:Ce

Peak intensity CdWO4:Ce

474 475

460 270

468 469

380 200

due to two possible mechanisms (1) excited state absorption and (2) energy transfer upconversion (ETU).[25] In the first mechanism a single ion is involved whereas two ions are involved in the second process.[26] In the ETU process the two excited ions which are present in intermediate state have a close affinity that are coupled by a non-radiative process in which one ion returns to the ground state while the other ion is promoted to the upper level. In most of the cases, these cross-relaxation processes are based on electric dipole– dipole interaction. In the present phosphors, the Cd2þ ions absorb energy through d–d transition and transfer to WO4 ions for intrinsic emission. It is also observed from Figure 5(b) and 5(c) and Table 2 that there is a minor peak shift of 6 nm which is mainly responsible due to the occupation of Ce3þ ions in the substitution positions of CdWO4 host crystal lattice. From XRD and TEM measurements, it is inferred that both the samples will have same phase and structure. Therefore the shifting of peak does not relate with phase and it is attributed to doping of cerium. The position of the peak changes with the increase in concentration of Ce3þ ions and shift occurred from blue to violet region.[27] 4. Conclusion CdWO4 was synthesised by hydrothermal process at 95 C for 12 h. XRD patterns reveals that the crystallinity decreases with cerium doping. TEM images infer that the doping of cerium does not affect the morphology of cadmium tungstate in terms of its shape and size. The emission spectra of CdWO4 nanophosphors under different excitations exhibit a broad intense peak at 474 and 468 nm wavelength in the blue–green region and the luminescence intensity decreases due to doping of cerium in CdWO4. The suppression in intensity suggests that the non-radiative traps are formed in the forbidden energy gap of CdWO4. Upconversion process occurred with reduced intensity of the main luminescence peaks at 475 nm. This upconversion emission is due to ETU process involving Cd2þ ions and [WO6]6 ions. It is concluded that Ce ion is responsible for the peak shift of 6 nm. Acknowledgements Dhaval Modi is thankful to Dr N.P. Lalla (UGCDAE CSR Indore) for TEM characterisation and Dr Mukul Gupta (UGCDAE CSR Indore) for XRD to carry out this research work.

References [1] Feldmann C, Ju€ ustel T, Ronda CR, Schmidt PJ. Inorganic luminescent materials: 100 years of research and application. Adv Funct Mater. 2003;13:511–516.



9

[2] Edelstein AS, Cammarata RC. Nanomaterials: synthesis, properties and applications. London: Taylor & Francis; 1998. [3] Blasee G, Grabmaier BC. Luminescent materials. Berlin: Springer-Verlag; 1994. [4] Chen SJ, Zhou JH, Chen XT, Li J, Li L-H, Hong J-M, Xue ZL, You X-Z. Fabrication of nanocrystalline ZnWO4 with different morphologies and sizes via hydrothermal route. Chem Phys Lett. 2003;375:185–190. [5] Kwan S, Kim F, Akana J. Synthesis and assembly of BaWO4 nano-roads. Chem Commun. 2001;447–448. [6] Liu B, Yu S-H, Zhang F, Li LJ, Zhang Q, Ren L, Jiang K. Nanorod-direct oriented attachment growth and promoted crystallization processes evidenced in case of ZnWO4. J Phys Chem. B 2004;108:2788–2792. [7] Hu XL, Zhu Y. Morphology control of PbWO4 nano- and microcrystals via a simple, seedless, and high-yield wet chemical route. J Langmuir. 2004;20:1521–1523. [8] Kloprogge JT, Weier ML, Duong LV, Frost RL. Microwave-assisted synthesis and characterisation of divalent metal tungstate nanocrystalline minerals: ferberite, h€ ubnerite, sanmartinite, scheelite and stolzite. Mater Chem Phys. 2004;88:438–443. [9] Yu SH, Liu B, Mo MS, Huang JH, Liu XM, Qian YT. General synthesis of single-crystal tungstate nanorods/nanowires: a facile, low-temperature solution approach. Adv Funct Mater. 2003;13:639–647. [10] Pankratov V, Grigorjeva L, Millers D, Chernov S, Voloshinovskii A. Luminescence center excited state absorption in tungstates. J Luminescence. 2001;94:427–432. [11] Lotem H, Burshtein Z. Method for complete determination of a refractive-index tensor by bireflectance: application to CdWO4. Opt Lett. 1987;12:561–563. [12] Greskovich CD, Cusano D, Hoffman D, Riedner R. Ceramic scintillators for advanced medical X-ray detectors. J Am Ceramic Soc Bull. 1992;71:1120–1130. [13] Yen WM. General factors governing the efficiency of luminescent devices. Phys Solid State. 2005;47:1393–1399. [14] Modi D, Srinivas M, Tawde D, Garg S, Vishwnath V, Khatri D, Patel N, Murthy KVR. Photoluminescence investigation of CdWO4: Ce, Er doped nano phosphors. Int J Luminescence its Appl. 2012;2:27–29. [15] DosSantos ML, Lima RC, Riccardi CS, Tranquilin RL, Bueno PR, Varela JA, Longo E. Preparation and characterization of ceria nanospheres by microwave-hydrothermal method. Mater Lett. 2008;62:4509–4511. [16] Zawadzksi M. Preparation and characterization of ceria nanoparticles by microwave-assisted solvothermal process. J Alloys Compounds. 2008;454:347–351. [17] Anicete-Santos M, Orhan E, de Maurera MAMA, Sim~ oes LGP, Souza AG, Pizani PS, Leite ER, Varela JA, Andres J, Beltran A, Longo E. Contribution of structural order-disorder to the green photoluminescence of PbWO4. Phys Rev B. 2007;75:165105. [18] Nikl M, Laguta VV, Vedda A. Complex oxide scintillators: material defects and scintillation performance. Phys Status Solidi B. 2008;245:1701–1722. [19] Annenkov AN, Vasil’chenko VG, Korzhik MV, Ligun VN, Lobko AS, Pavlenko VB, Timoshchenko TN. Luminescence of PbWO4 single crystals. J Appl Spectrosc. 1994;61:495–499. [20] Itoh M, Alov DL, Fujita M. Exciton luminescence of scheelite- and raspite-structured PbWO4 crystals. J Luminescence. 2000;87:1243–1245. [21] Alov DL, Rybchenco SI. Luminescence properties of raspite PbWO4. Mater Sci Forum 1997;279:239–241. [22] Nikl M. Presented at the CCC meeting, May 1995, CERN, W. Van Loo, Phys. Stat. Sol. a. 27, 1979, 565. [23] Groenink JA, Blasse G. Some new observations on the luminescence of PbMoO4 and PbWO4. J Sol State Chem. 1980;32:9–20.

10

D. Modi et al.


[24] Lecoq P, Dafinei I, Auffray E, Schneegans M, Korzhik V, Missevitch OV, Pavlenko VB, Fedorov AA, Annenkov AN, Kostylev VL. Lead tungstate (PbWO4) scintillators for LHC EM calorimetry. Nucl Instruments Methods. A 1995;365:291–298. [25] Tawde D, Srinivas M, Murthy KVR. Effect of lead source and cerium (III) doping on structural and photoluminescence properties of PbWO4 microcrystallites synthesized by hydrothermal method. Phys Status Solidi A. 2011;208:803–807. [26] Chai C, Yang S, Liu Z, Liao M, Chen N, Wang Z. The PL “violet shift” of cerium dioxide on silicon. Chin Sci Bull. 2001;46(24):2046–2048. [27] Huang Y, Jang KH, Jang K, Seo HJ. Up-conversion fluorescence of Pr3+ ions in PbWO4 single crystal. J Luminescence. 2007;122–123:47–50.