Effect of Ce doping on microstructural, morphological and optical properties of ZrO2 nanoparticles

Materials Science in Semiconductor Processing 30 (2015) 518–526

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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp

Effect of Ce doping on microstructural, morphological and optical properties of ZrO2 nanoparticles K. Gnanamoorthi a, M. Balakrishnan a,n, R. Mariappan b, E. Ranjith Kumar c a b c

PG & Research Department of Physics, Government Arts College, Tiruvannamalai 606 603, Tamilnadu, India Department of Physics, Adhiyamaan College of Engineering, Hosur 635 109, Tamilnadu, India Sri Ramakrishna Mission Vidyalaya Swami Shivananda Higher Secondary School, Coimbatore 641 020, Tamilnadu, India

a r t i c l e in f o

Keywords: Ce doped ZrO2 nanoparticles Structural Surface Optical properties

abstract Pure and Ce doped ZrO2 nanostructures have been synthesized by the microwave irradiation method. The prepared nanoparticles were characterized by various analytical techniques like Thermogravimetric and Differential Thermal Analysis (TG–DTA), X-Ray Diffraction (XRD), Fourier Transform Infra-Red Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Energy Dispersive Spectrum (EDS) and Transmission Electron Microscopy (TEM). The XRD pattern of Ce doped ZrO2 nanoparticles have been confirms that the tetragonal structure. TEM observations indicated that the average particle size of the pure ZrO2 some particles spherical shaped and some particles agglomeration in the range of 16–44 nm. Whereas on addition of Ce agglomeration in the range of 32–56 nm. The pure ZrO2 and Ce doped ZrO2 nanoparticles were further characterized for their optical properties by UV–vis reflectance spectra (DRS) and Photoluminescence (PL) spectroscopy. & 2014 Elsevier Ltd. All rights reserved.

1. Introduction ZrO2 has likely the richest relatives of nanostructures between all materials, such as nanoparticles, wires, tubes, etc. Recently, nanocrystalline ceramics and the application of nanoparticles to develop the ceramic properties have concerned great interest, as the mechanical, electrical and magnetic properties are crystallite size responsive [1–3]. Nanoparticles have expected much consideration in the field of material science because of their smart mechanical and physico-element properties which are completely different from their bulk counterparts. Semiconductor nanoparticles are of large importance suitable to their electronic and optical properties [4]. Ceria and zirconia are greatly reactive rare-earth metal oxides, which have been broadly studied for their applications in catalysts,

n

Corresponding author. Tel.: 91 9445140029. E-mail address: [email protected] (M. Balakrishnan).

http://dx.doi.org/10.1016/j.mssp.2014.10.054 1369-8001/& 2014 Elsevier Ltd. All rights reserved.

fuel cell, and gas sensors [5–7]. Co precipitation [8], sol–gel [9,10] and the ultrasound assisted method [11]. The use of microwave irradiation for the preparation of nanoparticles has been industrial in latest years [12]. The microwave synthesis route has the advantages of producing slighter particle size metal oxides with high purity, within tiny reaction time than the other conventional methods [13,14]. It has been reported that La (III), Ce (IV), Y (III), and Zr (IV) oxides had high adsorption ability for fluoride [15–17]. Among these adsorbents, zirconium-based resources have been rewarded more awareness in modern investigations due to their high binding attraction with fluoride [18–21]. In this work, we have prepared pure ZrO2 and different concentrations of Ce doped ZrO2 nanoparticles by a microwave irradiation method. Microwave systems are more solid and thus need slighter tools hole than conventional synthesis equipment. Microwave synthesis is capable due to its single property such as power reduction, top reaction charge, fast volumetric heating, higher selectivity and top

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yields of products. Here conventional heating techniques, the process time is restricted by the rate of temperature stream into the body of the objects from the plane as resolute by its precise heat, thermal conductivity, thickness and stickiness. Surface heating is not only measured, but also non-regular with the surfaces, boundaries and corners being much hotter than the surrounded by the substance. Equally, with microwave heating techniques the size of a material heated significantly at the equal rate is possible. This is recognized as volumetric heating. Since volumetric heating is not needy on heat convey by transmission or convection, it is promising to use microwave heating for applications where convectional heat convey is insufficient. The force is transferred the microwave heating technique from first to last the material (into the size) electromagnetically, not as a thermal warm change. Hence, the rate of heating is not restricted to the surface and the regularity of heat circulation is significantly better. To the best of our knowledge, there has been no work on the preparation of Ce doped ZrO2 nanoparticles by the microwave irradiation method. Effect of Ce doping on microstructural and optical properties of ZrO2 nanoparticles were analyzed by X-ray diffraction (XRD), Fourier transform

Fig. 1. (a and b) TG–DTA analysis of pure ZrO2 and Ce doped ZrO2 15% nanoparticles annealed at 400 1C.

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infrared spectroscopy (FT-IR), Scanning electron microscopy (SEM), Transmission electron microscope (TEM) techniques, and Energy-dispersive spectrum (EDS). Furthermore, their optical properties were assessed by UV–vis (DRS) – spectra and photoluminescence (PL) measurements.

2. Experimental procedure 2.1. Materials Cerium nitrate (Sigma Aldrich) and Zirconium acetate (Sigma Aldrich) and ammonia solution (Merk, 98%) were used for the synthesis of Ce doped ZrO2 nanoparticles. All other chemicals used were of reagent grade and double distilled water was used as solvent.

2.2. Synthesis Pure ZrO2 and Ce doped ZrO2 samples have been prepared by microwave irradiation method as following. A 0.1 M solution was first prepared by dissolving zirconium acetate in deionized water and stirred at room temperature for 15 min. Ce-doped ZrO2 powders were prepared adding the suitable amount of a 0.1 M solution of cerium nitrate to obtain the 5, 10 and 15 wt% Ce-doped zirconium oxide samples. Then, NH3 was added, under constant stirring conditions, until pH level at 8. The stirred mixture was irradiated by the microwave radiation of frequency 2.45 GHZ, for 5 min continuously. The precipitates were collected and, washed with distilled water for several times until the extracts turns into a white product. The final product was annealed at 400 1C for 3 h.

Fig. 2. X-ray diffraction patterns of pure ZrO2 and Ce doped ZrO2 nanoparticles annealed at 400 1C.

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Table 1 The lattice spacing (d), full width at half maximum (FWHM) and (hkl) planes for pure ZrO2 and Ce doped ZrO2 nanoparticles. Ce doped ZrO2 5%

Pure ZrO2

Ce doped ZrO2 10%

Ce doped ZrO2 15%

2θ

d

FWHM

(hkl)

2θ

d

FWHM

(hkl)

2θ

d

FWHM

(hkl)

2θ

d

FWHM

(hkl)

30.0795 34.6034 34.9913 42.9324 44.9964 50.0373 59.8561 62.5792 74.1014 – – –

2.97098 2.59223 2.56437 2.10666 2.01471 1.82292 1.54524 1.48439 1.27846 – – –

0.2952 0.1968 0.1574 0.6298 0.4723 0.1771 0.1771 0.2755 0.3840 – – –

101 002 110 102 102 112 211 202 220 – – –

34.6013 35.0387 42.8707 50.0845 50.4433 59.1834 59.8669 62.6421 74.1458 – – –

2.59238 2.56102 2.10955 1.82131 1.80920 1.56118 1.54498 1.48305 1.27780 – – –

0.3149 0.1968 0.2362 0.4723 0.1574 0.3149 0.5117 0.2755 0.5760 – – –

101 002 110 102 102 112 211 202 220 – – –

30.0311 34.4587 35.0117 42.9176 50.0092 50.4468 59.0885 59.8821 62.5351 72.6999 74.0440 –

2.97566 2.60278 2.56293 2.10735 1.82388 1.80908 1.56346 1.54463 1.48533 1.30069 1.27931 –

0.3542 0.1968 0.1181 0.3542 0.1378 0.2755 0.4330 0.4723 0.6298 0.4723 0.2880 –

101 002 110 102 102 112 211 211 202 004 220 –

29.9590 34.3788 35.0028 42.6704 49.8823 50.0816 50.3387 58.9784 59.7638 62.4913 72.8685 74.0751

2.98266 2.60865 2.56356 2.11899 1.82671 1.81990 1.81271 1.56612 1.54740 1.48626 1.29809 1.27885

0.2952 0.2362 0.2362 0.3936 0.1200 0.1680 0.3149 0.3542 0.4330 0.4723 0.4723 0.5760

101 002 110 102 112 220 220 103 211 202 004 220

Table 2 Calculated a and c lattice constants for the pure ZrO2 and Ce doped ZrO2 nanoparticles. Lattice constant

Ref

Pure

5%

10%

15%

a (Å) c (Å)

3.6190 5.1862

3.6138 5.1875

3.6083 5.1817

3.6022 5.1871

3.6058 5.1891

Fig. 3. Average crystalline size versus doping concentrations of pure ZrO2 and Ce doped ZrO2 nanoparticles.

Fig. 4. FT-IR Spectrum of pure ZrO2 and Ce doped ZrO2 nanoparticles annealed at 400 1C.

2.3. Characterization The microstructure of the sample was analyzed by XRD using a Bruker AXS D8 Advance instrument and the monochromatic CuKα1 wavelength of 1.5406 Å. The average crystalline size of the crystallites was evaluated using Scherrer's formula, d¼Kλ/β cos θ, where d is the mean crystalline size, K is a grain shape dependent constant (0.9), λ is the wavelength of the incident beam, θ is a Bragg reflection angle, and β is the full width at half maximum (FWHM) of the main diffraction peak. The sample morphology was observed by scanning electron microscopy (SEM), using a JEOL 5600LV microscope at an accelerating voltage of 10 kV. High resolution transmission

electron microscopy (HRTEM) and selected-area electron diffraction (SAED) were recorded on a Tecnai G20-stwin an accelerating voltage of 200 kV. The Fourier transform infrared spectra (FT-IR) of the samples were recorded by using a Nicolet 5DX FTIR spectrometer. Thermal analysis was carried out by using a thermogravimetric and differential scanning calorimeter apparatus (TG–DSC Netzsch-Model STA 409PC). The analyses were carried out with a heating rate of 10 1C/min in static air up to 800 1C. The ultraviolet (UV) spectrum of the ZrO2 samples was recorded on a Perkin Elmer UV–visible DRS spectrophotometer. The room-temperature PL spectrum was performed on a spectrofluorometer instrument (JY Fluorolog-FL3-11).

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3. Results and discussion 3.1. Thermogravimetric analysis (TG–DTA) The thermal behavior of the samples prepared under microwave irradiation method has been first investigated by TG–DTA. The samples are tested with a heating rate of 10 1C/ min in N2 gas atmosphere. Fig. 1(a and b) shows the TG–DTA curve of pure ZrO2 and Ce-doped ZrO2 at a higher dopent ratio of 15%. Differential thermal analysis (DTA) of pure ZrO2

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powder is presented in Fig. 1(a). It is observed that the oxidation is occurred at the temperature of 543 1C. The thermo gravimetric analysis (TGA) is indicated that the weight loss is observed from two steps. The first step in the range between 100 and 600 1C is ascribed to the removal of absorbed water molecules. The process resulted in the synthesis of ZrO2 with the total weight loss as 3.8%. Fig. 1(b) shows the DTA curve for Ce-doped ZrO2 at 15% indicates oxidation temperature of 671 1C. The TGA curve shows that the weight loss is observed from only one step at the

Table 3 Structural, morphological and optical properties of pure ZrO2 and Ce doped ZrO2 nanoparticles. Sample code

Temperature (1C)

Crystalline phase

Crystallite Size (nm)n

Shape and particle size (nm)nn

Band gap (eV)

Pure ZrO2 Ce doped ZrO2 (5%) Ce doped ZrO2 (10%) Ce doped ZrO2 (15%)

400 400 400 400

Tetragonal Tetragonal Tetragonal Tetragonal

40 38 34 32

Spherical shaped 16–44 – – 32–56

4.68 3.84 3.79 3.75

n

XRD. TEM.

nn

Fig. 5. SEM images. (a) Pure ZrO2, (b) 5% Ce doped ZrO2, (c) 10% Ce doped ZrO2 and (d) 15% Ce doped ZrO2.

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temperature range between 100 1C and 650 1C. This is recognized due to removal of absorbed water molecules and impurities. Also there is no significant weight loss at and above 300 1C. It is further indicating that no decomposition has been occurred at 300 1C. The process resulted in the synthesis of Ce-doped ZrO2 with the total weight loss as 5.9%. Pure ZrO2 and Ce-doped ZrO2, the small weight loss variation is observed. It may be recognized to the higher concentration of Ce component in ZrO2 powders.

planes of are given in Table 1. It shows that the value of full width at half maximum (FWHM) of the 5%, 10% and 15% of Ce doped ZrO2 and pure ZrO2 crystalline planes. Thus this increase in FWHM confirms the reduction in crystalline size. The lattice parameter of a and c calculated using Eq. (1), 1 d

2

2

¼

2

2

h þk l þ 2 a2 c

ð1Þ

These values are presented in Table 2. The crystalline size (D) of these samples are estimated using following Scherrer's formula (2),

3.2. X-ray diffraction (XRD) X-ray diffraction patterns of pure ZrO2 and Ce doped ZrO2 samples annealed at 400 1C are presented in Fig. 2. The presence of sharp peaks in the XRD patterns indicates the good crystalline structure of samples obtained. The pattern of pure ZrO2 shows the diffraction peaks of crystalline Ce doped ZrO2 corresponding to main diffraction planes, namely (101), (002), (112), (211), (202) and (220). These peaks are also show the formation of polycrystalline nature with a tetragonal structure (JCPDS 89-7710). The lattice spacing (d), angle of diffraction (2θ), full width at half maximum (β) and the identified nanocrystalline pure ZrO2 and Ce doped ZrO2 samples (hkl)

Element OK Zr

Mass% 33.29 66.71

Atom% 73.99 26.01

Element OK Zr Ce

Mass% 37.76 59.81 2.43

Atom% 77.81 21.62 0.57

D¼

0:9λ β cos θ

ð2Þ

where D is the crystalline size, λ is the wavelength of X-rays used; β is the broadening of diffraction line measured at half its maximum intensity and θ is the angle of diffraction [22]. The calculated average crystallite sizes of pure ZrO2 in 40 nm. Similarly, the average crystalline size of Ce doped ZrO2 is 38 nm for 5% dopent level, 34 nm for 10% dopent level and 32 nm for 15% dopent level. XRD graphs also show that the intensities of diffraction peaks of ZrO2 decreased as the Ce concentrations increased,

Element OK Zr Ce

Element OK Zr Ce

Mass% 40.92 57.71 1.37

Atom% 79.93 19.77 0.31

Mass% 35.08 61.91 3.01

Fig. 6. EDS spectrum. (a) Pure ZrO2, (b) 5% Ce doped ZrO2, (c) 10% Ce doped ZrO2 and (d) 15% Ce doped ZrO2.

Atom% 75.8 23.46 0.74

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i.e., ZrO2 doping with Ce caused the crystallinity to degenerate. The decrease in the crystallite size with respect to increase in Ce concentration is shown in Fig. 3. This calculated average crystalline size reveals that there is a decrease in crystalline size at the higher doping Ce concentration in ZrO2. The decrease in crystalline size with respect to increase in Ce concentration is depicted in Fig. 3 (Table 3). 3.3. Fourier transform infra-red spectroscopy (FT-IR) Fig. 4 shows the IR spectra recorded for pure ZrO2 and Ce doped ZrO2 (5%, 10%, and 15%) samples in the range 4000– 400 cm  1. The broad peak in the range 3950–3232 cm  1 corresponds to the vibrational mode of O–H bond. Also stretching modes of vibrations in asymmetric and symmetric C¼O bonds are observed at 1465–1543 cm  1, respectively. The peaks located at 2283–2924 cm  1 are due to symmetric

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and asymmetric C–H bonds, respectively. Furthermore, broad absorption peaks centered at around 1373 cm  1 is caused by the O–H stretching of the absorbed water re-absorption through the storage of the sample in ambient air. The presence f the ZrO2 bond is indicated by the absorption peak at 478 cm  1 for pure ZrO2; similarly, for 5% Ce doped ZrO2 sample it is pointed out from absorption peak in the range of 470 cm  1. New absorption peaks at 447 cm  1, 532 cm  1, 617 cm  1, 624 cm  1, 663 cm  1, 408 cm  1, 439 cm  1 and 470 cm  1 appear as Ce concentration increases from 10% to 15%. A peculiar peak at 1056 cm  1 is presented in the 10% and 15% Ce concentration with these peaks [23]. 3.4. Scanning electron microscopy (SEM) The synthesized pure ZrO2 and Ce doped ZrO2 samples have been characterized by SEM (see Fig. 5a–d). The more agglomerated crystallites in all the doping concentration

16.32 nm

112

18.27 nm

101 211

002

20 nm

20 nm

112 101

211

002 220

32.78 nm

50 nm

20 nm

Fig. 7. (a and b) TEM micrograph of pure ZrO2 inset in (b) shows the corresponding SAED pattern; (c and d) 15% Ce doped ZrO2; inset in (d) shows the corresponding SAED pattern.

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such as 5%, 10% and 15% are observed. The morphological are agglomerated shape for the pure ZrO2 sample are shown in Fig. 5(a). The morphological are agglomerated shape for three Ce doping levels in ZrO2 samples are shown in Fig. 5(b–d). Agglomerated smaller crystallites are observed in all the Ce doped ZrO2 samples.

3.5. Energy dispersive spectrum (EDS) Energy-dispersive spectrum analysis (Fig. 6a–d) is performed to investigate the elemental composition of pure ZrO2 and Ce doped ZrO2 nanostructures. EDS analysis confirmed the presence of Zr, O and Ce elements in the nanostructures. Fig. 6(a) exhibits pure ZrO2 spectrum with single intense peaks and two single small peaks which are associated with O, Zr atoms, respectively. The measured atomic percentage of these elements is about 73.99% and 26.01%. Fig. 6(b–d) shows EDS analysis of Ce doped ZrO2 powders. The measured Ce contents are about 0.31%, 0.57% and 0.74% respectively, for the three different nominal compositions of 5%, 10% and 15%, indicating that Ce

percentage into the ZrO2 nanostructure increases according the nominal loading of Ce. 3.6. Transmission electron microscopy (TEM) Fig. 7(a–d) shows the TEM micrographs of pure ZrO2 and 15% Ce doped ZrO2 nanoparticles. Insets in Fig. 7(b and d) shows the corresponding SAED patterns. TEM micrographs in Fig. 7(a and b) show some particles spherical shaped and some particles agglomeration of ZrO2 nanoparticles, having size about 16–44 nm. Fig. 7(c and d) shows that the some particles spherical shaped and some particles agglomeration of nano assemblies present on the 15% Ce doped ZrO2 sample, with size in the range of 32–56 nm, which is in good agreement with the crystallite size obtained from XRD line broadening. 3.7. UV–visible – Diffused reflectance spectroscopy The optical properties of the samples synthesized have been investigated. The UV–vis reflectance spectra of the pure ZrO2 and Ce doped ZrO2 nanoparticles are shown in

Fig. 8. UV–vis reflectance spectrum. (a) Pure ZrO2, (b) 5% Ce doped ZrO2, (c) 10% Ce doped ZrO2 and (d) 15% Ce doped ZrO2.

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Fig. 8(a–d). The reflectance values were transformed to absorbance by use of the Kubelka–Munk translation, Eq. (3) [24]. K¼

ð1  RÞ2 2R

ð3Þ

where K is the reflectance transformed according to Kubelka Munk and R is the reflectance
1=2(%) The relationship between knhϑ ¼ f ðhnϑÞis shown in Fig.8(a–d). This plot gives band gap energies of 4.68 eV for pure ZrO2 and 3.84 eV, 3.79 eV and 3.75 eV for Ce doped ZrO2 with Ce concentrations 5%, 10% and 15%, respectively. It can be noticed that the band gap decreased on adding 5%, 10% and 15% of Ce. This performance is the result of a slightly decreases in the free carrier concentration due to Ce doping and the corresponding descending shift of the Fermi level to less than of the band edge [25].

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3.8. Photoluminescence Fig. 9(a–d) shows the Photoluminescence studies are influential for investigating Ce doped ZrO2 nanostructures, because they are estimated to have different optoelectronic properties than room temperature PL spectra of pure ZrO2 and Ce doped ZrO2 (5%, 10% and 15%) samples. Fig. 9(a) exhibits broad spectrum in pure ZrO2 was monochromatic line and all the samples demonstrate the UV emission peak at 367 nm and blue emission peak at  441 nm which corresponds to the near- band-edge (NBE) emissions and originates due to the recombination of the free excitons of ZrO2. Fig. 9(b–d) exhibits Ce doped ZrO2 spectrum the blue emission at 424 nm,  428 nm and  432 nm. It exhibits strong green emission of the Ce doped ZrO2 samples at  469 nm is related to the rich defects like dislocations, which are helpful for fast oxygen transportation [26]. Fig. 9(b–d) the three additional peaks presence of green

Fig. 9. PL spectrum. (a) Pure ZrO2, (b) 5% Ce doped ZrO2, (c) 10% Ce doped ZrO2 and (d) 15% Ce doped ZrO2.

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emission peak at  535 nm,  542 nm and  548 nm and it is observed that the band becomes broader and very intense with doping of Ce doped ZrO2 nanostructures. It is often attributed to the radiative recombination of photogenerated holes with electrons occupying the singly ionized oxygen vacancy [27]. The decrease in the intensity of UV emission and increase in blue emission intensity indicates that the quantity of defects may increase with the increase in Ce- doping, which suppresses the excitonic emission of the doped samples. Maximum reduction capacity was obtained at 20% Ce but at 80% ZrO2 even more of the ZrO2 was reduced and the total amount of Ce was increased. The reduction is shifted to higher temperatures, as ZrO2 is decreased and Ce is increased [28]. 4. Conclusion Pure ZrO2 and Ce doped ZrO2 nanoparticles with different nominal Ce doping loading such as 5%, 10%, 15% were prepared by microwave irradiation method. The change in structural and morphological properties due to the effect of doping level of Ce doped ZrO2 were investigated by XRD and UV–DRS study. XRD patterns indicated that the Ce doped ZrO2 nanoparticles had polycrystalline nature. The average crystalline size of pure ZrO2 is 40 nm. Similarly, the average crystalline size of Ce doped ZrO2 is 38 nm for 5% dopent level, 34 nm for 10% dopent level and 32 nm for 15% dopent level respectively. The FT-IR spectrum confirmed, the new absorption peaks appeared at higher level of doping concentration and a peculiar peak at 1056 cm  1 is indicates the presented in the doping level at 10% and 15%. SEM analysis confirmed the doping of Ce concentration have been agglomerated and smaller crystallites. The presence of Zr, O and Ce elements in prepared samples are confirmed from EDS spectrum. In summary, the simple synthesis method proposed represents an interesting approach to produce Ce doped ZrO2 nanoparticles for optical and ceramics applications. Reference [1] S.J. Lee, M. Waltraud Kriven, J. Am. Ceram. Soc. 81 (1998) 2605–2612. [2] En.-Hai. Sun, Takafumi Kusunose, Tohru Sekino, Koichi Niihara, J. Am. Ceram. Soc. 85 (2002) 1430–1434.

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