Ni co-doping in CeO2on structural, optical and vibrational properties

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ScienceDirect Materials Today: Proceedings 5 (2018) 17641–17647

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ICMPC_2018

Effect of Co/Ni co-doping in CeO2on structural, optical and vibrational properties Saurabh Tiwaria, Sajal Biringb, Somaditya Sena* a

Metallurgical Engineering and Material Sciences, Indian Institute of Technology Indore, India b Electronic Engg., Ming Chi University of Technology, New Taipei City, Taiwan

Abstract Ce1-x-yCoxNiyO2 (x=0.0125, y=0.0125) is synthesized by sol-gel method. The structural, optical and vibrational properties of synthesized nanoparticles have been analyzed by X-ray diffraction (XRD), UV-Vis spectroscopy and Raman spectroscopy. XRD and Raman spectroscopy are pointing out the generation of strain in lattice and reduction of crystallite size. Raman broad peak at ~600 cm-1 attributes to presence of oxygen vacancies in lattice. X-ray absorption near edge structure (XANES) is used for observing the oxidation state of Ce in prepared samples. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization. Keywords: Sol-gel; Strain; Bandgap; Urbach energy

1. Introduction To improve the functionality of nanomaterials, doping process is a very important tool. In doping parent element is substituted by dopant which causes tuning of various properties of nanomaterials like structural, optical, electrical, and magnetic etc. Cerium oxide based nano materials has got greater interest in researchers. In recent time there are many applications found for multifunctionality and applicability like catalysis, biomedical, solid oxide fuel cells, gas

* Corresponding author.

E-mail address: [email protected] 2214-7853© 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization.

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sensors and in oxygen storage [1−5].TiO2, CeO2are used in photocatalysis for degradation of dye pollutants but there bandgap lies in UV range (~3.2 eV) which restrict its application in solar radiation which has only ~3-5 % UV radiation and ~43-45% visible light range. Hence to bring the bandgap down of this photo catalyst in visible range will be very vital for enhancing their functionality. Tuning of optical properties in nanomaterials is very efficiently achieved by doping.Wu et al [6] reported excellent reduction of bandgap of CeO2 with Co doping, George et al [7] reported effective tuning of bandgap with Fe doping [7]. Enhanced photocatalytic activity is reported by Channei et al [8] with Fe doping in CeO2, photocatalytic activity increases mainly due to decrease in bandgap and increases in surface area with Fe doping. Ranjith, et al [9] reported ferromagnetism increases of CeO2 with Co doping, Abbas et al [2] reported enhanced anticancer activity of CeO2 with Ni doping. Hence transition element doping tunes the electronic, optical, magnetic and bio-medical properties of CeO2. But there are very less work has been reported on the co-doping of transition elements in CeO2. In the present work we have synthesized Co/Ni co-doped CeO2nanoparticles by sol-gel technique. The structural, optical and vibrational properties have been investigated with the help of X-ray Diffraction (XRD), UV-Vis DRS (Diffused reflectance spectroscopy) and Raman spectroscopy. Increase in strain, oxygen vacancy and reduction in crystallite size and bandgap is observed. XANES analysis is used for observing the change in Ce oxidation state (Ce3+/Ce4+) with Co/Ni co-doping. 2. Experimental Cerium (III) nitrate hexahydrate [{Ce(NO3)3.6H2O}; 99.9%, Alfa Aesar], Cobalt (II) nitrate hexahydrate [{Co(NO3)2.6H2O};99.9%, Alfa Aesar], and Nickel (II) nitrate hexahydrate [{Ni(NO3)2.6H2O};99.9%, Alfa Aesar] were dissolved in double distilled de-ionized water. The three solutions were mixed and stirred for 2 h. Citric acid and glycerol was used as a chelating agent and fuel. The homogeneously mixed ionic solution was gradually poured in the chelating solution allowing ions to be homogeneously added to the chelating chains. The final aqueous mixture was heated and stirred for two hour at 800C to form a gel, a black powder that was obtained and was further calcined at 450oC for 6 h. Using the above procedure CeO2 and Ce1-x-yCoxNiyO2samples were prepared and coded as S1andS2for x, y = 0, 0;0.0125, 0.0125 respectively. The structural properties of nanoparticles have been investigated by XRD using a Bruker D2 Phaser X-ray Diffractometer. UV-vis diffused reflectance spectroscopy (DRS) analysis is performed using Shimadzu UV-vis spectrophotometer (UV-2600).High Resolution (HR) Raman Micro Spectrometer, (HORIBA Scientific, excitation wavelength 632.8nm) was used for examination of the vibrational modes of the samples. The XANES measurements on the samples at Ce L3-edge have been carried out at room temperature at EXAFS beamline (BL-9) at INDUS-2 synchrotron source (2.5 GeV, 300 mA) at RRCAT, Indore, India.

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3. Result and discussion XRD of samples S1 and S2, reveal pure cubic fluorite phase of CeO2 (Cif file-4343161; Crystallography open database). No oxide impurity phases of Co or Ni are present. This ensures successful substitution of Ce by Co and Ni. Reitveld refinement [fig. 1(a) and (b)] was performed using GSAS. Lattice parameter decreases with substitution [fig. 1 (c)]; a signature of substitution. Ionic radii of Co2+/3+ (0.75-0.9Å) and Ni2+/3+/4+ (0.6-0.83 Å) are lesser than

(a)

70

Intensity (a.u.)

60

331 420

400

311

S2

obs calc bckgr diff

(b)

S2

222

200

220

111

Normalized Intensity (a.u.)

that of Ce4+ (0.97 Å).

S1

20

30

40

50 o

2 ( )

80

20

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50 o 2( )

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Fig.1. (a) XRD spectra of pure (S1) and Co-Ni co-doped CeO2 nanoparticles (S2); (b) Rietvild refinement of Co-Ni co-doped CeO2 (S2) and (c) effect of Co/Ni- co-doping on lattice parameter, crystallite size and strain of CeO2.

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Hence, shrinkage of lattice is expected. This shrinkage and the lesser valence state of the dopant ions may lead to oxygen vacancies. A consequence may be the transformation of a Ce4+to a Ce3+ (1.143 Å) ion. Lattice parameter decreases from 5.4144 Å (S1) to 5.3396Å. Lattice shrinkage causes lattice distortion leading to lattice strain. Lattice strain was estimated from Williamson-Hall [10] equation:

=ε

D

; where, D is the effective

crystallite size, θ is diffraction angle, λ is wavelength of x-ray diffractometer, β is full width at half maxima (FWHM) of the XRD peaks and εhkl is induce strain in lattice. Strain increases with substitution [fig. 1 (c)]. DebyeScherer’s equation reveals reduction of crystallite size [Fig.1 (c)] most probably due to disruption of crystallite growth by lattice strain.

Fig. 2. (a) UV-Vis absorption spectra of S1 and S2 (inset showing Urbach fitting); (b) Variation of bandgap and Urbach energy with Co-Ni codoping

Absorption spectra (UV-vis DRS) reveals bandgap decrement due to Co-Ni co-doping in Ce0.975Co0.0125Ni0.0125O2 samples [Figure 2 (a)]. Bandgap was calculated using Kubelka-Munk function [11], F(R) = (1-R)2/2R. Bandgap of CeO2 (S1) is 3.19 eV while that of Ce0.975Co0.0125Ni0.0125O2 (S2) is 2.99 eV [Figure 2 (b)]. Doping may create impurity bands, between valance band (VB) and conduction band (CB). Some defect states are due to Ce3+ states and oxygen vacancies instigated by external impurities [9]. Distortions and disorder in lattice is best understood by calculating Urbach energy (EU): α = α0*exp(E/Eu), where, α is absorption coefficient. EU is calculated by fitting a straight line to natural logarithm plot of the absorption edge (i.e. tailing part of absorption spectra) [Fig.2 (a) inset]. Defect states increases with substitution [Figure 2 (b)]. These deformed electronic states reduce the effective gap between VB and CB, thereby reducing bandgap.

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Disorder and presence of impurities modify Raman phonon modes. CeO2 has cubic fluorite structure with a F2g symmetrical stretching mode of Ce-O. The oxygen atoms vibrate around Ce cations. S1 and S2 has F2g mode at 462.48 cm-1. The mode becomes weaker and broader with substitution [Fig. 3 (a), (b) and (c)] indicating decrease of crystallite size and increasing strain. Associated with oxygen vacancies, a broad feature at ~ 600 cm-1 intensifies with substitution indicating increasing charge imbalance due to lower valence Co2+/3+ and Ni2+/3+ in CeO2 (Ce4+) [9].

Fig. 3. (a) Raman spectra of undoped and Co-Ni doped CeO2; (b) Normalized Raman spectra; (c) Change in peak position and in FWHM

Room temperature Ce L3 XANES edge (transmission mode) reveals a mixed state of Ce. The edge consists of two major peaks [Fig. 4 (a), (b)]. The shapes are similar for both samples and were fitted by a combination of an arctangent and four Gaussian peaks, A (~5730.9eV), B (~5724.1eV), C (~5719.3eV) and D (~5712 eV). A, B  mixture of multi-electron with final states of 2p4f05d* and 2p4f15d*L, where 2p refer to a Ce 2p hole, while 5d* refer to excited electron in 5d state and L refer to a hole in anion ligand orbital (O 2p), in Ce4+ valance state. C  Ce in Ce3+ valance state. D  final states of 2p5d due to crystal field splitting of Ce 5d states cause delocalization of d character at the bottom of the conduction band [12]. Quantification of Ce valence states was determined using area ratio of Ce3+ (C) and Ce4+ (A+B) as [13]: [Ce3+] = [C / (A+B+C)]*100 and [Ce4+] = [(A+B) / (A+B+C)]*100

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Analysis of the XANES data confirms presence of Ce in Ce4+ and Ce3+ states and increase of Ce3+ state with CoNi co-doping from 16.7% (S1) to 18.4% (S2).

a

b

Fig. 4 .XANES of Ce L3-edge spectra of the undoped (S1) and Co-Ni co-doped CeO2(S2).

4. Conclusion CeO2 and Ce0.975Co0.0125Ni0.0125O2 is synthesized by sol-gel method. XRD confirms a pure phase and incorporation of Co-Ni in CeO2. Lattice parameter and crystallite size decreases with increases in strain with increasing doping. This signifies lattice disorder. Band-gap is reduced and shifted in visible region from UV region; and Urbach energy confirms strain and disorder increased doping. Raman spectroscopy is in agreement with the results. The F2g mode loses intensity and broadens, due to the reduction in crystallite size. Increase in oxygen vacancies related defects is evident from a Raman feature at ~600 cm-1. XANES analysis confirms the presence of Ce in Ce3+ and Ce4+. Acknowledgements The authors would like to thank IIT Indore for the funding,Prof. A.L.Verma and Dr. V. K. Jain (Amity University, Noida, India) for UV-Vis measurement facility, Dr. S. Saha (IISER, Bhopal, India) for Raman spectroscopy of the samples and to Dr. S. N. Jha (RRCAT Indore) for providing XANES facility at BL-9.

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