Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 556–563
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Synthesis and characterization of ZrO2–CuO co-doped ceria nanoparticles via chemical precipitation method G. Viruthagiri ⇑, E. Gopinathan, N. Shanmugam, R. Gobi Department of Physics, Annamalai University, Annamalai Nagar, 608 002 Tamil Nadu, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
We have synthesized nanostructures
of ZrO2–Cuo doped with CeO2 by simple chemical precipitation method. The PL emission has emission bands at UV and visible regions as a result of 2p ? 4f transition. In Surface analysis crystalline size of nanosized powders were calculated using BET techniques.
a r t i c l e
i n f o
Article history: Received 26 February 2014 Received in revised form 11 April 2014 Accepted 22 April 2014 Available online 30 April 2014 Keywords: Ceria Nanocrystals Fluorite Quantum confinement UV-DRS
a b s t r a c t In the present study, the fluorite cubic phase of bare and ZrO2–CuO co-doped ceria (CeO2) nanoparticles have been synthesized through a simple chemical precipitation method. X-ray diffraction results revealed that average grain sizes of the samples are within 5–6 nm range. The functional groups present in the samples were identified by Fourier Transform Infrared Spectroscopy (FTIR) study. Surface area measurement was carried out for the ceria nanoparticles to characterize the surface properties of the synthesized samples. The direct optical cutoff wavelength from DRS analysis was blue-shifted evidently with respect to the bulk material and indicated quantum-size confinement effect in the nanocrystallites. PL spectra revealed the strong and sharp UV emission at 401 nm. The surface morphology and the element constitution of the pure and doped nanoparticles were studied by scanning electron microscope fitted with energy dispersive X-ray spectrometer arrangement. The thermal decomposition course was followed using thermo gravimetric and differential thermal analyses (TG-DTA). Ó 2014 Elsevier B.V. All rights reserved.
Introduction Nanomaterials contain particles with one dimension in the nanometer regime. Recently, there is a growing interest from the scientific community in the applications of these nanomaterials, which are sometimes referred to as ‘‘the next industrial revolution’’ [1]. Nanoparticles have received much attention in the field of ⇑ Corresponding author. Tel.: +91 9486223626. E-mail address: [email protected] (G. Viruthagiri). http://dx.doi.org/10.1016/j.saa.2014.04.117 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
material science because of their fascinating mechanical and physico-chemical properties which are entirely different from their bulk counterparts. Semiconductor nanoparticles are of great interest due to their electronic and optical properties [2]. Among these semiconductor nanoparticles, cerium oxide has been of great interest in versatile applications due to its chemical stability and close lattice parameter with silicon [3]. It is a noticeable functional material with an extraordinary capacity to store and release oxygen with cubic fluorite structure [4]. Among oxides, the cubic CeO2 phase (fluorite) has long been considered as one of the most
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promising materials because of high refractive index, good transmission in visible and infrared regions, strong adhesion, and high stability against mechanical abrasion, chemical attack and high temperatures [5]. Cerium oxide (CeO2) particles have been extensively studied owing to their potential uses in many applications, such as UV absorbents and filters [6,7], buffer layers with silicon wafer [8], gas sensors [9], photo catalysts in the fuel cell technology [10–12], catalytic wet oxidation [13], engine exhaust catalysts [14], NO removal [15], and photo catalytic oxidation of water [16]. However, the material performances in practical uses are strongly influenced by the properties of constituent CeO2 particles. Many methods have been employed to produce dopedcerium oxide nanoparticles such as the chemical precipitation, sol–gel method, microwave-assisted hydrothermal processes [17–23], mechano-chemical processing [24], polyvinyl pyrrolidone (PVP) solution route [25], electrochemical synthesis [26], combustion method [27], direct sono-chemical route [28] and gas–liquid co-precipitation [29]. In this study, we have used the chemical-precipitation method to synthesize bare and co-doped CeO2 powders. The main advantage of the chemical precipitation technique has the capability of producing ultrafine powders with high purity and homogeneous phase composition at lower temperatures. Recently, simultaneous doping of two kinds of atoms (co-doping) into semiconductor materials has attracted considerable interest, as it could result in a higher photocatalytic activity and special characteristics compared with single element doping into semiconductor oxides, such as Ag+ and La3+, metallic silver and V, Ga/ Al/Co co-dopants [30–32]. In the present work, we have synthesized ZrO2–CuO co-doped ceria nanoparticles and the effects of doping on the structural and optical properties of CeO2 were analyzed.
(NH4)4[Ce(SO4)4].2H2O
C4H6CuO4.H2O
C2H2O4.2H2O
Zr (NO3)2.H2O
Stirring for 5hrs+Heat 80°C
Precipitation
Washing with De-ionized water
Temp 400 °C 6hrs
Calcinations
CeO2/Doped nanopowder
Materials and method Scheme 1. Schematic representation for preparation of ceria Nps.
In the chemical precipitation method the, stoichiometric ratios of the materials Ammonium Ceric Sulfate ((NH4)4 [Ce(SO4)4]2H2O), Oxalic acid (C2H2O42H2O), Copper acetate (C4H6CuO4H2O), Zirconium nitrate (Zr(NO3)2H2O) were used as precursors. All chemicals were used as received in analytical reagent (AR) grade with 99% purity. 0.2 M of ((NH4)4 [Ce(SO4)4]2H2O) was dissolved in 25 ml of deionized water and then stirred the solution vigorously. Next the solution 0.2 M of (C4H6CuO4H2O) and (Zr(NO3)2H2O) was dissolved in 10 ml of deionized water was added drop wise to the solution. Finally 0.3 M of (C2H2O42H2O) was dissolved in 25 ml of deionized water was added drop wise to solution under vigorous stirring. The solution was heated at 60 °C and continuously stirred for 5 h using magnetic stirrer. By gradually mixing these of solutions a pale blue precipitate were formed. After the obtained precipitate was washed repeatedly with deinosied water and then filtered. The precipitate was dried in hot air oven at 100 °C for 1 h, and then the product was annealed at 400 °C in muffle furnace for 6 h to get phase pure and doped ceria nanopowders. The possible chemical reactions are given below:
The XRD pattern of bare and ZrO2–CuO–CeO2 is given in Fig. 1(a and b). All the marked diffraction peaks of CeO2 can coincidently be indexed by the standard CeO2 (JCPDS card No: 81-0792). The crystallographic phase of bare CeO2 belongs to the face-centered cubic (FCC) fluorite type and the space group Fm-3m. The relatively high intensity of the plane (1 1 1) is indicative of anisotropic growth and implies a preferred orientation of the crystallites. Fig. 1b shows the XRD pattern of ZrO2–CuO–CeO2 nanoparticles. It is different from that of bare CeO2 pattern. In the ZrO2–CuO–CeO2 system, there is a shift in the diffraction peaks which correspond to ZrO2–CuO. These shift at high angle side due to the dopant ions take interstitial positions of host matrix (or) placed on a metal surface. This confirms the loading of ZrO2–CuO on CeO2. Broadening of diffractograms indicate that the reduction of the size of the particle when compared to bare CeO2.
The preparation of bare and co-doped CeO2 nanoparticles by the chemical precipitation method is illustrated in Scheme 1.
The crystallite size of the ceria nanoparticles was determined using Scherrer’s equation,
Results and discussion Powder X-ray diffraction study (XRD)
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Fig. 1. XRD pattern of: (a) bare CeO2 and (b) ZrO2–CuO–CeO2 nanoparticles.
D¼
Kk Å b cos h
ð1Þ
where D is the particle diameter in Å, k is a constant (shape factor) with a value of 0.89, b is the half maximum line-width, and k is the wavelength of the X-ray (1.5406 Å). It could be noticed that the crystallite size of the particles was about 6 nm for CeO2 and about 5.2 nm for the doped cerium oxides. The difference in size between pure and doped ceria was probably due to sensitive dopant incorporation in the structure and resulted in changing crystal growth kinetics. Subsequently, the lattice constant (a) values have been found to be 5.41 and 5.40 Å for bare and doped ceria nanoparticles respectively, which are in good accordance with the standard value of 5.41 Å (JCPDS 81-0792). The obtained values of present study are matched well with the values reported by earlier researchers [33,34]. The possible structural parameters are calculated from the following equations,
Microstrain ¼
b cos h 4
ð2Þ
Lattice strain ¼ 4e tan h
qx ¼
ð3Þ
ZM
ð4Þ
Na3
where M is the molecular weight of the sample (kg), N is the Avogadro’s number (6.022 1023 per mole) and a is the lattice constant (Å). The calculated values are listed in Table 1.
Table 1 Structural properties of bare and doped cerium oxide nanoparticles. Parameters
CeO2
ZrO2–CuO–CeO2
Crystallite size (nm) Lattice constant (Å) Volume (A3) Strain Stacking fault X-Ray density, Dx (g/cm3) Dislocation density, d
6.05 5.41 158.34 0.0034 0.72 1.443 0.002
5.28 5.40 157.46 0.0038 0.77 1.451 0.003
Fourier Transform Infrared Spectroscopy (FTIR) Fig. 2(a and b) shows the FTIR spectra of the bare and co-doped (ZrO2–CuO) ceria nanoparticles annealed at 400 °C for 6 h. The FTIR spectrum of the ceria also exhibits a strong, broad band below 700 cm1 which is due to the d (Ce–O–C) mode. Specifically, the strong absorptive peaks at 437 and 462 cm1 are attributed to the Ce–O stretching vibration [35]. Moreover, the band observed in the region 500–400 cm1 is related to the metal–oxygen bonds, is more intense for the sample synthesized at lower temperatures indicating a higher amount of ceria formed. Additional bands around 1110–1120 cm1 and 985–850 cm1 are most probably associated to the presence of residual organic or the formation of ‘‘carbonate-like’’ species on the ceria surface [33]. The bands at 2374 and 2376 cm1 may arise from the absorption of atmospheric CO2 [36]. The bands at 2929 and 2924 cm1 are due to the stretching vibration of the C–H bond of organic compounds [37]. Water incorporation is found with the peak in the range 1600– 1631 cm1 for all powders. It is known that the broad band in the range 3408–3385 cm1 is due to the stretching vibration of the hydroxyl (O–H) group [38]. Existances of these peaks have strongly confirmed the presence of hydroxyl ions in the structure of the samples. The obtained FT-IR results are in good agreement with the XRD analysis. Surface analysis BET was performed to determine the specific surface area of the synthesized powders. The surface area of bare and co-doped ceria nanoparticles was determined using the nitrogen gas adsorption method. N2 adsorption–desorption isotherms of bare CeO2 and ZrO2–CuO–CeO2 are shown (Figs. 3a and 3b). The evaluated BET surface area of ZrO2–CuO–CeO2 (73.91 m2/g) is smaller than the bare CeO2 (93.08 m2/g) [39]. The average crystalline size of the nanosized powders can be figured out by using the following equation
dBET ¼
6000
qSBET
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559
Fig. 2. FTIR spectra of (a) bare CeO2 and (b) ZrO2–CuO–CeO2 nanoparticles.
Fig. 3a. N2 adsorption–desorption isotherms of bare CeO2 nanoparticles.
SBET ¼
6000
qdBET
ð5Þ
where dBET is the crystalline size in (nm), q is the density of nanosized CeO2 powder (in gm/cm3) SBET is the BET specific surface area. The calculated crystalline size was 8.9 and 11.2 nm respectively for bare and doped ceria nanoparticles. After doping with ZrO2–CuO, the crystallite sizes of nanoparticles were increased and resulted in decrease of the surfaces of nanoparticles. This happened because the inversely relationship between the SBET and dBET as shown in Eq. (5). The agglomeration coefficient can be calculated using the following formula:
CF ¼
dBET D
ð6Þ
where CF is agglomeration coefficient, dBET and D is the average crystalline size determine by the BET method and the XRD Scherer
formula respectively. If the CF is bigger than the agglomeration is more evident. The agglomeration coefficient values are calculated using Eq. (6) and are 1.6 and 2.1, respectively, for the bare and doped nano CeO2 powders.
Ultra Violet-Diffuse Reflectance Spectra (UV-DRS) Direct band gap energy (plot of (a h m)2 vs. hm) Fig. 4a shows UV-DRS spectra of bare and ZrO2–CuO doped CeO2 nanoparticles annealed at 400 °C for 6 h. From the figure, it is clear that the spectrum of CeO2 showed an absorption peak at 344 nm in the UV region. However, on curve shape of the DRS spectrum of CeO2 nanoparticles remarkable changes after ZrO2–CuO was added. Then doped with metal ions, there is considerable shift in peak observed towards the UV range Fig. 4a for the samples.
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Fig. 3b. N2 adsorption–desorption isotherms of ZrO2–CuO–CeO2 nanoparticles.
Fig. 4a. Plot of direct band gap energy for (a) bare CeO2 and (b) ZrO2–CuO–CeO2 nanoparticles.
The fundamental absorption, which corresponds to the transmission from valence band to the conduction band, is employed to determine the band gap of the material. The direct band gap energy can be estimated from a plot of (a h m)2 vs. photon energy (hm) by using the relationship, 2
ðahmÞ ¼ Aðhm Eg Þ
n
ð7Þ
The optical absorbance coefficient (a) of a semiconductor close to the band edge can be expressed by the following equation,
a ¼ Aðhm Eg Þn =hm
ð8Þ
where a = 4pk/k (k is the absorption index or absorbance) and A depicts constant factor. The exponent n depends on the nature of the transitions; n may have values 1/2, 2, 3/2 and 3 corresponding to allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions, respectively. In this case n = 1/2 for allowed direct transition. The plot of (a h m)2 vs. hm was presented in Fig. 4a. The value of the energy band gap was determined by extrapolating the straight line portions of (a h m)2 = 0 axis (i.e. the
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zero co-efficient).The intercept of the tangent to the plot will give a good approximation of the direct band gap energy of the materials. The calculated direct band gap value of bare CeO2 was 3.81 eV, with the incorporation of ZrO2–CuO content, the band gap value would be shifted into 4.22 eV. The blue shift in the DRS spectrum attributed to the quantum size effect. Moreover, it could also be seen that in ZrO2–CuO doped CeO2 nanoparticles, a sharp increase in reflectance was noted than the undoped CeO2 sample due to strong absorption. On the effect of dopant content, the absorption edge suffered a gradual blue shift. The spectra distinctly exhibited a strong absorption band in the UV region, which is due to the charge-transfer from O 2p to Ce 4f, which overruns the well-known f–f spin orbit splitting of the Ce 4f state [40–42]. From the UV spectrum of these samples shows most of the UV light (200–350 nm) is blocked, allowing ceria nanoparticles to be used as a UV blocker. In other words, the absorbing band of the doped CeO2 caused a blue shift comparing with that of bare CeO2. The blue shifting phenomenon is due to the fact that the particle size of the doped CeO2 was smaller than that of the bare CeO2. These results reveal that the band gap energy increases when the particle size is reduced. Since ceria is a direct band gap semiconductor, a decrease in particle size is exhibited to be manifested by a blue shift of the absorption edge [43]. Indirect band gap energy (Kubelka–Munk plot) The reflectance spectra were analyzed using the Kubelka–Munk relation (Eq. (9)). To convert the reflectance data into a Kubelka– Munk function (equivalent to the absorption coefficient) F(R), the following relation was used.
FðRÞ ¼
ð1 RÞ2 2R
ð9Þ
where R is the reflectance value. Bandgap energies of the samples were estimated from the variation of the Kubelka–Munk function with photon energy. Fig. 4b shows the Kubelka–Munk plots for the undoped and ZrO2–CuO doped CeO2 samples used to determine their band gap energy associated with the indirect transitions. The pure CeO2 exhibits indirect Eg of 3.84 eV, while doping the absorption edge blue shifted and the value of Eg = 4.29 eV for doped CeO2.
Fig. 4b. Plot of indirect band gap energy (a) bare CeO2 and (b) ZrO2–CuO–CeO2 nanoparticles.
Determination of crystallite size (Brus model) The absorption edge of the CeO2 nanoparticles (Eg = 3.81 eV) is blue shifted when compared to that of the bulk cerium oxide (Eg = 3.15 eV). Already it was proposed a semi-empirical equation [44] that quantitatively describes the dependence of the CeO2 band gap for direct transition on the particle size, Eq. (8)
EgðnanoÞ ¼ EgðbulkÞ þ
h
2
p2 1
2R2 me
þ
1 1:8e2 mh eR
ð10Þ
where Eg (nano) is the measured band gap of CeO2 nanoparticles, Eg (bulk) is the band gap width for coarsely crystalline CeO2, h is the Planck’s constant, R is the nanoparticle radius, l is the reduced mass and me and mh are effective masses of electron (me/mh = 0.42 for CeO2) in the conduction band and holes in valance band respectively. ‘e’ is the electron charge and e is the relative dielectric constant of CeO2 and it is 24. The calculated particle diameter of the bare and doped cerium oxides are 5.7 and 4.6 nm respectively. Semiconductor nanocrystals of size smaller than the bulk exciton Bohr radius constitute a class of materials of an intermediate nature between molecular and bulk forms of matter. Quantum confinement of both electrons and holes in all the three dimensions leads to increase the effective band gap of the material by decreasing the particle size. As the nanoparticle size of CeO2 obtained from three methods (X-ray diffraction, BET and optical absorption studies) is smaller than Bohr radius, which is strong enough to cause a quantum confinement effect of CeO2 nanoparticles [45]. Photoluminescence (PL) Fig. 5 shows the room temperature PL spectra of bare and ZrO2– CuO doped CeO2 nanoparticles with excitation wavelengths of 320 and 290 nm. PL emission spectra have been widely used to investigate the efficiency of charge carrier trapping and migration, and to understand the fact of electron-hole pairs in semiconductors. As shown in Fig. 5, four emission peaks are observed for bare CeO2. It exhibits strong and sharp blue emission peaks observed at 401 and 468 nm respectively, with relative low intense UV-emission peak at 344nm and similarly, a strong green emission peak observed at 527 nm as shown in Fig. 5 [46,47]. The investigation showed that the emission bands ranging from 400 to 500 nm for cerium oxide samples are attributed to the trapping from different levels of the range from Ce 4f and O 2p band [40]. The PL intensity of peaks 344, 401, 468 and 527 nm emissions dramatically increases on doping for UV and PL emission, which indicates the abundant defects like dislocations, which are helpful for fast
Fig. 5. PL spectra of (a) bare CeO2 and (b) ZrO2–CuO–CeO2 nanoparticles.
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oxygen transportation. The defects, energy levels between Ce 4f and O 2p are dependent on the temperature and density of defects in the crystal. Morphology analysis Scanning electron microscopy (SEM) has been used to investigate surface morphology of the pure and doped ceria nanoparticles are shown in Fig. 6(a and c). SEM observation reveals that material consists of non-uniform distribution of particles was found and they consist of either some single particle or cluster of particles. As can be seen, powders consist of some agglomerates which are due to the ultrafine size of particles and their high surface area. However on ZrO2–CuO doping the sizes of the particles are increased in spherical morphology as shown in Fig. 6c. on close observation rather agglomerated uniform spherical grains of average size approximately below 10 nm. The confirmation of dopant existence is given by the EDX Spectrum of ZrO2–CuO doped CeO2 (Fig. 6d). The elemental constitutions of bare and doped CeO2 are shown in (Fig. 6c and d) These results confirmed that Ce, O, Zr and Cu are exist in crystal structure. Thermal analysis The TG and DTA traces of the as-prepared ceria powder are shown in Fig. 7. The DTA curve exhibits an exothermic at 570 °C and endothermic at 405 °C. The exothermic peak corresponds to desorption of physically absorbed water and organic solvent, and the endothermic peak corresponds to decomposition of some residual organic species and oxygen loss at higher temperature. However, we suggested in three-steps. The initial weight loss of 12.84% obtained between room temperature and 230 °C could be
Fig. 7. TGA /DTA curve for as-prepared ceria powders.
attributed to desorption of free and physically adsorbed water on ceria powders. The second weight loss of 4.36% recorded between 230 and 383 °C is attributed to the removal of chemisorbed water; the monolayer of H2O molecules that directly interact with the solid surface such as cerium cations and hydroxyls; and to the de-hydroxylation (release of OH from the structure). The final step of 35.63% of weight is due to the thermal decomposition of the intermediate CeO (OH) from its oxide CeO2. The thermal analysis results showed that there were no weight losses at temperatures higher than 630 °C, indicating the crystalline CeO2 formation as the final product.
Fig. 6. SEM images of (a and b) bare CeO2 and ZrO2–CuO–CeO2 nanoparticles and (c and d) corresponding EDX spectra.
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Conclusion Nano-particles of ceria have been synthesized using simple precipitation method. Cubic-fluorite ceria structure can be obtained in 6 hrs at 400 °C with relatively good crystallinity. The ZrO2–CuO doping modifies the structural and optical properties of pure CeO2. The crystalline size of ZrO2–CuO–CeO2 (5.2 nm) is lower than bare CeO2 (6 nm). The FTIR spectra signified that the characteristic vibrational frequencies of nanoceria and the chemical composition of the material belong to the (O–Ce–O) metal oxygen bond. The surface areas of pure (93.0839 m2/g) and doped (73.9188 m2/g) ceria nanopowders were measured by using BET equipment. In UV-DRS, the cutoff wavelengths of ZrO2–CuO–CeO2 were blue shifted towards the lower wavelength side, resulting larger band gap energies compared to pure CeO2. Further, the crystallite sizes were calculated by Brus model and it showed the good agreement with XRD measurements. From the PL emission spectra of pure and doped CeO2, the emission bands between 344 and 401 nm were attributed to the blue emission, whereas the bands emission at 468 and 527 nm were attributed to the lower signal green emission. Morphology of the products revealed spherical structure of bare and ZrO2–CuO doped CeO2 nanoparticles. The synthesized ceria has dimensions suitable for many applications such as oxide fuel cells, catalysts, cosmetics, optical or electrical devices, insulators, high refractive index materials, UV filters, polishing materials, gas sensors, including biomedical applications. Acknowledgements The authors thank Dr. S. Bharathan, Professor and Head, Department of Physics, Annamalai University, for his constant support and encouragement and B. Subash, Department of chemistry, Annamalai University, for support in BET measurement. References [1] I.R. Larramendi, N. Ortiz-Vitoriano, B. Acebedo, D.J. Aberasturi, I.G. Muro, A. Arango, E. Rodriguez Castellon, J.I.R. Larramendi, T. Rojo, Int. J. Hydrogen Energy 36 (2011) 10981–10990. [2] N.K. Renuka, J. Alloys Compd. 513 (2012) 230–235. [3] S. Wang, W. Wang, J. Zuo, Y. Qian, Mater. Chem. Phys. 68 (2001) 246–248. [4] J.R. Vargas-Garcia, L. Beltran-Romero, R. Tu, T. Goto, Thin Solid Films 519 (2010) 1–4. [5] F. Zhang, S.W. Chan, J.E. Spanier, E. Apak, Q. Jin, R.D. Robinson, I.P. Herman, Appl. Phys. Lett. 80 (2002) 127–129. [6] S. Tsunekawa, T. Fukuda, A. Kasuya, J. Appl. Phys. 87 (3) (2000) 1318–1321. [7] M. Yamashita, K. Kameyama, S. Yabe, S. Yoshida, Y. Fujishiro, T. Kawai, J. Mater. Sci. 37 (4) (2002) 683–687. [8] J. Tashiro, A. Sasaki, S. Akiba, S. Satoh, T. Watanabe, H. Funakubo, M. Yoshimoto, Thin Solid Films 415 (1/2) (2002) 272–275.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Synthesis and characterization of ZrO2–CuO co-doped ceria nanoparticles via chemical precipitation method G. Viruthagiri ⇑, E. Gopinathan, N. Shanmugam, R. Gobi Department of Physics, Annamalai University, Annamalai Nagar, 608 002 Tamil Nadu, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
We have synthesized nanostructures
of ZrO2–Cuo doped with CeO2 by simple chemical precipitation method. The PL emission has emission bands at UV and visible regions as a result of 2p ? 4f transition. In Surface analysis crystalline size of nanosized powders were calculated using BET techniques.
a r t i c l e
i n f o
Article history: Received 26 February 2014 Received in revised form 11 April 2014 Accepted 22 April 2014 Available online 30 April 2014 Keywords: Ceria Nanocrystals Fluorite Quantum confinement UV-DRS
a b s t r a c t In the present study, the fluorite cubic phase of bare and ZrO2–CuO co-doped ceria (CeO2) nanoparticles have been synthesized through a simple chemical precipitation method. X-ray diffraction results revealed that average grain sizes of the samples are within 5–6 nm range. The functional groups present in the samples were identified by Fourier Transform Infrared Spectroscopy (FTIR) study. Surface area measurement was carried out for the ceria nanoparticles to characterize the surface properties of the synthesized samples. The direct optical cutoff wavelength from DRS analysis was blue-shifted evidently with respect to the bulk material and indicated quantum-size confinement effect in the nanocrystallites. PL spectra revealed the strong and sharp UV emission at 401 nm. The surface morphology and the element constitution of the pure and doped nanoparticles were studied by scanning electron microscope fitted with energy dispersive X-ray spectrometer arrangement. The thermal decomposition course was followed using thermo gravimetric and differential thermal analyses (TG-DTA). Ó 2014 Elsevier B.V. All rights reserved.
Introduction Nanomaterials contain particles with one dimension in the nanometer regime. Recently, there is a growing interest from the scientific community in the applications of these nanomaterials, which are sometimes referred to as ‘‘the next industrial revolution’’ [1]. Nanoparticles have received much attention in the field of ⇑ Corresponding author. Tel.: +91 9486223626. E-mail address: [email protected] (G. Viruthagiri). http://dx.doi.org/10.1016/j.saa.2014.04.117 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
material science because of their fascinating mechanical and physico-chemical properties which are entirely different from their bulk counterparts. Semiconductor nanoparticles are of great interest due to their electronic and optical properties [2]. Among these semiconductor nanoparticles, cerium oxide has been of great interest in versatile applications due to its chemical stability and close lattice parameter with silicon [3]. It is a noticeable functional material with an extraordinary capacity to store and release oxygen with cubic fluorite structure [4]. Among oxides, the cubic CeO2 phase (fluorite) has long been considered as one of the most
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promising materials because of high refractive index, good transmission in visible and infrared regions, strong adhesion, and high stability against mechanical abrasion, chemical attack and high temperatures [5]. Cerium oxide (CeO2) particles have been extensively studied owing to their potential uses in many applications, such as UV absorbents and filters [6,7], buffer layers with silicon wafer [8], gas sensors [9], photo catalysts in the fuel cell technology [10–12], catalytic wet oxidation [13], engine exhaust catalysts [14], NO removal [15], and photo catalytic oxidation of water [16]. However, the material performances in practical uses are strongly influenced by the properties of constituent CeO2 particles. Many methods have been employed to produce dopedcerium oxide nanoparticles such as the chemical precipitation, sol–gel method, microwave-assisted hydrothermal processes [17–23], mechano-chemical processing [24], polyvinyl pyrrolidone (PVP) solution route [25], electrochemical synthesis [26], combustion method [27], direct sono-chemical route [28] and gas–liquid co-precipitation [29]. In this study, we have used the chemical-precipitation method to synthesize bare and co-doped CeO2 powders. The main advantage of the chemical precipitation technique has the capability of producing ultrafine powders with high purity and homogeneous phase composition at lower temperatures. Recently, simultaneous doping of two kinds of atoms (co-doping) into semiconductor materials has attracted considerable interest, as it could result in a higher photocatalytic activity and special characteristics compared with single element doping into semiconductor oxides, such as Ag+ and La3+, metallic silver and V, Ga/ Al/Co co-dopants [30–32]. In the present work, we have synthesized ZrO2–CuO co-doped ceria nanoparticles and the effects of doping on the structural and optical properties of CeO2 were analyzed.
(NH4)4[Ce(SO4)4].2H2O
C4H6CuO4.H2O
C2H2O4.2H2O
Zr (NO3)2.H2O
Stirring for 5hrs+Heat 80°C
Precipitation
Washing with De-ionized water
Temp 400 °C 6hrs
Calcinations
CeO2/Doped nanopowder
Materials and method Scheme 1. Schematic representation for preparation of ceria Nps.
In the chemical precipitation method the, stoichiometric ratios of the materials Ammonium Ceric Sulfate ((NH4)4 [Ce(SO4)4]2H2O), Oxalic acid (C2H2O42H2O), Copper acetate (C4H6CuO4H2O), Zirconium nitrate (Zr(NO3)2H2O) were used as precursors. All chemicals were used as received in analytical reagent (AR) grade with 99% purity. 0.2 M of ((NH4)4 [Ce(SO4)4]2H2O) was dissolved in 25 ml of deionized water and then stirred the solution vigorously. Next the solution 0.2 M of (C4H6CuO4H2O) and (Zr(NO3)2H2O) was dissolved in 10 ml of deionized water was added drop wise to the solution. Finally 0.3 M of (C2H2O42H2O) was dissolved in 25 ml of deionized water was added drop wise to solution under vigorous stirring. The solution was heated at 60 °C and continuously stirred for 5 h using magnetic stirrer. By gradually mixing these of solutions a pale blue precipitate were formed. After the obtained precipitate was washed repeatedly with deinosied water and then filtered. The precipitate was dried in hot air oven at 100 °C for 1 h, and then the product was annealed at 400 °C in muffle furnace for 6 h to get phase pure and doped ceria nanopowders. The possible chemical reactions are given below:
The XRD pattern of bare and ZrO2–CuO–CeO2 is given in Fig. 1(a and b). All the marked diffraction peaks of CeO2 can coincidently be indexed by the standard CeO2 (JCPDS card No: 81-0792). The crystallographic phase of bare CeO2 belongs to the face-centered cubic (FCC) fluorite type and the space group Fm-3m. The relatively high intensity of the plane (1 1 1) is indicative of anisotropic growth and implies a preferred orientation of the crystallites. Fig. 1b shows the XRD pattern of ZrO2–CuO–CeO2 nanoparticles. It is different from that of bare CeO2 pattern. In the ZrO2–CuO–CeO2 system, there is a shift in the diffraction peaks which correspond to ZrO2–CuO. These shift at high angle side due to the dopant ions take interstitial positions of host matrix (or) placed on a metal surface. This confirms the loading of ZrO2–CuO on CeO2. Broadening of diffractograms indicate that the reduction of the size of the particle when compared to bare CeO2.
The preparation of bare and co-doped CeO2 nanoparticles by the chemical precipitation method is illustrated in Scheme 1.
The crystallite size of the ceria nanoparticles was determined using Scherrer’s equation,
Results and discussion Powder X-ray diffraction study (XRD)
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Fig. 1. XRD pattern of: (a) bare CeO2 and (b) ZrO2–CuO–CeO2 nanoparticles.
D¼
Kk Å b cos h
ð1Þ
where D is the particle diameter in Å, k is a constant (shape factor) with a value of 0.89, b is the half maximum line-width, and k is the wavelength of the X-ray (1.5406 Å). It could be noticed that the crystallite size of the particles was about 6 nm for CeO2 and about 5.2 nm for the doped cerium oxides. The difference in size between pure and doped ceria was probably due to sensitive dopant incorporation in the structure and resulted in changing crystal growth kinetics. Subsequently, the lattice constant (a) values have been found to be 5.41 and 5.40 Å for bare and doped ceria nanoparticles respectively, which are in good accordance with the standard value of 5.41 Å (JCPDS 81-0792). The obtained values of present study are matched well with the values reported by earlier researchers [33,34]. The possible structural parameters are calculated from the following equations,
Microstrain ¼
b cos h 4
ð2Þ
Lattice strain ¼ 4e tan h
qx ¼
ð3Þ
ZM
ð4Þ
Na3
where M is the molecular weight of the sample (kg), N is the Avogadro’s number (6.022 1023 per mole) and a is the lattice constant (Å). The calculated values are listed in Table 1.
Table 1 Structural properties of bare and doped cerium oxide nanoparticles. Parameters
CeO2
ZrO2–CuO–CeO2
Crystallite size (nm) Lattice constant (Å) Volume (A3) Strain Stacking fault X-Ray density, Dx (g/cm3) Dislocation density, d
6.05 5.41 158.34 0.0034 0.72 1.443 0.002
5.28 5.40 157.46 0.0038 0.77 1.451 0.003
Fourier Transform Infrared Spectroscopy (FTIR) Fig. 2(a and b) shows the FTIR spectra of the bare and co-doped (ZrO2–CuO) ceria nanoparticles annealed at 400 °C for 6 h. The FTIR spectrum of the ceria also exhibits a strong, broad band below 700 cm1 which is due to the d (Ce–O–C) mode. Specifically, the strong absorptive peaks at 437 and 462 cm1 are attributed to the Ce–O stretching vibration [35]. Moreover, the band observed in the region 500–400 cm1 is related to the metal–oxygen bonds, is more intense for the sample synthesized at lower temperatures indicating a higher amount of ceria formed. Additional bands around 1110–1120 cm1 and 985–850 cm1 are most probably associated to the presence of residual organic or the formation of ‘‘carbonate-like’’ species on the ceria surface [33]. The bands at 2374 and 2376 cm1 may arise from the absorption of atmospheric CO2 [36]. The bands at 2929 and 2924 cm1 are due to the stretching vibration of the C–H bond of organic compounds [37]. Water incorporation is found with the peak in the range 1600– 1631 cm1 for all powders. It is known that the broad band in the range 3408–3385 cm1 is due to the stretching vibration of the hydroxyl (O–H) group [38]. Existances of these peaks have strongly confirmed the presence of hydroxyl ions in the structure of the samples. The obtained FT-IR results are in good agreement with the XRD analysis. Surface analysis BET was performed to determine the specific surface area of the synthesized powders. The surface area of bare and co-doped ceria nanoparticles was determined using the nitrogen gas adsorption method. N2 adsorption–desorption isotherms of bare CeO2 and ZrO2–CuO–CeO2 are shown (Figs. 3a and 3b). The evaluated BET surface area of ZrO2–CuO–CeO2 (73.91 m2/g) is smaller than the bare CeO2 (93.08 m2/g) [39]. The average crystalline size of the nanosized powders can be figured out by using the following equation
dBET ¼
6000
qSBET
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559
Fig. 2. FTIR spectra of (a) bare CeO2 and (b) ZrO2–CuO–CeO2 nanoparticles.
Fig. 3a. N2 adsorption–desorption isotherms of bare CeO2 nanoparticles.
SBET ¼
6000
qdBET
ð5Þ
where dBET is the crystalline size in (nm), q is the density of nanosized CeO2 powder (in gm/cm3) SBET is the BET specific surface area. The calculated crystalline size was 8.9 and 11.2 nm respectively for bare and doped ceria nanoparticles. After doping with ZrO2–CuO, the crystallite sizes of nanoparticles were increased and resulted in decrease of the surfaces of nanoparticles. This happened because the inversely relationship between the SBET and dBET as shown in Eq. (5). The agglomeration coefficient can be calculated using the following formula:
CF ¼
dBET D
ð6Þ
where CF is agglomeration coefficient, dBET and D is the average crystalline size determine by the BET method and the XRD Scherer
formula respectively. If the CF is bigger than the agglomeration is more evident. The agglomeration coefficient values are calculated using Eq. (6) and are 1.6 and 2.1, respectively, for the bare and doped nano CeO2 powders.
Ultra Violet-Diffuse Reflectance Spectra (UV-DRS) Direct band gap energy (plot of (a h m)2 vs. hm) Fig. 4a shows UV-DRS spectra of bare and ZrO2–CuO doped CeO2 nanoparticles annealed at 400 °C for 6 h. From the figure, it is clear that the spectrum of CeO2 showed an absorption peak at 344 nm in the UV region. However, on curve shape of the DRS spectrum of CeO2 nanoparticles remarkable changes after ZrO2–CuO was added. Then doped with metal ions, there is considerable shift in peak observed towards the UV range Fig. 4a for the samples.
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Fig. 3b. N2 adsorption–desorption isotherms of ZrO2–CuO–CeO2 nanoparticles.
Fig. 4a. Plot of direct band gap energy for (a) bare CeO2 and (b) ZrO2–CuO–CeO2 nanoparticles.
The fundamental absorption, which corresponds to the transmission from valence band to the conduction band, is employed to determine the band gap of the material. The direct band gap energy can be estimated from a plot of (a h m)2 vs. photon energy (hm) by using the relationship, 2
ðahmÞ ¼ Aðhm Eg Þ
n
ð7Þ
The optical absorbance coefficient (a) of a semiconductor close to the band edge can be expressed by the following equation,
a ¼ Aðhm Eg Þn =hm
ð8Þ
where a = 4pk/k (k is the absorption index or absorbance) and A depicts constant factor. The exponent n depends on the nature of the transitions; n may have values 1/2, 2, 3/2 and 3 corresponding to allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions, respectively. In this case n = 1/2 for allowed direct transition. The plot of (a h m)2 vs. hm was presented in Fig. 4a. The value of the energy band gap was determined by extrapolating the straight line portions of (a h m)2 = 0 axis (i.e. the
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zero co-efficient).The intercept of the tangent to the plot will give a good approximation of the direct band gap energy of the materials. The calculated direct band gap value of bare CeO2 was 3.81 eV, with the incorporation of ZrO2–CuO content, the band gap value would be shifted into 4.22 eV. The blue shift in the DRS spectrum attributed to the quantum size effect. Moreover, it could also be seen that in ZrO2–CuO doped CeO2 nanoparticles, a sharp increase in reflectance was noted than the undoped CeO2 sample due to strong absorption. On the effect of dopant content, the absorption edge suffered a gradual blue shift. The spectra distinctly exhibited a strong absorption band in the UV region, which is due to the charge-transfer from O 2p to Ce 4f, which overruns the well-known f–f spin orbit splitting of the Ce 4f state [40–42]. From the UV spectrum of these samples shows most of the UV light (200–350 nm) is blocked, allowing ceria nanoparticles to be used as a UV blocker. In other words, the absorbing band of the doped CeO2 caused a blue shift comparing with that of bare CeO2. The blue shifting phenomenon is due to the fact that the particle size of the doped CeO2 was smaller than that of the bare CeO2. These results reveal that the band gap energy increases when the particle size is reduced. Since ceria is a direct band gap semiconductor, a decrease in particle size is exhibited to be manifested by a blue shift of the absorption edge [43]. Indirect band gap energy (Kubelka–Munk plot) The reflectance spectra were analyzed using the Kubelka–Munk relation (Eq. (9)). To convert the reflectance data into a Kubelka– Munk function (equivalent to the absorption coefficient) F(R), the following relation was used.
FðRÞ ¼
ð1 RÞ2 2R
ð9Þ
where R is the reflectance value. Bandgap energies of the samples were estimated from the variation of the Kubelka–Munk function with photon energy. Fig. 4b shows the Kubelka–Munk plots for the undoped and ZrO2–CuO doped CeO2 samples used to determine their band gap energy associated with the indirect transitions. The pure CeO2 exhibits indirect Eg of 3.84 eV, while doping the absorption edge blue shifted and the value of Eg = 4.29 eV for doped CeO2.
Fig. 4b. Plot of indirect band gap energy (a) bare CeO2 and (b) ZrO2–CuO–CeO2 nanoparticles.
Determination of crystallite size (Brus model) The absorption edge of the CeO2 nanoparticles (Eg = 3.81 eV) is blue shifted when compared to that of the bulk cerium oxide (Eg = 3.15 eV). Already it was proposed a semi-empirical equation [44] that quantitatively describes the dependence of the CeO2 band gap for direct transition on the particle size, Eq. (8)
EgðnanoÞ ¼ EgðbulkÞ þ
h
2
p2 1
2R2 me
þ
1 1:8e2 mh eR
ð10Þ
where Eg (nano) is the measured band gap of CeO2 nanoparticles, Eg (bulk) is the band gap width for coarsely crystalline CeO2, h is the Planck’s constant, R is the nanoparticle radius, l is the reduced mass and me and mh are effective masses of electron (me/mh = 0.42 for CeO2) in the conduction band and holes in valance band respectively. ‘e’ is the electron charge and e is the relative dielectric constant of CeO2 and it is 24. The calculated particle diameter of the bare and doped cerium oxides are 5.7 and 4.6 nm respectively. Semiconductor nanocrystals of size smaller than the bulk exciton Bohr radius constitute a class of materials of an intermediate nature between molecular and bulk forms of matter. Quantum confinement of both electrons and holes in all the three dimensions leads to increase the effective band gap of the material by decreasing the particle size. As the nanoparticle size of CeO2 obtained from three methods (X-ray diffraction, BET and optical absorption studies) is smaller than Bohr radius, which is strong enough to cause a quantum confinement effect of CeO2 nanoparticles [45]. Photoluminescence (PL) Fig. 5 shows the room temperature PL spectra of bare and ZrO2– CuO doped CeO2 nanoparticles with excitation wavelengths of 320 and 290 nm. PL emission spectra have been widely used to investigate the efficiency of charge carrier trapping and migration, and to understand the fact of electron-hole pairs in semiconductors. As shown in Fig. 5, four emission peaks are observed for bare CeO2. It exhibits strong and sharp blue emission peaks observed at 401 and 468 nm respectively, with relative low intense UV-emission peak at 344nm and similarly, a strong green emission peak observed at 527 nm as shown in Fig. 5 [46,47]. The investigation showed that the emission bands ranging from 400 to 500 nm for cerium oxide samples are attributed to the trapping from different levels of the range from Ce 4f and O 2p band [40]. The PL intensity of peaks 344, 401, 468 and 527 nm emissions dramatically increases on doping for UV and PL emission, which indicates the abundant defects like dislocations, which are helpful for fast
Fig. 5. PL spectra of (a) bare CeO2 and (b) ZrO2–CuO–CeO2 nanoparticles.
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oxygen transportation. The defects, energy levels between Ce 4f and O 2p are dependent on the temperature and density of defects in the crystal. Morphology analysis Scanning electron microscopy (SEM) has been used to investigate surface morphology of the pure and doped ceria nanoparticles are shown in Fig. 6(a and c). SEM observation reveals that material consists of non-uniform distribution of particles was found and they consist of either some single particle or cluster of particles. As can be seen, powders consist of some agglomerates which are due to the ultrafine size of particles and their high surface area. However on ZrO2–CuO doping the sizes of the particles are increased in spherical morphology as shown in Fig. 6c. on close observation rather agglomerated uniform spherical grains of average size approximately below 10 nm. The confirmation of dopant existence is given by the EDX Spectrum of ZrO2–CuO doped CeO2 (Fig. 6d). The elemental constitutions of bare and doped CeO2 are shown in (Fig. 6c and d) These results confirmed that Ce, O, Zr and Cu are exist in crystal structure. Thermal analysis The TG and DTA traces of the as-prepared ceria powder are shown in Fig. 7. The DTA curve exhibits an exothermic at 570 °C and endothermic at 405 °C. The exothermic peak corresponds to desorption of physically absorbed water and organic solvent, and the endothermic peak corresponds to decomposition of some residual organic species and oxygen loss at higher temperature. However, we suggested in three-steps. The initial weight loss of 12.84% obtained between room temperature and 230 °C could be
Fig. 7. TGA /DTA curve for as-prepared ceria powders.
attributed to desorption of free and physically adsorbed water on ceria powders. The second weight loss of 4.36% recorded between 230 and 383 °C is attributed to the removal of chemisorbed water; the monolayer of H2O molecules that directly interact with the solid surface such as cerium cations and hydroxyls; and to the de-hydroxylation (release of OH from the structure). The final step of 35.63% of weight is due to the thermal decomposition of the intermediate CeO (OH) from its oxide CeO2. The thermal analysis results showed that there were no weight losses at temperatures higher than 630 °C, indicating the crystalline CeO2 formation as the final product.
Fig. 6. SEM images of (a and b) bare CeO2 and ZrO2–CuO–CeO2 nanoparticles and (c and d) corresponding EDX spectra.
G. Viruthagiri et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 556–563
Conclusion Nano-particles of ceria have been synthesized using simple precipitation method. Cubic-fluorite ceria structure can be obtained in 6 hrs at 400 °C with relatively good crystallinity. The ZrO2–CuO doping modifies the structural and optical properties of pure CeO2. The crystalline size of ZrO2–CuO–CeO2 (5.2 nm) is lower than bare CeO2 (6 nm). The FTIR spectra signified that the characteristic vibrational frequencies of nanoceria and the chemical composition of the material belong to the (O–Ce–O) metal oxygen bond. The surface areas of pure (93.0839 m2/g) and doped (73.9188 m2/g) ceria nanopowders were measured by using BET equipment. In UV-DRS, the cutoff wavelengths of ZrO2–CuO–CeO2 were blue shifted towards the lower wavelength side, resulting larger band gap energies compared to pure CeO2. Further, the crystallite sizes were calculated by Brus model and it showed the good agreement with XRD measurements. From the PL emission spectra of pure and doped CeO2, the emission bands between 344 and 401 nm were attributed to the blue emission, whereas the bands emission at 468 and 527 nm were attributed to the lower signal green emission. Morphology of the products revealed spherical structure of bare and ZrO2–CuO doped CeO2 nanoparticles. The synthesized ceria has dimensions suitable for many applications such as oxide fuel cells, catalysts, cosmetics, optical or electrical devices, insulators, high refractive index materials, UV filters, polishing materials, gas sensors, including biomedical applications. Acknowledgements The authors thank Dr. S. Bharathan, Professor and Head, Department of Physics, Annamalai University, for his constant support and encouragement and B. Subash, Department of chemistry, Annamalai University, for support in BET measurement. References [1] I.R. Larramendi, N. Ortiz-Vitoriano, B. Acebedo, D.J. Aberasturi, I.G. Muro, A. Arango, E. Rodriguez Castellon, J.I.R. Larramendi, T. Rojo, Int. J. Hydrogen Energy 36 (2011) 10981–10990. [2] N.K. Renuka, J. Alloys Compd. 513 (2012) 230–235. [3] S. Wang, W. Wang, J. Zuo, Y. Qian, Mater. Chem. Phys. 68 (2001) 246–248. [4] J.R. Vargas-Garcia, L. Beltran-Romero, R. Tu, T. Goto, Thin Solid Films 519 (2010) 1–4. [5] F. Zhang, S.W. Chan, J.E. Spanier, E. Apak, Q. Jin, R.D. Robinson, I.P. Herman, Appl. Phys. Lett. 80 (2002) 127–129. [6] S. Tsunekawa, T. Fukuda, A. Kasuya, J. Appl. Phys. 87 (3) (2000) 1318–1321. [7] M. Yamashita, K. Kameyama, S. Yabe, S. Yoshida, Y. Fujishiro, T. Kawai, J. Mater. Sci. 37 (4) (2002) 683–687. [8] J. Tashiro, A. Sasaki, S. Akiba, S. Satoh, T. Watanabe, H. Funakubo, M. Yoshimoto, Thin Solid Films 415 (1/2) (2002) 272–275.
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