CeO2 nanoscale particles: Synthesis, characterization ...

Journal of Rare Earths 36 (2018) 575e587

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Journal of Rare Earths journal homepage: http://www.journals.elsevier.com/journal-of-rare-earths

CeO2 nanoscale particles: Synthesis, characterization and photocatalytic activity under UVA light irradiation Laouedj Nadjia a, Elaziouti Abdelkader b, *, Benhadria Naceur a, Bekka Ahmed a a

Laboratory of Inorganic Materials and Application L.I.M.A, University of Science and Technology Oran Mohammed Boudiaf (USTO M.B), PB 1505 El M'naouar, 31000 Oran, Algeria b Laboratory of Electron Microscopy and Materials Science L.E.M.M.S, University of Science and Technology Oran Mohammed Boudiaf (USTO M.B), PB 1505 El M'naouar, 31000 Oran, Algeria

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 August 2017 Received in revised form 30 December 2017 Accepted 2 January 2018 Available online 13 March 2018

CeO2 nanoparticles (NPs) were synthesized in alkaline medium via the homogeneous precipitation method and were subsequently calcined at 80 C/24 h (assigned as CeO2-80) and 500 C/2 h (assigned as CeO2-500). The as-prepared materials and the commercial ceria (assigned as CeO2-com) were characterized using TGA-MS, XRD, SEM-EDX, UVevis DRS and IEP techniques. The photocatalytic performances of all obtained photocatalysts were assessed by the degradation of Congo red azo-dye (CR) under UVAlight irradiation at various environmental key factors (e.g., reaction time and calcination temperature). Results reveal that CeO2 compounds crystalize with cubic phase. CeO2-500 exhibits smaller crystallite size (9 nm vs 117 nm) than that of bare CeO2-com. SEM analysis shows that the materials are sphericallike in shape NPs with strong assembly of CeO2 NPs observed in the CeO2-500 NPs. EDX analysis confirms the stoichiometry of CeO2 NPs. UVevis DRS measurement reveals that, CeO2-500 NPs exhibits a red-shift of absorption band and a more narrow bandgap (2.6 eV vs 3.20 eV) than that of bare CeO2-com. On the contrary, Urbach energy of Eu is found to be increased from 0.12 eV (CeO2-com) to 0.17 eV (CeO2-500), highlighting an increase of crystalline size and internal microstrain in the CeO2-500 NPs sample. Zeta potential (IEP) of CeO2-500 NPs is found to be 7.2. UVA-light-responsive photocatalytic activity is observed with CeO2-500 NPs at a rate constant of 10 103 min1, which is four times higher than that of CeO2-com (Kapp ¼ 2.4 103 min1) for the degradation of CR. Pseudo-first-order kinetic model gives the best fit. On the basis of the energy band diagram positions, the enhanced photocatalytic performance of CeO2-500 nano-catalyst can be ascribed to O2 $ , · OH and R$þ as the primary oxidative species involved in the degradation of RC under UVA-light irradiation. © 2018 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.

Keywords: CeO2 NPs Congo red Photocatalytic activity Bandgap Urbach energy of Eu Band theory

1. Introduction The release of recalcitrant organic pollutants into the environment from industrial activities such as the textile, dyeing, leather, pharmaceutical, paper, cosmetic, plastic and synthetic detergent is a primary concern. Among organic pollutant compounds, the organic AZO dye such as Congo red (CR) is one of the most widely used in almost all these industries. It has a highly stable complex structure, carcinogenic, hazardous, mutagenic and toxic to the human's health as well as ecosystem, potential bioaccumulation and persistence in sediments of their degradation/

* Corresponding author. E-mail addresses: [email protected], [email protected] (E. Abdelkader).

biotransformation by-products. Thus, the treatment and refinement of wastewaters via a judicious route is crucial for environmental pollution control and industrial applications. In recent years, various technologies for treating organic dyes, such as oxidereduction and the exchanging resins of ions, coagulation/flocculation, sedimentation, filtration, adsorption, membrane separation, have been explored. However, the most conventional methods either can only transfer pollutants to other phases rather than destroy them completely or generate by-product toxic pollutants during the treatment process.1 Hence, researchers are looking forward for alternative inexpensive and suitable technologies to solve this drawback. Recently, advanced oxidation processes (AOPs) have received significant attention in terms of ecology, sustainable development and environmental protection. Among the various advanced oxidation processes, heterogeneous photocatalysis using

https://doi.org/10.1016/j.jre.2018.01.004 1002-0721/© 2018 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.

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L. Nadjia et al. / Journal of Rare Earths 36 (2018) 575e587

UV and visible light is one of the best prominent, efficient, low-cost, potentially advantageous and green environmentally friendly techniques for this purpose. The major semiconductor photocatalysts employed for the effective remedy of organic pollutants are TiO2, SnO2, ZrO2, CeO2, Fe2O3, Bi2O3, Al2O3, WO3 and ZnO metal oxides and CdS, CdSe, CdTe, ZnS, PbS and HgS metal chalcogenides. Their environment applications take benefit of oxide's high selectivity and stability conditions, nontoxicity and sufficient energies of their band gap.2 Cerium (electron configuration [Xe] 4f15d16s2) is a lanthanide series rare earth element (Z ¼ 58) and exists in both the þ3 (Ce3þ ¼ [Xe]4f1) and þ4 (Ce4þ ¼ [Xe]) oxidation states. Nano-sized CeO2, as an n-type semiconductor, is a cubic fluoritetype oxide in which each cerium site is surrounded by eight oxygen sites in FCC arrangement and each oxygen site has a tetrahedron cerium site. As an important inorganic rare earth metal oxide, CeO2 has been attracting great interest in the past decade as potential substitutes of TiO2 owing to their widespread variety of environment and energy-related applications including solid-state electrolytes for electrochemical devices,3 catalysts for three-way automobile exhaust systems (TWC),4 polishing agents for chemicalemechanical planarization process,5 and ultraviolet (UV) blocking materials in UV shielding,6 the adsorption and reaction of formaldehyde,7 oxygen storage capacity,8 hybrid solar cells,9 H2S removal,10 luminescent materials for violet/blue fluorescence11 as well as antioxidants in biotechnology and medicine.12e14 These applications depend on their inherent physicochemical properties of nanoscale materials and the specific ability of CNPs to uptake and release oxygen. CeO2 NPs is associated with rich oxygen vacancies, Ce3þ defects present in CeO2x and the relative thermodynamic efficiency of redox cycling between Ce3þ and Ce4þ ions on the surface of CeO2 NPs, therefore promoted as a higher capacity oxygen storage material.15 Under lean fuel conditions, CeO2x adsorbs oxygen in the forms of O2 $ and O2 2 . Under rich fuel conditions, the stored oxygen in CeO2 is released with regeneration of CeO2x.16 In CeO2 NPs, non-stoichiometric oxygen atoms are present at the grain boundary, and these vacancies play an important role in the stable grain boundary structure of CeO2 NPs.17 Recently, CeO2 has been proven to be a promising photocatalyst for environmental applications owing to the strong redox potential of the Ce4þ/Ce3þ couple, nontoxicity, high chemical stability, strong light absorption in the UV region (absorption edge, 385e400 nm) and commercial availability. For instance, Pouretedal et al. reported that CeO2 NPs synthesized by a simple precipitation method showed the highest photocatalytic reactivity for methylene blue degradation.18 Khan et al. developed a newly synthesized CeO2 NPs which exhibited high electrochemical sensitivity for the detection of ethanol and excellent photo-catalytic activity for the degradation of acridine orange.19 Anitha et al. reported that CeO2 NPs showed significantly higher photocatalytic efficiency for methylene blue degradation.20 Recently, Channei et al. reported that the CeO2 and Fe-doped CeO2 NPs displayed excellent photocatalytic performance toward methyl orange degradation under visible light irradiation.21 Parvathya et al. reported that CeO2 NPs prepared by co-precipitation and green synthesis method, using Azadirachta indica leaf extract, possessed a higher antibacterial activity than chemical CeO2 NPs.22 Thammadihalli et al. synthesized CeO2 NPs via solution combustion method and reported that these NPs exhibit good photocatalytic degradation and antibacterial activity against trypan blue and pseudomonas aeruginosa.23 Reddy Yadav et al. 24 reported that photocatalytic degradation of methylene blue dye using CeO2 nanoparticles shows 98% of degradation in UV irradiations. Furthermore the antibacterial properties of CeO2 NPs were investigated by their bacterial activity against two bacterial strains using the agar well diffusion method.

Unfortunately, as the primary drawback of CeO2 NPs, the broad band gap energy (3.2 eV)25 and fast recombination rate of photoinduced electronehole pairs, hence resulting in a low efficiency for visible or sunlight-driven photocatalytic applications.26 In recent years, several synthesis approaches have been developed as alternative effective methods to overcome this issue, such as precipitation,27 sonochemical,28 hydrothermal crystallization,29 microemulsion,30 mechanochemical processing,31 thermal 32 33 decomposition, spray pyrolysis, solvothermal,34 direct synthesis,35 electro-synthesis,36 flux method,37 oxalate,38 carbonate,39 peroxide,40 hydroxide,41 citric acid,42 chitosan,43 bioorganic polymer precursors,44 organometallic decomposition,45 microwaveassisted hydrothermal,46 homogeneous precipitation technique,47 solegel48 and microwave-assisted solvothermal49 methods. Nevertheless, the above methods have some disadvantages such as, high-energy consuming, need of complicated equipment, expensive and hazardous, requirement of a strong base, higher processing temperature and pressure, long reaction time caused by the multiple steps to complete the crystallization of final product and growth of crystallite size with severe loss of effective surface area. In view of better, safe and sustainable utilization of solar energy by photocatalysts, very recently, our group proposed a route and catalyst system using ethylene glycol (EG) which aimed to control particle size and surface chemistry of ceria NPs and to extend its spectral response to the visible light range. Aside from efficiency and stability, the technique is effective to synthesize ceramic powders with high crystallinity, compositional homogeneity, precise stoichiometry and ultrafine particles. In the present study, CeO2 NPs were first synthesized via the homogeneous precipitation method using ethylene glycol (EG) as a solvent and reducing agent. All prepared materials were fully characterized by TGA-MS, XRD, SEM-EDX, UVevis-DRS and IEP techniques technique. The photodegradation of Congo red (CR) azodye catalyzed by CeO2 nano-catalysts was studied under UVA-light irradiation as a function of two environmental key factors (e.g., reaction time and calcination temperature). Moreover, three preliminary experiments were conducted according to the optimum condition with UVA alone (UVA process), CeO2-NPs catalyst alone (CeO2 NPs system) and CeO2 NPs along with UVA irradiation (CeO2 NPs/UVA process) under UVA-light irradiation, for comparison. The photodegradation reactions are correlated with the pseudo-firstorder kinetic model. A feasible photocatalytic degradation pathway for CR in the presence of the CeO2 NPs together with UVA light irradiation is discussed by the charge separation model and the valance band theory. The commercial ceria (CeO2-com NPs) is also reported for a comparison purpose. 2. Experimental 2.1. Materials Cerium (III) nitrate hexahydrate (Ce(NO3)3$6H2O) (99.99%), commercial CeO2 (99.99%) were provided from Aldrich chemical Company Ltd. Ethylene glycols (EG) (C2H6O2, molecular weight (MW) ¼ 62.07 g/mol, density ¼ 1.1132 g/cm3, boiling point (BP) ¼ 197.3 C), Congo red; 1-naphthalene sulfonic acid, 3, 30-(4,40biphenylenebis (azo)) bis (4-amino-) disodium salt is a benzidinebased anionic diazo dye, that is, a dye with two azo groups (C32H22N6O6S2Na2, MW ¼ 696.67 g/mol, chemical index (C.I.) 22020, maximum wavelength (lmax) ¼ 497 nm and pKa ¼ 4) and other chemicals used in the experiments (NH4OH and H2SO4) were purchased from I.S.A Espagne. All the chemicals used in this study were of analytical grade without further purification. The Congo red dye molecular structure and its characteristics are given in Table 1.


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Table 1 The molecular structure and chemical properties Congo red azo-dye (CR). Molecular structure

Chemicals properties Molecular formula Absorption wavelength (lmax) Chemical class Molecular weight (g/mol) Molecular surface area (nm2) Density (g/cm3) Dye class used for food Melting point Color pKa

2.2. Preparation of CeO2 nano-catalysts In this one-pot synthesis, 10.91 g of Ce (NO3)3$6H2O was first dissolved in 100 mL of 80 vol% ethylene glycol (EG) solution, where EG acted as a solvent and reducing agent, with a constant stirring for 1 h and heated at 50 C until a homogeneous solution was obtained. About 25 mL of 3.0 moL/L NH4OH was added dropwise in the preheated Ce (NO3)3$6H2O/EG/H2O solution up to pH > 9 with constant stirring. The transparent solution immediately changed to a yellowish suspension. The temperature was kept between 60 and 70 C for a further 24 h to complete the reaction. Finally, the yellow colored Ce(OH)4 precipitate was collected, which were cooled to room temperature. It was washed with deionized water and ethanol and centrifuged three times to remove EG, chloride ions and unreacted reagents. It was then dried at 80 C for 24 h (labeled as CeO2-80). The obtained powder was consequently calcined at 500 C for 2 h (labeled as CeO2-500). The commercial CeO2 sample (labeled as CeO2-com) was used for comparison raison. The main reactions occurring during the experimental procedure can be written briefly as Eqs. (1e5):

C32H22N6Na2O6S2 497 nm Diazo dye 696.665 5.576 0.995 at 25 C Azo >360 C Blue (pH 3.0) to blue red (pH 5) 3e4.1

width at half maximum FWHM of the peaks in radians and Bragg angle in radians. The morphologies of the commercial and prepared CeO2 NPs were characterized by a S4800 field emission SEM (FESEM, Hitachi, Japan) using an accelerating voltage of 20 kV. Energy dispersive X-ray (EDX) analysis was carried out on a Jeol JSM 6360LV electron microscope with an attached JED-2300 Analysis Station Software. UVevis-IR spectrophotometer (Perckin Elmer Lambda 920) equipped with an integrating sphere assembly was employed to determine the UVevis diffuse reflection spectra in the wavelength range 200e800 nm. The band gap energy of the samples can be evaluated from the Eg measurements using Schuster-KubelkaMunk model.51 According to the Planck's Law and some further calculation, we can find that the absorption wavelength of the photoreactor can be done by determining its band gap value as shown in Eq. (7):

Eg ¼ h

c

l

(7)

CeðNO3 Þ3 $6H2 OðsÞ /Ce3þ ðaqÞ þ 3NO3 ðaqÞ þ 6H2 OðaqÞ

(1)

4NH4 OHðsÞ /4NH4 þ ðaqÞ þ 4OH ðaqÞ

(2)

Ce3þ ðaqÞ þ 4OH ðaqÞ þ xH2 OðaqÞ /CeðOHÞ4 $xH2 OðsÞ

(3)

where h is Planck's constant (4.13566733 1015 eV s), c the speed of light (2.99792458 1017 nm/s) and l the UVA-light wavelength (355e375 nm). From the calculation, in order to absorb a UVA-light wavelength, the band gap value of the photoreactor has to be below 3.49 eV and above 3.30 eV.

CeðOHÞ4 $xH2 OðsÞ /CeðOHÞ4 $xH2 OðsÞ

(4)

2.4. Photocatalytic study

CeðOHÞ4ðsÞ /CeO2ðsÞ þ 2H2 OðgÞ

(5)

In standard experiment, 100 mg of CeO2 catalyst was added in 200 mL of CR (20 mg/L) aqueous solution into a flask with constant stirring. The mixture, previously adjusted at pH ¼ 7, was agitated in dark with constant stirring for 30 min at 298 K to complete the adsorption/desorption equilibrium reaction. The photocatalytic reaction was initiated by irradiating the system with 35 W mercury lamp using BLX-E365 photoreactor under continuous stirring. At given intervals, 5 mL aliquots were collected, centrifuged at 3500 r/ min for 15 min, and then filtered to remove the catalyst particles for analysis. The filtrates were finally monitored using a Shimadzu UV mini-1240 UVevis spectrophotometer at the maximum absorption wavelength of Congo red (497 nm) using 1 cm optical pathway cells. The efficiency of CeO2 catalysts in terms of adsorption percentage h (%) and heterogeneous Fenton-like oxidation percentage h0 (%) were calculated using the following equations Eqs. (8) and (9):

2.3. Characterization The thermogravimetric analysis coupled with mass spectroscopy (TG-MS) was performed with a Setaram Instrument, at a heating rate of 10 C/min, at temperature range from 25 to 800 C, in air gas purge of 30 mL/min with a sample mass of 22.4 mg. The crystal structures of the commercial and as-prepared CeO2 catalysts were analyzed by Bruker D8 Advance XRD with Cu Ka radiation (l ¼ 0.154178 nm) for 2q over 10 e80 and a scanning rate of 10 /min and accelerating voltage of 40 kV at room temperature. The crystalline size of CeO2 catalysts involved in the researched solid established on X-ray diffraction line broadening by using Scherrer equation50 as follows Eq. (6):

dXRD ¼

0:9l b sinq

(6)

where dXRD, l, b, and 2q are the average crystallite size of the phase under exploration, the X-ray wavelength 0.15406 nm, the full-

hð%Þ ¼

ðCi C60 Þ 100 Ci

(8)

578


2

h0 ð%Þ ¼ 4

C60 Cf

3

C60

5 100

(9)

where Ci denotes dye initial concentration (mg/L), C60 dye residual concentration after adsorption/desorption equilibrium (mg/L) and Cf dye residual concentration under oxidation conditions after certain intervals (mg/L). The photocatalytic activities of commercial and prepared CeO2 catalysts were quantified by measurement of dye apparent first order rate constants in Eq. (10):

ln

C0 ¼ Kapp t C

(10)

where kapp is the apparent pseudo-first order rate constant, C and C0 are the concentration at time ‘t’ and ‘t ¼ 0’, respectively. The plot of ln Co/C against irradiation time t should give straight lines, whose slope is equal to Kapp. The half-life of dye degradation at various process parameters was raised from Eq. (11).

t1=2 ¼

0:693 Kapp

3.3. XRD analysis

(11)

where half-life time, t1/2, is defined as the amount of time required for the photocatalytic degradation of 50% of CR dye in a aqueous solution by catalyst. 3. Results and discussion 3.1. TGA analysis TGA was used to clarify the appropriate heat treatment temperature for the cerium complex; the thermal properties of obtained compounds were studied by the TG method in oxidative atmosphere (synthetic air). Fig. 1 shows the TG curve of the CeO280 sample heated up to 800 C. The decomposition process of the precursor in oxidative atmosphere occurs in three well defined steps. The first decomposition step (Step I) was observed from room temperature to 120 C with mass loss of 3.8%. The second sequential mass loss (Step II) occurred from 120 to 360 C, displayed an even rapid speed and was found to be 8.2%. The third step of decomposition (Step III) with mass loss of 2% occurred at T 360 C. 3.2. TG-MS analysis To explain the thermal degradation mechanism of studied CeO2 compound, the TG-MS analysis was additionally carried out. The

Fig. 1. TG-MS of the as-prepared CeO2 catalyst (CeO2-80 precursor).

example MS spectra of gaseous products emitted during decomposition of studied compound in oxidative atmosphere are depicted in Fig. 1. At Tmax1 ¼ 120 C, MS analysis showed the beginning of the evaporation of the six H2O molecules from Ce(NO3)3$6H2O. Their existence is confirmed by the presence of corresponding ions: m/z ¼ 16, 17, 18. The presence of alcohol fragments (m/z ¼ 29, 44, 15), is indicated at Tmax2 ¼ 240 C from MS spectra, as a result of ethylene glycol decomposition reaction during its pyrolysis in the second decomposition step. The dehydration of the Ce(OH)4 gel also takes place along with the removal of the coordinated water as reported and decomposition of nitrates (m/z ¼ 30, 46, 14). At third decomposition step, removal of organics in the form of oxides of carbon, the most likely dioxide of carbon (m/z ¼ 12, 44) are indicated at Tmax3 ¼ 300 C. At Tmax4 ¼ 460 C, removal of the remained nitrates and the beginning of nucleation process of the formation of cubic ceria, with phase transformation occurs at 460 C.52 The TGA result indicated that the appropriate temperature for heat treatment was T ¼ 500 C. Hence, cerium complex was then heat treated at T ¼ 500 C for 2 h to obtain a yellowish color of CeO2.53

The XRD analysis was carried out to study the crystallographic structure and phase purity of the CeO2 samples and results are shown in Fig. 2. All XRD peaks of CeO2 catalysts, located at 2q values of 28.02 , 33.11, 47.45 56.33 , 59.08 , 69.4 and 76.69 could be indexed to (111), (200), (220), (311), (222), (400), (331) and (420) crystal planes cubic fluorite-type CeO2 phase, as identified using standard data (Fm-3m, JCPDS file No. 340394). From Fig. 2, it can be seen that no other peak associated to impurities was found in the XRD analysis, indicating high purity of the all samples. For the bare CeO2-com, the XRD patterns showed very intense and sharp peaks, indicating its crystalline nature. While the successive decrease in the intensities of both catalysts, CeO2-80 and CeO2-500, indicated a decline in the crystallinity of nanostructures. The obtained lattice parameters “a ¼ b ¼ c” using Fullprof program were found to be 0.54110 (2), 0.5431 (3) and 0.5413 (2) nm for CeO2-com, CeO2-80 and CeO2-500 NPs, respectively. It is observed that the lattice parameter of 0.5431 (3) nm, which corresponds to CeO2-80, was slightly decreased compared to the CeO2-500 (0.5413 (2) nm) owing to the decrease in the concentration of O vacancies, which leads to lattice contraction.54 Besides, broadened peaks with a very slight shift toward lower 2q ( ) of the XRD patterns, in the 2q range from 26 to 31 (Fig. 3), were observed for both CeO2-80 and CeO2-500 NPs, which could be attributed to oxygen vacancies, defects and Ce3þ species that contribute to the development of strain in the XRD pattern, which leads to a change in the particle size.

Fig. 2. XRD patterns of CeO2 nano-catalysts.


Based on the Scherrer's formula, the average crystallite size (dXRD) was calculated from the full-width-half-maximum (FWHM), using the broadening of the hkl ¼ 311, as prominent reflection peak.55 As displayed in Table 2, the estimated average particle size of the ceria samples was found to be 117 nm (bare CeO2-com), 4 nm (CeO2-80) and 9 nm (CeO2-500). From a methodological point of view, the designated synthesis process of ceria NPs, using organic solvent like ethylene glycol as reducing agent, have proved to be crucial in minimizing particles aggregation through an inhibition of crystallite nucleation and retardation of crystallite growth processes.56 Moreover, by increasing temperature up to 500 C, the specific surface area was decreased, indicating that the growth rate of the product is dominated by the calcination temperature.57 The highest broadening of the diffraction peaks was observed for CeO280 NPs sample, suggesting the presence of nano-sized powders.

3.4. Theoretical density The theoretical density (Dt) and specific surface area (SS) of CeO2 nano-catalysts were calculated using the following equation Eq. (12):

Dt ¼

Z Mc a3exp N

(12)

where Z is the number of chemical species in the unit cell of spinel lattice (Z ¼ 8), Mc the molecular weight of the sample (g/mol), aexp the experimental lattice parameter of the ceria (nm) and N Avogadro's number (6.022 1023 mol1). The theoretical density Dt depends on the lattice constant and molecular weight of the sample. The theoretical density Dt as a function of calcination temperature is reported in Table 2. It can be observed that the theoretical density Dt rose from 7.14 to 7.21 g/cm3 as the calcination temperature was increased from 80 to 500 C, which is inversely proportional to the lattice constant and decreases with increasing calcining temperature.

Fig. 3. XRD patterns of CeO2 nano-catalysts in the 2q range from 26 to 31.

579

3.5. Specific surface area Assuming that the particles have spherical shape and uniform size, the specific surface area average particle size can be estimated by BET equation Eq. (13) in the form:

SS ¼

6 Dt dXRD

(13)

where dXRD (nm) is the average particle size of CeO2 NPs sample and Dt is the theoretical density of CeO2 NPs (7.215 gm/cm3). It can be observed from Table 2 that the specific surface area dropped from 355.87 m2/g (CeO2-80 NPs) to 158.17 m2/g (CeO2-500 NPs) with an increase in temperature from 80 to 500 C, signaling a high acceleration of crystallite growth process in CeO2-500 NPs. Pristine CeO2-com NPs showed a low specific surface area at around 12.17 m2/g. Higher calcination temperatures gave rapid growth of crystals and construction to larger clusters.58 3.6. SEM analysis The morphology and size of CeO2 nano-catalysts are described by SEM images as shown in Fig. 4. According to the low and high resolution SEM images (Fig. 4(a) and (b)), CeO2-com shows spherical shaped nanoparticles with an average diameter of ~100e200 nm. Fig. 4(c) and (d), illustrate typical SEM images of CeO2-80 and CeO2-500 samples, respectively. It can be seen from SEM images that nanoparticle are randomly shaped aggregates. The aggregates are formed by accumulation of nanoparticles with dimension of about 50e100 (Fig. 4(c)) between 100 and 400 nm (Fig. 4(d)). A strong assembly of the nanoparticles is observed in CeO2-500 sample. Groups of ultrafine particles assemble into bigger particles with size up to 400 nm. The irregularly shape and the ultrafine particle size favours the formation of larger spherical particles to further reduce the surface energy.59 3.7. EDX analysis EDX analysis of the as-prepared CeO2 nano-catalyst was carried out at 20 kV. As reported in Fig. 5, the effective atomic concentration of Ce and O species on top surface layers of the CeO2-500 investigated was determined by EDX technique. The relative atomic abundance of Ce and O species present in the uppermost surface and bulk layers of CeO2-500 are tabulated in Table 3. Inspection of Table 3 revealed that the surface O/Ce ratio (2.13) is very close to the bulk O/Ce ratio that is 2.00, indicating that the samples have intrinsic defect, which would affect their photocatalytic activity. Oxygen vacancy defects, which are usually present in impurity and dopant-controlled regimes of slightly sub-stoichiometric pure or transition-metal-doped oxides, such as TiO2, WO3 and CeO2 are of great importance as a separation or recombination center for the photogeneration electronehole pairs. A smaller amount of oxygen vacancy defects and Ce3þ ions (as mentioned in XRD analysis), acting as electrons traps, are beneficial to delaying the recombination of hole-electron pairs and enhancing quantum efficiency,

Table 2 Crystallographic parameters of the commercial and the as-prepared CeO2 nano-catalysts. Abbreviation

Catalysts

UC parameter a (nm)

dXRD (nm)

Dt (g/cm3)

Ss (m2/g)

CeO2 CeO2-com CeO2-80 CeO2-500

JCPDS No. 34-0394 Commercial ceria Dried ceria at 80 C/24 h Calcined ceria at 500 C/2 h

0.54113 0.54110 (2) 0.5431 (3) 0.5413 (2)

e 117 4 9

7.23 7.14 7.21

12.17 355.87 158.17

UC: Unit cell; dXRD: Crystallite size; Dt: Theoretical density; SS: Specific surface area.

580


Fig. 4. SEM images of CeO2 nano-catalysts. (a) Low-resolution of CeO2-com; (b) High-resolution of CeO2-com; (c) CeO2-80; (d) CeO2-500 NPs.

which are the primary factors for high photocatalytic efficiency of CeO2-500 NPs activity, even under visible-light domain.60 3.8. UVevis DRS analysis Fig. 6 shows the UVeVis DRS absorption spectra of the synthesized CeO2-500 NPs and CeO2-com NPs samples. Both of the samples exhibited two strong absorption bands in UV region, located at z245 nm (5.06 eV) and z345 nm (3.59 eV) for CeO2com, and at z255 nm (4.86 eV) and z355 nm (3.49 eV) for CeO2500 NPs. Hence, the absorption bands at z245e255 and 345e355 nm of CeO2 NPs may be attributed to the transitions from the O 2p

state (VB) to Ce 5d state (CB) and from the O 2p state (VB) to the localized Ce 4f state, respectively. As shown in Fig. 6, bare CeO2-com exhibited UV-light absorption edge at z387.5 nm. For CeO2-500 NPs, the absorption edge appeared in the visible region, ranging between 450 and 550 nm, which gave the first notation that our catalyst could be expected to act efficiently as a visible light responsive photocatalyst. The red shift in CeO2-500 NPs could be ascribed to the presence of the additional localized energy levels (oxygen vacancies) within the bandgap, absorbing with relatively low energy photons. Moreover, presence of two absorption edges in UVevis DRS of CeO2-500 NPs has been correlated to two crystallite phases belonging to CeO2-500 NPs. The optical properties of the CeO2-com and CeO2-500 nano-catalysts are displayed in Table 4. The energy band gap (Eg) was then calculated from the plot of the modified Schuster-Kubelka-MunkK-M function, Eq. (14):

ahvn=2 xb hv Eg

Fig. 5. EDX pattern of CeO2-500 nano-catalyst.

(14)

where a, h, n, Eg and b are the linear absorption coefficient, Planck's constant, light frequency, band gap energy of the material and a constant involving properties of the bands respectively. The exponent of n is the power factor of the transition mode, which is dependent upon the nature of the material, whether it is crystalline or amorphous. To obtain the band gap energies of the CeO2 NPs, Schuster-Kubelka-Munk absorption function (ahv)1/n was plotted against the photon energy (hn) according to Eq. (7) for indirect (n ¼ 2) and direct (n ¼ 0.5) transitions. The approximated band gap can then be determined from the straight line x-intercept, as indicated in Figs. 7 and 8, respectively. The estimated band gap energies of the CeO2 NPs are listed in Table 4.


581

Table 3 EDX data of CeO2-500 nano-catalyst. Element

Calculated (bulk) (at %)

Found (surface) (at %)

Surface O/Ce ratio

Bulk O/Ce ratio

Ok Ce k

66.66 33.33

68.07 31.93

2.13

2

shows that, the empirical equations of this linear relationship are given as Eqs. (16) and (17):

For CeO2 com

lnðahvÞ ¼ 2:2612 ln hv Eg þ 4:8006 R2 ¼ 0:9964

i:e: nz2 (16)

For CeO2 500

lnðahvÞ ¼ 0:4079 ln hv Eg þ 3:4915 R2 ¼ 0:9859

i:e: nz0:5 (17)

Fig. 6. UVevis DRS spectra of CeO2 nano-catalysts.

Table 4 Approximated allowed indirect and direct band gaps of CeO2, nano-catalysts. Power factor of the transition mode n

2 0.5

Band gap energies Eg (eV) CeO2-com

CeO2-500

3.20 3.20

2.70 3.10

To verify the optical band transition mode, whether it is direct or indirect for our present CeO2-com and CeO2-500 nano-catalysts, one can use Eq. (15), which can be rearranged to become as follows:

lnðahvÞ ¼ lnb þ ln hv Eg

(15)

Hence, plotting of ln (ahn) vs. ln (hveEg), gives a straight line whose slope determines the power factor n, which identifies the type of the optical transition mode. Fig. 8 indicates the results of the above analysis for CeO2-com and CeO2-500 samples. This figure

Fig. 7. (ahv)1/2 vs. hv ploting of CeO2 nano-catalysts.

The calculated values of the transition power factor (n) of CeO2com and CeO2-500 matrix are tabulated in Table 4, they are approximately equal to 2 and 0.5, respectively. Thus, for crystalline and non-crystalline materials, indirect and direct transitions are effective for CeO2-com and CeO2-500 nano-catalyst, respectively. On the basis of the above results, as reported in Figs. 7e9 and Table 4, for indirect allowed transition band gap of 3.20 eV was obtained for CeO2-com while the value of 3.10 eV was found for direct allowed transition, corresponding to CeO2-500. Moreover, the plot of (ahv)2 vs. (hv) showed a significant tail and absorption below the extrapolated values possibly due to defects. Comparison of these values with the reported values for bulk CeO2 (3.15 eV), confirms blue shift for obtained band gap potential of CeO2-com nano-catalyst due to quantum confinement effect and the reduction of structural defects.61 However, the abundant surface and interface defects in the agglomerated CeO2 NPs were supposed to be responsible for the red-shift of absorption edge of CeO2-500 NPs catalyst, as compared with the corresponding value for the pristine CeO2-com (3.20 eV). Furthermore, the valence and conduction band edge energy of a semiconductor can be assessed through the following empirical formulae Eqs. (18) and (19)62:

EVB ¼ X Ee þ 1 2Eg

(18)

ECB ¼ EVB Eg

(19)

where X is the absolute electronegativity of the atom semiconductor, which is the geometric mean of the electronegativity of the constituent atoms. Herein, the electronegativity of an atom is

Fig. 8. (ahv)2 vs. hv ploting of CeO2 nano-catalysts.

582


Fig. 9. Representation of ln (ahv) against ln (hveEg) for CeO2 nano-catalysts.

the arithmetic mean of the atomic electron affinity and the first ionization energy; Ee is the energy of free electrons on the hydrogen scale (about 4.5 eV), Eg the band gap of the semiconductor, ECB the conduction band potential and EVB the valence band potential. The X value for CeO2 NPs is 5.56 eV. Thus, the EVB of CeO2-com and CeO2-500 are estimated to be 2.66 and 2.61 eV, and their corresponding ECB ¼ 0.54 and 0.49 eV (vs. NHE), respectively. Along the absorption coefficient curve and near the optical band edge there is an exponential part called Urbach tail. This exponential tail appears in the low crystalline, poor crystalline, the disordered and amorphous materials because these materials have localized states which extended in the band gap. In the low photon energy range, the spectral dependence of the absorption coefficient (a) and photon energy (hn) is known as Urbach empirical rule, which is expressed by Eq. (20),63:

a ¼ a0 eðEuÞ hv

(20)

where a0 is a constant, hv is the photon energy and EU denotes the energy of the band tail or sometimes called Urbach energy, which refers to the optical transition between occupied states in the VB tail and the CB edge. Urbach energy is weakly dependent upon temperature and is often interpreted as the width of the band tail due to localized states available in the normally band gap that is associated with the disordered or low crystalline materials. Taking the logarithm of the two sides of Eq. (21), one can get a straight line equation. It is given as follows:

ln a ¼ ln a0 þ

hv EU

Fig. 10. ln (a) vs. hv ploting of CeO2 nano-catalysts from which the Urbach energy can be obtained.

quantum confinement effect. For spherical NPs with an substantially high potential energy outside the sphere, the band-gap value is dependent on the particle radius R, and can be determined from the effective mass approximation approach using a simple particle in a box model following Eq. (22),65:

E ¼ Eg þ

h2 1 1 1:8e2 þ 2 4pεε0 R 8R me mh

(22)

where Eg is the bulk bandgap, R is the radius of the NPs, me and mh are the effective masses of the electron and hole, respectively, and ε is the relative dielectric constant of CeO2. The above equation indicates that the bandgap value increases with decreasing particle radius R. We have calculated the approximate effective band gap values for each sample from the above equation by taking Eg ¼ 3.15 eV, me ¼ mh ¼ 0.4 m, where m is the mass of a free electron, and e ¼ 24.5, and by replacing R by the crystallite size obtained from the XRD results. The approximate effective band gap values of CeO2 nanocatalysts are listed in Table 6, using the effective mass approximation approach, the theoretical band gap was increased by 0.09 eV for CeO2-80 (Eg ¼ 3.24 eV), by 0.03 eV for CeO2-com (Eg ¼ 3.18 eV) and by 0.01 eV for CeO2-500 (Eg ¼ 3.16 eV) compared to that of bulk CeO2 powder (Eg ¼ 3.15 eV). In this regard, for the calcined samples, this blue-shift towards higher energies can be explained as a consequence of either the quantum size effect originating from the diminution of ceria particle size, or the existence of larger contribution of

(21)

Therefore, the band tail energy or Urbach energy (EU) can be obtained by plotting ln(a) against the incident photon energy (hn) for CeO2-com and CeO2-500 nano-catalysts, as shown in Fig. 10. The values of Urbach energy were estimated from the reciprocal of the slope of the linear portion of the curve with a straight line, as shown in Fig. 10 and in Table 5. It can be clearly seen that CeO2-500 NPs displayed a higher value of Urbach energy (0.17 eV vs 0.13 eV) than that of bare CeO2-com. This highlight was ascribed to an increase of crystalline size, the degree of preferred orientation and internal microstrain on the product. Moreover, the increase in the Urbach energy was due to increased acceptor levels of interstitial oxygen atoms, in good agreement with those of literature.64 3.9. Approximated effective band gap energy For nanostructure materials with particle sizes down to a few nanometers, the band-gap value is modified because of the

Table 5 n, c, lmax, Eg, EVB, ECB and EU for CeO2 nano-catalysts. Catalysts abbr.

n

c (eV) lmax (nm) Eg (eV) ECB (eV) EVB (eV) EU (eV)

CeO2-com CeO2-500

2 0.5

5.56

387.5 400

3.20 3.10

0.54 0.49

2.66 2.61

0.13 0.17

Table 6 The crystallite size, theoretical and experimental optical properties of CeO2 nanocatalysts. Catalysts abbr.

CeO2-bulk CeO2-com CeO2-80 CeO2-500

Approximate effective band gap

Experimental band gap

dXRD (nm)

lmax (nm)

Eg (eV)

lmax (nm)

Eg (eV)

e 117 4 9

394 390 383 392

3.15 3.18 3.24 3.16

e 387.5 e 400

e 3.20 e 3.10

dXRD: Crystallite size; lmax: Absorption edge.


charge transfer transitions between the O (2p6) and Ce (4f0) states in O2 and Ce4þ (Ce4þ ) O2). Yin et al.66 reported the band gaps of 5.8 and 3.6 nm CeO2 NPs synthesized using sonochemical synthesis to be 3.03 and 3.68 eV, respectively. Ho et al.67 have reported direct band gap values ranging from 3.36 to 3.62 eV for mesoporous CeO2 nanostructures prepared using a polyol method. Most recently, Chen and Chang have reported direct band gap values ranging from 3.56 to 3.71 eV for CeO2 NPs prepared using a precipitation method.27 Some previous studies reported the quantum size effects in CeO2 systems. Masui et al.68 reported the band gaps of 4.1 and 2.6 nm CeO2 NPs prepared using reverse micelles to be 3.38 and 3.44 eV, respectively. Zhang et al.69 reported the increase in band gap by a value of 0.05 eV when the crystal size of CeO2 NPs was reduced from 6.9 to 4.6 nm. The approximate effective band gap values of CeO2-500 NPs showed a decrease in band gap energy (Eg) from 3.18 eV of CeO2com to 3.16 eV, indicating similar trends of the reported experimental results. The red-shift in the band gap has been attributed to the presence of Ce3þ at the grain boundaries, which forms some localized gap states in the band gap.

583

Fig. 11. The IEP of CeO2-500 nano-catalyst.

3.10. The isoelectric point or IEP Zeta potential (i.e. the isoelectric point, or IEP), an indicator of any dispersion stability, is influenced by the surface chemistry which can be changed by any number of means including a change in the pH, salt concentration, surfactant concentration, and other formulation options. An isoelectric point measurement studies how pH influences zeta potential and determine at which pH the zeta potential equals zero. The isoelectric point of CeO2-500 nanocatalyst was determined by the pH drift method. The pH of a solution of 0.01 moL/L NaCl was adjusted between pH 3 and pH 11 by adding either HCl or NaOH. Then, 0.15 g of CeO2-500 powder was added to 50 mL of the solution and the final pH (pHf) was measured after 48 h. The IEP is defined as the point where the electrical charge density on the surface of the nanoparticles is zero. The DpH versus pHi plot for CeO2 NPs is illustrated in Fig. 11, The IEP of CeO2 NPs was determined to be 7.2, which is consistent with reported values of pH 6.7e8.6,70 implying that they are positively charged under acidic and neutral pH conditions ( hCR=CeO2 80=UVA > hCR=CeO2 com=UVA > hCR=UVA According to Eq. (10), the linear plot of (ln C0/C) against irradiation time (t) is a straight line with a slope equal to Kapp. The firstorder rate constants are shown in Table 7, where regression coefficients values (R2 ¼ 0.892e0.97), confirm that the dye discoloration efficiency of CeO2 nano-catalytic systems under UVA-light irradiation follows apparently a pseudo first-order kinetics model. The rate constants were calculated to be 2.4 103 and 10 103 min1 for CeO2-com NPs and CeO2-500 NPs catalytic system respectively. From the crystallographic, physical, optic (a, DXRD, Dt, SS, Eg exp, Eg th, EU and IEP) and photocatalytic (h) properties point of view, as shown in Fig. 13 and reported in Table 8, the synthesized CeO2-500 nano-catalyst possessed excellent physical performances and exhibited superior degradation efficiency for the UVA-light driven photocatalytic degradation of CR azo-dye molecules. 3.13. Possible reaction mechanism The photocatalytic degradation mechanism is generally elucidated on the basis of the band edge positions of VB and CB of the semiconductor and the positions of the redox couple potentials.76 Based on the characteristic and experimental results discussed above, we briefly describe the proposed mechanism for the photodegradation of CR by CeO2 nano-catalysts under UVA-light irradiation. The relative positions of the redox potentials of Ce4þ/Ce3þ (þ1.31.8 V/NHE), superoxide radical O2 =O2 $ (0.28 V/NHE), hydroxyl radicals (H2 O=· OH (þ1.9 V/NHE)) and organic cationradicals R=R$þ (þ1 V/NHE) couples along with the energy bands of CeO2 nano-catalysts, are shown in Fig. 14. According to the predicted band edge positions of CeO2 nano-catalysts, as displayed in

Fig. 13. Crystallographic, physic, optic, sorption and photocatalytic parameters of CeO2 nano-catalysts.

Table 6, under visible light irradiation, both CeO2-com and CeO2-500 NPs, can be activated by UVA-light (355e375 nm / 3.49e3.30 eV) to produce electron and holes, since their band gap energies were 3.20 and 3.10 eV, respectively. As presented in Fig. 14(a), the photodegradation efficiency was significantly reduced in the presence of the bare CeO2-com NPs. Considering the more negative potential of CB (0.54 eV/NHE) than that of E0 (O2 =O2 $ ), the photo-generated electrons in the CB could reduce the dissolved O2 through multi-electron reduction pathways resulting in the generation of O2 $ and · OH via hydrogen peroxide (H2O2). Meanwhile, the photo induced holes in the VB with high oxidation ability through interfacial charge transfer process could react with water to produce hydroxyl radicals which could directly involve in the photocatalytic degradation process. Unfortunately, the present photocatalytic efficiency (14.92%) was in contrary trend for more anodic potential of CB. During the photocatalytic reaction process, the recombination of photo-induced carriers easily takes place, therefore reducing the quantum efficiency because of low specific surface area and wide band gap, in good agreement with XRD and UVevis DRS measurements. On the other hand, the possible pathways highlighting the different electron-hole pair separation in the CeO2-500 NPs sample under UVA light irradiation, as illustrated in Fig. 14(b), were proposed by Eqs. (23e29):

CeO2 500 þ hvð365 nmÞ/CeO2 500 e CB þ hþ VB

(23)

e CB þ O2 /O2 $

(24)

O2 $ þ H2 O/· OH þ OH þ O2

(25)

hþ VB þ H2 O/· OH

(26)

R þ hþ VB /R$þ

(27)

Ce4þ þ e 4Ce3þ

(28)

O2 $ ; $ OH; R$þ þ CR azoedye/Intermediates products/Mineralized products

(29)

L. Nadjia et al. / Journal of Rare Earths 36 (2018) 575e587 Table 8 a, DXRD, Dt, SS, Eg

exp,

Eg

th,

585

EU, IEP, h0 and h of CeO2 nano-catalysts.

Crystallographic, physic, optic and photocatalytic parameters

CeO2-com

CeO2-80

CeO2-500

CR dye

a (nm) DXRD (nm) Dt (g/cm3) SS (m2/g) Eg exp (eV) Eg th (eV) EU (eV) IEP h (%) h0 (%) CR-self photolysis process

0.5411 117 7.23 12.17 3.2 3.18 0.13 6.7e8.670 8.17 14.93 e

0.5431 4 7.14 355.87 3.24 3.24 e e 6.07 42.4 e

0.5413 9 7.21 158.17 2.7 3.16 0.17 7.2 50.00 76.44 e

e e e e e e e e 0.49

Fig. 14. Schematic diagram of the photocatalytic mechanism of CeO2-com NPs (a) and CeO2-500 NPs (b).

When the CeO2-500 catalyst was irradiated by UVA-light, the electrons (e) in the VB position were excited to CB position, simultaneous generation of the same amount of holes (hþ) left in the VB Eq. (23). According to the position of band energy as depicted from UVevis-DRS results (Table 5), the photo-induced electrons in the CB can directly reduce O2 into O2 $ (Eq. (24)) because electronic potential of the CB is more negative (0.49 vs 0.28 V/NHE) than that of E0 (O2 =O2 $ ). The O2 $ can react with H2O to form · OH radicals (Eq. (25)). At the same time, the VB is more positive (þ2.61 vs þ 1.9 V/NHE) than these of E0 (H2 O=· OH) and E0 (R=R$þ ), indicating that the photo-induced holes at the VB can oxidize H2O to · OH radicals Eq. (26) and CR dye directly, forming R$þ (Eq. (27)). These radicals species (O2 $ , · OH and R$þ ) can subsequently transfer the charge to the present in the reaction medium. Thus, they suppress the recombination of the charge carriers and enhance the photocatalytic activity in the UVA light as well. Moreover, the presence of Ce3þ, as illustrated in XRD and UVevis DRS observations, may act as electron acceptor (from Ce4þ to Ce3þ) and/or hole donor (from Ce3þ to Ce4þ) (Eq. (28)) to facilitate charge carrier localization, reducing their recombination and hence prolonged separation by trapping at energy levels close to the CV and VB, respectively. These findings are in good agreement with trends reported in the literature.57,77 On the basis of the above re-

sults, the enhanced photocatalytic performance of CeO2-500 NPs catalyst could be ascribed to O2 $ , · OH and R$þ as the primary oxidative species responsible for the reduction of the CR dye solution under UVA-light irradiation Eq. (29). Thus, the improvement in UVA-light activity was attributed to defects (Ce3þ and oxygen vacancy), resulting in band gap narrowing and a high separation efficiency of photo-induced electronehole pairs. 4. Conclusions In summary, a series of CeO2 NPs photocatalyst (i.e. CeO2-80 and CeO2-500) were synthesized by a facile homogeneous precipitation method using EG as a reducing agent. The as-prepared materials were characterized using TGA-MS, XRD, SEM-EDX, UVevis DRS and IEP techniques. The photocatalytic performances of these materials were evaluated on Congo red azo-dye solution under UVA-light irradiation as a function of two environmental key factors: reaction time and calcination temperature. The XRD results reveal that all CeO2 samples crystalize with cubic fluorite-type CeO2 phase. CeO2-500 exhibits smaller crystallite size (9 nm vs 117 nm) than that of bare CeO2-com. The morphological analysis shows that CeO2 NPs are spherical-like in shape NPs with strong assembly of the CeO2 NPs observed for CeO2-500 NPs. EDX analysis confirms the stoichiometry of CeO2

586


NPs. UVevis DRS measurements show that the presence of oxygen vacancy defects in CeO2-500 NPs can efficiently suppress the recombination rate of photogenerated electron-hole pairs and subsequently improve the quantum efficiency. Urbach energy of EU is found to be increased from 0.12 eV (CeO2-com) to 0.17 eV (CeO2-500), highlighting the increase of crystalline size, the degree of preferred orientation and internal microstrain in the CeO2-500 sample. IEP of CeO2-500 NPs is found to be 7.2. CeO2500 NPs exhibit higher rate constant (Kapp ¼ 10 103 min1 vs 2.4 103 min1) than of bare CeO2-com for the degradation of CR. The photodegradation of CR by CeO2-500 follows a pseudo first-order rate law. On the basis of the above physical properties, the improved photocatalytic performance of CeO2-500 nanocatalyst can be credited to the fine particles size, high homogeneous size of spherical CeO2 NPs and favorable bandgap energy. Additionally, oxygen vacancy defects and Ce3þ centers can be responsible for enhanced quantum absorption efficiency and UVA-light photocatalytic activity.

Acknowledgments The authors thank the Unit of Catalysis and Solid State Chemistry of Lille 1, France, Dr. Moulay Tahar from University of Saida, Algeria, Mohammed Boudiaf from the Sciences and Technology University of Oran, Algeria and the Catalysis and Materials Research s, Tunisia for their Unit for the Environment and Processes, Gabe materials support of this research.

References 1. Sukon P, Auppatham N, Duangdao C. Photocatalytic activity of the binary composite CeO2/SiO2 for degradation of dye. Appl Surf Sci. 2016;387:214. 2. Navarro P, Sarasa J, Sierra D, Esteban S, Ovelleiro JL. Degradation of wine industry wastewater by photocatalytic advanced oxidation. Water Sci Technol. 2005;51:113. 3. Yashima MT. Internal distortion in ZrO2-CeO2 solid solutions: neutron and high-resolution synchrotron X-ray diffraction study. Appl Phys Lett. 1998; 72:182. 4. Nikolaou K. Emissions reduction of high and low polluting new technology vehicles equipped with a CeO2 catalytic system. Sci Total Environ. 1999;235:71. 5. Feng X, Sayle DC, Wang ZL, Paras MS, Santora B, Sutorik AC, et al. Converting ceria polyhedral nanoparticles into single-crystal nanospheres. Science. 2006;312:1504. 6. Imanaka N, Masui T, Hirai H, Adachi GY. Amorphous cerium-titanium solid solution phosphate as a novel family of band gap tunable sunscreen materials. Chem Mater. 2003;15:2289. 7. Zhou J, Mullins DR. Adsorption and reaction of formaldehyde on thin-film cerium oxide. Surf Sci. 2006;600:1540. 8. Kakuta N, Morishima N, Kotobuki M, Iwase T, Mizushima T, Sato Y, et al. Oxygen Storage Capacity (OSC) of aged Pt/CeO2/Al2O3 catalysts, roles of Pt and CeO2 supported on Al2O3. Appl Surf Sci. 1997;121/122:408. 9. Lira-Cantu M, Krebs FC. Hybrid solar cells based on MEH-PPV and thin film semiconductor oxides (TiO2, Nb2O5, ZnO, CeO2 and CeO2eTiO2): performance improvement during long-time irradiation. Sol Energy Mater Sol Cells. 2006;90: 2076. 10. Flytzani-Stephanopoulos M, Sakbodin M, Wang Z. Regenerative adsorption and removal of H2S from hot fuel gas streams by rare earth oxides. Science. 2006;312:1508. 11. Morshed AH, Moussa ME, Bedair SM, Leonard R, Liu SX, El-Masry N. Violet/blue emission from epitaxial cerium oxide films on silicon substrates. Appl Phys Lett. 1997;70:1647. 12. Celardo I, Pedersen JZ, Traversa E, Ghibelli L. Pharmacological potential of cerium oxide nanoparticles. Nanoscale. 2011;3:1411. 13. Das S, Dowding JM, Klump KE, McGinnis JF, Self W, Seal S. Cerium oxide nanoparticles: applications and prospects in nanomedicine. Nanomedicine. 2013;8:1483. 14. Walkey C, Das S, Seal S, Erlichman J, Heckman K, Ghibelli L, et al. Catalytic properties and biomedical applications of cerium oxide nanoparticles. Environ Sci Nano. 2015;2:33. 15. Lin K-S, Chowdhury S. Synthesis, characterization, and application of 1-D cerium oxide nanomaterials: a review. Int J Mol Sci. 2010;11:3226. 16. Wu Z, Li M, Howe J, Meyer HM, Overbury SH. Probing defect sites on CeO2 nanocrystals with well-defined surface planes by Raman spectroscopy and O2 adsorption. Langmuir. 2010;26:16595.

17. Hartridge A, Bhattacharya AK. Preparation and analysis of zirconia doped ceria nanocrystal dispersions. J Phys Chem Solid. 2002;63:441. 18. Pouretedal HR, Kadkhodaie A. Synthetic CeO2 nanoparticle catalysis of methylene blue photodegradation: kinetics and mechanism. Chin J Catal. 2010;31: 1328. 19. Khan SB, Faisal M, Rahman MM, Akhtar K, Asiri AM, Khan A, et al. Effect of particle size on the photocatalytic activity and sensing properties of CeO2 nanoparticles. Int J Electrochem Sci. 2013;8:7284. 20. Anitha R, Ramesh KV, Sudheerkumar KH. Photocatalytic activity of CeO2 nanoparticles: synthesis, using Atocarpusgomezianus fruit mediated facile green combustion method. Int J Pharm Biol Sci. 2017;8:B933. 21. Channei D, Inceesungvorn B, Wetchakun N, Ukritnukun S, Nattestad A, Chen J, et al. Photocatalytic degradation of methyl orange by CeO2 and Feedoped CeO2 films under visible light irradiation. Sci Rep. 2014;4:5757. 22. Parvathya S, Venkatramanb BR. In vitro antibacterial and anticancer potential of CeO2 nanoparticles prepared by Co-precipitation and Green synthesis method journal of nanosciences: current research. J Nanosci Curr Res. 2017;2:2. 23. Thammadihalli NR, Thippeswamy R, Ganganagappa N, Hanumanaika R. Synthesis and characterization of CeO2 nanoparticles via solution combustion method for photocatalytic and antibacterial activity studies. ChemistryOpen. 2015;4:146. 24. Reddy Yadav LS, Manjunath K, Archana B, Madhu C, Raja Naika H, Nagabhushana H, et al. Fruit juice extract mediated synthesis of CeO2 nanoparticles for antibacterial and photocatalytic activities. Eur Phys J Plus. 2016;131:154. 25. Ozer N. Optical properties and electrochromic characterization of sol-gel deposited ceria films. Sol Energy Mater Sol Cells. 2001;68:391. 26. Li HB, Liu GC, Duan XC. Monoclinic BiVO4 with regular morphologies: hydrothermal synthesis, characterization and photocatalytic properties. Mater Chem Phys. 2009;115:9. 27. Chen HI, Chang HY. Synthesis of nanocrystalline cerium oxide particles by the precipitation method. Ceram Int. 2005:31795. 28. Qi RJ, Zhu YJ, Huang YH. Sonochemical synthesis of single-crystalline CeOHCO3 rods and their thermal conversion to CeO2 rods. Nanotechnology. 2005;16: 2502. 29. Shuk P, Greenblatt M. Hydrothermal synthesis and properties of mixed conductors based on Ce1xPrxO2d solid solutions. Solid State Ionics. 1999;116:217. 30. Bumajdad A, Zaki MI, Eastoe J, Pasupulety L. Microemulsion-based synthesis of CeO2 powders with high surface area and high-temperature stabilities. Langmuir. 2004;20:11223. 31. Tsuzuki T, Robinson JS, McCormick PG. UV-shielding ceramic nanoparticles synthesised by mechanochemical processing. J Aust Ceram Soc. 2002;38:15. 32. Wang SQ, Maeda KH, Awano MN. Direct formation of crystalline gadoliniumdoped ceria powder via polymerized precursor solution. J Am Ceram Soc. 2002;85:1750. 33. Navarrete EL, Caballero A, Elipe ARG, Ocana M. Low temperature preparation and structural characterization of Pr-doped ceria solid solutions. J Mater Res. 2002;17:797. 34. Sun CW, Li H, Zhang HR, Wang ZX, Chen LQ. Controlled synthesis of CeO2 nanorods by a solvothermal method. Nanotechnology. 2005;16:1454. 35. Hu CG, Zhang Z, Liu H, Gao P, Wang LZ. Direct synthesis and structure characterization of ultrafine CeO2 nanoparticles. Nanotechnology. 2006;17:5983. 36. Mukherjee A, Harrison D, Podlaha J. Electrosynthesis of nanocrystalline ceriazirconia. Solid State Lett. 2001;4:D5. 37. Bondioli F, Corradi AB, Leonelli C, Manfredini T. Nanosize powders obtained by flux method mater. Res Bull. 1999;34:2159. 38. Zha SW, Xia CR, Meng GY. Effect of Gd (Sm) doping on properties of ceria electrolyte for solid oxide fuel cells. J Power Sources. 2003;115:44. 39. Li JG, Ikegami T, Wang YR, Mori T. 10-mol%-Gd2O3-doped CeO2 solid solutions via carbonate coprecipitation: a comparative study. J Am Ceram Soc. 2003;86: 915. 40. Djuri cic B, Pickering S. Nanostructured cerium oxide: preparation and properties of weakly-agglomerated powders. J Eur Ceram Soc. 1999;19:1925. 41. Godinho MJ, Goncalves R, Santos LPS Varela JA, Longo E, Leite ER. Room temperature co-precipitation of nanocrystalline CeO2 and Ce0.8Gd0.2O1.9ed powder. Mater Lett. 2007;61:1904. 42. Rocha RA, Muccillo ENS. Preparation and characterization of Ce0.8Gd0.2O1.9 solid electrolyte by polymeric precursor techniques. Mater Sci Forum. 2003;711:416. ndez FJ, Oviedo O, Zoltan T. Effect of calcination 43. Sifontes AB, Rosales M, Me temperature on structural properties and photocatalytic activity of ceria nanoparticles synthesized employing chitosan as template. J Nanomater. 2013;2013:9. 44. Kargar H, Ghasemi F, Darroudid M. Bioorganic polymer-based synthesis of cerium oxide nanoparticles and their cell viability assays. Ceram Int. 2015;41: 1589. 45. Song HZ, Wang HB, Zha SW, Peng DK, Meng GY. Aerosol-assisted MOCVD growth of Gd2O3-doped CeO2 thin SOFC electrolyte film on anode substrate. Solid State Ionics. 2003;156:249. 46. Riccardi CS, Lima RC, dos Santos ML, Bueno PR, Varela JA, Longo E. Preparation of CeO2 by a simple microwaveehydrothermal method. Solid State Ionics. 2009;180:288. 47. Chaiwichian S, Inceesungvorn B, Pingmuang K, Wetchakun K, Phanichphant S, Wetchakun N. Synthesis and characterization of the novel BiVO4/CeO2 nanocomposites. Eng J. 2012;16:153.

L. Nadjia et al. / Journal of Rare Earths 36 (2018) 575e587 48. He HW, Wu XQ, Ren W, Shi P, Yao X, Song ZT. Synthesis of crystalline cerium dioxide hydrosol by a solegel method. Ceram Int. 2012;38:S501. 49. Raubach CW, Polastro L, Ferrer MM, Perrin A, Perrin C, Albuquerque AR, et al. Influence of solvent on the morphology and photocatalytic properties of ZnS decorated CeO2 nanoparticles. J Appl Phys. 2014;115, 213514. 50. Cullity BD. Elements of X-ray diffraction. In: vol. 98. Boston: Addison-Wesley Publishing Co.; 1956. 51. Boonprakob N, Wetchakun N, Phanichphant S, Waxlert D, Sherrell P, Nattestad A, et al. Enhanced visible-light photocatalytic activity of g-C3N4/TiO2 films. J Colloid Interface Sci. 2014;417:402. 52. Tok AIY, Boey FYC, Dong Z, Sun XL. Hydrothermal synthesis of CeO2 nanoparticles. J Mater Process Technol. 2007;190:217. 53. Ketzial JJ, Nesaraj AS. Synthesis of CeO2 nanoparticles by chemical precipitation and the effect of a surfactant on the distribution of particle sizes. J Ceram Process Res. 2011;12:74. 54. Saravanan R, Thirumal E, Gupta VK, Narayanan V, Stephen A. The photocatalytic activity of ZnO prepared by simple thermal decomposition method at various temperatures. J Mol Liq. 2013;177:394. 55. Saravanan R, Shankara H, Prakasha T, Narayanan V, Stephen A. ZnO/CdO composite nanorods for photocatalytic degradation of methylene blue under visible light. Mater Chem Phys. 2011;125:277. 56. Reverberi AP, Salerno M, Lauciello S, Fabiano B. Synthesis of copper nanoparticles in ethylene glycol by chemical reduction with vanadium (þ2) salts. Materials. 2016;9:809. 57. Elaziouti A, Laouedj N, Benhadria N, Bettahar N. SnO2 foam grain-shaped nanoparticles: synthesis, characterization and UVA light induced photocatalysis. J Alloys Compd. 2016;679:408. 58. Elaziouti1 A, Laouedj N, Bekka A. Synergetic effects of Sr-doped CuBi2O4 catalyst with enhanced photoactivity under UVA-light irradiation. Environ Sci Pollut Res. 2016;16:15862. 59. Abdulkarem AM, Li J, Aref AA, Ren L, Elssfah EM, Wang H, et al. CuBi2O4 single crystal nanorods prepared by hydrothermal method: growth mechanism and optical properties. Mater Res Bull. 2011;46:1443. 60. Yang K, Li DF, Huang WQ, Liang X, Huang GF, Wen SC. Origin of enhanced visible-light photocatalytic activity of transition-metal (Fe, Cr and Co)-doped CeO2: effect of 3d orbital splitting. Appl Phys A. 2017;123(96):1. 61. Tnunekawa S, Fukuda T, Kasuya A. Blue shift in ultraviolet absorption spectra of monodisperse CeO2-x nanoparticles. J Appl Phys. 2000;87:1318. 62. Phoka S, Laokul P, Swatsitang E, Promarack V. Synthesis, structural and optical properties of CeO2 nanoparticles synthesized by a simple polyvinyl pyrrolidone (PVP) solution route. Mater Chem Phys. 2009;15:423. 63. Urbach F. The long-wavelength edge of photographic sensitivity and of the electronic absorption of solids. Phys Rev. 1953;92:1324.

587

64. Hassanien AS, Aki AA. Influence of composition on optical and dispersion parameters of thermally evaporated non-crystalline Cd50S50-xSex thin films. J Alloys Compd. 2015;648:280. 65. Truffault L, Minh-Tri T, Devers T, Konstantinov K, Harel V, Simmonard C, et al. Application of nanostructured Ca doped CeO2 for ultraviolet filtration. Mater Res Bull. 2010;45:527. 66. Yin L, Wang Y, Pang G, Koltypin Y, Gedanken A. Sonochemical synthesis of cerium oxide nanoparticles-effect of additives and quantum size effect. J Colloid Interface Sci. 2002;246:78. 67. Ho C, Yu JC, Kwong T, Mak AC, Lai S. Morphology-controllable synthesis of mesoporous CeO2 nano- and microstructures. Chem Mater. 2005;17:4514. 68. Masui TY, Fujiwara KY, Machida KI, Adachi GY. Characterization of cerium (IV) oxide ultrafine particles prepared using reversed micelles. Chem Mater. 1997;9: 2197. 69. Zhang YW, Si R, Lia CS, Yan CH, Xiao CX, Kou Y. Facile alcohothermal synthesis, size-dependent ultraviolet absorption, and enhanced CO conversion activity of ceria nanocrystals. J Phys Chem B. 2003;107:10159. 70. Pitchaimuthu S, Velusamy P, Rajalakshmi S, Kannan N. Enhanced photocatalytic activity of titanium dioxide by b-cyclodextrin in decolorization of acid yellow 99 dye. Desalin Water Treat. 2014;52:3392. pez Y, García-Rosales G, Jime nez-Becerril J. Synthesis and character71. Lara-Lo ization of carbon-TiO2-CeO2 composites and their applications in phenol degradation. J Rare Earths. 2017;35(6):551. 72. Hahn NT, Holmberg VC, Korgel BA, Mullins CB. Electrochemical synthesis and characterization of p-CuBi2O4 thin. film photocathodes. J Phys Chem C. 2012;116:6459. 73. Elaziouti A, Laouedj N, Bekka A, Vannier RN. Preparation and characterization of pen heterojunction CuBi2O4/CeO2 and its photocatalytic activities under UVA light irradiation. J King Saud Univ Sci. 2015;27:120. 74. Elaziouti A, Laouedj N, Bekka A, Vannier RN. Preparation and characterization of pen heterojunction CuBi2O4/CeO2 and its photocatalytic activities under UVA light Irradiation. Sci Technol A. 2014;39:9. 75. Laouedj N, Elaziouti A, Bekka A. Photodegradation study of Congo Red in aqueous solution using ZnO/UV-a: effect of pH and band gap of other semiconductor groups. J Chem Eng Process Technol. 2011;2:1. 76. Chen SF, Zhao W, Liu W, Zhang HY, Yu XL, Chen YH. Preparation, characterization and activity evaluation of pen junction photocatalyst p-CaFe2O4/nAg3VO4 under visible light irradiation. J Hazard Mater. 2009;172:1415. 77. Latha P, Karuthapandian S. Novel, facile and swift technique for synthesis of CeO2 nanocubes immobilized on zeolite for removal of CR and MO dye. J Cluster Sci. 2017;28:3265.