Cassava-starch-assisted sol–gel synthesis of CeO2 nanoparticles

Materials Letters 165 (2016) 139–142

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Cassava-starch-assisted sol–gel synthesis of CeO2 nanoparticles N.S. Ferreira a,n, R.S. Angélica b, V.B. Marques a, C.C.O. de Lima a, M.S. Silva c a b c

Departamento de Física, Universidade Federal do Amapá, 68902-280 Macapá, AP, Brazil Instituto de Geociências, Universidade Federal do Pará, 66075-110 Belém, PA, Brazil Instituto Federal de Educação, Ciência e Tecnologia do Sertão Pernambucano, 56000-000 Salgueiro, PE, Brazil

art ic l e i nf o

a b s t r a c t

Article history: Received 15 August 2015 Received in revised form 23 October 2015 Accepted 23 November 2015 Available online 2 December 2015

Cerium oxide (CeO2) nanoparticles were successfully synthesized using native cassava starch as a sizelimiting chelating agent. X-ray diffraction (XRD) and Raman spectroscopy analysis revealed that all synthesized samples exhibited a cubic CeO2 structure. Transmission electron microscopy (TEM) and XRD observations showed that increasing the calcination temperature increased the average crystallite particle sizes from 8.1 to 12.7 nm. X-ray photoelectron spectroscopy (XPS) and Raman results also revealed that reduction in the valence of Ce4 þ to Ce3 þ caused an increase in the molar fraction of oxygen vacancies, resulting in the presence of structural defects in the CeO2 lattice. The cassava-starch-assisted sol–gel method proved to be highly efficient for the synthesis of highly crystalline CeO2 nanoparticles with potential for industrial applications. & 2015 Elsevier B.V. All rights reserved.

Keywords: Nanoparticles Cassava Starch CeO2

1. Introduction Metal oxide nanoparticles are widely used in practical applications in several industrial sectors [1,2]. Many of those, such as CeO2 nanoparticles, are receiving considerable attention because of their significant potential applications as agents for cancer therapy [3], photocatalysts for the purification and decomposition of pollutants [4], and humidity sensors [5]. Moreover, the synthesis of CeO2 nanoparticles with different morphologies and sizes is of significant importance both from the viewpoint of basic fundamental research and in advanced applications such as the development of novel devices (e.g., dye-sensitized solar cells) [6]. To date, various chemical methods have been developed to synthesize CeO2 nanoparticles [3,6,7]. However, only a few authors have focused on the development of new strategies for low-cost nanoparticle synthesis. For example, Macêdo et al. [8] provided a promising and simple route named proteic sol–gel process, which does not need complicated equipment and expensive raw materials. This method differs from traditional sol–gel processes because it uses salts (nitrates, chlorates, and sulfates) as the starting material and in natura coconut water as a chelating agent instead of expensive alkoxides. However, this method is limited by the difficulty in storing the coconut fruit and its limited availability in some regions of the world. Thus, we aimed to develop a lowtemperature, simple, and cost-effective approach for the synthesis of oxide nanoparticles that overcomes these limitations. Our main n

Corresponding author. E-mail address: [email protected] (N.S. Ferreira).

http://dx.doi.org/10.1016/j.matlet.2015.11.107 0167-577X/& 2015 Elsevier B.V. All rights reserved.

strategy is the use of the native starch of cassava (Manihot esculenta), also called tapioca, as an inexpensive chelating agent; to the best of our knowledge, this has not yet been reported. Starch is a natural, renewable, and biodegradable polymer that presents biochemical complexity at both the molecular and supramolecular levels similar to those of coconut water. Moreover, this extender presents two main advantages. First, it is an abundant biomass material in nature and can be easily stored, allowing it to be used in areas where fresh coconuts are not available; second, it exhibits a standardized composition with a low-ordered crystalline structure in which each uncompleted biding between two glucose molecules has a certain affinity to metal ions. This can control the size and morphology of oxide nanoparticles. Herein, we report a new approach to directly synthesize CeO2 nanoparticles using Ce(NO3)3  6H2O as a cerium source and cassava starch cultivated in the Brazilian Amazon region as a sizelimiting chelating agent.

2. Experimental The typical procedure used to prepare the CeO2 nanoparticles was as follow. First, a 0.5 M Ce(NO3)3  6H2O homogeneous solution was slowly added to a 500 g L  1 solution of native cassava starch at 25 °C. Subsequently, the resultant mixture was maintained under continuous magnetic stirring for 1 h at 70 °C to complete starch gelatinization. Afterwards, the obtained gel was kept in an oven at 100 °C overnight to obtain an amorphous solid (xerogel). Finally, the xerogels were calcined for 1 h at temperatures ranging from 200 °C to 500 °C. The formation of CeO2

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Table 1 F2g mode position, average crystallite size, lattice parameters, and unit cell volume of CeO2 synthesized at different calcination temperatures. Lattice Temperature (°C) F2g (cm  1) Average crystallite size (nm) constants

Unit cell volume (nm3)

a¼ b¼ c (nm) 200 300 400 500

Fig. 1. (a) TGA-DTA curves of a synthesized precursor sample (dried gel). (b) XRD patterns of CeO2 nanoparticles calcined at different temperatures. (c) Typical Rietveld refined XRD pattern for sample calcined at 200 °C.

nanoparticles was confirmed using thermogravimetric and differential thermal analyses (TGA-DTA, TA Instruments) and Raman spectrometry (SENTERRA, Bruker). X-ray Powder Diffraction (XRD) analysis (X’Pert PRO-MPD, PANalytical) was performed using a CuKα-radiation tube operated at 40 kV and 40 mA, a scan range from 20 to 85° (2θ), a step size of 0.02°, and a time/step of 60 s. Rietveld refinement procedures were applied to XRD patterns using the FullProf program [9]. The crystallite size of the samples was calculated by using the Scherrer equation [10]. The particle size and morphology were observed using transmission electron

464.61(5) 464.51(5) 466.06(1) 466.56(1)

8.11 8.73 9.39 12.69

0.54158(12) 0.54137(12) 0.54131(12) 0.54131(12)

15.885 15.867 15.861 15.861

Fig. 2. (a) Raman spectra (open circles) for the CeO2 nanoparticles calcined at different temperatures, together with the fitted spectra (solid line) using the Lorentzian line shape. (b) High-resolution XPS spectrum for the Ce 3d region of the CeO2 sample calcined at 200 °C. The inset also shows the high-resolution XPS spectrum for the O 1s region.

microscopy (TEM, JEOL-100F). The chemical composition of the samples was investigated by X-ray photoelectron spectroscopy (XPS) using an X-ray photoelectron spectrometer (ESCALAB-250, Thermo-VG Scientific) with Al-Kα radiation (1486.6 eV). 3. Results and discussion The TGA-DTA analysis of the xerogel is shown in Fig. 1(a). The TGA-DTA curve shows that the xerogel continually lost weight in the range of 30 °C–600 °C. The weight loss in the temperature

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Fig. 3. TEM images of CeO2 nanoparticles synthesized at 200 °C. (a–b) Images of CeO2 nanoparticles spread on a holey carbon grid; (c) SAED patterns (d) TEM image showing an interval of about 0.31 nm between two lattice fringes. The insert is the corresponding FFT pattern.

range of 30 °C–190 °C is attributed to the evaporation of water adsorbed onto the xerogel surface, which corresponds to the exothermic peak at 164 °C in the DTA curve. In the temperature range of 190 °C–220 °C, which corresponds to the exothermic peak at 213 °C in the DTA curve, the weight loss (  12%) was attributed to the decomposition of non-carbonized anions NO−3 , the collapse of the xerogel network, and subsequent combustions of the organic complex. The large weight loss (  31%) could be attributed to the complete decomposition of the nitrate precursor into CeO2. A faint peak at 344 °C, accompanied by a weight loss of  13%, is indicative of the release of CO2 due to the combustion of residual organic groups (remaining organic mass of starch). Therefore, the temperatures in the 200 °C–500 °C range were chosen for the calcination of the xerogel in order to obtain the CeO2 particles. Fig. 1(b) shows XRD patterns of the CeO2 nanoparticles calcined at different temperatures. All the intense and sharp reflection peaks can be indexed to the fluorite structure, space group Fm3¯ m (ICDD#00-004-0593). Rietveld refinements were used to refine the structural parameters of all samples. The Bragg peaks are modeled with a pseudo-Voigt function, and the background is estimated by linear interpolation between selected background points. Fig. 1(c) represents the typical Rietveld refinement for the sample calcined at 200 °C. The average crystallite size, lattice parameters, and unit cell volume of the samples calculated from the Rietveld refinement are provided in Table 1. The values of lattice parameters are 0.54158(12), 0.54137(12), 0.54131(12), and 0.54131(12) nm for the samples calcined at 200 °C, 300 °C, 400 °C, and 500 °C, respectively, higher than the values for bulk CeO2

(a ¼0.5411 nm, ICDD#00-004-0593). This lattice expansion is explained in terms of a decrease in electrostatic force caused by the valence reduction of Ce4 þ to Ce3 þ , which caused an increase of Ce3 þ ions and the molar fraction of oxygen vacancies on the nanoparticle surface. This resulted in higher lattice constant expansions for successively smaller CeO2 particles [11]. The average crystallite size of the CeO2 nanoparticles increased from 8.1 70.3 nm to 12.7 70.3 nm when the calcination temperature was increased from 200 °C to 500 °C. At higher temperatures, the crystallization rate of solid phases increases due to the large movement of atoms, which facilitates a rapid arrangement of the crystalline structure and subsequent aggregation of the crystallites to minimize the interfacial surface energy [10]. The structure of the CeO2 nanoparticles was further confirmed by Raman spectroscopy. Fig. 2(a) shows the Raman spectra of the samples calcined at various temperatures, measured in the 350– 600 cm  1 range. There is only one Raman active mode (F2g) for all the CeO2 samples. F2g is the lattice mode and is correlated to the first-order symmetrical stretching mode of the Ce–O8 vibrational unit, which is very sensitive to any disorder in the oxygen sublattice as a result of thermal or grain size effects [12]. Herein, the F2g position shifts to lower frequencies as the calcination temperature increases from 200 °C to 500 °C (see Table 1). Moreover, the F2g peak half-width (ΓF2g) becomes more asymmetrically broadened for successively smaller CeO2 nanoparticles due to the existence of oxygen vacancies on the surface [11,13]. In order to obtain further insights into the chemical composition and valence state of the chemical species present in the

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samples, XPS analysis was carried out on the CeO2 nanoparticles. The XPS survey spectra (not shown here) showed that no elements other than C, Ce, and O were present in the samples, confirming the high chemical purity of the CeO2 nanoparticles. Representative high-resolution XPS spectrum taken from the Ce 3d and O 1s levels for the CeO2 sample calcined at 200 °C are shown in Fig. 2(b). The Ce 3d spectrum exhibits complex features with eight peaks labeled as u and v, which is represented by spin-orbits of Ce 3d3/2 and Ce 3d5/2, respectively. Spin-orbit doublet (v‴,u‴), (v″,u″), and (v,u) are attributed to the Ce4 þ final state, while (v′,u′) are assigned to the Ce3 þ final state [14]. This result indicates that both Ce4 þ and a small amount of Ce3 þ ions coexist in the sample. The inset of Fig. 2 (b) displays the O1s peaks centered at 526.4 (LBE) and 528.1 (HBE) eV. The LBE component corresponds to the stoichiometrically bonded O2  , and the HBE component is attributed to the O2  ions in the oxygen deficient regions within the CeO2 matrix [14]. These results corroborate the assumption that there are oxygen vacancies on the surface of the CeO2 nanoparticles. The TEM image of the sample calcined at 200 °C (Fig. 3(a)) shows that CeO2 nanoparticles are mostly spherical in shape and highly agglomerated. Fig. 3(b) shows the high-resolution TEM image of the CeO2 nanoparticles, and the inset shows the corresponding fast Fourier transform (FFT) image. The average particle size was 8.947 0.09 nm, which is consistent with the values obtained using the Scherrer equation. Moreover, the spacing between the lattice fringes is found to be 0.3127 0.004 nm (Fig. 3 (d)), which is consistent with the interplanar spacing between the (111) lattice planes of the CeO2 phase. Furthermore, selected area electron diffraction (SAED) ring patterns in Fig. 3(c) are identified as (111), (200), (220), (311), (222), and (400) planes of a CeO2 fcc phase, which corroborates to the FFT pattern inserted in Fig. 3(b). These results confirm that by using cassava starch as a chelating agent, highly crystalline CeO2 nanoparticles can be successfully produced using low-cost reagents in an environmentally friendly reaction medium. Moreover, the samples do not require any postwashing treatments to remove soluble phases containing sodium or potassium, which are inorganic compounds present in fresh coconut water and are frequently present as impurities in samples synthesized by the proteic sol–gel process [15].

4. Conclusion In summary, CeO2 nanoparticles were successfully synthesized

at low temperature by a cassava-starch-assisted sol–gel approach. The structural analysis confirms that all CeO2 nanoparticles calcined in the range of 200 °C–500 °C exhibited a fluorite structure and an average crystallite size between 8 and 16 nm. XPS, Rietveld refinements, and Raman spectroscopy also revealed that reduction in the valence of the Ce4 þ ions to Ce3 þ ions caused an increasing molar fraction of oxygen vacancies, which resulted in a higher lattice constant expansion for successively smaller CeO2 nanoparticles. The results of this study demonstrate that cassava starch is a useful chelating agent to prepare CeO2 nanoparticles.

Acknowledgments This work was supported by the FAPEAP (Project No. 800062/ 2013-2). The author specially acknowledges Prof. A.S. Góes whose inspiration gave shape to this research project. V.B. Marques gratefully acknowledges financial support from the CNPq (Grant No. 143642/2014-6).

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