ARTICLE pubs.acs.org/IECR
A Novel Aqueous Route To Fabricate Ultrasmall Monodisperse Lipophilic Cerium Oxide Nanoparticles Thadathil S. Sreeremya, Kunnambeth M. Thulasi, Asha Krishnan, and Swapankumar Ghosh* Nanoceramics Division, National Institute for Interdisciplinary Science & Technology (NIIST), Council of Scientific & Industrial Research (CSIR), Trivandrum-695 019, India
bS Supporting Information ABSTRACT: Ultrasmall monodisperse 2.3 nm CeO2 nanoparticles have been synthesized by simple ammonia precipitation of cerium nitrate in a mixed glycolwater solvent and phase transferred into apolar solvents. Cerium oxide crystal surfaces were passivated with oleic acid (OA) by reflux under ambient pressure conditions. OA molecules were chemisorbed on the ceria nanoparticle surfaces. Preferential growth of the {100} planes has been observed, ascribed to restricted growth of {111} faces due to adsorbed OA. The surfacted CeO2 nanoparticles were dispersed into a stable, colloidal suspension of fine particles in hydrocarbon solvents. The CeO2 nanocrystals were characterized by X-ray diffraction, Fourier transform infrared spectra, and Raman spectral methods. Transmission electron microscopic images and photon scattering studies proved that the colloidal particles in hydrocarbon solvent were indeed monocrystalline with one crystallite per particle, monodispersed with a narrow size distribution.
Several methods have been employed to synthesize fine CeO2 particles of sizes ranging from 2 to 4 nm.16,2528 Nonaqueous methods using a metaloleate complex are already reported for the synthesis of organophilic CeO2 NPs.29 Inoue et al. produced 2 nm CeO2 particles using a hydrothermal method, requiring a steam pressure of 40 kg/cm,2 which constitutes a serious hurdle for adoption in industry.26 CeO2 particles, in the range 2.8 3.3 nm, have also been synthesized by sonochemical treatment27,30 as well as microwave-assisted heating.30 Adachi and co-workers synthesized 2.6 and 4.1 nm ceria particles using reverse micelles.25 However, all of these methods involve toxic and/or expensive organometallic compounds/complexes, or high pressure unit operations in the synthetic process flow sheet. For obvious reasons it is difficult to synthesize OA capped, organophilic CeO2 nanocrystals by aqueous methods and, to the best of our knowledge, there is only one patent reporting the fabrication of OA capped ultrasmall CeO2 crystals under ambient pressure conditions.31 We demonstrate a synthetic strategy to prepare 2.3 nm ultrasmall cerium oxide crystals through homogeneous ammonia precipitation in a mixed wateralcohol solvent. The CeO2 crystals were subsequently capped with oleate ions by reflux under ambient pressure conditions. The proposed aqueous method has produced ultrasmall CeO2 NPs at relatively low reaction temperatures at ∼100 °C without the use of any organometallic precursor. Furthermore, monodisperse, stable transparent suspensions of 2.3 nm CeO2 particles have been achieved by suspending the capped CeO2 in nonpolar solvents.
1. INTRODUCTION Materials of nanodimension have been attracting increasing interest worldwide for their unique size- and shape-dependent chemical and physical properties1 compared to bulk materials, owing to the enhanced surface area to volume ratio,2 quantum confinement, as well as their potential self-assembly for device applications.3 Over the last two decades cerium-oxide-based materials have been extensively studied and employed in various applications including oxygen storage capacitors,4 catalysts,5,6 UV blockers,7,8 gas sensors,9,10 solid oxide fuel cells,11 and in chemical mechanical planarization.12 Capped ultrasmall CeO2 is the precursor for embedded quantum dots (QDs) in spectral bar coding.13 Oleate-capped CeO2 is also used in illumination and in tricolor display devices as a composite with polymer,14 and as photovoltaic material.15 Numerous techniques have been reported to synthesize nanosized CeO2 particles with promising control of size and properties. In recent years, the aqueous synthetic method has been intensively studied, modified, and improved to find economic and environmentally friendly pathways for fabrication of controlled metal oxide nanostructures.16 Homogeneous precipitation methods are largely investigated for the industrialscale synthesis of CeO2 nanocrystals. Hydrothermal treatments have been successfully used for shape-controlled synthesis of CeO2 nanomaterials, such as nanopolyhedra,17 nanowires,18 and nanotubes.19 All of these investigations highlighted the possibility of a convenient aqueous synthetic route to highly controlled CeO2 nanocrystals. Ammonia precipitation under ambient conditions is undoubtedly the most utilized method to synthesize CeO2 for economic reasons.2022 A notable limitation of ammonia precipitation of cerium nitrate is that the minimum size of the nanocrystals achievable is larger than 45 nm. Synthesis methods of ultrasmall and monodispersed CeO2 nanoparticles (NPs) are challenging16 and are covered by patents.23,24 r 2011 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Materials. All the syntheses were carried out with double distilled water using a quartz glass distillation unit, and the chemicals Received: August 31, 2011 Accepted: November 18, 2011 Revised: November 18, 2011 Published: December 06, 2011 318
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utilized in this study were used as received without further purification. Ce(NO3)3 3 6H2 O (99.9%) was purchased from Indian Rare Earths, India, ethylene glycol (analytical reagent) was obtained from Merck, India, 25% ammonia solution (analytical grade) was procured from Qualigens Fine Chemicals, oleic acid (90%) from Alfa Aesar, UK. Common solvents such as acetone, ethyl alcohol, and toluene (analytical grade) were procured from Merck, India. 2.2. Precipitation of CeO2 Nanoparticles. A 5.8 mmol portion of Ce(NO3)3 3 6H2O (CN) was dissolved in 100 mL of 1:1 waterethylene glycol mixture in a 250 mL beaker. The mixed solvent was heated to 60 °C and 16 mL of 25% ammonia solution was added to it under strong mechanical agitation using a 1/8 hp overhead stirrer. The fresh suspension turned blackish purple immediately after the ammonia addition and finally to yellow after about 1 h stirring indicating the formation of particles containing cerium in the +4 state. The crystal growth was allowed to proceed further for 2 h at 60 °C with constant stirring. The slurry was subsequently cooled slowly to room temperature. The precipitate was separated by centrifugation and was washed with distilled water until free from nitrates. 2.3. Capping with Oleic Acid Surfactant. The precipitate collected from the above precipitation step was dispersed in 120 mL of distilled water in a 500 mL beaker under strong mechanical stirring, and the temperature of the suspension was increased to ∼60 °C. A 10 mL portion of ammonia (25%) was added to the suspension which was immediately followed by the addition of 5 mmol oleic acid to the reaction mixture. The pH of the reaction mixture at this point was ∼10. The temperature of the slurry was kept constant at ∼60 °C for total 45 min with continuous mechanical agitation. The resultant slurry was poured into a 250 mL three-neck round-bottom flask, an additional 5 mL of ammonia was added to it, and the suspension was refluxed for 1 h. Deprotonated oleate ions were chemisorbed onto the surface of CeO2 NPs. After cooling the reaction mixture to room temperature, the slurry was precipitated with the addition of acetone and ethyl alcohol (∼1:1) one after the other, and the precipitates were isolated from the solvent by centrifugation. The precipitate was washed twice with acetone and dried at 60 °C overnight in an air oven. A small amount of dried precipitate was dispersed in toluene which produced transparent straw-colored suspension of CeO2 nanocrystals, this material will be referred to as OleicAcidNP. Oversized particles/clusters in the suspension were removed by centrifugation at ∼5000 g rcf and the centrifuged product in suspension will be referred to as OleicAcidNPr. The yield of OleicAcidNPr was estimated gravimetrically as ∼79.5%. The yield is relatively high as the loss of nanomaterials during washing in section 2.2 and 2.3 as well as resizing of OleicAcidNP by centrifugation was low. The OleicAcidNPr nanofluids were stable for over six months. To identify the effect of the surface functionalization on the crystal growth, control CeO2 samples were also prepared. Cerium oxide nanocrystals were synthesized in the mixed solvent, and the reflux was performed in alkaline suspension without the addition of OA in the second step, this material will be referred to as EthyleneGlycolNP. CeO2 crystals synthesized in pure water by ammonia precipitation following conditions in the precipitation step (2.2) were collected with no further reflux treatment, this material will be referred to as WaterNP. The motivation behind this approach was also to investigate the influence of solvent media and its dielectric strength on nanoparticle formation characteristics.
2.4. Instruments and Characterization. The crystalline phase composition of the solid products were determined from the powder X-ray diffraction (XRD) patterns using a Philips X’PERT PRO diffractometer with Ni-filtered Cu Kα1 radiation (λ = 1.5406 Å) in the 2θ range 20100 degree at a scanning rate 2° min1 with a step size 0.04°. Standard Harris analysis was performed on X-ray data of CeO2 powders.32,33 The preferred orientation of the crystallographic planes was estimated and expressed as texture coefficient C (hikili), following the equation
Iðhi ki li Þ 1 Cðhi ki li Þ ¼ Io ðhi ki li Þ n
∑
Iðhi ki li Þ Io ðhi ki li Þ
1
ð1Þ
where, I(hikili) is the diffraction intensity of the (hikili) plane of the particular sample under investigation, Io(hikili) is the intensity of the (hikili) plane from the standard JCPDS powder diffraction pattern for the corresponding peak i, and n is the number of reflections taken in to account. The morphology, average size of the CeO2 nanocrystals and crystal structure were determined by high resolution transmission electron microscopy (HR-TEM) using a FEI Tecnai 30 G2 S-Twin microscope operated at 300 kV and equipped with a Gatan CCD camera. A small amount of capped CeO2 was dispersed in toluene and ultrasonicated to get a stable suspension. Samples for TEM study were prepared by dropping a microdroplet of suspension in isopropyl alcohol on to a 400 mesh copper grid and drying the excess solvent naturally. Size measurements for the colloidal cerium oxide NPs in suspensions were performed at 25 °C by photon correlation spectroscopy (PCS) on a Zetasizer 3000 HSA, Malvern Instruments, Worcestershire, UK using a 60 mW HeNe laser operating at a wavelength of 633 nm with General Purpose algorithm with Dispersion Technology Software (v. 1.61) at 90° detection angle. Fourier transform infrared (FTIR) spectra of the as prepared products were recorded at room temperature using the KBr (Sigma Aldrich, g 99%) pellet method on a Nicolet Magna IR-560 spectrometer ranging from 400 to 4000 cm1 with 50 scans. Raman spectra were acquired using a WITec alpha 300R confocal Raman spectrometer (Ulm, Germany) with excitation by a 40 mW, 532 nm air-cooled solid state laser. The sample was scanned through the laser focus in a raster pattern at a constant stage speed of fractions of a micrometer per second. Raman scattered light was focused through a holographic notch filter onto a multimode fiber (50 mm diameter) and into a 300 mm focal length monochromator incorporating a 600/mm grating blazed at 500 nm. Detection of the Raman spectrum was provided by a Peltier cooled charge coupled device (CCD) with an integration time of 3 s. The specific surface area of CeO2 powders were calculated from the N2 adsorption data using the BrunauerEmmettTeller (BET) technique. Nitrogen adsorption isotherms on nanocrystalline CeO2 samples were determined at liquid nitrogen temperature (77 K) using a Micromeritics Gemini 2360 surface area analyzer. Powder samples were degassed at 200 °C for 2 h in flowing dry nitrogen gas prior to the adsorption studies. The thermogravimetric studies of the powders were carried out using a Shimadzu 50 H analyzer in ambient atmosphere heated at a constant ramp of 10 °C min1 under air purge. The reflectance spectra of the CeO2 powders (EthyleneGlycolNP and OleicAcidNP) were obtained using a UVvisible 2401 PC spectrophotometer (Shimadzu, Japan) in the wavelength range 200800 nm, corresponding to photon energy varying from 6.19 to 1.55 eV. 319
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3. RESULTS AND DISCUSSION
Table 1. The Effect of Solvent Composition on the Cerium Oxide Particle Sizes
3.1. Synthesis of 2.3 nm CeO2 Nanocrystals by OA Assisted Precipitation Method. Ultrasmall nanocrystals of CeO2 were
solvent
synthesized by ammonia precipitation in glycolwater mixed solvent and subsequent capping the crystals with OA in an alkaline suspension using a simple reflux technique with inexpensive chemicals and common laboratory glasswares. Formation of CeO2 from CN involves several reaction steps involving intermediates. Ce(OH)3 is first produced as soon as the ammonia is added to the aqueous CN solution as the hydroxide has an extremely low solubility constant (Ksp = 6.3 1024 at 25 °C).34 Subsequently, the Ce(OH)3 precipitate is oxidized to Ce(IV) in the presence of atmospheric oxygen.35 The alkaline pH condition favors the formation of Ce(IV) (Ksp of Ce(OH)4 = 2 1048).3638 Finally, precipitates of cerium(IV) hydroxide are dehydrated to CeO2 3 nH2O during the drying step,21,39 according to the overall equation 1 O2 þ 6OH f 2CeO2 3 nH2 O 2 þ ð3 2nÞH2 O
a
þ 6NO3
zþ z e2 4πε0 εðrþ þ r Þ
ð2Þ
ð3Þ
ð5Þ
2mγ rkTF
6.1 ( 0.98 3.7 ( 0.91a
Calculated size of CeO2 nanoparticles using eq 8 is 2.6 nm.
ð7Þ
ð8Þ
where A = (kTF)/(2mγ) ln C and B = (Fz+ze2)/(8πmγ ε0(r+ + r)). A and B are constants for mixed solvents of homologous alkyl alcohol and water. Replacing F = 1033.8 kg m3 for a 1:1 water EG mixed solvent at 60 °C, γ = 0.0557 N m1, C = 6.3 1024, ε0 = 8.85 1012 F m1- r+ (ionic radius of Ce4+) = 1.02 1010 m and r = 1.32 1010 m, in eq 8 gives the size of CeO2 as 2.6 nm, which is of the same order as the TEM size 3.7 nm obtained experimentally for EthyleneGlycolNP, validating the equation. The influence of solvent media, its dielectric strength, surface tension, etc. on the particle size obtained is summarized in Table 1. Stable, transparent straw-yellow color dispersion of colloidal CeO2 nanocrystals (OleicAcidNPr) of average TEM size 2.3 nm were obtained by dispersing the OA-capped CeO2 in toluene. The precursor solution of the same concentration (5.8 mmol CN) was reported to produce 6 nm CeO2 particles at 60 °C under identical conditions.22 Remani et al. reported that the presence of glycol during the homogeneous precipitation prevented crystal growth and produced 3.5 nm CeO2 nanocrystals as a result of lowering of the nucleation rate.40 3.3. TEM Analysis. Representative bright field HR-TEM images of WaterNP, EthyleneGlycolNP, and OleicAcidNPr are provided in Figure 1. The samples WaterNP and EthyleneGlycolNP are highly crystalline as evidenced from the sharp crystal facets and the pseudopolyhedral morphology can be viewed in the micrographs (Figure 1a,b). The sample WaterNP is composed of particles in the size range 49 nm with the presence of large clusters. The calculated average size on 100 particles of WaterNP is ∼6.1 ( 0.98 nm. Figure 1b shows the TEM images of EthyleneGlycolNP consisting of primary CeO2 nanoparticles in the range 26 nm with average size 3.7 nm (σTEM = 0.91 nm). The particles are found to be agglomerated to some extent, with a network structure apparent. The TEM images of OleicAcidNPr show rather spherical, monodisperse, and well-formed ultrasmall primary nanoparticles of CeO2 in the size range 1.53.6 nm with no evidence of agglomerates. No differences in particle morphology were apparent in any of the samples (OleicAcidNPr) at the magnification used (∼300 000). The polydispersity index of TEM sizes (PDITEM) is calculated from the statistical analysis of the detailed size data derived from the crystal dimensions of over 100 particles from multiple TEM
Supersaturation can also be correlated with the radius of stable nuclei (r) using the Kelvin equation:34 ln S ¼
0.0662 0.0557
1 Β ¼Α þ r ε
So the solvents with higher dielectric constant will have more solubility (Cl) which can be modulated by varying the composition of mixed solvent. Formation (precipitation) of particles is strongly dependent upon the supersaturation (S) of solute following classical nucleation theory. Supersaturation can be achieved by the lowering of temperature, solvent evaporation, pH change, chemical reaction, alteration in solvent composition, etc. and is defined as the ratio of solute concentration (C) and saturation concentration (Cs); that is, C Cs
66.7 54.1
On rearranging eq 7, the dependence of the radius (r) of particles produced in a precipitation reaction on ε value of the solvent can be expressed as
where ε0 is the permittivity in vacuum and ε is the dielectric constant of the solvent used. The symbols r+ and r represent the radii of charged z+ and z ions, respectively, and e represents the charge of electron (1.602 1019 C).34 The equilibrium concentration of solute in the saturated solution can be expressed as " # zþ z e2 ð4Þ Cl ≈ exp 4πε0 εkTðrþ þ r Þ
S¼
water 1:1 waterEG
2mγ zþ z e2 ¼ ln C þ rkTF 4πε0 εkTðrþ þ r Þ
The measured BET surface area for EthyleneGlycolNP is high at ∼175 m2 g1, equivalent to a diameter of 4.7 nm for nonporous spherical particles, suggesting the cluster formation of fine nanoparticle. 3.2. Effect of Dielectric Constant of Solvents on Particle Size. The nature of the solvents changes significantly with variation in dielectric constant (ε). Dissolution of an ionic compound (solute) in a solvent changes the compound from the associated state to dissociated charged ions. The resultant change of chemical potential is mainly a result of Coulomb’s interaction34 as Δμ0 ¼
DTEM/nm
where m is the molecular weight of the solute and γ is the interfacial energy between solute and solution phases. Combining eqs 4, 5, and 6, we get
2CeðNO3 Þ3 þ
dielectric constant surface tension/N m1
ð6Þ 320
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Figure 1. TEM images of ceria nanocrystals (a) WaterNP, (b) EthyleneGlycolNP, and (c), OleicAcidNPr, and (d) high resolution TEM image of OleicAcidNPr. Particle size distributions as histogram, evaluated from TEM images of OleicAcidNPr yielding DTEM = 2.3 ( 0.43 nm is shown as inset of Figure 1c. Selected area electron diffraction pattern (SAED) of OleicAcidNPr is shown as inset of Figure 1d.
images from the relation22 PDITEM ¼ σ TEM =DTEM
3.4. Size Analysis by Photon Correlation Spectroscopy. The monodispersity of OleicAcidNPr suspension is further substantiated with the size measurement data by light scattering. Figure 2 shows the hydrodynamic sizes of OleicAcidNPr colloid and the aqueous suspension of WaterNP. The optical image of transparent OleicAcidNPr suspension in toluene is shown in the inset of Figure 2. PCS data shows that OleicAcidNPr contains colloidal CeO2 particles in the size range 215 nm with an average hydrodynamic diameter of 4.3 nm with PDI 0.159. The width of the distribution is ∼1 nm. The carboxylic headgroup of the fatty surfactant molecule is chemisorbed onto the NP surface41 as shown in Scheme 1. Lowering of the dielectric constant of water from 80 at 20 °C to 55 at 100 °C during reflux process leads to a homogeneous waterorganic phase providing a suitable environment for achieving efficient OA attachment on CeO2 nanocrystal surfaces.16 The hydrophobic tail of the surfactant extends out of the NP surface (inset of Figure 2) and stabilizes the particles in hydrophobic environments producing stable colloidal suspension. The PDI in size data by light scattering techniques represents the narrowness of the distribution and a size distribution with PDI value of below 0.3 is considered as monodisperse.42 TEM images (Figure 1c,d) provided the average crystal size of 2.3 nm with 18% deviation (σTEM, 0.43 nm; PDITEM = 0.187) in OleicAcidNPr. The low PDITEM as well as PDIPCS also confirms that the nanoparticles in OleicAcidNPr are
ð9Þ
where σTEM is the standard deviation of the TEM sizes and DTEM is the average size from TEM. The average size (DTEM) of CeO2 crystals in OleicAcidNPr is 2.3 nm with the standard deviation (σTEM) being 0.43 nm. In normal statistical parlance, PDITEM is also referred to as the coefficient of variation.23 Using eq 7, PDITEM of OleicAcidNPr is estimated as 0.187. The average size (DTEM) of CeO2 crystals in OleicAcidNPr is smaller than that of EthyleneGlycolNP as apparently the presence of OA on the crystal facets retards crystal growth during reflux.3 The visual examination of the exposed crystal facets in the high resolution images indicate that the nanoparticles are single crystals and fringes corresponding to predominant (111) planes could be identified. Selected area diffraction (SAED) patterns of OleicAcidNPr (inset of Figure 1d) gave (111), (200), (220), and (311) DebyeScherrer rings with corresponding interplanar spacings of 0.31, 0.27, 0.19, and 0.16 nm, respectively, which can be indexed to cubic CeO2 of Fm3m space group (JCPDS No. 34-0394). TEM images of CeO2 crystals in OleicAcidNPr (Figure 1c,d) show low contrast, apparently due to the presence of surfactant and relatively high crystal lattice strain associated with the presence of oxygen vacancies (discussed in the following sections also). 321
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Figure 3. XRD patterns of the CeO2 nanocrystals (a) EthyleneGlycolNP and (b) OleicAcidNP. The vertical drop-lines, corresponding to cubic fluoritic CeO2 (JCPDS No. 34-394) are shown for comparison of reflection positions and intensity.
Figure 2. Particle size distribution as measured by photon scattering (intensity distribution) of suspensions of OleicAcidNPr in toluene and WaterNP in water. Schematic representation of the surfacted CeO2 NPs, hydrodynamic diameter of suspended particles (DPCS) and average CeO2 core size (DTEM) are shown in the inset. Image of transparent colloidal dispersion of CeO2 in toluene is also shown as inset.
Scheme 1. Carboxylic Acid Group of OA, Chemisorbed in a Bidentate Fashion on Cerium Oxide Particle M
indeed, monodisperse. DPCS and DTEM have shown a difference of ∼2 nm. These differences arise because TEM is sensitive to the heavier elements (here Ce), and so gives the core size, while PCS gives the hydrodynamic size (volume responsible for the photon scattering). The difference is mainly due to the surfactant coating, a value of ∼1 nm is very close to 0.9 nm for the length of OA chain.43 The clustering of capped nanocrystals is prevented due to an increase in configurational entropy of capped nanocrystals as the surfactant chains begin to compress one another when two particles each containing a chemisorbed surfactant layer approach. The average size (DPCS) for the uncapped WaterNP is 76 nm with a PDI of 1 which indicates clustering of nanoparticles in aqueous suspension and the sample is polydisperse as DTEM for the same is 6.1 nm (Figure 1a). 3.5. X-ray Diffraction. Figure 3 shows the XRD patterns of the OleicAcidNP and EthyleneGlycolNP powders. All detectable reflection peaks were indexed to a pure fluorite cubic phase (space group: Fm3m) of CeO2 with lattice constant, a = 0.541 nm (JCPDS 34-0394).44 No peaks from possible intermediate phases, such as Ce(OH)4/Ce(OH)3 were detected in the XRD pattern, supporting the phase analysis from the SAED (Figure 1d). The full-width at half maxima for all the peaks of sample OleicAcidNP were broader due to the combined effect of small crystal dimension and associated relatively higher crystal lattice strain and defects.45 The crystallite sizes for EthyleneGlycolNP and OleicAcidNP were estimated by using the Debye Scherrer formula22 from the line broadening of the (111) XRD reflection (line-broadening analysis) as 3.4 and 1.8 nm, respectively. The preferential orientation of the crystallites along a crystal plane (hkl) in the CeO2 nanocrystals was measured as
Figure 4. The texture coefficient of small CeO2 nanocrystals calculated from their powder X-ray diffraction patterns (reference JCPDS card no. 34-394 is used for the calculations).
texture coefficient and presented in Figure 4. A sample with randomly oriented crystallites presents a C(hkl) of 1, while a larger value indicates an abundance of crystallites oriented to that (hkl) plane.46 The order of surface energy of CeO2 is γ {100} > γ {110} > γ (111).47 WaterNP appears to be very close to random, and EthyleneGlycolNP and OleicAcidNP have preferentially grown {100} planes to texture coefficient values of 1.375 in EthyleneGlycolNP and 1.4 in OleicAcidNP, because of the growth confinement of (111) crystal facets due to preferential solvation of the (111) face by EG and OA.3,47 CeO2 crystals in EthyleneGlycolNP has a C(111) = 0.679, very close to that of OleicAcidNP (0.63), and (111) surface passivation did not have an appreciable effect on development of the {110} plane. CeO2 with predominant active {100} planes (EthyleneGlycolNP) is of great potential in catalytic applications. 3.6. Thermogravimetry Analysis. Figure 5 shows the thermogravimetric data of WaterNP, EthyleneGlycolNP, and OleicAcidNP powders. EthyleneGlycolNP and WaterNP follow a similar profile up to ∼185 °C losing ∼7% by weight which constitutes 322
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Figure 5. Thermogravimetric plots against temperature for WaterNP, EthyleneGlycolNP, and OleicAcidNP nanocrystals.
mostly adsorbed moisture. WaterNP shows a total loss of ∼10.5% in two stages. The loss of ∼6%, at 70 °C, is due to the removal of associated moisture. The second loss (4.5%) which occurred in the temperature range 150600 °C is due to the removal of the crystallized water of CeO2 3 nH2O. EthyleneGlycolNP shows a total loss of ∼16% confined in two major steps, a 6% loss with peak at ∼70 °C is due to removal of moisture and a further ∼10% loss, with the peak at ∼255 °C, is extended up to 600 °C due to the removal of crystallized water as well as remaining coordinated ethylene glycol from the CeO2 nanoparticle surface. The thermogram of OleicAcidNP shows a total loss of ∼42.3% by weight confined to three major steps up to 1000 °C. The 12% loss, at 185 °C, is associated with the removal of structural water and the remaining ∼30% is spread over two small steps in the temperature range 180550 °C (at 215 and 360 °C) due to the removal of ethylene glycol and decomposition of capped OA, respectively, from the CeO2 particles. The average surface cover by OA ligand molecules (foot-print) on the CeO2 nanoparticle surface was calculated using TG data and BET surface area. Using the BET surface area as 175 m2 g1 for EthyleneGlycolNP, r is the mean radius of the CeO2 nanoparticles (based on TEM size data), F is the density of the nanoparticles (7.28 g cm3), and w is the mass loss of OA in percent (26.3%) from TG, the coverage on the CeO2 nanoparticle surface in OleicAcidNP per OA ligand is thus calculated as 23 Å2. This is in good agreement with values reported in literature43 and also proves that a surfacted particle surface is effectively covered with a monolayer of OA surfactant. 3.7. Fourier Transform-Infrared Spectroscopy (FT-IR). Interaction of OA and glycol with the CeO2 particles in OleicAcidNP and EthyleneGlycolNP were investigated by Fourier transform infrared (FTIR) spectroscopy and shown in Figure 6. The CeO2 crystals with chemisorbed OA showed two strong vibrational bands at 2845 and 2927 cm1 due to the methyl νs(CH3) and the νas(CH) groups.48,49 These peaks are known to be the characteristic modes of the methylene (CH2) chains that are present in OA.16,50 The rocking vibration at 715 cm1 is also typical of (CH2)n chains with n > 3.51 The sharp peak characteristic of ν(CdO) in free OA at 1710 cm1 is replaced by two new intense and broad peaks at 1540 and 1410 cm1 in
Figure 6. FTIR spectra of (a) CeO2 crystals synthesized in water (WaterNP), glycolwater mixed solvent (EthyleneGlycolNP) and OA stabilized (OleicAcidNP), and (b) pure OA.
OleicAcidNP, which are characteristic of the asymmetric and symmetric stretches of COO, respectively.47,51 The separation of 127 cm1 confirms that the COO group of OA is covalently bonded to cerium atoms on the CeO2 NP surface of OleicAcidNP in a bidentate fashion.17,51 The FTIR spectra of EthyleneGlycolNP and WaterNP in the 1300400 cm1 region have shown vibrations at ∼550 cm1 (sh), which is a characteristic phonon mode for cubic cerium oxide.52 The peak corresponding to the CeO stretch is observed at 475 cm1 and the bands at 1054 and 1350 cm1 are due to ν(CeOCe) vibration.49 The band at 1054 cm1 is assigned to the first overtone of the fundamental vibration52 at 550 cm.1 The peaks appearing at 3440 and 3750 cm1 correspond to the OH stretching frequency of unidentate CeOH and tridentate Ce(OH)3, respectively.53 3.8. Raman Spectra. The crystal phase identification and the effect of OA surfactant on the size of the CeO2 NPs were investigated by Raman scattering. Figure 7 shows the visible Raman spectra of EthyleneGlycolNP and OleicAcidNP CeO2 crystals. The strong Raman peak at ∼462 cm1 is the fingerprint of F2g Raman-active mode from the space group Fm3m of CeO2 323
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Figure 7. Room temperature Raman spectra of different CeO2 powders: (a) OleicAcidNP and (b) EthyleneGlycolNP.
with the cubic fluorite structure which shifts to lower energies ca. 454 cm1, with asymmetric broadening of the Raman peak as the particle size decreases.15,54 XRD has already confirmed the CeO2 phase, structure, and space group in EthyleneGlycolNP and OleicAcidNP (Figure 3). Several other factors like phonon confinement, strain, nonhomogeneity of the size distribution, defects, and variations in phonon relaxation with particle size can contribute to the changes in the Raman peak position and linewidh of F2g mode.54 In addition, the weak peak at about 1175 cm1 in EthyleneGlycolNP which shifted to 1166 cm1 in OleicAcidNP is assigned to the second order Raman mode feature of surface superoxide species (O2) due to the presence of oxygen vacancies.15 3.9. Optical Properties. Figure 8 shows the reflectance spectra of the EthyleneGlycolNP and OleicAcidNP powders in the UVvisible range 200800 nm. The strong absorption band with an edge below 400 nm is due to charge-transfer transitions from O 2p to Ce 4f.15 Interestingly, the UV absorption edge wavelength shows a red shift from ∼358 nm for 3.7 nm EthyleneGlycolNP toward the visible region to ∼456 nm in 2.3 nm OleicAcidNP. The extended tail of the oleic acid functionalized surface is due to the presence of more Ce3+ in OleicAcidNP as was supported by Raman analysis also (Figure 7). UVvisible spectrum of WaterNP reduced by the addition of NaBH4 in suspension showed a similar extended tail following the absorption edge (see Supporting Information, Figure S1). The optical band gap Eg of a semiconductor material can be calculated from the equation of (αhν)n = B (hν Eg), where hν is the photon energy, α is the absorption coefficient, B is a constant for the material, and n is 2 for a direct transition or 1/2 for an indirect transition.55 As the size of nanocrystals decreases from 3.7 to 2.3 nm, the calculated direct band gap energy Ed decreases from 3.1 eV for EthyleneGlycolNP to 2.25 eV for OleicAcidNP, all of which are smaller than 3.19 eV for CeO2 single crystal and polycrystalline films.55 The band gap of nanocrystals is generally influenced by the particle size and the defect in the crystal structure. Since CeO2 is a direct band gap semiconductor, a blue shift of the absorption edge is normally observed as a result of decrease in its crystal dimension due to quantum confinement effects.15,26,47,56 In fact, as size reduces, the Ce3+ and oxygen vacancy concentrations increase. The presence of defects in ceria is evidenced from the Raman shift (Figure 7) and X-ray peak broadening (Figure 3). It is known that electronphonon coupling can determine the energy of excitons and the effective mass of carriers scattering by lattice. CeO2 is a strong electronphonon coupling
Figure 8. Electronic band gap measurement of ultrasmall CeO2 nanocrystals: (a) UVvis absorption spectra and (b) plot of (αhν)2 versus photon energy.
system. We suggest that in the case of OleicAcidNP and EthyleneGlycolNP, the presence of a significant fraction of Ce atoms (in either the 3+ or 4+ state) causes electronphonon relaxation and leads to oxygen vacancies and defects whose influence on the band gap overcomes the expected influence of the regular quantum size effect.47,56
4. CONCLUSIONS We present a simple, novel, and scaleable method for preparing ultrasmall nanocrystals of CeO2 by aqueous precipitation of cerium nitrate with ammonium hydroxide, without using expensive chemicals or hazardous high pressure apparatus. The surfaces of CeO2 nanocrystals were coated with a full layer of lipophilic stabilizer exemplified by OA as evidenced by TG and FTIR. The resulting materials could be well dispersed in hydrocarbon solvents producing suspensions which were stable for over 6 months. Estimation of texture coefficients from the X-ray data have shown that the {111} and {110} planes are preferentially passivated by amphiphilic OA surfactant and {100} plane has preferentially grown, which has great potential in catalytic applications. Ultrasmall CeO2 nanocrystals capped with OA have shown significant spectral red shift toward the visible region, which is of great importance in spectral bar coding and composite display applications. 324
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bS
Supporting Information. UVvis absorption spectra of CeO2 (a) WaterNP, and (b) WaterNP reduced by the addition of NaBH4. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
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[email protected]. Tel: +91-471-2515385. Fax: +91-471-2491712.
’ ACKNOWLEDGMENT This work was supported by the Indian Rare Earths Limited Technology Development Council (IRELTDC), DAE, India. The authors are grateful to the Director, National Institute for Interdisciplinary Science & Technology (NIIST), CSIR for providing the necessary facilities for the work. We thank the Department of Science & Technology (DST) and CSIR, India, for providing HRTEM facility to NIIST. The MMD (X-ray), HR-TEM staffs are kindly acknowledged for their assistance in obtaining XRD and electron microscopy data. Special thanks to Dr. Dermot Brougham of Dublin City University, Dublin, and Dr. Eoin Murray, IPRI, ARC, University of Wollongong, Australia, for technical discussions. We thank the staffs of Biomedical Wing, SCTIMST, Trivandrum, for providing Raman spectra. T.S.S. thanks Indian Rare Earths Limited, India, for the fellowship and A.K. acknowledges CSIR for the CSIR-UGC JRF fellowship. ’ REFERENCES (1) Corradi, A. B.; Bondioli, F. B.; Ferrari, A. M.; Manfredini, T. Synthesis and Characterization of Nanosized Ceria Powders by MicrowaveHydrothermal Method. Mater. Res. Bull. 2006, 41, 38. (2) Kamruddin, M.; Ajikumar, P. K.; Nithya, R.; Tyagi, A. K.; Raj, B. Synthesis of Nanocrystalline Ceria by Thermal Decomposition and SoftChemistry Methods. Scr. Mater. 2004, 50, 417. (3) Si, R.; Zhang, Y. W.; You, L. P.; Yan, C. H. Rare-Earth Oxide Nanopolyhedra, Nanoplates, and Nanodisks. Angew. Chem., Int. Ed. 2005, 44, 3256. (4) Mai, H. X.; Sun, L. D.; Zhang, Y. W.; Si, R.; Feng, W.; Zhang, H. P.; Liu, H. C.; Yan, C. H. Shape-Selective Synthesis and Oxygen Storage Behavior of Ceria Nanopolyhedra, Nanorods, and Nanocubes. J. Phys. Chem. B 2005, 109, 24380. (5) Murugan, B.; Ramaswamy, A. V. Defect-Site Promoted Surface Reorganization in Nanocrystalline Ceria for the Low-Temperature Activation of Ethylbenzene. J. Am. Chem. Soc. 2007, 129, 3062. (6) Kobayashi, M.; Flytzani-Stephanopoulos, M. Reduction and Sulfidation Kinetics of Cerium Oxide and Cu-Modified Cerium oxide. Ind. Eng. Chem. Res. 2002, 41, 3115. (7) Zhang, Y. W.; Si, R.; Liao, C. S.; Yan, C. H. Facile Alcohothermal Synthesis, Size-Dependent Ultraviolet Absorption, and Enhanced CO Conversion Activity of Ceria Nanocrystals. J. Phys. Chem. B 2003, 107, 10159. (8) Duan, W. D. W.; Xie, A. J.; Shen, Y. H.; Wang, X. F.; Wang, F.; Zhang, Y.; Li, J. L. Fabrication of Superhydrophobic Cotton Fabrics with UV Protection Based on CeO2 Particles. Ind. Eng. Chem. Res. 2011, 50, 4441. (9) Lyons, D. M.; Ryan, K. M.; Morris, M. A. Preparation of Ordered Mesoporous Ceria with Enhanced Thermal Stability. J. Mater. Chem. 2002, 12, 1207. (10) Zou, H.; Lin, Y. S.; Rane, N.; He, T. Synthesis and Characterization of Nanosized Ceria Powders and High-Concentration Ceria Sols. Ind. Eng. Chem. Res. 2004, 43, 3019. 325
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