PLASMA-ASSISTED SYNTHESIS OF NON-STOICHIOMETRIC NANOCERIA POWDER FROM CERIUM CARBONATE HYDROXIDE ( CeCO 3 OH )

Highly non-stoichiometric nanoceria was synthesized for the first time by thermal plasma from the precursor cerium carbonate hydroxide. The particle size was approximately 60 nm according to measurement by TEM. The nanoceria synthesized with 25 kW plasma power with argon as the carrier gas had the largest concentration of oxygen vacancies, follow by that produced with 20 kW with hydrogen as the carrier gas. XRD results indicated that the CeO1.66 phase was present with mostly nonstoichiometric ceria CeO2-x in these two products. SEM and TEM images showed that most of the particles were of irregular shape, while some triangular particles were also present. Raman spectra revealed that the F2g mode of synthesized nanoceria powders had a remarkable downshift of 7.9 10.5 cm-1 relative to the peak for single crystal ceria located at 466.0 cm-1. The Raman downshift was explained by the increase in ionic radius upon Ce4+ reduction to Ce3+. XPS results indicated that the Ce3+ content on the surface of the synthesized nanoceria was in the range of 15-30 %, depending on the plasma power and carrier gas composition. Both the Raman and XPS spectra showed numerous oxygen vacancies in the nanoceria. The results of this work indicated that the oxygen vacancy formation occurred when the CeO2 formed from the oxidation of cerium carbonate hydroxide was reduced by the hydrogen as well as the high temperature of the plasma. This investigation has verified that plasma treatment provides a promising method for the synthesis of nanoceria powder with high oxygen vacancies. * Corresponding author: Hong Yong Sohn, h.y.sohn@utah.edu 214 Metall. Mater. Eng. Vol 23 (3) 2017 p. 213-225


Introduction
Cerium has a ground state electron in the 4f orbital (Xe 4f 1 5d 1 6s 2 ), which is responsible for the reduction/oxidation behavior when cycling between its two ionic states, Ce 4+ (the Xe ground state), and Ce 3+ (Xe 4f 1 ) [1].Ceria has an excellent catalytic activity for converting CO in automobile exhaust gas to CO 2 based on its redox property [2].Ceria is also used to produce CO or H 2 with solar energy [3].It is also used for chemomechanical polishing and planarization.It has been theorized that the Ce 4+ to Ce 3+ transition and the presence of surface cerium hydroxyl group are responsible for the catalytic property of ceria [4,5].Additionally, ceramic materials based on ceria hold much promise for the production of electrolyte for solid oxide fuel cells, and reportedly have a better ion conductance than Y 2 O 3 stabilized ZrO 2 (YSZ) [6,7].
Because of the redox property due to the transition between its cations, ceria easily forms a non-stoichiometric state, CeO 2-x , in which 0 < x < 0.5.Nonstoichiometric ceria can be produced by chemical reduction [8], X-ray exposure [9], or a high temperature [10].Mudiyanselage et al. [11] noted that the Ce3+ plays an important role on CO oxidation by providing an adsorption site for CO.Recently, Gao and coworkers [8] reported that ceria with a larger Ce 3+ fraction enhanced CO conversion.The oxygen mobility in these materials is based on vacancy hopping [12,13].The energy levels associated with the formation of single and double vacancies were found to be equal to 4.1 and 4.7 eV, respectively [14].The electron carrier concentration Ce´C e , which determines the conductive property of non-stoichiometric ceria, is proportional to x in CeO 2-x [15].The oxygen vacancies present in non-stoichiometric ceria are related to many unique properties of ceria: For example, the oxygen storage capacity increases with a number of oxygen vacancies [16].Chemical stability and high mobility of oxygen vacancies [17] make ceria a good candidate for fast oxygen sensors at high temperatures.Additionally, the oxygen vacancy in ceria also plays an important role in its stable grain boundary structures [18].
A thermal plasma is an excellent tool for synthesizing nanostructured materials because its high temperature can rapidly vaporize or decompose precursors and form nanostructure materials.Nanoparticles of oxides and ceramic materials, such as TiO 2 [26] and TiO 2 -based materials [27], Y 2 O 3 stabilized ZrO 2 (YSZ) [28] and spherical alumina particles [29], SiC nano-fibers [30] and boron nitride (BN) nanotubes [31], have been synthesized by the use of thermal plasma.It was reported [32] that stable non-stoichiometric ceria was prepared using thermal plasma, but little has been reported on the plasma synthesis of such particles from precursors.
On the other hand, cerium carbonate hydroxide CeCO 3 OH has been reported to be decomposed into spherical nanoceria particles by hydrothermal treatment in glucose solutions [33] and into micro-sized particles by simply heating it in the air [34].Based on this previous work, thermal plasma was considered to offer an excellent means to decompose cerium carbonate hydroxide for the synthesis of nanoceria particles.Using this method, nanoceria powder with a high concentration of oxygen vacancies was prepared in this work.The effects of plasma power and gas atmosphere were investigated, and the properties and oxygen vacancy levels of the synthesized nonstoichiometric nanoceria were studied by the use of several instrumental analysis techniques.

Materials
Ce 2 (CO 3 ) 3 •xH 2 O from Sigma-Aldrich (St. Louis, Missouri, U.S.A.) was used as the precursor after first milling it to powder with an average particle size of less than 20 μm followed by drying the milled powder at 100 °C for 12 hours to form cerium carbonate hydroxide, CeCO 3 OH, following the procedure reported in the literature [35].

Methods
The DC plasma reactor used in this work consisted of a plasma generator with a downward plasma torch, a cylindrical reactor, a cooling chamber, a precursor feeding system, and a powder collector, as shown in Fig. 1.More details of the plasma reactor can be found in previous publications [28,36].The CeCO 3 OH powder was fed into the reactor at a rate of 0.75 ± 0.04 g/min, carried by a 3.5 L/min flow of argon (25 °C and 86.1 kPa atmospheric pressure at Salt Lake City).Another 39 L/min argon flow was used as the plasma gas.The plasma torch power was set at 15 kW, 20 kW, and 25 kW.After the powder was fed into the reactor for 12 ± 1 minutes, blue non-stoichiometric ceria particles were collected on a Teflon-coated polyester filter with a pore size of 1 μm that was placed in the off-gas stream.
Additional experiments were conducted with carrier gases of different compositions under 20 kW plasma power.One carrier gas was a mixture of 1.75 L/min H 2 and 1.75 L/min Ar, and the other was just H 2 at a flow rate of 3.5 L/min.Fig. 1.A schematic of the plasma reactor system.(1) powder feeding system, (2) plasma gun, (3) reactor chamber, (4) cooling chamber, (5) powder collector, and ( 6) scrubber.

Analysis methods
The nanoceria products were subjected to several types of instrumental analysis.X-ray powder diffraction (XRD) analysis was conducted using Rigaku D/Max-2200V X-ray diffractometer with Cu Kα radiation (λ = 1.5406Å) from 10.00° to 90.00° at a rate of 0.02°/second.SEM images were taken by FEI Quanta 600 FEG, USA, while TEM images were collected by JEOL USA JEM-2800.The surface binding energy was obtained using a Kratos Axis Ultra DLD XPS instrument.Raman spectra were recorded using an R3000 QE Raman spectrometer (PhotoniTech, Singapore) with a 785 nm laser for excitation in the backscattering geometry.

XRD results
Results of the XRD analysis performed on the product nanoceria are shown in Fig. 2. Firstly, CeO 2 diffraction peaks (PDF # 04-0593) represent a face-centered cubic (fcc) structure.The XRD patterns for the samples obtained at 25 kW plasma power with a 3.5 L/min Ar flow as the carrier gas and at 20 kW plasma power with a 3.5 L/min hydrogen flow show somewhat different patterns compared with the other samples: Around the peaks for the fcc crystal faces (111), ( 200), ( 220) and (311), small peaks marked with triangles are present for these two samples, which were determined to represent CeO 1.66 (PDF# 89-8434).This means that a more reduced cerium sub-oxide phase/crystal was formed under the above conditions than the other samples.

Fig. 2. XRD patterns of nanoceria synthesized under different conditions.
The crystal sizes calculated from the XRD results using the following Scherrer equation [37] and other crystal information are listed in Table 1: In this equation, D represents the crystallite size, K the Scherrer constant (0.89), λ the wavelength of the X-ray radiation (1.5418 Å), β the full width at half maximum (FWHM), and θ the diffraction angle.
The results listed in Table 1 show that the crystal sizes of the nanoceria powders obtained with only Ar as the carrier gas were similar, and the crystal sizes of the two samples produced with the carrier gases containing hydrogen were of the same average size and smaller than that of the nanoceria synthesized by using only argon as the carrier gas.

SEM and TEM images
The SEM micrographs of CeCO 3 OH and nanoceria products are shown in Fig. 3.The images show the particle sizes of nanoceria obtained under different conditions are similar in the range of 40 -80 nm.
The TEM and high-resolution TEM (HRTEM) images of nanoceria powders are shown in Fig. 4. Most particles are of irregular shape, but some particles are triangular (Fig. 4d), which was reported to form after high-temperature exposure [38].
The average particle sizes were measured as 40 nm, 54 nm, and 61 nm, respectively, for the 15 kW, 20 kW and 25 kW experiments with argon only as the carrier gas.Increasing plasma power provided a higher temperature and a larger particle size [28,36].The average particle size of the product obtained with a plasma power of 20 kW with the argon+hydrogen (1:1) mixture as the carrier gas was 57 nm, while it increased to 60 nm when the only hydrogen was used as the carrier gas.
The spacing of lattice fringes of Fig. 4d was measured at 0.19 nm, a value for (220) CeO 2 crystal face.The HRTEM image in Fig. 4e shows a lattice spacing of 0.32 nm of the (111) face of ceria.This is the most stable [39] and least active face [13].

Fig. 4. TEM and HRTEM images of nanoceria samples. A -15 kW with argon only, b -20 kW with argon only, c -25 kW with argon only, d -20 kW with argon + hydrogen (1:1), e -15 kW with hydrogen only.
Raman spectra Raman spectroscopy yields information about the interaction between the ions [40] and has been used for examining ceria or ceria-based materials in terms of defects and oxygen vacancies [40][41][42][43].The Raman spectroscopy results for the synthesized ceria samples are shown in Fig. 5.
For single crystal ceria, the Raman peak located at 466.0 cm -1 is due to the F 2g mode of vibration, which represents the symmetric stretching of Ce-8O [40].This mode is sensitive to any disorder in the oxygen sub-lattice resulting from heat, doping, or grain-size induced effects.In Fig. 5 it is seen that for each sample there was a peak near but below 466.0 cm -1 , and it is shown in the magnified figure that all nanoceria powders had a downshifted peak from 466.0 cm -1 .The largest downshift of 10.5 cm -1 was found for nanoceria synthesized at 25 kW with Ar as the carrier gas.The downshift increased with plasma power, from 7.9 cm -1 to 10.5 as the power increased from 15 to 25 kW with Ar as the carrier gas.The downshift also increased with hydrogen addition in the carrier gas from 7.9 cm -1 to 9.6 cm -1 as hydrogen concentration changed from 0 to 100 % under 20 kW power.

Fig. 5. Raman spectra of nanoceria synthesized under different conditions.
The downshift of Raman spectra occurs when the oxygen vacancies form upon the reduction of Ce 4+ (ionic radius 0.970 Å) to Ce 3+ (1.143Å), in which two trivalent cerium ions replace a tetravalent cerium ion due to the lattice expansion and mode softening [43].The relationships between the increase in vibration frequency and the changes in the lattice parameter and lattice volume are given by [41,43] where ω is the vibration frequency, γ is the Grüneisen parameter for the F 2g mode (1.24, assumed to be independent of temperature), Δa is the increase in lattice parameter, a o is the reference lattice parameter, ΔV is the volume change and V o is the reference volume of the lattice.The reduced ion Ce 3+ has a longer ionic radius than Ce 4+ , which is partially offset by the effective compression due to the loss of O 2-(1.380Å) and the creation of oxygen vacancies (1.164 Å) [44].This results in the overall changes in lattice parameter as shown in Table 1.
The relationship between x in CeO 2-x and ΔV/V o is essentially linear, as follows [43, 45]: Then, based on Eqs.( 2) and ( 3), x for the non-stoichiometric ceria is given by [43]: According to Eq. ( 4), the chemical formulas for the samples with downshifts of 10.5 cm -1 , 9.6 cm -1 , and 7.9 cm -1 were determined, respectively, to be CeO 1.941 , CeO 1.946 , and CeO 1.956 .These results are in agreement with those of Gilman and co-workers [32], who obtained nonstoichiometric ceria CeO 1.940±0.005by treating ceria with air plasma.
The Ce 3+ fractions determined by the following equation [46] are shown in Fig. 6b: Results show that the Ce 3+ fractions of nanoceria synthesized at 15 kW, 20 kW and 25 kW with argon as the carrier gas was 15, 20, and 30 %, respectively.The nanoceria synthesized at 20 kW with a 1:1 argon/hydrogen mixture had a Ce 3+ fraction of 22 % while it was 26 % for the nanoceria obtained with hydrogen as the carrier gas.
As mentioned before, XPS analyzes a surface layer (top 0-10 nm), and thus the results from this examination determined that the chemical formulas of the surface layer were CeO 1.925 , CeO 1.900 and CeO 1.850 , respectively, for ceria synthesized at 15 kW, 20 kW and 25 kW with argon and CeO 1.890 and CeO 1.870 , respectively, for nanoceria produced at 20 kW with 1:1 Ar/H 2 and pure H 2 as the carrier gas.

Formation of non-stoichiometric nanoceria and oxygen vacancies
The formation of non-stoichiometry and oxygen vacancies in samples prepared in thermal plasma is summarized in Fig. 7.The crystal structure was constructed by the VESTA software [47].Ce 2 O 3 has a hexagonal (space group: p3 m1) structure while CeO 2 has a face-centered cubic structure (space group: Fm3 m).
A plasma torch can generate a temperature as high as 11,500 K at 16 kW [29].Due to the very high temperature, cerium carbonate hydroxide can be decomposed to nanoceria, according to the following reaction: The CO 2 and H 2 O produced provide enough oxidants to oxidize all Ce 3+ to Ce 4+ even without external supply, and nanoceria forms during the decomposition process by the rapid heating to a high temperature by thermal plasma.

Fig. 7. Formation of non-stoichiometry and oxygen vacancies in thermal plasma.
As the powder is further heated by plasma, ceria begins to be reduced.It has been reported [48] that at high temperatures, oxygen starts to be released from ceria around 1600 °C, according to Figure 6b indicates that the nanoceria synthesized with hydrogen in the carrier gas had a higher Ce 3+ fraction than without it.The CeO 1.66 phase, which has a cubic superstructure [49,50], was present at a higher temperature or under a more reducing atmosphere, according to the XRD results.
The crystal structure of non-stoichiometric nanoceria shown in Fig. 7 indicates different types of oxygen vacancies, including single, double and line vacancies.Ceria is reported to maintain a fluorite structure even after incorporating up to 14% oxygen vacancies [14].When oxygen is released, the two electrons previously occupying the p orbitals of the departed oxygen redistribute in the solid and an oxygen vacancy is formed [51].The energies associated with the formation of singly and doubly ionized vacancies were determined to be ~ 4.1 and 4.7 eV, respectively [14].Nörenberg and coworkers [52] calculated that oxygen vacancies are more stable at the surface than in the bulk, and their calculation suggested 0.19 eV energy gain for per vacancy pair if two initially separated oxygen vacancies form a line structure.
In non-stoichiometric ceria the concentration of oxygen vacancies is related to the deviation from stoichiometry x in CeO 2-x by the following equation [53]:

Table 1
Crystal information of nanoceria products.