Radiochim. Acta 101, 233–239 (2013) / DOI 10.1524/ract.2012.2014 © by Oldenbourg Wissenschaftsverlag, München Synthesis and characterization of thorium, uranium and cerium oxide nanoparticles By O. N. Batuk1,D.V.Szabo´ 2, M. A. Denecke3 , T. Vitova3 andS.N.Kalmykov1,∗ 1 Chemistry Department of Lomonosov Moscow State University, Moscow, Russia 2 Forschungszentrum Karlsruhe GmbH, Institut für Materialforschung III, Karlsruhe, Germany 3 Forschungszentrum Karlsruhe GmbH, Institut für Nukleare Entsorgung, Karlsruhe, Germany (Received July 12, 2011; accepted in final form July 14, 2012) (Published online March 18, 2013) Actinide oxides / Nanoparticle / Mesoporous silica / elevated radiation and thermal fields will prevail, forma- Ultrasound tion of various nanoparticles have been demonstrated in several studies. Utsunomiya et al. [1] observed the for- mation of uraninite (UO2+x ) nanoparticles upon irradi- Summary. We describe the synthesis of cerium, thorium ation of boltwoodite, (K,Na)[(UO2)(SiO3OH)]2(H2O)1.5,by and uranium oxide nanoparticles embedded in a mesoporous heavy ions and in-situ high resolution transmission elec- matrix as template in a kind of nanocasting technique. The tron microscopy (HR-TEM) measurements. In our previous solid matrix is used as a template to obtain and stabilize the studies [2], we demonstrate the formation of schoepite, actinide oxide nanoparticles. We apply high resolution trans- [(UO2)8O2(OH)12](H2O)12, nanocrystals on a UO2+x powder mission electron microscopy (HR-TEM) to show evidence of ◦ metal oxide incorporation into the matrix pores and analyze sample surface upon hydrothermal leaching at 150 C. The their structure. Measured interplanar distances and calculated corrosion of canisters for SNF or HLW with the formation lattice parameters for synthesized nanosized CeO2−x and ThO2 of green rust may lead to the reduction of U(VI) in solution, samples differ from their bulk crystalline counterparts. We with subsequent precipitation of uraninite nanoparticles as obtain with our synthesis CeO2−x particles containing both demonstrated by O’Loughlin et al. [3]. Ce4+ and larger sized Ce3+. The lattice parameter for these Tetravalent actinides, An(IV), and their analogues, as ceria nanoparticles is found to be larger than the bulk value highly hydrolyzable cations, have a strong tendency to 3+ due to the presence of Ce with its larger ionic radius. polymerize. The formation of Zr(IV) [4], Th(IV) [5]and 3+ The presence of Ce was established by means of high Pu(IV) [6] polymers in solution has been extensively resolution X-ray emission spectroscopy (HRXES), applied to studied [7, 8]. According to high resolution nano-electro- the investigation of nanoparticles for the first time. The ThO2 nanoparticles exhibit a decrease in interplanar distances, as spray mass spectrometry results, the hydrolysis of polynu- one might generally expected for these nanoclusters. However, clear An(IV) species is a continuous process and upon the lattice distance decrease for our particles is remarkable, exceeding the solubility limit An(IV) eigencolloids are up to 5%, indicating that contact with the surrounding silica formed. Associated investigations using extended X-ray matrix may exert a bond distance shortening effect such as absorption fine structure (EXAFS) and laser induced break- through significant external pressure on the particle surface. down detection (LIBD) have allowed development of a model for Pu(IV) eigencolloid formation, which includes ) ) 1. Introduction condensation of PunOp(OH 4n−2p(H2O z oligomers [7]. Ro- the et al. [8] reported that the white line intensity in X-ray Investigation of nanoparticles is significant to our under- absorption near edge structure (XANES) spectra of An(IV) standing of desirable, as well as undesirable, environmen- eigencolloids depends on the degree of condensation of tal, technical and basic solid state processes, in order to An(IV). To corroborate this finding well-defined An(IV) be able to control or predict their formation and stabil- nanoparticles with a defined size and narrow size distribu- ity. The evolution of a condensed solid often occurs via tion should be investigated. To this aim, we have developed formation and agglomeration of nanoparticles. Nanoparti- a method for synthesizing actinide oxide nanoparticles with cles are generally not stable, due to the reactivity of their defined and variable size. high surface area, and therefore have a tendency to ag- The goal of our research here is to synthesize thorium, glomerate into larger particles. Scientists and engineers uranium and cerium oxide nanoparticles and their charac- dealing with nanotechnology are faced with the challenge terization. We consider thorium dioxide as a simple model of preparation and stabilization of nanoparticles in vari- for other actinide dioxide compounds, since it has the same ous media. Under the conditions of proposed spent nuclear fluorite-type crystal structure, is non-redox sensitive and fuel (SNF) or high level waste (HLW) repositories, where does not form non-stoichiometric oxides. Uranium oxide is the main component of nuclear fuel and we synthesize *Author for correspondence (E-mail: [email protected]). uranium oxide nanoparticles as models for eigencolloids of 234 S. N. Kalmykov et al. this material. Cerium oxide is isostructural with thorium Table1. The pore size of synthesized mesoporous silica templates de- and uranium oxide but non-radioactive and therefore used termined from HR-TEM images and from BJH-analysis. to develop and refine the nanoparticle synthesis. We use an optimized method of oxide nanoparticle preparation and Matrix TEM fringes, nm Pore diameter, nm (BJH) stabilization by embedding them into the pore structure of MCM-41 (1) 2.9 2.0 mesoporous silica as a solid template. This allows us to ob- MCM-41 (2) 4.7 3.2 tain particles with a narrow size distribution and to stabilize SBA-15 9.5 4.8 them against agglomeration. To study the relationship be- tween particle size and structure, mesoporous matrixes of MCM-41 and SBA-15 families are used to synthesize par- HR-TEM is used to characterize the synthesized meso- ticles with a mean size of 2, 3 and 5 nm, which are then porous materials. HR-TEM investigations are performed characterized by means of HR-TEM. using a TECNAI F20 ST instrument (FEI Company, USA), operating at 200 kV. We prepare samples for microscopy by dropping sample suspensions in water onto a cop- 2. Experimental section per grid covered by Lacey carbon film and subsequent drying in air. The micrograph images show the regu- 2.1 Synthesis of mesoporous solid templates larity of the pore structure for the synthesized materi- Mesoporous silica templates having the hexagonal pore struc- als and the distance of the fringes are measured to be ture of MCM-41 and SBA-15 silica families are synthesized larger than inside pore diameter values and those deter- as described previously [9–11]. The scheme of synthesis is mined with BJH (Table 1). The measured fringes are larger schematically presented in Fig. 1. Briefly, the mesoporous compare to pore diameters determined by BJH due to silica synthesis is based on tetraethyloxysilane (TEOS) hy- the fact that HR-TEM gives a lattice parameter of meso- drolysis around micelles of surfactant (cetyltriammounium porous structure, considering pores themselves and walls in chloride, CTACl) and triblock copolymer (poly(ethylene oxi- between. de)-poly(propylene oxide)-poly(ethylene oxide), P123) in aqueous alkaline media for MCM-41 and in aqueous acidic solution for SBA-15. Cylindrical wire shaped micelles are 2.2 Synthesis of Ce, Th and U oxide nanoparticles ◦ obtained at room temperature for CTACl and at 90 Cfor To embed actinide oxide nanoparticles into the meso- P123. To increase the diameter of CTACl micelles, trimethyl- porous pore structure, we treat the mesoporous template benzene (TMB) is used as a swelling reagent. The following with aqueous solutions containing Ce(III), Th(IV) or U(VI) molar ratios of reagents are used for the silica matrix syn- at a total metal ion concentration of 5 × 10−3 M. We use / . / / thesis: 1TEOS 0 13CTACl 5NH3 140H2O for MCM-41 Ce(III) nitrite solution as a precursor for CeO2 nanopar- with pore size 2 nm; 1TEOS/0.13CTACl/0.9TMB/ ticles due to the instability of Ce(IV) compounds in so- / / 5NH3 140H2O for MCM-41 with pore size 3 nm; 1TEOS lution. Th(IV) and U(VI) chloride solutions are added as . / . / 0 017P123 5 7HCl 193H2O for SBA-15 with 5 nm pore precursor. We prepare samples under air; no special gas size. The suspensions are stirred 30 min after all reagents are atmosphere is used. In order to facilitate metal ion entry added. The solid and aqueous phases are then separated by into the MCM-41 pore structure, the suspension of silica filtration and washed with MilliQ water. The mesoporous and dissolved metal ions is subjected to ultrasonic treat- ◦ silica are dried for 24 h at 60 C and then annealed to re- ment at a frequency of 35 kHz. The starting pH values move organic surfactant material using the following regime: of suspensions in the both sets of matrices are pH 1 for ◦ ◦ heating up to 550 C with ramping 1 C/min, calcination for Th(IV), pH 2 for U(VI) and pH 6 for Ce(III), with step- 12 h and subsequent cooling down to room temperature with wise increase to pH values of 3.5, 4.5 and 8.5 for Th, U ◦ thesameramping1 C/min rate. The internal pore diam- and Ce, respectively. The pH values of the suspensions eters of the synthesized silica templates are determined using are adjusted using NaOH and HCl solutions with various an Autosorb-1 (Quantachrome KG, Germany). We meas- concentrations. After ultrasonic treatment or magnetic stir- ure N2 sorption and desorption at 77 K and evaluate data ring (for ∼ 5 nm particles in the SBA-15 template), the with the BET method to calculate free surface area and the solid material is filtered, washed with MilliQ water to re- BJH (Barrett-Joyner-Halenda) method to calculate pore size move any surface precipitates and then heated in air with distribution to be approximately 2 and 3 nm, in the case of a30◦C/min ramping rate to 250 ◦C and tempered for a dura- MCM-41, and 5 nm for SBA-15 (see Table 1).