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Nano Res

1

Thorium/uranium mixed oxide nanocrystals: Synthesis,

structural characterization and magnetic properties D. Hudry (), J.-C. Griveau (), C. Apostolidis, O. Walter, E. Colineau, G. Rasmussen, D. Wang, V. S. K.Chakravadhaluna, E. Courtois, C. Kübel, D. Meyer Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-013-0379-6

http://www.thenanoresearch.com on October 22, 2013

© Tsinghua University Press 2013

Just Accepted

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Nano Research DOI 10.1007/s12274‐013‐0379‐6

1

TABLE OF CONTENTS (TOC)

Thorium / Uranium Mixed Oxide Nanocrystals:

Synthesis, Structural Characterization and Magnetic

properties.

D. Hudry*(1), J.- C. Griveau*(1), C. Apostolidis(1), O.

Walter(1,6), E. Colineau(1), G. Rasmussen(1), D. Wang(2,3),

V. S. K. Chakravadhaluna(2,4), E. Courtois(2), C.

Kübel(2,3), D. Meyer(5)

(1) European Commission: Joint Research Centre,

Institute for Transuranium Elements, P. O. Box 2340,

76125 Karlsruhe, Germany.

(2) Karlsruhe Institute of Technology, Institute of

Nanotechnology, Hermann-von-Helmholtz-Platz 1,

76344 Eggenstein-Leopoldshafen, Germany.

(3) Karlsruhe Institute of Technology, Karlsruhe Nano

Micro Facility, Hermann-von-Helmholtz-Platz 1, 76344

Eggenstein-Leopoldshafen, Germany.

(4) Helmholtz Institute Ulm for Electrochemical Energy

Storage, Albert-Einstein-Allee 11, 89069 Ulm,

Germany.

(5) Institut de Chimie Séparative de Marcoule, UMR

5257, BP 17171, 30207 Bagnols sur Cèze Cedex,

France.

(6) Karlsruhe Institute of Technology, Institute for

Catalysis Research and technology,

Hermann-von-Helmholtz-Platz 1, 76344


We report on the non-aqueous synthesis of Th1-xUxO2 nanocrystals by the

controlled hot co-injection of Th(acac)4 and UO2(OAc)2.2H2O in a highly

coordinating organic medium. The synthesis, structure and magnetic

properties of the as-prepared nanocrystals are investigated.

D. Hudry, www.hudry.weebly.com

2

Thorium / Uranium mixed oxide nanocrystals: synthesis, structural characterization and magnetic properties.

D. Hudry1 (), J.- C. Griveau1 (), C. Apostolidis1, O. Walter1, 6, E. Colineau1, G. Rasmussen1, D. Wang2,3, V. S. K. Chakravadhaluna2,4, E. Courtois2, C. Kübel2,3, D. Meyer5 1 European Commission: Joint Research Centre, Institute for Transuranium Elements, P. O. Box 2340, 76125 Karlsruhe, Germany. 2 KIT, Institute of Nanotechnology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany. 3Karlsruhe Institute of Technology, Karlsruhe Nano Micro Facility, Hermann-von-Helmholtz-Platz 1, 76344

Eggenstein-Leopoldshafen, Germany. 4 Helmholtz Institute Ulm for Electrochemical Energy Storage, Albert-Einstein-Allee 11, 89069 Ulm, Germany. 5 Institut de Chimie Séparative de Marcoule, UMR 5257, BP 17171, 30207 Bagnols sur Cèze Cedex, France. 6 Karlsruhe Institute of Technology, Institute for Catalysis Research and technology, Hermann-von-Helmholtz-Platz 1, 76344


Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher) © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011

ABSTRACT One of the primary aims of the actinide community within nanoscience is to develop a good understanding

similar to what is currently done with stable elements. As a consequence, efficient, reliable and versatile

synthesis techniques dedicated to the formation of new actinide‐based nano‐objects (e.g. nanocrystals) are

necessary. Hence, a ʺlibraryʺ dedicated to the preparation of various actinide‐based nanoscale building blocks is

currently developed. Nanoscale building blocks with tunable sizes, shapes and compositions are of prime

importance. So far, the non‐aqueous synthesis method in highly coordinating organic media is the only

approach which has demonstrated the capability to provide size and shape control of actinide‐based

nanocrystals (both for thorium, uranium and recently extended to neptunium and plutonium). In this paper,

we demonstrated that the non‐aqueous approach is also well adapted to control the chemical composition of

the nanocrystals when mixing two different actinides. Indeed, the controlled hot co‐injection of thorium

acetylacetonate and uranyl acetate (together with additional capping agents) into benzyl ether can be used to

synthesize thorium / uranium mixed oxide nanocrystals covering the full compositional spectrum. Additionally,

we found that both size and shape are modified as a function of the thorium – uranium ratio. Finally, the

magnetic properties of the different thorium / uranium mixed oxide nanocrystals were investigated. Contrary

to several reports, we did not observe any ferromagnetic behavior. As a consequence, ferromagnetism cannot

be described as a universal feature of nanocrystals of non‐magnetic oxides as recently claimed in the literature.

KEYWORDS thorium, uranium, mixed oxide, non‐aqueous synthesis, nanoparticle, nanocrystal, magnetism

Nano Res DOI (automatically inserted by the publisher) Review Article/Research Article Research Article

3

1 Introduction

Nanocrystals (NCs) represent fundamental

building blocks in nanoscience and nanotechnology.

The small size of NCs modifies their physical and

chemical properties and size and shape effects are

generally observed at the nanoscale [1‐3]. Another

important feature characterizing NCs is related to

their high surface‐to‐volume ratio. Consequently

and compared to bulk materials, surface effects are

not negligible anymore [4]. Hence, NCs in the

range of few to tens of nanometers exhibit unusual

properties which are different to the ones of their

bulk counterparts [5]. Investigating the

fundamental chemical and physical properties of

these nanoscale building blocks opens up the way

to the design of functional nanomaterials with

innovative properties and high expectations in

fields as diverse as electronic and optoelectronic [6],

energy conversion [7, 8], magnetic storage [9], or

nanomedicine [8, 10‐12].

Investigations and developments related to NCs

have reached a high level of understanding and

complexity with stable elements. For example, the

synthesis of NCs with tunable sizes, shapes and

compositions is easily achievable for a wide variety

of compounds (chalcogenides, transition metal

oxides, noble metals and lanthanide‐based

compounds) [13‐17]. The self‐assembly of NCs into

superlattices is also under investigation in order to

take advantage of both individual and collective

properties of NCs due to their periodic

arrangement [18‐20]. Finally, over the last decade,

doped NCs have received a growing interest due to

the possibility to see the emergence of new

properties [21‐24]. All these fields are under active

investigations all around the world and stimulate

interactions between disciplines as diverse as

physics, chemistry, biology and engineering.

Comparatively, much less efforts have been done in

nanoscience within the actinide community [25, 26].

On one side, this is understandable when taking

into consideration difficulties in handling

radioactive elements and in particular

transuranium elements which requires the use of

dedicated facilities. Nevertheless, whereas some

efforts have been done in the fields of

actinide‐based colloids and molecular clusters

[27‐31], very little is known on the controlled

synthesis of actinide‐based NCs. Various methods,

mainly dedicated to uranium oxide NCs, have been

reported without any further development [32‐35].

The first controlled synthesis of uranium oxide

NCs has been proposed by Cao in 2006 according

to a non‐aqueous technique in highly coordinating

organic media [36]. The latter constitutes one of the

best methods towards the controlled synthesis of

NCs with tunable sizes, shapes and compositions.

This non‐aqueous technique has been developed

and applied to the synthesis of thorium oxide

nanocrystals [37]. The technique was further

successfully extended to the first

transuranium‐based NCs (NpO2) [Hudry et al. RSC

Advances, 2013, accepted manuscript].

Actinide‐based NCs could fill the gap between

molecular clusters, colloids and bulk materials and

constitute innovative building blocks both for

applied and fundamental research.

For example, the migration of radionuclides (and

particularly actinides) in the environment is of

major concern for the safety assessment of nuclear

waste disposal and legacy contamination sites

(nuclear accidents – e.g. Chernobyl, Fukushima,

atmospheric nuclear weapon testing). It has been

reported that plutonium transport through the

geosphere is much faster than predicted and

colloidal facilitated transport (with the potential

formation of nanoparticles) has been incriminated

[38, 39]. Hence, engineered (i.e. chemical

composition, surface chemistry, size and shape)

actinide‐based NCs could be used as model

systems. Actinide‐based NCs and particularly

actinide oxides have recently been proposed as

potential ʺprecursorsʺ to synthesize innovative

nanostructured nuclear fuels at low temperature

[40] with enhanced properties in terms of safety.

Finally, it has recently been demonstrated that

core‐shell nanoparticles doped with short half‐life

alpha emitters (e.g. actinium‐225) are of interest for

targeted alpha therapy (TAT) [41]. Additionally,

thorium‐227 and uranium‐230 have been proposed

as potential alpha emitters in TAT [42]. Hence,

understanding the formation of NCs with thorium

and uranium could be of major interests in

nanomedicine (e.g. short half‐life alpha emitters

doped NCs).

4

Table 1. Starting and final compositions of various mixed oxide (MOX) nanocrystals prepared in this study.

Starting Composition Final Composition

Sample U

(mol. %)

Th

(mol. %)

U

(mol. %)

Th

(mol. %)

MOX‐1 0 100 0 100

MOX‐2 4.5 95.5 7 93

MOX‐3 9.5 90.5 12 88

MOX‐4 19 81 28 72

MOX‐5 50 50 58 42

MOX‐6 72 28 70.5 29.5

MOX‐7 100 0 100 0

In terms of fundamental research, it would be

interesting to know whether size and shape effects

can give rise to modified or new properties of

actinide‐based NCs compared to their bulk

counterparts. It might bring new insights

concerning the solid state physics and the behavior

of 5f electrons whose nature (i.e. localized vs.

delocalized) varies throughout the actinide series

[43, 44].

In this article, we report on the synthesis of

thorium / uranium mixed oxide NCs, Th1‐xUxO2

(with 0 x 1). A hybrid method between

heating‐up and hot injection [45, 46] was used in

order to achieve a good homogeneity in terms of

composition as well as size and shape distributions.

The formation of mixed oxide NCs was studied by

powder x‐ray diffraction (PXRD), transmission

electron microscopy (TEM) techniques as well as

by global and local chemical analyses at the

nanometer scale. The obtained data confirmed the

formation of mixed oxide NCs over the entire

range of compositions. Additionally, the magnetic

properties of the as‐prepared NCs have been

characterized by superconducting quantum

interference device (SQUID) magnetometry.

Contrary to several reports, we did not observe any

ferromagnetic behavior. As a consequence,

ferromagnetism cannot be described as a universal

feature of NCs of non‐magnetic oxides as recently

claimed in the literature [47, 48].

2 Results and discussion

In previous articles dedicated to the non‐aqueous

synthesis of pure thorium and uranium oxide NCs

in highly coordinating organic media, it has been

shown that the reactivity of thorium and uranium

precursors is significantly different and hence

influences the final characteristics (i.e. size and

shape) of the as‐prepared NCs [37, 49]. Such a

feature can be a major drawback when considering

the formation of mixed oxide NCs. In order to

reach a homogeneous distribution of thorium and

uranium in the NCs, kinetics of reaction must be

similar. Indeed, in case the generation of active

thorium and uranium monomers (i.e. chemical

species involved to build up the oxide network)

would be characterized by kinetic constants which

are too different, various nucleation steps might be

involved. Consequently, such an effect would lead

to a phase segregation. To minimize such a kinetic

effect, we modified the controlled hot injection

technique which is usually applied to the growth of

core‐shell NCs [50]. Nevertheless, contrary to the

synthesis of core‐shell NCs, the main idea is to

trigger a homogeneous nucleation step by slowly

increasing the concentration of active thorium‐ and

uranium‐based monomers. Because thorium and

uranium oxides crystallize within the same

crystallographic structure (fluorite‐type) and only

exhibit a small cell parameter difference (< 5%),

enough material of both thorium and uranium

should be available to induce the formation of NCs

with a homogeneous chemical composition. In our

experiments, we apply the controlled hot

co‐injection of a mixture of thorium and uranium

precursors in a hot solvent.

5

edcba

fg

20 40 60 80 100 120

I /

a.u

.

2 / �

a

c

e

g

b

d

f

h(111)

(200)(220)(311)

40 45 50 55 602 / �

Figure 1. Powder x-ray diffraction (PXRD) patterns (left panel) and selected area electron diffraction (SAED) patterns (right panel) of various thorium / uranium mixed oxide (MOX) nanocrystals synthesized by the controlled hot co-injection technique a) MOX-1, b) MOX-2, c) MOX-3, d) MOX-4, e) MOX-5, f) MOX-6, g) MOX-7. The inset (left panel) shows an enlargement of the 2 theta area between 40° - 60°: MOX-1 (blue), MOX-2 (cyan), MOX-3 (black), MOX-4 (violet), MOX-5 (orange), MOX-6 (red), MOX-7 (green). Tick-marks indicate bulk thorium dioxide (green) and bulk uranium dioxide (magenta) Bragg peaks positions. The scheme (h) shows the indexation of the SAED patterns.

Therefore, thorium acetylacetonate (Th(acac)4) and

uranyl acetate (UO2(OAc)2.2H2O) are dissolved in a

mixture composed of oleic acid (OA),

tri‐n‐octylamine (N(Oct)3), tri‐n‐octyplhosphine

oxide (OP(Oct)3) and benzyl ether (BnOBn). The

resulting mixture (kept at room temperature) is then

slowly injected in BnOBn maintained at 260°C. After

injection, the resulting solution is kept at 260°C for

additional aging. During that time, the hot solution

gradually turns from colorless (i.e. pure BnOBn) to

light yellow and finally to black‐brown (depending

on the starting uranium concentration). When the

thorium precursor is injected without uranium, the

solution only turns from colorless to deep yellow.

Different mixed oxide (MOX) samples were

prepared with various starting thorium / uranium

ratios reported in Table 1. Additionally, Table S1 in

the Electronic Supplementary Material (ESM)

provides an overview of the global chemical

composition (i.e. molar quantities of the actinide

precursors and organics) for each MOX sample. For

simplicity the samples will be referred to in the

manuscript as MOX‐X (with X = 1, 2, 3, 4, 5, 6 or 7).

2.1 Structural characterization

The PXRD patterns of the different MOX samples are

given in Fig. 1 (left panel) as well as the Bragg peak

positions (tick‐marks in Fig. 1) of bulk thorium and

uranium dioxides. Although the peaks are very

broad (due to the small size of the crystallites), all

MOX samples are very well crystallized. Indeed,

Bragg reflections are visible in a wide 2 range (up to 120°). All PXRD patterns have been fitted using the

Rietveld method with the bulk structure of thorium

dioxide. The detailed results of the Rietveld

refinements are given in the supporting information

(Figures S1 to S7 and Table S2 in the ESM). Bulk

thorium and uranium dioxides crystallize in the

fluorite structure (space group Fm‐3m) with a

slightly different cell parameter namely 5.61 Ǻ and

5.43 Ǻ respectively. In the elementary cell,

thorium/uranium and oxygen atoms are placed on

6

the 4a or 8c special positions (0/0/0; 0.25/0.25/0.25),

respectively. As a consequence, only few parameters

can be refined. In our case, additionally to the

classical parameters (i.e. polynomial terms for the

background and scale factor), only parameters with a

physical meaning have been refined: the cell

parameter (a), the crystallite size (according to the

fundamental approach [51]) and the isotropic atomic

displacement parameters (Uiso). The site occupancy

of the 4a position (i.e. Th/U position) cannot be

refined because the atomic diffusion factors of

thorium and uranium are too close. In other words,

x‐rays are not suitable to distinguish thorium from

uranium. As a consequence, the site occupancy of the

4a position cannot be used to validate the formation

of a solid solution. The final composition of the

different MOX samples and particularly the Th:U

ratio was determined by inductively coupled plasma

mass spectrometry (ICP‐MS) and the results are

given in Table 1. The final composition slightly

differs from the nominal one revealing slight

differences between thorium and uranium reactivity.

Nevertheless, these results give an accurate idea

about the total thorium and uranium contents for

each MOX sample and were used to plot the

evolution of the cell parameter (i.e. a) as a function of

the uranium content (Fig. 2). The cell parameter

follows a linear evolution over the total range of

composition (i.e. from 0 mol.% of uranium up to 100

mol.%).

0 20 40 60 80 1005.42

5.44

5.46

5.48

5.50

5.52

5.54

5.56

5.58

5.60

5.62

cell

para

met

er /

�

U content / mol. %

MOX-1 MOX-2 MOX-3 MOX-4 MOX-5 MOX-6 MOX-71.0

1.5

2.0

2.5

3.0

FW

HM

/ �

0

20

40

60

80

100

U c

onte

nt /

mol

. %

Figure 2. Evolution of the cell parameter of the fluorite structure as a function of the uranium content determined by ICP-MS analysis of the as prepared thorium / uranium mixed oxide (MOX) nanocrystals. The inset shows the evolution of the full width at the half maximum (FWHM) of the (220) reflection of the as-prepared MOX nanocrystals and their corresponding uranium content.

According to Vegard’s law, the average parameter

should vary linearly with dopant concentration in

the crystal and deviations from linearity are

indications of phase transitions or segregation (i.e.

distinct phases). Because the Bragg peaks in XRD are

very broad, electron diffraction (ED) was used as an

additional proof to exclude significant amounts of

homo metal oxides. Because of the short wavelength

of the electrons, the electron diffraction pattern is

sharper and a superposition of different

nanocrystalline phases will be easier to detect by ED

than by PXRD. The selected area electron diffraction

(SAED) patterns of the MOX samples are given in

Fig. 1 (right panel). All SAED patterns were indexed

with the FCC structure in good agreement with the

bulk structure of AnO2 (An = Th, U). The (220)

reflection is particularly interesting because it is the

main non‐overlapped reflection with a high intensity.

For all MOX samples, the intensity of the (220)

reflection is homogenous and no splitting can be

seen, thus supporting the conclusion of the

formation of thorium / uranium mixed oxide NCs

without any significant phase segregation. Another

interesting feature from both the PXRD and SAED

patterns is related to the width of the Bragg peaks

(x‐rays) or electron diffraction rings (electrons). For

example in the case of PXRD, the full width at half

maximum of the (220) reflection is plotted in Fig. 2

(inset) and clearly indicates that the higher the

uranium content the sharper the peaks. A similar

feature is observed with the (220) electron diffraction

ring. As a consequence, when adding uranium to the

reactive mixture which contains thorium, bigger NCs

are obtained.

To determine the size and shape distributions, all

MOX NCs were characterized by transmission

electron microscopy (TEM). Panels in Fig. 3 show the

scanning transmission electron microscopy (STEM)

images (Fig. 3a) along with the transmission electron

microscopy (TEM) images (Fig. 3b) and high

resolution TEM (HRTEM) images (Fig. 3c) as well as

the energy‐dispersive x‐ray spectroscopy (EDX)

spectra (Fig. 3d) of the as‐prepared MOX samples.

The HRTEM pictures (Fig. 3c1 to 3c7) indicate that the

NCs are highly crystalline. Concerning MOX‐X (X = 4,

5, 6 and 7) samples, all prepared NCs are single

domain (Fig. 3c4 to 3c7).

7

0 5 10 15 20

Energy / keV

0 5 10 15 20

Energy / keV

0 5 10 15 20

Energy / keV

0 5 10 15 20

Energy / keV

0 5 10 15 20

Energy / keV

a1 b1 c1 d1

a2 b2 c2 d2

a3 b3 c3 d3

a4 b4 c4 d4

a5 b5 c5 d5

a6 b6 c6 d6

a7 b7 c7 d7

Th

oriu

m co

nten

t

0 5 10 15 20

Energy / keV

0 5 10 15 20

Energy / keV

Figure 3. The panels show STEM images (a1-a7), TEM images (b1-b7), HRTEM images (c1-c7) and EDX spectra of individual nanocrystals (d1-d7) of thorium / uranium mixed oxide (MOX) nanocrystals synthesized by the controlled hot co-injection in BnOBn (260°C) of Th(acac)4 and UO2(OAc)2.2H2O solved in a mixture of OA, N(Oct)3, OP(Oct)3 and BnOBn (RT). Each row represents the full TEM characterization (i.e. STEM, TEM, HRTEM and EDX) of a sample with a given thorium/uranium composition. The thorium content increases from the bottom to the top. Color code used for the EDX spectra: thorium (blue), uranium (green), oxygen (yellow), copper from the support grid (red) and silicon as an impurity on the support grid (orange).

8

In case of the MOX‐1 sample, the nanowires consist

of extended domains along the wire axis, but only

few nanowires are single domain (Fig. 3c1). Analysis

of the STEM images enables easy extraction of

information concerning the size and shape

distributions. In case of pure thorium oxide (MOX‐1

sample, Fig. 3a1), the as‐prepared NCs are highly

anisotropic nanowires. Although we previously

reported such a feature [49] the controlled hot

co‐injection dramatically changes the final

characteristics of the NCs and particularly the aspect

ratio between the long and short axes. Additionally,

narrower size and shape distributions are observed.

The mean size of the short length axis is 1.5 0.2 nm whereas the mean size of the long axis is 22.5 5.7 nm (Fig. 4a). It has to be noticed that although the

shape distribution is much better with the controlled

hot co‐injection technique compared to the

heating‐up technique, it is not perfect yet. Indeed, a

few small dog‐bones like thorium oxide nanocrystals

were also observed (Fig. S8 in the ESM). Adding small quantities of uranium during the controlled

hot injection is sufficient to prevent the growth of

anisotropic NCs. Indeed, when co‐injecting 4.5 mol.

% of uranium and 95.5 mol. % of thorium, isotropic

NCs are obtained (Fig. 3a2) although a negligible

percentage (< 1 %) of elongated particles is still

visible. The TEM and HRTEM images (Fig. 3b2 and

3c2) clearly show that the as‐prepared NCs are

characterized by highly irregular shapes. Despite

that, the NCs are essentially single crystalline (Fig.

3c2). Slightly increasing the uranium content (9.5 mol.

% of uranium) induces the formation of similar NCs

(Fig. 3a3, 3b3, 3c3) without any evidence of nanowires

(Fig. S10 in the ESM). Additionally, the size

distribution is improved compared to MOX‐2 (Fig.

4b and 4c). The co‐injection of 19 mol.% of uranium

and 81 mol.% of thorium still induces the formation

of isotropic NCs (Fig. 3a4). Nevertheless, the

as‐prepared NCs look like highly faceted. Compared

to the samples with a low uranium content (i.e.

MOX‐2 and MOX‐3), the NCs are characterized by a

similar size with a mean Feret diameter of 6.9 1.4 nm (Fig. 4d). The corresponding shape distribution is

homogeneous and only one population, without any

evidence of anisotropic NCs, was observed

throughout the TEM grid (Fig. S11 in the ESM).

0 2 4 6 8 10 12 14 16 18 200

20

40

Num

ber

of p

artic

les

Feret's diameter / nm

0 2 4 6 8 10 12 14 16 18 200

50

100

Num

ber

of

part

icle

s


0 1 2 3 4 50

20

40

60

80

Num

ber

of

part

icle

s

Short length axis / nm

0 10 20 30 40 500

10

20

30

Num

ber

of

part

icle

s

Long length axis / nm

0 2 4 6 8 10 12 14 16 18 200

100

200

Num

ber

of p

artic

les


0 2 4 6 8 10 12 14 16 18 200

50

100

150

Num

ber

of

part

icle

s

Feret's diameter / nm0 2 4 6 8 10 12 14 16 18 20

0

50

100

150

200

250

Num

ber

of

part

icle

s


a

c

b

d

fe

Figure 4. Size distribution of thorium / uranium mixed oxide (MOX) nanocrystals synthesized by the controlled hot co-injection in BnOBn (260°C) of Th(acac)4 and UO2(OAc)2.2H2O solved in a mixture of OA, N(Oct)3, OP(Oct)3 and BnOBn (RT). a) MOX-1 according to the short and long (inset) axes, b) MOX-2, c) MOX-3, d) MOX-4, e) MOX-5, f) MOX-7. Because of the agglomerated nanocrystals for the MOX-6 sample, the corresponding size distribution is not available.

Increasing the uranium content up to 50 mol.% does

not change the final characteristics of the as‐prepared

NCs (MOX‐5 sample, Fig. 3a5). The size distribution

shows slightly bigger NCs with a mean Feret

diameter of 7.1 1.5 nm (Fig. 4e) whereas the shape

is not altered and still homogeneous (Fig. S12 in the

ESM). For a higher uranium content (MOX‐6 sample,

72 mol.% of uranium) agglomerates of small NCs are

obtained (Fig. 3a6) similar to the formation of nano

flowers reported for In2O3 [52] or Mn‐doped ZnO

NCs [53]. Similarly to the others MOX samples, the

size and shape distribution are homogeneous (Fig.

S13 in the ESM). Nevertheless, in this particular case

a detailed size distribution analysis could not be

performed due to the aggregation of the primary

particles. Finally, in the case of pure uranium

(MOX‐7 sample), highly monodisperse nanodots (Fig.

3a7) with a mean Feret diameter of 10.7 0.6 nm (Figure 4f) were obtained. The STEM data clearly

show that the addition of uranium prevents the

formation of anisotropic NCs in favor of bigger

isotropic ones.

9

0 50 100 150 200 250 300-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

UO2 sc

UO2 nano

Th0.30

U0.70

O2 nano

Th0.42

U0.58

O2 nano

Th0.72

U0.28

O2 nano

ThO2 nano

ThO2 bulk

M/H

/ 1

0-3 e

mu.

mol

-1

T / K

31 K

a)

H = 70 kOe

0 10 20 30 40 50 60 70-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

-8 -6 -4 -2 0 2 4 6 8-0.03

-0.02

-0.01

0.00

0.01

0.02

0.03 UO

2 nano

Th0.30

U0.70

O2 nano

Th0.42

U0.58

O2 nano

Th0.72

U0.28

O2 nano

ThO2 nano

/

emu.

g-1

H / kOe

b)

T = 5 k

/ e

mu.

g-1

H / kOe

Th0.72

U0.28

O2 nano

T = 5 K

Figure 5. Magnetization measurements of the thorium / uranium mixed oxide (MOX) NCs. Figure 5a presents the molar magnetic response M/H = mol for all MOX samples at 70 kOe. References curves from UO2 single crystal (SC) and ThO2 powder (bulk) have been added for comparison. The magnetic transition is clearly visible at a temperature ~31 K for single crystal measurements. Neither anomaly nor magnetic features are observed on the NCs. We observe a progressive reduction of magnetization with Th doping from a Curie-Weiss like shape to a pure diamagnetic response for ThO2 based NCs. Th0.42U0.58O2 (MOX-5) NCs present an enhanced molar magnetic susceptibility while Th0.72U0.28O2 (MOX-4) NCs magnetic susceptibility crosses diamagnetic line above 150 K. Figure 5b presents the magnetization response with field at 5K. We do not observe any ferromagnetic features neither at low field nor at high field up to 70 kOe. Top left inset shows the hysteresis loop for Th0.72U0.28O2 (MOX-4) NCs displaying perfect linear field dependence typical of pure paramagnetic systems.

A modified molecular reactivity controlling the

concentration of active monomers in solution or

differences (doped vs. non‐doped) related to the

surface energies of the seeds could explain the shape

modification. Moreover, the homogeneous shape

distribution for all thorium / uranium mixed oxides

(Fig. S9 to S13 in the ESM) constitutes an additional

proof concerning the formation of mixed oxides

without any significant segregation (i.e. mixture of

thorium oxide and uranium oxide), which should

lead to a mixture of both anisotropic and isotropic

NCs. As previously demonstrated, thorium oxide

easily forms anisotropic NCs when Th(acac)4 is used

as a starting precursor and so far a shape

modification of thorium oxide NCs has only been

achievable by modifying the chemical nature of the

starting thorium precursor [49].

To strengthen the structural characterization, EDX

was used as a nanoscale local probe to get

information about the chemical composition of

individual NCs. For each MOX sample the

corresponding results are displayed in Figures 3c1 to

3c7. Although the EDX spectra are too noisy for a

quantitative analysis, they can be used qualitatively.

Accordingly, for each MOX sample (with the

exception of pure thorium and uranium dioxides),

the EDX spectra show the presence of both thorium

and uranium within a single MOX nanocrystal.

Additionally, the relative intensities of the thorium

and uranium lines follow the trend of the data

obtained by ICP‐MS analysis from the thorium‐rich

sample (MOX‐2) to the thorium‐poor sample

(MOX‐6). Hence, the EDX analysis used as a local

probe for single NCs corroborates the data obtained

by PXRD and ED and support the formation of

mixed oxides.

2.2 Magnetism

Magnetic molar susceptibility of the uranium –

thorium mixed oxide NCs are clearly different

compared to bulk uranium dioxide. Indeed, they

present reduced values and no anomaly reminiscent

of the 31 K antiferromagnetic transition can be

identified. The uranium rich NC magnetic

susceptibility M/H= can be described by a Curie Weiss behavior (=C/(T‐P)) with C, Curie constant

and P, Curie paramagnetic temperature. When

substituting uranium by thorium in the ThxU1‐xO2

NCs, a change of from Curie Weiss for UO2 NCs

(MOX‐7) to a pure TIP diamagnetic behavior for

ThO2 NCs (MOX‐1) is observed (Fig. 5 a). This trend

was already reported by Slowinski et al. for bulk

thorium / uranium solid solutions [54]. Parameters

10

such as effective moment eff and paramagnetic

Curie temperature P, exhibit a continuous change with increased percentage of thorium substituting

uranium in the lattice. Nevertheless, when

approaching 25 mol.% of uranium, a significant

discontinuity from the linear dependence for P with

the uranium content was observed. Curie Weiss fits

of our data for MOX‐7, MOX‐6 and MOX‐5 lead to

eff = 0.61, 0.58, and 0.8 B , with P = ‐31, ‐30, and ‐34 K, respectively. These values are clearly different

from the ones obtained for bulk crystals (3.11 B and

‐208 K). MOX‐4 has the particularity to display a

diamagnetic behavior at high temperature and an

important paramagnetic upturn at low temperature.

Finally, MOX‐1 is purely diamagnetic as the

reference bulk ThO2 we used. Interestingly its

diamagnetic signal is even enhanced and constant in

the full temperature range at d= ‐2.1x10‐4 emu.mol‐1

at 300 K.

Considering magnetic response with field on Fig. 5b,

it is obvious that no ferromagnetic response is

noticeable. Even at the lowest temperature achieved

(2 K) and at highest field achievable (70 kOe), no

hysteresis was observed. Figure 5b indicates the

magnetization per gram for representative MOX

systems. A slight curvature is visible for MOX‐7 to

MOX‐5 but no magnetic hysteresis was noticeable.

The curvature observed could possibly be associated

to a superparamagnetic state developing at low

temperature. MOX‐4 was examined at low field and

at low temperature (T= 5K) (Inset Fig. 5b) but no

difference for the magnetization curves is observed

for increasing and decreasing magnetic fields.

The absence of ferromagnetic behavior in all MOX

NCs studied in the present work is a strong

argument against the universality of ferromagnetism

in non‐magnetic metal oxide NCs as recently

proposed [47, 55]. On one side, recent works have

pointed out the importance of the preparation

process and the possible occurrence of magnetic

impurities during the synthesis as being a potential

source of misinterpretation [56]. It has also been

shown that Fe‐doped ZnO was necessary to observe

ferromagnetism. On the other side, several routes

have been proposed to explain the intrinsic

properties of these unconventional ferromagnetic

systems. For instance, despite the absence of clear

ferromagnetic magnetization curves of gold

nanoparticles, x‐ray magnetic circular dichroism

(XMCD) and x‐ray absorption spectroscopy spectra

(Au L3 and L2 edges) suggested the possibility of

spin polarization states developing within the

nanoparticles and being at the origin of a super

paramagnetic state [57]. The presence of defects due

to the high surface‐to‐volume ratio within NCs has

also been suggested to be at the origin of

ferromagnetism and especially the spin polarized

state induced by point defects [58]. Whereas

ferromagnetic features reported in the literature are

already observed at room temperature [48], no hint

of magnetic order is observed from room

temperature down to 2 K when considering thorium

/ uranium MOX NCs.

3. Conclusions

Efficient, reliable and versatile synthetic techniques

dedicated to the formation of new actinide‐based

nano‐objects are the cornerstone towards a good

understanding of actinide‐based nanoscience. To

date, the non‐aqueous method in highly

coordinating organic media is the only one having

demonstrated its capability concerning size and

shape control of actinide‐based NCs. In this paper,

we demonstrated that the non‐aqueous synthesis is

well adapted to control the chemical composition of

the as‐prepared NCs. Indeed, the controlled hot

co‐injection of Th(acac)4 and UO2(OAc)2.2H2O

(together with additional capping agents) into hot

BnOBn can be used to synthesize Th1‐xUxO2 NCs

within the entire range of compositions. Additionally,

both size and shape are modified as a function of the

Th:U ratio. Hence, the controlled hot co‐injection

technique opens up the way to synthesize various

doped (e.g. transuranium elements, lanthanides or

even transition metals) actinide‐based NCs. When

moving from UO2 to ThO2 based NCs, we observe

the disappearance of a Curie‐Weiss like magnetic

behavior replaced by a TIP diamagnetic feature as

for bulk materials. But contrary to reported magnetic

features of various nanocrystalline systems, no hint

of ferromagnetism has been observed down to 2 K in

our materials, especially for pure non‐magnetic ThO2

based NCs.

11

4. Materials and Method

Chemicals. Benzyl ether (BnOBn, 99%, Acros

Organics), trioctylphosphine oxide (OP(Oct)3, >98%,

Merck), oleic acid (OA, Ph. Eu., Fluka), oleylamine

(OAm, 80 – 90% C18 content, Acros Organics )

trioctylamine (N(Oct)3, >99%, Fluka ), ethanol

(absolute, Merck) and toluene (min 99.7%,

Sigma‐Aldrich) were used as received without

further purification. Thorium acetylacetonate

(Th(acac)4) was purchased from International

Bio‐Analytical Industries Inc. (Boca Raton, Florida,

USA). Th(acac)4 was purified prior to use (Electronic

Supplementary Material). Uranyl acetate

(UO2(OAc)2.2H2O) was synthesized as described in

the Electronic Supplementary Material.

Synthesis of Th/U mixed oxide NCs. All syntheses

were performed using air‐free (Schlenk) techniques

under purified argon atmosphere. First, a stock

solution containing thorium and uranium precursors

is prepared. Th(acac)4 and UO2(OAc)2.2H2O are

solved (T = 60°C) in a mixture of OA, N(Oct)3,

OP(Oct)3 and BnOBn. All reagents quantities for each

sample are given in Table S1 in the Electronic

Supplementary Material. The as‐obtained clear

yellow solution (the color intensity depending on the

uranium content) is degassed under vacuum (5.10‐2

mbar) at 100°C for 20 minutes. The solution is then

cooled down to room temperature and transferred

(under inert atmosphere) into a 10 mL dropping

funnel connected to a four‐neck flask. Additionally,

the four‐neck flask is equipped with a water

condenser, a PTFE stopcock connected to the Schlenk

line (Ar supply) and a Thermocouple. The second

step consists in transferring a given quantity of

BnOBn (previously degassed under vacuum and

stored over 4 Å molecular sieves) into the four

neck‐flask and heating up to 260°C. When the

temperature is stabilized at 260°C, the stock solution

is added (drop‐wise). After complete injection of the

stock solution, the mixture is kept at 260°C for

additional 30 minutes (Fig. S11 in the ESI).

Afterwards, the heating mantle is removed and the

flask is left to cool naturally to room temperature.

NCs recovering and purification procedure.

Absolute ethanol is added at room temperature to

the thermally treated mixture. The optically clear

solution turns turbid immediately. After

centrifugation (4000 rpm, 30 min) the clear

supernatant is discarded and the resulting

precipitate is dispersed in apolar solvents (e.g.

toluene). This purification procedure (ethanol

precipitation – centrifugation – toluene dispersion) is

repeated three times to remove all residual organics.

For the last purification cycle, the final toluene

dispersion is centrifuged 20 minutes at 8000 rpm in

order to remove eventual insoluble materials.

Powder X‐Ray Diffraction. PXRD measurements

were performed at room temperature in

Bragg‐Brentano geometry using a Bruker D8

Advance powder diffractometer with a copper

anticathode and a (111) Ge monochromator. The D8

Advance is equipped with a 1‐dimensional detector

(LynxEye). PXRD patterns were recorded in the

range 10° ‐ 120° with a step size of 0.03° and a

counting time of 15 s per step. Rietveld refinements

were performed using Jana 2006 software with the

fundamental approach method. XRD samples were

prepared by drop‐casting solutions of thorium /

uranium mixed oxide nanocrystals (precipitated in

ethanol) onto a (911)‐oriented silicon substrate.

Transmission Electron Microscopy. TEM analysis

was performed using an image corrected FEI Titan

80‐300 microscope operated at 300 kV and equipped

with a Gatan US1000 CCD camera for TEM imaging

and electron diffraction. Scanning transmission

electron microscopy (STEM) images were acquired

using a HAADF (High Angle Annular Dark Field)

detector with a nominal spot size of 0.14 nm. EDX

analysis was performed in STEM mode with a

nominal spot size of 0.5 nm using an EDAX S‐UTW

EDX detector.

TEM samples were prepared by drop‐casting diluted

suspensions of the thorium / uranium mixed oxide

nanocrystals in toluene onto carbon coated copper

grids (Quantifoil holey carbon grids coated with a 2

nm thickness carbon layer).

Inductively Coupled Plasma Mass Spectrometry.

The analyses were performed with a double focusing

magnetic sector field ICP‐MS (Finnigan Element2).

The dried thorium / uranium mixed oxide

12

nanocrystals were dissolved in concentrated nitric

acid (14 M). After complete dissolution and dilution

the thorium and uranium contents were determined

by ICP‐MS.

Magnetic properties.

Magnetization and d.c. magnetic susceptibility were

measured on the encapsulated (under Ar atmosphere)

fresh NCs in the temperature range 2 ‐ 300 K and in

magnetic fields up to 7 T using a Quantum Design

MPMS‐7 superconducting quantum interference

device (SQUID). Magnetic contribution of the argon

filled Plexiglass container was determined before

encapsulation and subtracted. Resulting data were

corrected for the diamagnetic contribution using

Pascal’s constants. Magnetic calibration was done

with a cylindrical palladium standard having

approximately the same geometry of the measured

samples.

Acknowledgements

This work was partially carried out with the support

of the Karlsruhe Nano Micro Facility (KNMF,

www.knmf.kit.edu) a large‐scale research

infrastructure of the Helmholtz Society at the

Karlsruhe Institute of Technology (KIT,

www.kit.edu). Daniel Bouexiere is acknowledged for

his help with powder x‐ray diffraction

measurements performed in a dedicated glove‐box

for radioactive samples.

Electronic Supplementary Material: Supplementary

material (purification of Th(acac)4, synthesis of

UO2(OAc)2.2H2O, composition of the reactive

mixtures, experimental thermal profiles, results of

the Rietveld refinements and additional TEM

pictures) is available in the online version of this

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