Chemical Vapor Deposition of Phase-Pure Uranium Dioxide Thin Films from Uranium(IV) Amidate Precursors

Article abstract of DOI:10.1002/anie.201901924

Homoleptic uranium(IV) amidate complexes have been synthesized and applied as single-source molecular precursors for the chemical vapor deposition of UO₂ thin films. These precursors decompose by alkene elimination to give highly crystalline ph

Full text of DOI:10.1002/anie.201901924

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Accepted Article
Title: Chemical Vapor Deposition of Phase-Pure Uranium Dioxide Thin
Films from Uranium(IV) Amidate Precursors.
Authors: Mark Straub, Jennifer Leduc, Michael Frank, Aida Raauf,
Trevor Lohrey, Stefan Minasian, John Arnold, and Sanjay
Mathur
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201901924
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Angewandte Chemie International Edition
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COMMUNICATION
Chemical Vapor Deposition of Phase-Pure Uranium Dioxide Thin
Films from Uranium(IV) Amidate Precursors
[
a,b]
[c]
[c]
[c]
[a,b]
[b]
Mark Straub , Jennifer Leduc , Michael Frank , Aida Raauf , Trevor Lohrey , Stefan Minasian,
[c]
[a,b]
Sanjay Mathur* , and John Arnold*
[15]
Abstract: Homoleptic uranium(IV) amidate complexes have been
mechanism to minimize the formation of side products.
synthesized and applied as single-source molecular precursors for
The last few decades have yielded an enormous amount
of research into volatile transition metal and lanthanide
complexes, which are used as molecular precursors for CVD of
chemical vapor deposition of UO
decompose by alkene elimination to give highly crystalline, phase-
pure UO films with an unusual branched heterostructure.
2
thin films. These precursors
[14,16–22]
2
metal oxide, nitride, sulfide, and carbide thin films.
In
contrast to the myriad quantity of CVD precursors that have
been described for these metals, only a small handful of uranium
oxide CVD precursors have ever been reported. The first gas-
phase synthesis of uranium oxide thin films was accomplished
The efficiency and safety profiles of nuclear reactors are
heavily impacted by the surface chemistry of the fuel materials
[
1]
used in the reactor core. Defect formation and void swelling in
UO fuel pellets dramatically reduce the lifetime and energy
o
by heating a uranyl β-diketonate precursor above 400 C in the
2
presence of O
2 2
or H O; however, the oxidizing environment of
output of these materials, but direct study of the surface
oxidation processes that cause these defects can be challenging
due to the chemical complexity and extreme radioactivity of
these reactive gases prevented the formation of phase-pure UO
2
[23]
films. In a more recent single-source process, films containing
a mixture of uranium oxides were synthesized via thermal CVD
[
1–3]
spent fuels.
pure UO thin films can serve as excellent model systems for the
chemical and physical changes that occur at the grain
boundaries of bulk UO fuel pellets. Uranium oxide thin films
have been fabricated using both solution
Due to their large active surface areas, phase-
[24]
from fluorinated uranium heteroarylalkenolate precursors. We
2
sought to develop a class of volatile, non-fluorinated precursors
with an easily-accessible thermal decomposition pathway to UO
2
,
2
thereby minimizing the heat required to form phase-pure UO
2
[
4–6]
[7–10]
and sputtering
[25]
films. Based on previous research in our group and promising
methods; however, achieving stoichiometric control in the
resulting films has been challenging due to the wide array of
accessible uranium oxide phases and the facile interconversion
[26–28]
results from related transition metal systems,
we turned to
uranium amidate complexes to meet these requirements.
Amidate ligand substituents can be readily varied to cover a
wide range of steric and electronic properties, enabling control
over the geometry, thermal stability, volatility, and decomposition
[
5,11]
of these phases at grain boundaries.
Consequently, many
published studies on uranium oxide have been performed using
materials with a varying degree of amorphous or polycrystalline
IV
mechanisms of the resulting metal complexes. Using new U
[12]
character. McClesky and coworkers have used an innovative
polymer-assisted deposition technique from UO (NO
precursors to produce UO and U thin films; epitaxial
amidate precursors with
a
well-defined decomposition
2
3 2
)
mechanism, we synthesized crystalline, phase-pure UO
via CVD.
2
films
2
3 8
O
matching of the desired uranium oxide phase to the substrate,
followed by annealing at 1000 °C, resulted in phase-pure
The amide proligands N-tert-butylisobutyramide (H(ITA))
(
1) and N-tert-butylpivalamide (H(TTA)) (2) were synthesized
[12,13]
films.
Chemical vapor deposition (CVD) is an additional
[29]
according to literature methods
Deprotonation of 1 and 2 with KN(SiMe )
3 2
and purified by sublimation.
in THF generated the
promising route, with the added benefit that direct synthesis of
uranium oxide films from a single-source molecular precursor
could facilitate enhanced tunability of the growth process
through rational control of the decomposition mechanism and
corresponding potassium amidates K(ITA) (3) and K(TTA) (4) in
high yield. As we have observed previously in related
[25]
systems,
using these potassium amidates directly for the
[
14]
rate.
However, the preparation of phase-pure thin films in a
metalation of uranium led to the formation of undesirable –ate
complexes, resulting in poor yields of the anticipated products.
This was overcome by adding 18-crown-6 to 3 and 4 to give the
crowned potassium amidates K(ITA)(18c6) (5) and K(ITA)(18c6)
CVD process can be challenging, as the precursors must be
both sufficiently volatile and decompose cleanly by a single
(
6) in near-quantitative yield (Scheme 1); these ligand salts
performed much more favorably for metalation of uranium.
[
[
[
a]
b]
c]
M. D. Straub, T. D. Lohrey, Prof. Dr. J. Arnold*
Department of Chemistry, University of California, Berkeley
Berkeley, CA 94720, USA
E-mail: arnold@berkeley.edu
M. D. Straub, T. D. Lohrey, S. G. Minasian, Prof. Dr. J. Arnold*
Heavy Element Chemistry Group, Chemical Sciences Division
Lawrence Berkeley National Lab
Berkeley, CA 94720, USA
J. Leduc, M. Frank, A. Raauf, Prof. Dr. S. Mathur*
Institute of Inorganic Chemistry, University of Cologne
Cologne, Germany 50939
Scheme 1. Synthesis of the crowned potassium amidates K(ITA)(18c6) (5)
and K(TTA)(18c6) (6).
Supporting information for this article is given via a link at the end of
the document.
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Table 1. Selected bond lengths in 7 and 8.
The homoleptic amidate complexes U(ITA)
U(TTA) (8) were synthesized by treating UI (1,4-dioxane)
4 4 2
4
(7) and
with
four equivalents of 5 or 6, respectively. (Scheme 2) We also
synthesized
(Me Si) N]
7
and
8
via
Si(Me)
a
protonolysis route with
Atoms
Bond lengths (Å)
2
[
3
2
2
U[κ -(C,N)-CH
2
2
N(SiMe )] and 1 or 2, but
3
7
8
yields were lower using this method. Green crystals of 7 and teal
crystals of 8 suitable for X-ray diffraction were grown from Et
and pentane, respectively.
2
O
U1 – O1
U1 – O2
U1 – O3
U1 – O4
U1 – N1
U1 – N2
U1 – N3
U1 – N4
C1 – O1
C2 – O2
C3 – O3
C4 – O4
C1 – N1
C2 – N2
C3 – N3
C4 – N4
2.333(2)
2.350(2)
2.366(2)
2.346(2)
2.499(3)
2.507(2)
2.502(2)
2.493(3)
1.307(4)
1.311(3)
1.296(4)
1.303(4)
1.305(4)
1.292(4)
1.296(4)
1.310(4)
2.123(5)
2.135(4)
2.284(4)
2.296(4)
-
-
2.495(5)
2.457(5)
1.375(9)
1.361(7)
1.323(7)
1.323(7)
1.141(11)
1.240(9)
1.297(8)
1.309(9)
The U-O and U-N bond lengths in 7 are comparable to the
analogous U-O and U-N bonds of the chelated amidates in 8
(2.284(4) and 2.296(4) Å for U-O). However, the U-O bonds of
the O-bound amidates in 8 are substantially shorter (2.123(5)
and 2.135(4) Å), suggesting increased localization of electron
4 4
Scheme 2. Synthesis of U(ITA) (7) and U(TTA) (8).
Although the proligands 1 and 2 are comparable sterically,
we observed different molecular geometries for 7 and 8.
Crystallographic analysis of 7 showed this molecule to be eight-
coordinate and D2d symmetric, with all four amidate ligands
density on the oxygen atoms of these ligands when bound κ -O.
Providing further support for this claim, the O-bound ligands in 8
also possess longer C-O bonds (1.375(9) and 1.361(7) Å) and
shorter C-N bonds (1.141(11) and 1.240(9) Å for C-N) than their
1
chelated (κ
coordinate and C
and two (κ -O) amidate ligands bound to the uranium center.
2
-N,O) to the uranium center. By comparison, 8 is six-
chelated counterparts (1.323(7) and 1.323(7)
1.297(8) and 1.309(9) Å).
Å for C-O;
1
symmetric, with two (κ -N,O) amidate ligands
2
1
The thermal decomposition of the precursors was further
investigated using thermogravimetric (TG) analysis (Figure 2).
Precursor 7 showed an onset of decomposition at 85 C, while
precursor 8 showed an onset of decomposition at 70 C. The
experimentally detected overall weight losses of 7 (65.0%) and 8
(62.7%) are lower than the theoretical values of 66.5% and
This difference in geometry can possibly be attributed to the
higher electron donating effect of the tert-butyl vs. iso-propyl
substituents on the amidate backbone, disfavoring electron
donation from the lone pairs on all four nitrogen atoms to the
uranium center in 8; however, it is also possible that the larger
o
o
t
steric bulk of the C- Bu substituent contributes to the lower
2
68.8% for the formation of UO , suggesting the potential
coordination number of 8.
incorporation of carbon impurities; this mass difference is
substantially more prominent for precursor 7 than for precursor 8.
An in-depth discussion on the TG analysis can be found in the
supporting information.
Figure 1. 50% probability thermal ellipsoid view of U(ITA)
U(TTA) (8) (right). Hydrogen atoms are omitted for clarity.
4
(7) (left) and
4
Figure 2. Thermograms of 7 (left) and 8 (right) collected under nitrogen at a
heating rate of 5 C/min.
o
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Scheme 3. Proposed decomposition mechanism of 7 and 8. Alkene, amide, and nitrile byproducts were observed by NMR; intermediate decomposition products
D-1 and D-2 are postulated.
To elucidate the decomposition mechanism of these
precursors to uranium oxide, solid samples of 7 and 8 were
o
heated to 300 C in sealed J-Young tubes under a nitrogen
atmosphere. The tubes were then cooled with liquid nitrogen to
condense volatile decomposition products, and C D was added
6 6
for NMR analysis. Three main products were visible in the
1
resulting H NMR spectra of the decomposed precursors: the
amides 1 or 2, isobutyronitrile (from 7) or pivalonitrile (from 8),
and isobutylene. A small amount of insoluble black uranium
oxide precipitate was also observed. These products are
consistent with an alkene elimination mechanism (Scheme 3),
[26]
as reported previously for related homoleptic Zr(IV) amidates.
We also observed slow formation of the same products by
o
8
heating solutions of 7 and 8 to 150 C in d -toluene over the
course of multiple days.
In the first decomposition step, isobutylene elimination
from a nitrogen atom generates a hemiamidate intermediate (D-
1). This hemiamidate then undergoes protonolysis with another
amidate ligand to promote the elimination of one equivalent each
of amide and nitrile, leaving an oxygen atom bound to the
uranium center (D-2). Because this is a low-coordinate system, it
is likely that the intermediate D-2 aggregates prior to elimination
of additional ligands. A second iteration of this process yields
2
UO and one more equivalent each of isobutylene, amide, and
nitrile.
Figure 3. XPS survey spectrum (top) and high-resolution U 4f XPS spectrum
bottom) of crystalline UO films prepared via CVD using 7.
(
2
2
Complexes 7 and 8 were both tested as UO thin film
precursors in a cold wall thermal CVD reactor. We were
interested to determine if the oxidation state of the precursors
would be retained in the CVD-generated materials to form
stoichiometric UO
2
films, as suggested by the thermal
decomposition experiments. As the sublimation temperatures
-3
and decomposition temperatures at a pressure of 10 mbar
were in very close vicinity (~130 °C for 7, ~120 °C for 8), CVD
-6
experiments were performed at a pressure of 10 mbar to favor
sublimation of the precursors. In order to ensure a proper
precursor flow, the precursor temperatures were set to 160 °C.
Since TG analysis displayed a complete decomposition of both
precursors at a temperature of 500 °C, substrate temperatures
of 500 °C were chosen for both CVD processes. Precursor 7
sublimed without prior decomposition in the precursor flask, and
black films were generated during the CVD process. In contrast,
the deposition process using complex 8 was not successful; we
Figure 4. Powder XRD pattern of the film deposited using 7 at a precursor
temperature of 160 °C and a substrate temperature of 500 °C.
postulate this may be due to the weakly bound κ
1
-O coordinated
ligands and thus thermal instability of the precursor.
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Following the deposition process, the films were
characterized without an additional annealing step to determine
The vapor-deposited UO
2
films exhibited a homogeneous
distribution of fir tree-like structures at the surface and good
adhesion to the silicon substrate, as depicted in the top-view
(Figure 5) and side-view (Figure 6) SEM images, respectively.
The side-view SEM images additionally revealed the formation
of a dense layer with a thickness of ~400 nm at the substrate
if UO
XRD (Figure 4) analyses, the film surface and the bulk are
composed of phase-pure UO . This finding is noteworthy given
2
was prepared directly. As shown by XPS (Figure 3) and
2
the pronounced tendency of uranium dioxide to exhibit surface
oxidation resulting in hyperstoichiometric compositions (O:U >
interface, which seamlessly continues into
a branch-like
2
:1). The XPS survey spectrum exhibited signals attributable
structure with a thickness of ~2.5 µm. This change in the
microstructure from 2-D to 1-D growth is likely due to the good
solely to uranium, oxygen and carbon (Figure 3, left). The high-
resolution U 4f XPS spectrum showed two main signals at 380.4
eV and 391.4 eV, corresponding to the U 4f7/2 and U 4f5/2 orbitals,
respectively (Figure 3, right). The peak positions of the main
signals and the satellites (binding energy distance to the main
lattice match between silicon (a = 5.431 Å) and UO
(a = 5.471 Å), which facilitates epitaxial growth of UO onto Si up
to a thickness of ca. 400 nm. Beyond this thickness, the cubic
UO crystals act as seeds for 1-D nanostructures that grow with
2
2
2
[
5]
signals of DEsat = 6.9 eV) are consistent with the reported data.
The Bragg reflections in the XRD pattern were assigned to cubic
fluorite-type UO (Figure 4). The peaks at 2θ = 13.0°, 15.0°,
1.2°, 24.9° and 26.0° were indexed to the (111), (200), (220),
311) and (222) planes, respectively. As the diffraction peaks are
equal probability in multiple directions, leading to the formation
of branch-like structures.
2
In summary, we have developed an effective single-source
2
(
route to fabricate crystalline, phase-pure UO
2
films through
IV
chemical vapor deposition of U amidate molecular precursors,
which decompose cleanly via alkene elimination. Small changes
in the ligand substituents were seen to affect both the molecular
geometry and the decomposition behavior of the precursors,
broadened anisotropically, the crystallite sizes were calculated
using the Scherrer equation and amounted to 146 nm, 195 nm
and 99 nm for the (111)/(222), (200) and (311) planes,
respectively. Whereas the anisotropic broadening suggests the
formation of anisotropic crystallite shapes, the predominant
intensity of the reflection indexed to the (111) plane hints
towards a preferred growth direction that was confirmed by the
surface topography (Figure 5). Taken together, these results
with the 8-coordinate complex
7 performing much more
favorably for thin film deposition than the 6-coordinate complex 8.
XRD and XPS measurements confirmed the vapor-deposited
uranium oxide films to be stoichiometric UO
showed good epitaxial growth of the UO
2
, and SEM images
layer onto the Si
2
strongly suggest that phase-pure UO
directly via decomposition of the molecular precursor.
2
films were prepared
substrate. Above a film thickness of ca. 400 nm, the UO
2
crystals formed fir tree-like structures with isotropic growth of 1-
D branches and a large accessible surface area. Given the
anisotropic microstructure and high surface area of these films,
2
in conjunction with high charge carrier mobilities in UO , we plan
to investigate the performance of these films as photoanodes in
photoelectrochemical water splitting reactions.
Acknowledgements
This work was supported by the Director, Office of Science,
Office of Basic Energy Sciences, Division of Chemical Sciences,
Geosciences, and Biosciences Heavy Element Chemistry
Program of the U.S. Department of Energy (DOE) at LBNL
under Contract No. DE-AC02-05CH11231. The Advanced Light
Source (ALS) is supported by the Director, Office of Science,
Office of Basic Energy Sciences, of the U.S. Department of
Energy under Contract No. DE-AC02-05CH11231. We thank Dr.
Simon J. Teat for oversight of our crystallographic studies at the
ALS. We also thank M. A. Boreen, N. S. Settineri, and L. M.
Moreau for helpful discussions. J.L., M.F., A.R. and S.M. are
thankful to the University of Cologne for providing the
infrastructural support. J.L. is thankful to Fonds der Chemischen
Industrie for a PhD fellowship. A.R. acknowledges the DAAD for
a travel grant. The financial support in the framework of the DFG
priority programs (SPP 1613; “Fuels Produced Regeneratively
Through Light-Driven Water Splitting: Clarification of the
Elemental Processes Involved and Prospects for Implementation
in Technological Concepts” and SPP 1959 "Manipulation of
Figure 5. Top-view SEM images with different magnifications of crystalline
UO films, prepared via CVD from precursor 7.
2
Figure 6. Side-view SEM images with different magnifications of crystalline
UO films, prepared via CVD from precursor 7. The thickness of these films is
.9 µm.
2
2
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matter controlled by electric and magnetic field: Towards novel
synthesis and processing routes of inorganic materials") are
gratefully acknowledged.
Keywords: Uranium • Inorganic materials • Chemical
vapor deposition • Thin films • Actinides
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Entry for the Table of Contents (Please choose one layout)
Layout 2:
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M. D. Straub, J. Leduc, M. Frank, A.
Raauf, T. D. Lohrey, S. G. Minasian, S.
Mathur*, and J. Arnold*
Page No. – Page No.
Chemical Vapor Deposition of Phase-
Pure Uranium Dioxide Thin Films
from Uranium(IV) Amidate Precursors
From molecules to materials: Volatile uranium(IV) amidate complexes are used
as single-source molecular precursors to uranium oxide films. Chemical vapor
deposition (CVD) of these single-source precursors yields crystalline, phase-pure
2
UO films with a fir tree-like microstructure and a high surface area.
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