Intrinsic Semiconducting Behavior in a Large Mixed-Valent Uranium(V/VI) Cluster

Article abstract of DOI:10.1002/anie.202017298

We disclose the intrinsic semiconducting properties of one of the largest mixed-valent uranium clusters, [H₃O⁺][U^V(U^VIO₂)₈(μ₃-O)₆(PhCOO)₂(Py(CH₂O)₂)₄(DMF)₄] (Ph=phenyl, Py=pyridyl, DMF=N,N-dimethylformamide) (1). Single-crystal X-ray crystallography demonstrates that U^V center is stabilized within a tetraoxo core surrounded by eight uranyl(VI) pentagonal bipyramidal centers. The oxidation states of uranium are substantiated by spectroscopic data and magnetic susceptibility measurement. Electronic spectroscopy and theory corroborate that U^V species serve as electron donors and thus facilitate 1 being a n-type semiconductor. With the largest effective atomic number among all reported radiation-detection semiconductor materials, charge transport properties and photoconductivity were investigated under X-ray excitation for 1: a large on-off ratio of 500 and considerable charge mobility lifetime product of 2.3×10^?4 cm² V^?1, as well as a high detection sensitivity of 23.4 μC Gy_air^?1 cm^?2.

Full text of DOI:10.1002/anie.202017298

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Uranium Clusters
Hot Paper
Intrinsic Semiconducting Behavior in a Large Mixed-Valent Uranium-
(V/VI) Cluster
Mingxing Zhang+, Chengyu Liang+, Guo-Dong Cheng+, Junchang Chen, Yumin Wang,
Linwei He, Liwei Cheng, Shicheng Gong, Duo Zhang, Jiong Li, Shu-Xian Hu,* Juan Diwu,
Guozhong Wu, Yaxing Wang,* Zhifang Chai, and Shuao Wang*
Abstract: We disclose the intrinsic semiconducting properties
of one of the largest mixed-valent uranium clusters, [H3O+]-
[UV(UVIO2)8(m3-O)6(PhCOO)2(Py(CH2O)2)4(DMF)4] (Ph =
phenyl, Py = pyridyl, DMF = N,N-dimethylformamide) (1).
Single-crystal X-ray crystallography demonstrates that UV
center is stabilized within a tetraoxo core surrounded by
eight uranyl(VI) pentagonal bipyramidal centers. The oxida-
tion states of uranium are substantiated by spectroscopic data
and magnetic susceptibility measurement. Electronic spectros-
copy and theory corroborate that UV species serve as electron
donors and thus facilitate 1 being a n-type semiconductor. With
the largest effective atomic number among all reported
radiation-detection semiconductor materials, charge transport
properties and photoconductivity were investigated under X-
ray excitation for 1: a large on-off ratio of 500 and considerable
charge mobility lifetime product of 2.3 ꢀ 10À4 cm2 VÀ1, as well
as a high detection sensitivity of 23.4 mCGyairÀ1 cmÀ2.
6d/5f orbitals endows uranium-based materials with many
distinct properties not observed in those of transition metals
and lanthanides. The common oxidation states of uranium
span from III to VI, each with distinct chemical species,
physicochemical properties, and related functionalities.[2] For
instance, UIII complexes are substantially utilized for catalytic
transformation of small molecules owing to their high
chemical reactivity.[3] The intrinsic luminescence from UVI
compounds is fertilized in designing sensors and optical
materials.[4] Intriguing magnetism have been shown in several
UV compounds.[5] UIV compounds often exhibit large electric
current outputs and outstanding superconductivity under
external stimulation.[6] Mixed-valent uranium compounds are
unique class where nonidentical metal centers potentially
combine disparate physicochemical properties together, and
therefore distinct functionalities can be envisioned.
Over the years, several synthetic strategies have been
developed to stabilize mixed-valent uranium in solids. Lii
et al. demonstrated that UV/VI compounds can be prepared by
high-temperature and high-pressure supercritical hydrother-
mal methods.[7] UIV/VI compounds are readily afforded
through in situ partial reduction of UVI under solvothermal
methods.[8] Oxidation of UIII/UIV organometallic compounds
(or reduction of UVI organometallic compounds) also leads to
the formation of mixed-valent uranium cluster compounds.[9]
To the best of our knowledge, previous efforts have been
devoted to crystal structures, magnetism, and spectroscopy of
mixed-valent uranium compounds aiming at understanding
their coordination behaviors and electronic structures while
their potential applications have been substantially over-
looked since their discovery. In this work, for the first time, we
disclose the semiconducting nature of a large mixed-valent
uranium cluster compound, [H3O+][UV(UVIO2)8(m3-O)6-
(PhCOO)2(Py(CH2O)2)4(DMF)4] (Ph = phenyl, Py = pyridyl,
DMF = N,N-dimethylformamide) (1). Furthermore, with the
combination of a large density of the heaviest naturally
abundant element within the molecular structure and its high
attenuation coefficient towards high-energy photons, we
demonstrated that compound 1 is an ideal semiconducting
material for efficient direct conversion of X-ray photons into
electrical signals, endowing mixed-valent uranium com-
pounds with a decent functionality with application poten-
tials.
E
xtending the potential applications of uranium beyond
nuclear industry provides useful hint in disposal and recycling
of piled depleted uranium.[1] The bonding involvement of
[*] M. Zhang,[+] C. Liang,[+] J. Chen, Y. Wang, L. He, L. Cheng, S. Gong,
Dr. D. Zhang, Prof. J. Diwu, Prof. Y. Wang, Prof. Z. Chai, Prof. S. Wang
State Key Laboratory of Radiation Medicine and Protection
School for Radiological and interdisciplinary Sciences (RAD-X) and
Collaborative Innovation Center of Radiation Medicine of
Jiangsu Higher Education Institutions, Soochow University
Suzhou 215123 (China)
E-mail: yxwang@suda.edu.cn
shuaowang@suda.edu.cn
M. Zhang,[+] Prof. G. Wu
Shanghai Institute of Applied Physics, Chinese Academy of Sciences
Shanghai 201800 (China)
M. Zhang,[+]
University of Chinese Academy of Sciences
Beijing 100049 (China)
Dr. G.-D. Cheng[+] Prof. S.-X. Hu
School of Mathematics and Physics
University of Science and Technology Beijing
Beijing 100083 (China)
E-mail: hushuxian@csrc.ac.cn
Dr. J. Li
Shanghai Synchrotron Radiation Facility
Shanghai Advanced Research Institute
Chinese Academy of Sciences, Shanghai 201210 (China)
Compound 1 was synthesized by solvothermal method
through heating the mixture of UO2(NO3)2·6H2O, macro-
cyclic oligoboronate, and benzoic acid in mixed solvents of
DMF, methanol, and ethanol (Scheme 1, detailed synthetic
procedure can be found in Supporting Information, PXRD
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202017298.
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v.u., further confirming its pentavalent assignment (Support-
ing Information, Table S3).[11] All uranyl(VI) groups display
a pentagonal bipyramidal geometry with five coordinated
atoms in the equatorial plane. The equatorial atoms for U3
and U4 are two m3-O atoms as well as two oxygen atoms and
one nitrogen atom from a Py(CH2OÀ)2 ligand. Equatorial
atoms for U1 and U2 are m3-O atoms and oxygen atoms from
PhCOOÀ ligand and DMF molecule (Figure 1a). All bond
Scheme 1. Synthetic route for 1.
=
À
À
lengths of U O, U O, and U N are listed in the Supporting
Information, Table S2, within the range of those found in
previous UVI and UV compounds.[5a,10,12] Furthermore, 1 is the
third case of UV/VI oxo cluster and the largest tetraoxo core
based UV/VI oxo cluster.[12a,b] Importantly, the crystallinity of
1 remains intact after being stored in a screw-capped vial for
six months, making it sufficiently stable for acting as a func-
tional material. Moreover, 1 is thermally stable up to 2618C
(Supporting Information, Figure S3).
data is shown in the Supporting Information, Figure S1). The
macrocycle oligoboronate is crucial for the synthesis of 1,
which in situ decomposes forming the ligand of Py(CH2OÀ)2
during the solvothermal reaction. The deep brown block
crystals were isolated, where the coloration initially suggests
the presence of mixed-valent uranium (Supporting Informa-
tion, Figure S2). X-ray crystallography analysis shows 1 crys-
tallizes in the monoclinic space group of P21/c (Supporting
Information, Table S1), and its structure can be best described
as a large rhombus-shaped cluster containing nine uranium
centers protected by a series of organic ligands (Figure 1a).
Figure 1b shows that the central UV (labeled as U5), located
at the intersection of two diagonals, is encompassed by eight
uranyl(VI) groups (Figure 1c). This unique structural
arrangement efficiently stabilizes UV, which generally exhibits
a large tendency of disproportionation to UVI and UIV. The UV
center is six-coordinated by four m3-O atoms and two oxygen
atoms from DMF molecules, forming a UO6 tetragonal
bipyramid geometry. The bond lengths are typical for
tetraoxo core based pentavalent uranium with two relatively
To further study the electronic structure of 1, spectro-
scopic and magnetic characterizations were performed. Fig-
ure 2a illustrates the solid-state UV/Vis absorption spectrum
of 1. The pronounced absorption peaks from 300 to 550 nm
are typical absorption bands of uranyl, originating from the
vibronically coupled transitions from ligand-to-metal charge
transfer.[2] Two additional peaks centered at 617 nm and
757 nm are ascribed to the characteristic electronic transitions
of UV.[9a] The infrared spectrum of 1 contains the character-
VI
V
=
=
=
=
istic stretching vibration bands of O U O and O U O at
900, 885 and 800 cmÀ1 (Supporting Information, Figures S4
and S5). X-ray photoelectron spectroscopy further verifies the
existence of UV. The U 4f high-resolution core-level energy
spectrum is presented in Figure 2b. Two strong and broad
peaks appear in the range of 375 to 395 eV, which are the U
4f7/2 (381 eV) and U 4f5/2 (392 eV) peaks, respectively. The
peaks were further fitted with two components with dispro-
portionate satellite peaks, ascribed to UVI (381.2, 392.1 eV)
and UV (380.2, 391.2 eV), respectively.[13] Furthermore, we
investigated the X-ray absorption near edge structure
(XANES) spectroscopy at U L3 edge. Figure 2c shows
À
longer (2.327(14) ꢀ) U O bonds in the axial direction and
À
four shorter (2.046(11) and 2.069(10) ꢀ) U O bonds in the
equatorial plane, respectively (Supporting Information,
Table S2).[10] The calculated bond valence sum of U5 is 5.13
Figure 2. a) UV/Vis spectrum of 1. b) XPS spectra of 1. c) U LIII-edge
XANES of (UEt4)2UCl6, 1, and UO2(NO3)2·6H2O. d) Temperature-
dependent magnetic susceptibility data for 1.
Figure 1. a) Molecular structure of 1.[19] b) The polyhedral view of 1.
c) A view of the UVUVI8(m3-O)6 O6 core.
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normalized XANES spectra for 1 and UO2(NO3)2·6H2O. The
absorption features correspond to electric-dipole-allowed
transitions from U 2p3/2 core level to unoccupied 6d/7s level,
in which the energy of infection point in absorption edge is
generally correlated with oxidation state of absorbing
atoms.[14] From the comparison results, the energy for 1 is
17176.95 eV, lower than that of the hexavalent uranium
compound of UO2(NO3)2·6H2O (17177.30 eV). Moreover,
we collected the variable-temperature magnetic susceptibility
for 1, showing a paramagnetic behavior (Figure 2d), in
agreement with the structural feature of isolated paramag-
netic UV surrounded by diamagnetic UVI. Fitting of the
experimental data using modified Curie–Weiss law (c = C/
(TÀq) + cTIP) yields the effective moment (meff) of 1.76 mB, in
agreement with the reported values for UV compounds (1.4–
3.0 mB).[5a,12d,15]
To understand the semiconducting nature of 1, density
functional theory (DFT) analysis was performed using PBE
method implemented in VASP for the electronic structure
calculations (computational details are summarized in the
Supporting Information). Figure 3a and the Supporting
Information, Figure S6 illustrates that band structure of 1 is
dominated by UVI centers. The calculated band gap is 1.83 eV,
close to the experimental value of 1.96 eV. The charge density
analysis in Figure 3b clarifies the spatial distribution of
electrons in UV/VI oxo cluster. Figure 3c illustrates that the
valence bands (VB) are mainly contributed by O 2p and UVI
5f orbitals, and conduction bands (CB) is formed by UVI 5f
orbitals. The distinctive feature of 1 is an intermediate band
(IB) within the forbidden gap. Further analyzing orbital
wavefunction illustrates that IB is mainly from the UV, and
electrons localized at 5fx(x2À3y2) orbitals (Figure 3c). Further-
more, the emergence of IB in mixed-valent uranium clusters
implies that UV species serve as electron donors endowing
1 with n-type semiconducting behavior. The donor level is
only 0.01 eV lower than that of CB, beneficial for ionization
and conductivity. The electronic transition feature can be
inferred from the UV/Vis absorption feature of 1, where two
additional electronic transitions emerge under low energy
photon excitation. In addition, the calculated energy for
allowed f–f transitions from UV to UVI is only 0.26 eV,
suggesting feasible charge transport within 1 (Figure 3d and
discussion in Supporting Information). The above results
clearly confirm that IB in 1 serves as a stepping stone to lower
the charge transport barrier, which facilitates the charge
carrier mobility.[16] The band structure of 1 highlights the
functionality of mixed-valent uranium in the charge transport
properties of 1.
Figure 4a shows statistical data on the density (1) and
effective atomic numbers (Zeff) of the commercial radiation
detection materials and recently emerged ones.[17] Impres-
sively, 1 shows the largest Zeff among all reported radiation
detection semiconductors, and thus provides intrinsic superi-
ority in absorbing high energy photons. Figure 4b illustrates
the role of chemical compositions on the X-ray mass
attenuation coefficient (MAC, calculated at 80 keV, data is
shown in the Supporting Information, Figure S7 and
Table S4). Overall, the combined merit of high X-ray MAC
and intrinsic semiconducting behavior endow 1 potential
capability for radiation detection.
To evaluate the X-ray detection performance of 1, we
investigated its optoelectronic properties. As shown in Fig-
Figure 3. a) Calculated energy gap structure for compound 1. b) 2D charge density analysis on the UVUVI8(m3-O)6O6 core of the unit cell. c) Wave
function analysis on conduction band minimum (CBM), valence band maximum (VBM), and intermediate band (IB), respectively. The isosurface
value is 0.03 eꢀÀ3. d) Representation for f–f transitions of the UVUVI8(m3-O)6 O6 core within the primitive cell. The energy for UV 5fx(x2À3y2) orbitals is
set to 0 for clarity.
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Figure 4. a) Comparison of the density and effective atomic numbers (Zeff) of the commercial radiation detection materials, recently emerged
candidates, and 1. b) Comparison of the mass attenuation of the commercial radiation detection materials, recently emerged candidates, and 1.
c) I-V curves of 1-based detector under dark conditions and X-ray exposure. d) Photocurrent response of 1-based detector to the tungsten anode
X-ray source. e) X-ray-generated photocurrent versus dose rate. f) Mobility-lifetime product (mt) fitting based on the Hecht equation.
ure S8, an X-ray detector was fabricated by a sandwich
structure with an Au/1/Au configuration. Figure 4c shows the
I-V characteristics of the detector under dark and X-ray
irradiation conditions with the voltage ranging from À10 to
+ 10 V. Under the dark condition, we measured the leakage
current of the detector and calculated its resistivity to be
about 2.3 ꢁ 1011 Wcm, close to our previously reported semi-
conducting metal-organic framework SCU-12[18] and the
majority of commercial radiation detectors.[17a] The photo-
current increases upon X-ray irradiation, indicative of
response to X-ray photons and charge carrier generation.
Based on above results, we estimated its X-ray detection
performance by studying the detection sensitivity. Figure 4d
shows the photocurrent response of the detector under X-ray
irradiation (on/off) at different bias voltage. Impressively, the
on/off ratio reaches 500 at 1 V bias voltage, which suppresses
the responsive capacities of SCU-12 and lead-based perov-
skites,[17c,d,18] indicating the effective charge carriers are
generated under the interaction between uranium metals
and X-ray photons, resulting in a high sensitivity to X-ray
photons. The X-ray detection sensitivity of the detector is
23.04 mCGyÀ1 cmÀ2 at 30 V bias voltage as shown in Figure 4e,
on the same level of the SCU-12-based detector and the
commercial a-Se detector.[17a,b,18] Besides that, we measured
the charge carrier mobility and lifetime product (mt) to assess
the charge transport properties of 1. Figure 4 f shows the bias
dependence of the photonresponse. The fit of measured data
10À4 cm2 VÀ1, which is a considerable value by comparing
with reported radiation detection materials[17a,b,18] (about
range from 10À2 to 10À6 cm2 VÀ1 in the Supporting Informa-
tion, Table S5).
In summary, a distinct mixed-valent uranium oxo cluster
has been successfully synthesized and fully characterized. The
unique structural arrangement within the cluster substantially
stabilizes UV, providing an opportunity to expand the
potential applications of UV-based materials. The revealed
electronic structure and band structure of 1 suggest its
intrinsic semiconducting behavior. Since the uranium-based
oxo cluster contains a high spatial density of uranium leading
to a large effective atomic number and a high MAC, powerful
capabilities for direct conversion of X-ray photons into
electrical signals is disclosed for this class of compounds.
Acknowledgements
We are grateful for funding support from the National
Natural Science Foundation of China (21790374, 21825601,
21806118), the Science Challenge Project (TZ2016004), and
Defense Industrial Technology Development Program
(JCKY2019414D002). The computational work was finan-
cially supported by the National Natural Science Foundation
of China (21976014 and U1930402). We are grateful for
calculation resources provided by Tianhe2-JK, China.
using the Hecht model gives
a mt product of 2.3 ꢁ
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The authors declare no conflict of interest.
Keywords: mixed-valent uranium · radiation detection ·
semiconductors · uranium clusters
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Fachinformationszentrum Karlsruhe Access Structures service
www.ccdc.cam.ac.uk/structures.
Manuscript received: December 30, 2020
Accepted manuscript online: February 15, 2021
Version of record online: March 18, 2021
9890 www.angewandte.org
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 9886 –9890