Double-Exchange Effect in Two-Dimensional MnO2 Nanomaterials

Article abstract of DOI:10.1021/jacs.7b01903

Electronic state transitions, especially metal-insulator transitions (MIT), offer physical properties that are useful in intriguing energy applications and smart devices. But to-date, very few simple metal oxides have been shown to undergo electronic state transitions near room temperature. Herein, we demonstrate experimentally that chemical induction of double-exchange in two-dimensional (2D) nanomaterials brings about a MIT near room temperature. In this case, valence-state regulation of a 2D MnO₂ nanosheet induces a Mn(III)-O-Mn(IV) structure with the double-exchange effect, successfully triggering a near-room-temperature electronic transition with an ultrahigh negative magneto-resistance (MR). Double-exchange in 2D MnO₂ nanomaterials exhibits an ultrahigh MR value of up to -11.3% (0.1 T) at 287 K, representing the highest reported negative MR values in 2D nanomaterials approaching room temperature. Also, the MnO₂ nanosheet displays an infrared response of 7.1% transmittance change on going from 270 to 290 K. We anticipate that dimensional confinement of double-exchange structure promises novel magneto-transport properties and sensitive responses for smart devices.

Full text of DOI:10.1021/jacs.7b01903

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Article
Double-exchange effect in two-dimensional MnO2 nanomaterials
Xu Peng, Yuqiao Guo, Qin Yin, Junchi Wu, Jiyin Zhao, Chengming
Wang, Shi Tao, Wangsheng Chu, Changzheng Wu, and Yi Xie
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 17 Mar 2017
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Double-exchange effect in two-dimensional MnO2 nanomaterials
Xu Peng,§† Yuqiao Guo,§† Qin Yin,† Junchi Wu,† Jiyin Zhao,† Chengming Wang,† Shi Tao,‡
Wangsheng Chu,‡ Changzheng Wu*† and Yi Xie†
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† Hefei National Laboratory for Physical Sciences at the Microscale, iChEM(Collaborative Innovation Center of
Chemistry for Energy Materials), CAS Center for Excellence in Nanoscience, and CAS Key Laboratory of Mechanical
Behavior and Design of Materials, University of Science and Technology of China, Hefei, Anhui, 230026, P. R. China.
‡ National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, P.
R. China.
KEYWORDS: two-dimensional nanomaterials; chemical reduction process; manganese oxide; double-exchange; room-
temperature response.
ABSTRACT: Electronic state transition especially for metal-insulator transition (MIT) offers physical properties useful in
intriguing energy applications and smart devices. But to-date very few simple metal oxide possessed electronic transition
near room-temperature. Herein, we demonstrate experimentally that chemical introduction of double-exchange in 2D
nanomaterials brings a metal-insulator transition near room-temperature. In our case, valence-state regulation of 2D
MnO2 nanosheet induces Mn(III)-O-Mn(IV) structure with double-exchange effect, successfully triggering near-room-
temperature electronic transition with an ultrahigh negative magnetoresistance. Double-exchange in 2D MnO2 nano-
materials exhibits an ultrahigh MR value up to -11.3% (0.1T) at 287 K, representing the highest negative magnetoresistance
values in 2D nanomaterials approaching room-temperature. Also, MnO2 nanosheet displays infrared (IR) response of 7.1%
transmittance change from 270 to 290 K. We anticipate that dimensional confinement of double-exchange structure
promises novel magnetotransport properties and sensitive response for smart devices.
INTRODUCTION
Recently, reducing spatial dimensionality offers much
promise in the pursuit of electronic transition especially
for smart response behavior when compared to their bulk
counterparts.13-17 Recent has seen the advantages of one-
dimensional (1D) supermolecule18, V-V atomic chains19
and 2D Bi2Te3 nanosheet20 for ultrasensitive transport
behavior near room-temperature. On the other hand,
perovskite manganites were the prototype material sys-
tem for magnetic field induced MIT effect based on dou-
ble-exchange mechanism, and also possessed advantages
of enormous chemical and structural flexibility and stabil-
ity.21 In such system, double-exchange mechanism re-
quires unique electron configuration with the magnetic
ordering where neighboring Mn sites mutually aligned
spins.22 Inspired by these concepts, there has been a de-
manding challenge in realizing low-dimensional double-
exchange structure, opening a new window of opportuni-
ty for the development of novel magnetic field induced
MIT near room-temperature. However, crystal structural
characteristics of traditional perovskite manganites, usu-
ally with possessing highly-symmetric cubic structure
with the strong covalent bonds among them, hinders
available attempts to access their freestanding low-
dimensional structure by either top-down or bottom up
strategies; and up to date there are no report on double-
exchange structure in freestanding low-dimensional
structure.23 Therefore, double-exchange structure with
Electronic transition especially for metal-insulator tran-
sition (MIT), usually accompanies the benefits of huge
temperature-induced changes in resistivity and selective
IR optical switching, bringing intriguing applications of
thermoelectric conversion,1 energy harvesting,2 and smart
response devices.3 Driven by Mott localization or Peierls
distortion in correlated system, MIT materials has been
widely found in correlated transition-metal dichalcogeni-
des (TMDc), nickelate, transition-metal oxides (TMO)
and so on.4-6 For practical applications, an ideal MIT ma-
terial should have possessed electronic transition near
room-temperature, which is beneficial for the develop-
ment of nano-device with highly sensitive behavior.7,8
Among binary oxides, vanadium dioxides (VO2) is a pro-
totype MIT material with the slight distortion of the V-V
linear chains to the zigzag pattern across phase transi-
tion.9,10 However, phase-transition temperature of VO2 is
about 340K, it still has a significant space to reach room-
temperature.11 Although doping exotic metal ions offer
effective way to control transition temperature down to
approaching room-temperature,12 the intensity of transi-
tion and response would be sharply weaken as well.
Therefore, it remains us an open question how to pursue
intrinsic electronic state transition in the vicinity of
room-temperature.
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freestanding confined dimensionality is highly desirable thus a dark-brown high-quality dispersion of MnO2
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but remains a challenge for large MIT and sensitive re-
sponse near room-temperature.
nanosheet. Finally, the dispersion of highly exfoliated
MnO2 nanosheet was vacuum-filtered over a cellulose
membrane with a 0.22- µm pore to form a homogeneous
thin film for further characterizations.
Herein, we demonstrate experimental case of double-
exchange structure in 2D nanomaterials by a mild chemi-
cal reduction, bringing a metal-insulator transition near
room-temperature. In our case, valence-state regulation
of layered MnO2 induced Mn(III)-O-Mn(IV) structure
with double-exchange effect, successfully triggering near
room-temperature electronic transition with an ultrahigh
negative magnetoresistance. So far there were no cases of
binary oxides adopting a double-exchange structure.
Double-exchange in 2D MnO2 nanomaterials exhibits an
enhanced MR value up to -11.3% (0.1T) and -54.0% (5T) at
287 K, representing the highest negative magnetore-
sistance values in 2D nanomaterials approaching room-
temperature. Also, treated MnO2 nanosheet displays IR
response of 7.1% transmittance change from 270 to 290 K.
We anticipate that dimensional confinement of double-
exchange structure promises novel magnetotransport
properties and sensitive response for smart devices near
room-temperature.
Synthesis of treated MnO2 nanosheet. The treated
MnO2 nanosheet was obtained from pristine MnO2
nanosheet by heating them at 100°C for 30 min and fur-
ther up to 220°C for 2 h in N2 atomsphere. The obtained
film was collected for further characterizations. In our
case, the low-oxygen-pressure thermoannealing (N2-
annealing) play a vital role of successfully realizing the
surface-structural modulation of pristine MnO2
nanosheet as well as the preservation of 2D MnO2 frame-
work. While the other gas atmospheres (H2, O2, NH3 and
so on) have no ability to lead the structural destruction
with the formation of double-exchange in 2D nano-
materials.
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Characterizations. X-ray diffraction (XRD) was per-
formed using a Philips X’ Pert Pro Super diffractometer
with Cu Kα radiation (λ=1.54178 Å). The scanning speed
was set as 8 degree/min. Atomic-force microscope (AFM)
image was performed by Veeco DI Nano-scope Multi-
Mode V system. X-ray photoelectron spectra (XPS) were
acquired on an ESCALAB MKII with Mg Kα (Hν = 1253.6
eV) as the excitation source.
EXPERIMENTAL SECTION
Materials. Manganese chloride (MnCl2•4H2O), ammo-
nium persulfate ((NH4)2S2O8), Tetramethylammonium
hydroxide (TMAOH) were purchased from Sinopharm
Chemical Reagent Co. Ltd. (Shanghai, China). The water
used was ultrapure (18.2 MΩ) in all experiments. All
chemicals were used as received without further purifica-
tion.
X-ray absorption spectroscopy (XAS) measure-
ments. The X-ray absorption spectra at the Mn K-edge of
the samples were collected at room-temperature in the
transmission mode at the beam line BL14W1 of the
Shanghai Synchrotron Radiation Facility (SSRF, China).
The station was operated with a Si(111) double crystal
monochromator. During the measurements, the storage
ring was operated at energy of 3.5 GeV and with a current
between 150-210 mA. The photon energy was calibrated
with the first inflection point of Mn K-edge in Mn metal
foil.
Synthesis of bulk 2D MnO2. In a typical experiment, 3
mmol MnCl2•4H2O was added into 10 mL H2O. After vig-
orous stirring for 20 min, the MnCl2•4H2O was totally
dissolved becoming a homogenous solution, called “solu-
tion A”. Then, 4.0 g (NH4)2S2O8 was added into 10 mL
H2O, after vigorous stirring for 20 min, 4.375 g TMAOH
(25 % wt) was added into the mixed solution followed by
another 20 min stirring and adding amount of water to 20
mL, called “solution B”. Then “the solution B” was added
into “solution A” slowly in 10 min under vigorous stirring.
The dark brown MnO2 sample was obtained after vigor-
ously stirring overnight in the ambient atmosphere at
room-temperature (Figure S7). After this mild oxidation
reaction, the precipitate was washed with methanol and
water three times and centrifuged at 6000 rpm for 3 min,
then the precipitate was dried in a vacuum oven at 60°C.
Magnetotransport property measurements. The
magnetization was characterized by using a SQUID
(quantum design MPMS XL-7) magnetometer with a
temperature range of 10-340 K and an applied field range
of -50 to 50 kOe. The magnetotransport property was
measured by the four-probe technique using a Quantum
Design physical property measurement system (PPMS).
Gold wires contacts with the thin films using silver paste
to provide a small contact resistance. The magnetization
and magnetotransport property were performed on the
assembled thin film of ultrathin MnO2 nanosheets with
and without low-oxygen-pressure thermoannealing
treatment. The electrical transportion experiments were
tested with four-probe configuration on the assembled
thin film of MnO2 nanosheets with 360 nm thickness
(Figure S10).
Synthesis of pristine MnO2 nanosheet. The pristine
MnO2 nanosheet was obtained from adding bulk 2D
MnO2 nanosheet into water for ultrasonic treatment un-
der constant temperature at 25°C. After ultrasonic treat-
ment for 1h, the dispersion was centrifuged under the
speed of 1000 rpm, removing the un-exfoliated bulk 2D
MnO2 from dispersions. Afterwards, the supernatant was
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Scheme 1 Schematic illustration of synthesis strategy of 2D MnO2 nanosheet with Mn(III)-O-Mn(IV) double-exchange
structure.
In-situ variable temperature Fourier transform in-
frared spectra (FTIR) Monitoring. The Infrared (IR)
transmission spectra were recorded on Fourier-
transform infrared spectroscopy (Thermo Nicolet 8700)
between 4000-1000 cm-1 under variable temperature from
250-320K and N2 atmosphere with all samples dried.
Commercial Fe3O4 nanoparticle was added to pristine and
treated MnO2 nanosheet sample to provide effective
magnetic field. The mass ratio of MnO2 nanosheet sample
to commercial Fe3O4 nanoparticle is 5:1.
MnO2 nanosheet, possessing an average thickness of ca.
4.07 nm. All experiment results above revealed that the
2D δ-MnO2 structure was well preserved without obvious-
ly structural collapse (Figure S4 and S5). In order to ex-
plore the fine valence-state of treated MnO2 nanosheet
after low-oxygen-pressure thermoannealing treatment,
surface oxidation states of both pristine and treated MnO2
nanosheet were investigated by serial charaterizations,
giving solid evidence that Mn(III) was incorporated in 2D
MnO2 structure. Figure 1c shows the Mn 2p XPS spectra
of pristine and treated MnO2 nanosheet. The three peaks
in the Mn 2p3/2 binding energy range with maxima at
640.8 eV correspond to Mn(III), and at 642.2 and 643.2 eV
correspond to Mn(IV), respectively (Figure S8 and Table
S2).25 Of note, the Mn(III)/Mn(IV) intensity ratio shows
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a
RESULTS AND DISCUSSION
Double-exchange Mn(III)-O-Mn(IV) structure was
successfully produced by a low-oxygen-pressure thermo-
annealing of pristine MnO2 nanosheet (Scheme 1).
In our case, thermoannealing treatment of MnO2
nanosheet still well kept the pristine 2D MnO2 lattice
framework, only with valence-state regulation of ele-
mental Mn ion forming double-exchange Mn(III)-O-
Mn(IV) in 2D nanomaterials. Systematically characteriza-
tions were performed to investigate structural infor-
mation of both pristine and treated MnO2 nanosheet. X-
ray diffraction (XRD) patterns of pristine and treated
MnO2 nanosheet are shown in Figure 1a. The significant
XRD peaks can be well assigned to the MnO2 with a hex-
agonal birnessite structure (JCPDS 18-0802, a=5.82 Å,
c=14.62 Å). The peak broadening reveals the nanoscale
character. Notably, the peaks of (00l) can be detected,
indicating to some extent c-orientation of assembled ul-
trathin nanosheet of treated MnO2; while the only excep-
tion is (119) plane at 65.70°, which is the distinct peak for
MnO2 structure.24 Therefore, XRD pattern shows that
treated MnO2 is aligned in c axis oriented to some extent,
and revealing that the structural retention of 2D MnO2
after thermoannealing treatment (Figure S1). The atomic-
force microscopy (AFM) image in Figure 1b showed the
flat morphology with the size of ca. 2 µm of the treated
Figure 1 (a) XRD pattern of pristine and treated MnO2
nanosheets. (b) AFM image of treated MnO2 nanosheet.
Scale bar is 1 µm. Inset is corresponding height profile of
treated MnO2 nanosheets. (c) Mn 2p XPS spectra of pris-
tine and treated MnO2 nanosheets. (d) Mn 3s XPS spectra
of pristine and treated MnO2 nanosheets.
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obvious enhancement after treatment, demonstrating the Moreover, the obvious energy shift at the rising edge of
increase of the Mn(III) in treated MnO2 nanosheet.26
Moreover, Mn 3s XPS spectra analysis indicates the va-
lence state change of Mn element. The Mn 3s core level
spectra usually show a peak splitting due to the parallel
spin coupling of the 3s electron with the 3d electron dur-
ing the photoelectron ejection (Figure S9).27 As shown in
Figure 1d, the separation value of Mn 3s peak energies
(△Eb) of pristine MnO2 nanosheet and treated MnO2
nanosheet are ~4.82 and 5.02, respectively. Since the en-
ergy separation of the Mn 3s peaks and oxidation state of
manganese in the oxide has linear relationship, the corre-
sponding average Mn valences were calculated to be ap-
proximately 3.84 and 3.48.28 Therefore, XPS measure-
ments showed that Mn(IV) oxidation in treated MnO2
nanosheet was partially reduced into Mn(III/IV) species.
the XANES spectra was detected. The edge is shifted into
a lower energy by about 1 eV for treated MnO2 nanosheet.
This edge shift is a typical indication of valence-state vari-
ation for high-oxidation Mn(IV) to reduced Mn(III). It
should be noted that this edge shift is not so much,
demonstrating that only a partial amount of Mn(IV) has
been reduced. In short, systematic characterizations con-
firm that treated MnO2 nanosheet keeps well the 2D
MnO2 lattice framework with the partially incorporation
of Mn(III), holding promise for the producing of double-
exchange structure.
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To unveil the fine structure of MnO2 nanosheet after
treatment, the extended X-ray absorption fine structure
(EXAFS) was employed to probe the local geometrical
structures of pristine and treated MnO2 nanosheet. It is
important to check whether the reduction of MnO2 in-
duced the local lattice relaxation and how the sample
realized the charge balance again when it suffered from
low-oxygen-pressure thermoannealing. Some literatures
reported that the geometry and type of ligands were high-
ly conserved in redox-active Mn complexes when they
underwent one-electron oxidation state change.31,32 To
explore this issue at the atomic level, EXAFS data were
also collected and analyzed, as shown in Figure 2c and 2d.
FT curves, transformed from the k2-weighted EXAFS os-
cillations of both samples remains the similar pair radial
distribution functions (PDFs). It also indicates the MnO2
lattice framework does not change so much in the process
of thermoannealing. This kind of the PDFs is consistent
with the local structure from the space group P63/mmc,
which is the typical symmetry of MnO2. In this symmetry,
the absorbed Mn is coordinated by six oxygen atoms as
the nearest shell (Mn-O) and then by six manganese at-
oms as the second shell (Mn-Mn). The distinguishing
Besides, the HRTEM images of the treated MnO2
nanosheet as shown in Figure 2a showed two distinct
lattice fringes of 0.244 nm attributed to the (006) and
0.249 nm assigned to the (113) with 60° angles, which is
well consistent with the lattice framework of δ-MnO2.
Meanwhile, two distinct lattice fringes of 0.290 nm at-
tributed to the (020) and 0.276 nm indexed to the (103)
with 90° angles at the bottom of the HRTEM image,
which is well consistent with the lattice framework of
hausmannite Mn3O4 (JCPDS Card No.80-0382), indicating
that Mn(III) site was implanted in 2D MnO2
nanosheet.29,30 X-ray absorption near-edge spectroscopy
(XANES) gave further evidence to probe the structure and
oxidation state of pristine and treated MnO2 nanosheet,
as shown in Figure 2b. Mn K-edge XANES spectra of the
two samples presented the similar features either in the
position or in the intensity, indicating the lattice frame-
work of the MnO2 nanosheet was not sharply changed
after the low-oxygen-pressure thermoannealing treatment.
Figure 2 (a) HRTEM images of treated MnO2 nanosheet. Scale bar is 10 nm. (b) Mn K-edge XANES spectra of pristine and
treated MnO2 nanosheet. (c) Mn K-edge EXAFS oscillation functions k2χ(k) and (d) the corresponding Fourier transforms
(FT), where k=wave vector and χ(k)=oscillation as a function of the photoelectron wavenumber.
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feature of these FT curves is the significant decreasing of Mn pair in the process of low-oxygen-pressure thermoan-
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the Mn-Mn intensity, with the tiny decreasing of the Mn-
O shell. Notably, the intensity decreasing of the Mn-Mn
shell is not associated with this peak broadening. So this
intensity decreasing can be attributed to the reduction of
the coordinator number of the second shell around the
absorbed Mn. In other words, some Mn sites in treated
MnO2 nanosheet are coordinated by Mn metal atoms of
much less than six as the second shell. This local configu-
ration is consistent with that of the Mn octahedral site as
that in the Mn3O4 compound. As is known, Mn3O4 con-
tains two Mn sites. One is the tetrahedral site for Mn(II),
in which Mn is neighbored by four oxygen atoms at about
2.04 Å and eight manganese atoms at about 3.43 Å. The
other is the octahedral site for Mn(III), in which Mn is
neighbored by distorted six oxygen atoms (four at about
1.93 Å and two at about 2.28 Å) and then by totally dis-
torted manganese atoms (two at about 2.88 Å, four at 3.12
Å and four at 3.43 Å). Re-examining the distinguishing
features of FT curve, we can find that the local geometry
of some Mn site in the treated MnO2 nanosheet really
relax into one like the octahedral configuration of Mn3O4.
Therefore, the local configuration relaxation of the cluster
around some Mn ions obtained from EXAFS data is con-
sistent with the result of the XANES spectra, further con-
firming that Mn(III) was successfully incorporated in
treated MnO2 nanosheet. Moreover, EXAFS data were
analyzed quantitatively to extract the exact local struc-
tures. We can get following results from the EXAFS fit, as
shown in Table S1. First, both coordination numbers of
the Mn-O and Mn-Mn of the MnO2 nanosheet is a bit less
than six, which is the typical size effect of nanomaterials.
Second, the coordinator number of the Mn-O of the
treated MnO2 nanosheet reduces slightly but the distance
of the Mn-O remains. Last and most important one is the
significant change of the coordinator number of the Mn-
nealing, from initial 5.3 to final 3.5, associating with the
slight expansion of the Mn-Mn distance. This considera-
ble reduction indicates that a portion of the second
neighbored Mn ions was dissolved into higher shells and
finally a part of the MnO2 clusters were evolved into
Mn(III/IV) oxide domains (Mn(III)-O-Mn(IV) unit) in the
treated sample, showing promising signs to versatile
magnetotransport properties.
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Next, in order to disclose the magnetic properties of the
pristine MnO2 nanosheet and treated MnO2 nanosheet,
we measured the magnetizations curves (M-H) as a func-
tion of the applied magnetic field H (Figure 3a) at 10 K
from -50 to 50 kOe. The hysteresis loops indicate the fer-
romagnetic behavior of treated MnO2 nanosheet along
with the remanence (0.24 emu/g) and saturation magnet-
ization of (1.43 emu/g) at 10 K, different from that of pris-
tine MnO2 nanosheet. The high saturation magnerization
of treated MnO2 nanosheet was usually originated from
the double-exchange mechanism of Mn(III)-O-Mn(IV),
just like the Mn-Zn-O system.33 We also measured the
magnetization curves against temperature (M-T) under
zero-field-cooling (ZFC) and field-cooling (FC) modes for
pristine MnO2 nanosheet and treated MnO2 nanosheet, as
shown in Figure S2 and Figure 3b, respectively. These
measurements show that treated MnO2 nanosheet experi-
ence a ferromagnetism transition at about 50 K, and dis-
play spin-glass behavior under 22 K. On contrary, the
spin-glass state does not display in pristine MnO2
nanosheet. It has been recognized that the spin-glass
state usually emerges from the perovskite-type Mn(III)
and Mn(IV) clusters coexistence with antiferromagnetism
and ferromagnetism interactions.34 Thus, the divergence
Figure 3 (a) Isothermal magnetizations (MH curves) of
treated MnO2 nanosheet at 10 K from -50 to 50 kOe. (b)
Magnetization versus temperature (MT) curves of treat-
ed MnO2 nanosheet at an applied field of 500 Oe. Mag-
netic-field dependence of the MR for (c) pristine MnO2
nanosheet and (d) treated MnO2 nanosheet, revealing
negative magnetoresistance, as well as the latter reveal-
ing larger negative magnetoresistance and MIT at room-
temperature.
Figure 4 Schematic illustration of MR value of (a) pris-
tine and (b) treated MnO2 nanosheet under external
magnetic field of 0.1, 0.5, 1 and 5T near room tempera-
ture.
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between ZFC and FC curves confirm that Mn(III) and vealing that there is no transition founded. The tempera-
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Mn(IV) was coexisted in treated MnO2 nanosheet. Treat-
ed MnO2 nanosheet exhibited spin-glass state with intrin-
sic ferromagnetism phase, arising from double-exchange
of Mn(III)-O-Mn(IV) incorporated in 2D MnO2 structure.
Moreover, the existence of a short-range ferromagnetism
well above TC, which is revealed by the presence of an
downward deviation in the inverse susceptibility (1/χ)
from the paramagnetic Curie-Weiss law up to about 290ꢀK
(Figure 3b) and is proposed to be induced by the ferro-
magnetism double-exchange of Mn(III)-O-Mn(IV) against
the long-range antiferromagnetism superexchange of the
Mn(IV)-O-Mn(IV), further supports the ferromagnetism
double-exchange in the treated 2D MnO2 structure.
ture region is well accordant with that of the deviation in
the inverse susceptibility (1/χ) from the paramagnetic
Curie-Weiss law, indicating that the phenomenon is
closely related with the ferromagnetism double-exchange.
In order to further explore the application possibility of
the MR effect of treated freestanding MnO2 nanosheet
near room-temperature, in-situ Fourier transform infra-
red spectroscopy (FTIR) was used to monitor and verify
the sensitive room-temperature response under magnetic
field in treated MnO2 nanosheet. As shown in Figure 5a
and 5b, the FTIR spectra show absorbing peaks at 1116 and
1636 cm-1 can be indexed to the vibrations of residual hy-
droxyl groups. Meanwhile, the peak at 1401 cm-1 in the
spectrum is assigned to the absorption of TMA+ ions in
the pristine and treated MnO2 nanosheet.35 Notably, the
higher transmittance was observed in treated MnO2
nanosheet with increasing temperature under constant
magnetic field. In contrast, the transmittance has no ob-
vious change in pristine MnO2 nanosheet with various
temperatures. The commercial Fe3O4 also has no obvious
change with various temperatures, as shown in Figure S6.
Specifically, the transmittance displayed obvious change
with the temperature range from 270- 300K, as illustrated
in Figure 5c.36 Thus, it is noteworthy that the IR trans-
mittance is correlated with magnetotransport property
near room-temperature, with ca. 7.1% change at 2700 cm-1,
as highlighted in Figure 5d. Also, the transmittance
change could be reproducible undergoing warming and
cooling process at 1000-2000 cm-1 (Figure S12). Therefore,
the treated MnO2 nanosheet with double-exchange struc-
ture under magnetic field has a potential to become a
smart nanodevice with IR response near room-
temperature.
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The double-exchange of Mn(III)-O-Mn(IV) in treated
MnO2 nanosheet plays a crucial role in magnetic-
transport behavior. In order to further explore the mag-
netic-transport properties, the pristine MnO2 nanosheet
and treated MnO2 nanosheet were conducted by standard
four probe transport measurements in a commercial ap-
paratus (Quantum Design, PPMS). The direction of the
applied magnetic field was perpendicular to the sample
surface, and the dc resistivity was measured from 260 to
380 K under an applied field up to 5T. Figure 3c and 3d
show temperature dependence of the resistivity for the
pristine MnO2 nanosheet and treated MnO2 nanosheet
under different magnetic fields, respectively. Compared
with the smaller negative MR effect of -3.8% (0.1T) and
12.7% (5T) at 287 K for pristine MnO2 nanosheet sample,
the treated MnO2 nanosheet has much larger negative MR
effect of -11.3% (0.1T) and -54.0% (5T) at 287 K, along with
MIT near room-temperature (270-300 K), as highlighted
in Figure 4a and 4b. In contrast, the temperature de-
pendence of resistances of treated MnO2 nanosheet was
conducted in absence of magnetic field (Figure S3), re-
Figure 5 Variable-temperature FTIR spectra of (a) pris-
tine MnO2 nanosheet and (b) treated MnO2 nanosheet
under effective magnetic field. (c) The magnified FTIR
spectra of treated MnO2 nanosheet. (d) Room-
temperature response of FTIR at 2700 cm-1 along with
transmittance change, corresponding large negative
magnetoresistance and MIT of treated MnO2 nanosheet
near room-temperature.
Figure 6 (a) Diagrammatic representation of low-
oxygen-pressure thermoannealing treatment to achieve
double-exchange of Mn(III)-O-Mn(IV) in treated MnO2
nanosheet. (b) and (c) The double-exchange of Mn(III)-
O-Mn(IV) resulted in the magnetoresistance in 2D MnO2
nanosheet.
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the peak splitting of Mn 3s XPS spectra, The bond angle of
Hence, based on the above analysis on the local elec-
1
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3
4
5
6
7
8
Mn-O-Mn double exchange structure in treated MnO2
nanosheet, Reproducible transmittance change of FTIR test
at 1000-2000 cm-1 undergoing warming and cooling process.
This material is available free of charge via the Internet at
http://pubs.acs.org
tronic and geometrical structures as well as the applica-
tion of highly-sensitive response near room-temperature
of treated MnO2 nanosheet, we can deduce that coexist-
ence of Mn(III) and Mn(IV) domains are developed to
double-exchange Mn(III)-O-Mn(IV) structure in treated
MnO2 nanosheet, as illustrated in Figure 6a. The bond
angle of Mn-O-Mn double exchange structure in treated
MnO2 nanosheet is discussed in Figure S11 and Table S3.
Moreover, the transfer of eg electron from Mn(III) to
Mn(IV) in manganites is the basic mechanism of electrical
conduction, governed by double-exchange mechanism
(the simultaneous hopping of eg electron of Mn(III) to the
O p-orbital as well as the electron with same spin from
the O p-orbital to the empty eg orbital of Mn(IV)) leading
to an ferromagnetism state.22,23, as schematically shows in
Figure 6b and 6c. In our case, 2D MnO2 with spatial con-
fined double-exchange interaction enhanced spin-
dependent scattering, and the eg electrons can be signifi-
cantly enhanced in a moderate magnetic field by aligning
the spins at adjacent Mn(III)-O-Mn(IV), thus the conduc-
tivity will increase leading low-field negative magnetore-
sistance. As a result, dimensional confinement of double-
exchange from Mn(III)-O-Mn(IV) in treated MnO2
nanosheet answers for magnetic field induced MIT near
room-temperature with a large MR effect and IR response.
AUTHOR INFORMATION
Corresponding Author
9
czwu@ustc.edu.cn
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Author Contributions
§These authors contributed equally to this work.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was financially supported by the National Basic
Research Program of China (2015CB932302), the National
Natural Science Foundation of China (21501164, U1432133,
11321503, J1030412), National Young Top-Notch Talent Sup-
port Program, the Chinese Academy of Sciences
(XDB01020300), the Fok Ying-Tong Education Foundation,
China (Grant No.141042), and the Fundamental Research
Funds for the Central Universities (WK2060190027,
WK2340000065, WK2310000055), and Anhui Provincial Nat-
ural Science Foundation (1608085QA08). We would like to
thank beamline BL14W1 (Shanghai Synchrotron Radiation
Facility) for providing the beam time.
CONLUSIONS
In summary, we have experimentally reported double-
exchange structure in freestanding 2D nanomaterials for
the first time, successfully bringing metal-insulator transi-
tion near room-temperature. In our case, the chemical
introduction of Mn(III) into layered MnO2 via low-
oxygen-pressure thermoannealing route successfully in-
duces double-exchange of Mn(III)-O-Mn(IV) structure,
triggering novel electron transportation behavior. By di-
mensional confinement of double-exchange structure,
treated 2D MnO2 exhibits an enhanced MR value up to -
11.3% (0.1T) and -54.0% (5T) at 287 K, representing the
highest negative magnetoresistance values in 2D nano-
materials near room-temperature. Moreover, treated
MnO2 nanosheet displays IR response of 7.1% transmit-
tance change from 270 to 290 K. We anticipate that spa-
tial confinement would be powerful tool for triggering
versatile properties and sensitive response near room-
temperature.
REFERENCES
(1) Tan, G.; Zhao, L. D.; Kanatzidis, M. G. Chem. Rev. 2016, 116,
12123.
(2) Wu, C.; Feng, F.; Xie, Y. Chem. Soc. Rev. 2013, 42, 5157.
(3) Jeong, J.; Aetukuri, N.; Graf, T.; Schladt, T. D.; Samant, M. G.;
Parkin, S. S. Science 2013, 339, 1402.
(4) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.; Zhang, H.
Nature. Chem. 2013, 5, 263.
(5) Bisogni, V.; Catalano, S.; Green, R. J.; Gibert, M.; Scherwitzl, R.;
Huang, Y.; Strocov, V. N.; Zubko, P.; Balandeh, S.; Triscone, J.
M.; Sawatzky, G.; Schmitt, T. Nat. Commun. 2016, 7, 13017.
(6) Tokura, Y.; Nagaosa, N. Science 2000, 288, 462.
(7) Radisavljevic, B.; Kis, A. Nat. Mater. 2013, 12, 815.
(8) Imada, M.; Fujimori, A.; Tokura, Y. Rev. Mod. Phys. 1998, 70,
1039.
(9) Wu, C.; Feng, F.; Feng, J.; Dai, J.; Peng, L.; Zhao, J.; Yang, J.; Si,
C.; Wu, Z.; Xie, Y. J. Am. Chem. Soc. 2011, 133, 13798.
(10) Lee, S.; Cheng, C.; Guo, H.; Hippalgaonkar, K.; Wang, K.; Suh, J.;
Liu, K.; Wu, J. J. Am. Chem. Soc. 2013, 135, 4850.
(11) Berglund, C. N.; Guggenheim, H. J. Phys. Rev. 1969, 185, 1022.
(12) Kim, C.; Shin, J. S.; Ozaki, H. J. Phys.: Condens. Matter 2007, 19,
096007.
(13) Tan, C.; Zhang, H. Chem. Soc. Rev. 2015, 44, 2713.
(14) González-Herrero, H.; Gómez-Rodríguez, J. M.; Mallet, P.;
Moaied, M.; Palacios, J.; Salgado, C.; Ugeda, M. M.; Veuillen, J-
Y.; Yndurain, F.; Brihuega, I. Science 2016, 352, 437.
(15) Guo, Y.; Xu, K.; Wu, C.; Zhao, J.; Xie, Y. Chem. Soc. Rev. 2015, 44,
637.
ASSOCIATED CONTENT
Supporting Information
XRD pattern of the bulk pristine MnO2, magnetization
versus temperature (MT) curves of pristine MnO2 nanosheet,
PPMS test in absence of magnetic field, and data processing
of the extend X-ray absorption spectra (EXAFS), FE-SEM and
TEM image of treated MnO2 nanosheet, Variable-
temperature FTIR spectra of pristine Fe3O4 nanoparticle,
The color change in the process of the formation of MnO2
products, The explanation of the method of peak fitting and
three components of Mn 2p XPS spectra, The explanation of
(16) Zhu, X.; Guo, Y.; Cheng, H.; Dai, J.; An, X.; Zhao, J.; Tian, K.;
Wei, S.; Zeng, X. C.; Wu, C.; Xie, Y. Nat. Commun. 2016, 7, 11782.
(17) Cai, L.; He, J.; Liu, Q.; Yao, T.; Chen, L.; Yan, W.; Hu, F.; Jiang,
Y.; Zhao, Y.; Hu, T.; Sun, Z.; Wei, S. J. Am. Chem. Soc. 2015, 137,
2622.
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(18) Mahato, R. N.; Lülf, H.; Siekman, M. H.; Kersten, S. P.; Bobbert,
P. A.; de Jong, M. P.; De Cola, L.; van der Wiel, W. G. Science
2013, 341, 257.
(19) Li, Z.; Guo, Y.; Hu, Z.; Su, J.; Zhao, J.; Wu, J.; Wu, J.; Zhao, Y.;
Wu, C.; Xie, Y. Angew. Chem. Int. Ed. 2016, 55, 8018.
(20) Wang, X.; Du, Y.; Dou, S.; Zhang, C. Phys. Rev. Lett. 2012, 108,
266806.
1
2
3
4
5
(21) Jin, S.; Tiefel, T. H.; McCormack, M.; Fastnacht, R. A.; Ramesh,
R.; Chen, L. H. Science 1994, 264, 413.
(22) Zener, C. Phys. Rev. 1951, 82, 403.
6
7
8
9
(23) Millis, A. J. Nature 1998, 392, 147.
(24) Devaraj, S.; Munichandraiah, N. J. Phys. Chem. C 2008, 112, 4406.
(25) Nesbitt, H. W.; Banerjee, D. American Mineralogist, 1998, 83,
305.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(26) Ramírez, A.; Hillebrand, P.; Stellmach, D.; May, M. M.; Bog-
danoff, P.; Fiechter, S. J. Phys. Chem. C 2014, 118, 14073.
(27) Toupin, M.; Brousse, T.; Bélanger, D. Chem. Mater. 2004, 16,
3184.
(28) Guo, Y.; Tong, Y.; Chen, P.; Xu, K.; Zhao, J.; Lin, Y.; Chu, W.;
Peng, Z.; Wu, C.; Xie, Y. Adv. Mater. 2015, 27, 5989.
(29) Cheng, F.; Zhang, T.; Zhang, Y.; Du, J.; Han, X.; Chen, J. Angew.
Chem. Int. Ed. 2013, 52, 2474.
(30) Zhang, T.; Cheng, F.; Du, J.; Hu, Y.; Chen, J. Adv. Energy Mater.
2015, 5, 1400654.
(31) Yano, J.; Kern, J.; Sauer, K.; Latimer, M. J.; Pushkar, Y.; Biesiadka,
J.; Loll, B.; Saenger, W.; Messinger, J.; Zouni, A.; Yachandra, V.
K. Science 2006, 314, 821.
(32) Hocking, R. K.; Brimblecombe, R.; Chang, L. Y.; Singh, A.;
Cheah, M. H.; Glover, C.; Casey, W. H.; Spiccia, L. Nature Chem.
2011, 3, 461.
(33) García, M. A.; Ruiz-González, M. L.; Quesada, A.; Costa-Krämer,
J. L.; Fernández, J. F.; Khatib, S. J.; Wennberg, A.; Caballero, A.
C.; Martín-González, M. S.; Villegas, M.; Briones, F.; González-
Calbet, J. M.; Hernando, A. Phys. Rev. Lett. 2005, 94, 217206.
(34) Hauser, J. J.; Waszczak, J. V. Phys. Rev. B. 1984, 30, 5167.
(35) Yang, X.; Makita, Y.; Liu, Z.; Sakane, K.; Ooi, K. Chem. Mater.
2004, 16, 5581.
(36) Yao, T.; Liu, L.; Xiao, C.; Zhang, X.; Liu, Q.; Wei, S.; Xie, Y.
Angew. Chem. Int. Ed. 2013, 52, 7554.
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