Double-Exchange Effect in Two-Dimensional MnO2 Nanomaterials

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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 J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 17 Mar 2017 Downloaded from http://pubs.acs.org on March 17, 2017

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Journal of the American Chemical Society

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† † 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; roomtemperature 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-roomtemperature electronic transition with an ultrahigh negative magnetoresistance. Double-exchange in 2D MnO2 nanomaterials 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 Electronic transition especially for metal-insulator transition (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 dichalcogenides (TMDc), nickelate, transition-metal oxides (TMO) and so on.4-6 For practical applications, an ideal MIT material should have possessed electronic transition near room-temperature, which is beneficial for the development of nano-device with highly sensitive behavior.7,8 Among binary oxides, vanadium dioxides (VO2) is a prototype MIT material with the slight distortion of the V-V linear chains to the zigzag pattern across phase transition.9,10 However, phase-transition temperature of VO2 is about 340K, it still has a significant space to reach roomtemperature.11 Although doping exotic metal ions offer effective way to control transition temperature down to approaching room-temperature,12 the intensity of transition 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.

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 onedimensional (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 system for magnetic field induced MIT effect based on double-exchange mechanism, and also possessed advantages of enormous chemical and structural flexibility and stability.21 In such system, double-exchange mechanism requires unique electron configuration with the magnetic ordering where neighboring Mn sites mutually aligned spins.22 Inspired by these concepts, there has been a demanding challenge in realizing low-dimensional doubleexchange structure, opening a new window of opportunity for the development of novel magnetic field induced MIT near room-temperature. However, crystal structural characteristics of traditional perovskite manganites, usually with possessing highly-symmetric cubic structure with the strong covalent bonds among them, hinders available attempts to access their freestanding lowdimensional structure by either top-down or bottom up strategies; and up to date there are no report on doubleexchange structure in freestanding low-dimensional structure.23 Therefore, double-exchange structure with

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freestanding confined dimensionality is highly desirable but remains a challenge for large MIT and sensitive response near room-temperature. Herein, we demonstrate experimental case of doubleexchange structure in 2D nanomaterials by a mild chemical 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 magnetoresistance values in 2D nanomaterials approaching roomtemperature. Also, treated MnO2 nanosheet displays IR response of 7.1% transmittance change from 270 to 290 K. We anticipate that dimensional confinement of doubleexchange structure promises novel magnetotransport properties and sensitive response for smart devices near room-temperature. EXPERIMENTAL SECTION Materials. Manganese chloride (MnCl2•4H2O), ammonium 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 purification. Synthesis of bulk 2D MnO2. In a typical experiment, 3 mmol MnCl2•4H2O was added into 10 mL H2O. After vigorous stirring for 20 min, the MnCl2•4H2O was totally dissolved becoming a homogenous solution, called “solution 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 vigorously 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. Synthesis of pristine MnO2 nanosheet. The pristine MnO2 nanosheet was obtained from adding bulk 2D MnO2 nanosheet into water for ultrasonic treatment under constant temperature at 25°C. After ultrasonic treatment 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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thus a dark-brown high-quality dispersion of MnO2 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. 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 further 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 (N2annealing) play a vital role of successfully realizing the surface-structural modulation of pristine MnO2 nanosheet as well as the preservation of 2D MnO2 framework. 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 nanomaterials. Characterizations. X-ray diffraction (XRD) was performed 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 MultiMode V system. X-ray photoelectron spectra (XPS) were acquired on an ESCALAB MKII with Mg Kα (Hν = 1253.6 eV) as the excitation source. X-ray absorption spectroscopy (XAS) measurements. 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. 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).


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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 infrared spectra (FTIR) Monitoring. The Infrared (IR) transmission spectra were recorded on a Fouriertransform 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. RESULTS AND DISCUSSION Double-exchange Mn(III)-O-Mn(IV) structure was successfully produced by a low-oxygen-pressure thermoannealing 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 elemental Mn ion forming double-exchange Mn(III)-OMn(IV) in 2D nanomaterials. Systematically characterizations were performed to investigate structural information of both pristine and treated MnO2 nanosheet. Xray 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 hexagonal 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 ultrathin nanosheet of treated MnO2; while the only exception 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 atomicforce microscopy (AFM) image in Figure 1b showed the flat morphology with the size of ca. 2 µm of the treated

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 obviously structural collapse (Figure S4 and S5). In order to explore 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

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 pristine 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 increase of the Mn(III) in treated MnO2 nanosheet.26 Moreover, Mn 3s XPS spectra analysis indicates the valence 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 during 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 energy separation of the Mn 3s peaks and oxidation state of manganese in the oxide has linear relationship, the corresponding average Mn valences were calculated to be approximately 3.84 and 3.48.28 Therefore, XPS measurements showed that Mn(IV) oxidation in treated MnO2 nanosheet was partially reduced into Mn(III/IV) species. 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 attributed 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 framework of the MnO2 nanosheet was not sharply changed after the low-oxygen-pressure thermoannealing treatment.

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Moreover, the obvious energy shift at the rising edge of 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 variation 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 confirm that treated MnO2 nanosheet keeps well the 2D MnO2 lattice framework with the partially incorporation of Mn(III), holding promise for the producing of doubleexchange structure. 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 induced 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 highly 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 oscillations 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 atoms as the second shell (Mn-Mn). The distinguishing

Figure 2 (a) HRTEM images of treated MnO2 nanosheet. Scale bar is 10 nm. (b) Mn K-edge XANES spectra of pristine and 2 treated MnO2 nanosheet. (c) Mn K-edge EXAFS oscillation functions k χ(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 the Mn-Mn intensity, with the tiny decreasing of the MnO 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 configuration is consistent with that of the Mn octahedral site as that in the Mn3O4 compound. As is known, Mn3O4 contains 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 distorted 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 consistent with the result of the XANES spectra, further confirming that Mn(III) was successfully incorporated in treated MnO2 nanosheet. Moreover, EXAFS data were analyzed quantitatively to extract the exact local structures. 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-

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 treated MnO2 nanosheet at an applied field of 500 Oe. Magnetic-field dependence of the MR for (c) pristine MnO2 nanosheet and (d) treated MnO2 nanosheet, revealing negative magnetoresistance, as well as the latter revealing larger negative magnetoresistance and MIT at roomtemperature.

Mn pair in the process of low-oxygen-pressure thermoannealing, from initial 5.3 to final 3.5, associating with the slight expansion of the Mn-Mn distance. This considerable 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. 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 function of the applied magnetic field H (Figure 3a) at 10 K from -50 to 50 kOe. The hysteresis loops indicate the ferromagnetic behavior of treated MnO2 nanosheet along with the remanence (0.24 emu/g) and saturation magnetization of (1.43 emu/g) at 10 K, different from that of pristine 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 experience a ferromagnetism transition at about 50 K, and display 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 4 Schematic illustration of MR value of (a) pristine and (b) treated MnO2 nanosheet under external magnetic field of 0.1, 0.5, 1 and 5T near room temperature.



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between ZFC and FC curves confirm that Mn(III) and Mn(IV) was coexisted in treated MnO2 nanosheet. Treated MnO2 nanosheet exhibited spin-glass state with intrinsic 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 290 K (Figure 3b) and is proposed to be induced by the ferromagnetism 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. The double-exchange of Mn(III)-O-Mn(IV) in treated MnO2 nanosheet plays a crucial role in magnetictransport behavior. In order to further explore the magnetic-transport properties, the pristine MnO2 nanosheet and treated MnO2 nanosheet were conducted by standard four probe transport measurements in a commercial apparatus (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 dependence of resistances of treated MnO2 nanosheet was conducted in absence of magnetic field (Figure S3), re-

Figure 5 Variable-temperature FTIR spectra of (a) pristine MnO2 nanosheet and (b) treated MnO2 nanosheet under effective magnetic field. (c) The magnified FTIR spectra of treated MnO2 nanosheet. (d) Room-1 temperature response of FTIR at 2700 cm along with transmittance change, corresponding large negative magnetoresistance and MIT of treated MnO2 nanosheet near room-temperature.

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vealing that there is no transition founded. The temperature 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 infrared 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 hydroxyl 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 obvious 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 transmittance 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 structure under magnetic field has a potential to become a smart nanodevice with IR response near roomtemperature.

Figure 6 (a) Diagrammatic representation of lowoxygen-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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Hence, based on the above analysis on the local electronic and geometrical structures as well as the application of highly-sensitive response near room-temperature of treated MnO2 nanosheet, we can deduce that coexistence 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 confined double-exchange interaction enhanced spindependent scattering, and the eg electrons can be significantly enhanced in a moderate magnetic field by aligning the spins at adjacent Mn(III)-O-Mn(IV), thus the conductivity will increase leading low-field negative magnetoresistance. As a result, dimensional confinement of doubleexchange 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. CONLUSIONS In summary, we have experimentally reported doubleexchange structure in freestanding 2D nanomaterials for the first time, successfully bringing metal-insulator transition near room-temperature. In our case, the chemical introduction of Mn(III) into layered MnO2 via lowoxygen-pressure thermoannealing route successfully induces double-exchange of Mn(III)-O-Mn(IV) structure, triggering novel electron transportation behavior. By dimensional 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 nanomaterials near room-temperature. Moreover, treated MnO2 nanosheet displays IR response of 7.1% transmittance change from 270 to 290 K. We anticipate that spatial confinement would be powerful tool for triggering versatile properties and sensitive response near roomtemperature.

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, Variabletemperature 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

the peak splitting of Mn 3s XPS spectra, The bond angle of Mn-O-Mn double exchange structure in treated MnO2 nanosheet, Reproducible transmittance change of FTIR test -1 at 1000-2000 cm undergoing warming and cooling process. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author [email protected]

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 Support 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 Natural Science Foundation (1608085QA08). We would like to thank beamline BL14W1 (Shanghai Synchrotron Radiation Facility) for providing the beam time.

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Double-Exchange Effect in Two-Dimensional MnO2 Nanomaterials

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