A Study on the Weak Ferromagnetism of Nanocrystalline Stannous

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A Study on the Weak Ferromagnetism of Nanocrystalline Stannous Oxide Induced by L-Shaped O-Sn-O Vacancies Yadong Lian, Xiaokun Huang, and Min Gu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10562 • Publication Date (Web): 12 Dec 2018 Downloaded from http://pubs.acs.org on December 13, 2018

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The Journal of Physical Chemistry

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A Study on the Weak Ferromagnetism of Nanocrystalline

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Stannous Oxide Induced by L-Shaped O-Sn-O Vacancies

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Yadong Lian1, Xiaokun Huang2, and Min Gu1,*

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National Laboratory of Solid State Microstructures and Department of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, P.R. China 2

School of Materials Science and Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333001, P.R. China

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Abstract

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Nanocrystalline stannous oxide (SnO) powder has been prepared by ball-milling of

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micron-SnO in a nitrogen atmosphere. A cation deficiency of 6.5% was found in the

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nano-SnO sample by X-ray photoelectron spectroscopy. After annealing at 350, 450, or

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550℃ under dynamic vacuum conditions, oxygen vacancies were found in the nano-

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SnO samples, as verified by electron paramagnetic resonance experiments. The samples

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displayed weak ferromagnetism; that with both 6.5% Sn and at least 7.5% O vacancies

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showed the maximum saturation magnetization, reaching almost 1.7×10−3 emu/g at

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room temperature. The O vacancies tended to gather around Sn vacancies. An L-shaped

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defect model composed of one Sn and two O vacancies has been established, and its

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ferromagnetism has been calculated by a first-principles method. We found that the L-

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shaped defect induced a magnetic moment of 0.55 μB in the SnO lattice, the main

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contribution to which came from a spin-polarized electron in the p orbital of a corner

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Sn atom.

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*Corresponding author: Tel.: +86 25 83593508; Fax: +86 25 83595535; e-mail:

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[email protected]. 1 ACS Paragon Plus Environment

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1. INTRODUCTION

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Stannous oxide (SnO) is a polymorph of the tin oxides, which is metastable at room

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temperature, but unstable above 573 K, undergoing a disproportionation reaction to

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metallic -Sn, Sn3O4, and SnO21,2. SnO has recently attracted much attention and is

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finding ever more applications. It can serve as an anode material in rechargeable

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lithium-ion batteries3–5, gas sensors6, and catalysts7, and is also useful for broadband

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electromagnetic interference shielding8 and hydrogen storage9. It has also been found

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to be a promising p-type semiconductor with high mobility, making it a candidate for

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applications in flexible circuits10 and thin-film transistors11.

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SnO has a tetragonal unit cell with space group P4/nmm and lattice constants of a

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= b = 3.802 Å and c = 4.836 Å. It is isostructural with α-PbO at normal pressures, and

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is constructed from layered pyramids with one Sn atom at the apex and a square base

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of four O atoms. The lone-pair electrons spread into the interlayer spatial region, and

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this is thought to be responsible for the characteristic physical properties12. The indirect

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band gap has been found to be 0.7 eV11 and the direct optical band gap varies from 2.7

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to 3.4 eV11,13,14. However, the O/Sn ratio of SnO has been found to be off-stoichiometric.

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A cation deficiency has been detected by Moreno et al.15 and Pan et al.16 in single

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crystals, powder, and epitaxial thin films, with a deficit of up to 10% Sn. The formation

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energies of a tin vacancy (VSn) are below 3.1 and 2 eV at the Sn- and O-rich limits,

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respectively12,17.

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Another familiar polymorph of the tin oxides, stannic oxide (SnO2), a thermally

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stable and n-type semiconductor, has been extensively studied. Due to its potential 2 ACS Paragon Plus Environment

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application in spintronic devices, the ferromagnetism of SnO2 has been investigated in

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both its undoped form and doped by transition metals18–20. Obvious ferromagnetism has

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been observed at room temperature for both pristine SnO2 films and nanocomposites,

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which is believed to stem from surface and lattice defects as well as oxygen vacancies21–

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25

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ferromagnetism in SnO226,27. Recently, the ferromagnetism of stannous oxide has also

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become of concern. SnO powder samples doped with Mn, Co, Ni, and other 3d

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transition metals28–32, and with a hole-doped SnO monolayer17, have been studied both

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experimentally and theoretically. To the best of our knowledge, however, there have not

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. Theoretical calculations further suggest that neutral cation vacancies induce

been any reports on the ferromagnetism of undoped SnO nanoparticles.

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In the present work, we fabricated SnO nanoparticles by a mechanochemical

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method. We have carefully avoided the influence of iron impurities, keeping them at a

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level below 10 ppm. A small amount of tin proved to be missing from the prepared

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samples, making them off-stoichiometric. Some oxygen was then removed by a further

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annealing treatment in an oxygen-free atmosphere, producing a small amount of oxygen

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vacancies. Obvious ferromagnetism was observed for these samples at room

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temperature. Density functional theory (DFT) calculations showed that the

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ferromagnetism is mainly induced by L-shaped defect O-Sn-O vacancies in the SnO

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lattice.

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2. MATERIALS AND METHODS

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2.1.Sample Preparation 3 ACS Paragon Plus Environment


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Micron-size SnO powder with a purity of 99.9% was purchased from Aladdin

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(Shanghai, China) and was used as the precursor of nano-SnO without further

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purification. Nano-SnO powder samples were produced by ball-milling of the micron-

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size SnO for 0.5 h at 25℃ in an agate jar filled with N2, using a ball-to-powder weight

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ratio of 10:1. Thermal processing of the samples was conducted in a tube furnace for

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4 h at either 350, 450, or 550℃ under a dynamic vacuum of ca. 10 Pa. The purchased

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micron-SnO was designated as m-SnO; the nano-SnO prepared at 25℃ was designated

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as S25; and the nano-SnO samples annealed at 350, 450, and 550℃ were designated as

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S350, S450, and S550, respectively.

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2.2.Characterization

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Crystal structures were investigated by X-ray diffractometry (XRD) with a Bruker

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D8 diffractometer operated with a Cu-K1 (1.5406 Å) source. X-ray photoelectron

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spectrometry (XPS) analyses were performed on samples in a PHI Versa II microprobe,

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employing a raster-scanned micro-focused Al-Kα X-ray (1486.7 eV) beam, and the

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results were analyzed with XPSPEAK41 software. Electron paramagnetic resonance

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(EPR) spectroscopy with a Bruker EMX-10/12 spectrometer was applied to identify

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oxygen vacancies in our samples. A Quantum Design (MPMS-XL-7) superconducting

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quantum interference device (SQUID) magnetometer was used to characterize the

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ferromagnetism of the samples. Trace 3d metals were detected by inductively coupled

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plasma optical emission spectrometry (ICP-OES).

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2.3.Theoretical Calculations

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The ferromagnetism induced by defects in the SnO lattice was investigated 4 ACS Paragon Plus Environment

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theoretically through a plane-wave pseudopotential DFT approach, as implemented in

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the Vienna ab initio simulation package33,34. The projector-augmented wave potentials

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explicitly included the 14 valence electrons for Sn (4d10, 5s2, 5p2) and 6 for O (2s2, 2p4).

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Their wavefunctions were expanded on a plane-wave basis with an energy cut-off of

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600 eV. The energy convergence criterion was set at 10−6 eV for electronic structure

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calculations. Geometry optimization was stopped once a convergence criterion for the

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forces on the atoms of less than 0.01 eV/Å was reached. Spin polarization calculations

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utilized the generalized gradient approximation with Perdew-Burke-Ernzerhof

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parametrization for the xc functional35. Gamma-centered 12 × 12 × 8 and 4 × 4 × 4

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Monkhorst-Pack k-point meshes were used for the SnO unit cell (4 atoms) and 3 × 3 ×

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2 supercell, respectively36. Total energy was calculated using the linear tetrahedron

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method with Bloch corrections37. Since SnO is a two-dimensional layered material with

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layers maintained by van der Waals forces, we used the DFT-D3 correction method

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suggested by Grimme to describe the van der Waals interactions38. For an SnO unit cell

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containing four atoms, we obtained optimized crystal constants of a = b = 3.848 Å and

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c = 4.818 Å, in accordance with the experimental results (a = b = 3.802 Å, c = 4.836 Å)

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and previous reports30,39.

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3.

RESULTS

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Sample S25 was produced by ball-milling of m-SnO for 0.5 h under an N2

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atmosphere. The obtained S25 was then annealed at elevated temperatures of 350, 450,

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or 550℃ under dynamic vacuum conditions, and the XRD patterns of the respective 5 ACS Paragon Plus Environment


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products (Fig. 1) revealed a process of disproportionation: SnO → SnO2 + Sn3O4 + -

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Sn1,2. At 450℃, only a very weak (110) peak of the rutile structure was observed, which

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resulted from the minor phase of SnO2. At 550℃, the contents of the SnO2, Sn3O4, and

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-Sn phases increased markedly. The average particle size of the nano-SnO powder

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prepared at 25℃ was 20 nm, calculated by the Scherrer equation. After further

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annealing for 4 h at 350 or 450℃, the average sizes were 27 and 34 nm, respectively.

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Figure 1. XRD patterns of S25, S350, S450, and S550.

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The stoichiometries of the samples were determined by XPS, and the results are

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shown in Fig. 2. Binding energies were calibrated with reference to the C1s peak at

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284.6 eV. Survey scans from 0 to 900 eV showed only Sn, O, and C in the samples

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(Fig. 2b). High-resolution Sn3d scans (Fig. 2a) showed a doublet with a spin-orbit

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splitting of 8.4 eV, corresponding to Sn3d3/2 at 494.6 eV and Sn3d5/2 at 486.2 eV40. The

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symmetric peak of Sn3d5/2 at 486.2 eV could be ascribed to Sn2+16, and no sign of Sn of

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other valence was detected except for S550. All three O1s peaks at around 530 eV 6 ACS Paragon Plus Environment

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appeared slightly asymmetrical in the scans (Fig. 2a), and each could be deconvoluted

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into two components at 530 and 531.7 eV. The O1s peak at 530 eV was ascribed to

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lattice O2−, and that at 531.7 eV to adsorbed O species41. The O/Sn molar ratio was

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calculated from the peak areas of lattice O2− and Sn2+. The O/Sn ratios of m-SnO and

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S25 were evaluated as 1.01 and 1.07, respectively, indicating that a deficiency of Sn

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cations or intrinsic Sn vacancies appeared in the SnO lattice. After annealing at 350 or

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450℃ under dynamic vacuum conditions, the O/Sn molar ratio decreased to 1.00 and

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0.99, indicating a deficit of oxygen in the latter case; in other words, oxygen vacancies

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were generated in the SnO lattice. For S550, the existence of -Sn with an Sn3d5/2

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binding energy of 485 eV and an increased O/Sn molar ratio of 1.25 indicated that a

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disproportionation reaction had occurred in this sample.

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Figure 2. XPS patterns: high-resolution O1s and Sn3d scans (a) and survey scans (b)

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for m-SnO, S25, S350, S450, and S550.

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The samples m-SnO, S25, S350, S450, and S550 were subjected to EPR analysis 7 ACS Paragon Plus Environment


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(Fig. 3). The signal centered at g = 2.003 is attributed to localized electrons around

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oxygen vacancies40, and so the signal intensity is proportional to the number of oxygen

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vacancies. S450 clearly had the largest number of oxygen vacancies, followed by S550

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and then S350. Almost no signals were detected for the other two samples.

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Figure 3. EPR results for samples m-SnO, S25, S350, S450, and S550.

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Figure 4 shows plots of the magnetic inductions of m-SnO, S25, S350, S450, and

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S550 vs. external magnetic field at room temperature, with the induction corrected by

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subtraction of the contribution from the diamagnetic sample holder. Samples m-SnO

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and S25 showed negligibly small hysteresis loops. After annealing S25 at 350, 450, or

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550℃, small but distinct hysteresis loops indicated weak ferromagnetism. The sample

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annealed at 450℃ showed the strongest saturation magnetization of 1.7 × 10−3 emu/g

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with a coercivity field of 60 Oe, followed by that annealed at 350℃, with a saturation

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magnetization of 0.22 × 10−3 emu/g and a coercivity field of 110 Oe; the weakest effect

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was seen for the sample annealed at 550℃, with corresponding values of 0.13 ×

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10−3 emu/g and 140 Oe. To further confirm the weak ferromagnetism observed in the 8 ACS Paragon Plus Environment

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samples, other samples were prepared by ball-milling for 2 or 8 h and annealed at

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various temperatures. Their hysteresis loops were observed by SQUID magnetometry,

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as shown in Fig. S1 (Supporting Information).

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The contents of the trace 3d metals Mn, Fe, Co, and Ni were below 10 ppm in our

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samples, as measured by ICP-OES, sufficiently low not to affect the observed induction

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values.

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Figure 4. Magnetic hysteresis loops for samples m-SnO, S25, S350, S450, and S550.

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In order to calculate ferromagnetism in the nano-SnO samples annealed at elevated

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temperatures, we considered a supercell containing 3 × 3 × 2 unit cells and removed

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atoms to simulate the defect model. We designed three kinds of defects: (1) removal of

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one oxygen atom to form four three-coordinated Sn centers (Fig. 5a); (2) removal of

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two nearest-neighbor O atoms to form four three-coordinated Sn and two two-

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coordinated Sn centers (Fig. 5b); and (3) removal of an Sn atom (Fig. 5c). After

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removing the atoms to form the defect structures, we further optimized the atomic 9 ACS Paragon Plus Environment


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positions until all atoms reached the force convergence criterion. However, these

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defects had no magnetic moment. We further considered that an O vacancy was

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generated near an Sn vacancy. The configuration shown in Fig. 5d is that with the lowest

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energy calculated for a series of Sn-O vacancy pairs, which verifies that the O vacancy

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tends to appear near the Sn vacancy. Secondly, we studied the presence of two O

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vacancies near the Sn vacancy. We removed a second O atom starting from the

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configuration shown in Fig. 5d. There are two configurations: one is an L-shaped O-

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Sn-O defect, as shown in Fig. 5e, while the other is a linear O-Sn-O defect, as shown

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in Fig. 5f. The results showed that only an L-shaped defect with lower energy can

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provide a magnetic moment of 0.55 μB.

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Figure 5. Diagrams of defect models.

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We then proceeded to analyze the electronic structure of SnO with L-shaped defects.

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As shown in Fig. 6a, the electronic density of states (DOS) after introduction of the L-

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shaped defect was characterized by electron doping at the bottom of the conduction

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band. The DOS at the bottom of the conduction band is mainly derived from the Sn,

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and this part of the DOS is spin polarized, which indicates that the magnetic moment 10 ACS Paragon Plus Environment

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induced by the defect is mainly derived from the contribution of the Sn. In order to

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investigate the distribution of magnetic moments, we generated a three-dimensional

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spatial distribution map of spin density (Fig. 6b). The distribution of 0.55 μB magnetic

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moments in space can be intuitively discerned. The magnetic moments are mainly

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derived from two-coordinated Sn atoms at the corners of L-shaped defects. The spindle-

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shaped electron cloud pointing to the L-shaped defect indicates that the spin-polarized

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electron cloud belongs to the p orbital.

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Figure 6. Spin-polarized density of states of a (3 × 3 × 2) supercell containing an L-

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shaped defect (a), and distribution map of spin density in three dimensions (b).

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Different colors denote majority and minority spins.

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We further expanded the cell to calculate the magnetic exchange interaction

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between the magnetic moments of defects. We designed two types of cell expansion.

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Fig. 7a shows the first structure of intraplanar chain-like antiferromagnetism, which is

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based on a supercell of 3 × 6 × 2, that is, in a plane; the defect magnetic moment forms

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a chain of ferromagnetic alignment, and the chain is antiparallel. The energy of this 11 ACS Paragon Plus Environment


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structure is higher than that with ferromagnetic ordering by 6 meV. The other (Fig. 7b)

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shows interlaminar antiferromagnetism, based on a supercell of 3 × 3 × 4, meaning that

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all defect magnetic moments are arranged parallel in one plane. The magnetic moment

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in one plane conforms to antiferromagnetic coupling, with another magnetic plane

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crossing a nonmagnetic SnO plane. The energy of this antiferromagnetic structure is

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4 meV higher than that of the ferromagnetic structure. The results show that the

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ferromagnetic structure is the ground state of the whole system.

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Figure 7. Expanded supercell and two types of antiferromagnetic ordering: (a) intraplanar and (b) interlaminar antiferromagnetism. The opposite spin is marked with different color.

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4.

DISCUSSION

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Sample S25 has a tetrahedral structure (Fig. 1), and its O/Sn ratio is off-

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stoichiometric due to a cation deficiency. XPS measurements showed that the tin

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content in the lattice corresponded to a deficit of 6.5% relative to the stoichiometric

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ratio (Fig. 2). We also found a slight decrease in the spacing of the (001) face (Fig. 1),

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which was consistent with the reduced amount of tin. No evidence for oxygen vacancies 12 ACS Paragon Plus Environment

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in S25 was detected by EPR (Fig. 3). After annealing for 4 h at 450℃ under dynamic

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vacuum conditions, some oxygen atoms in the sample were driven off, mainly those

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near the surface of the nanoparticles. At the same time, a small amount of tin was likely

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to be removed. The O/Sn ratio decreased from 1.07 (S25) to 0.99 (S450), indicating

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that more oxygen atoms became missing than tin atoms. At this point, there were more

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oxygen vacancies in the sample than tin vacancies, and a strong signal of the O

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vacancies in S450 was observed by EPR (Fig. 3). Assuming that the number of Sn

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vacancies remained unchanged after annealing, at least 7.5% O vacancies could be

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produced, as calculated from the XPS data. DFT calculations showed that oxygen

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vacancies tend to gather around Sn vacancies to reduce energy. Therefore, L-shaped

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defects, with twice as many oxygen vacancies as tin vacancies, could be found in the

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sample, giving rise to the magnetic moment of 0.55 μB.

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The introduction of L-shaped defects will lead to a readjustment of the electronic

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distribution. The DOS was characterized by electron doping at the bottom of the

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conduction band. Based on the calculated DOS, we found intact SnO to be an insulator

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but that electron doping provided by the defects imparts SnO with a semi-metallic band.

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This would improve the electrical conductivity of the samples, as has been verified by

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impedance spectroscopy experiments (Fig. S2).

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In our designed structure of magnetic order, the distance between nearest-neighbor

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defects, whether in the in-plane direction (ab plane) or in the vertical inter-plane (c

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direction), is about 1 nm. Even at such a long distance, the energy difference between

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the parallel and anti-parallel arrangements of the magnetic moment can be maintained 13 ACS Paragon Plus Environment


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at several meV per defect. If the density of defects was increased and the distance

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between magnetic moments was decreased, the ferromagnetic coupling strength would

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increase accordingly, and thus the ferromagnetic ordering can be maintained at above

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room temperature.

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After annealing at 550℃, the magnetic induction of sample S550 was weakened

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(Fig. 4). The additional phases of SnO2, -Sn, and Sn3O4 evidently appeared in S550

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due to the disproportionation reaction (Fig. 1). We propose that these additional phases

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weaken the ferromagnetism of the sample (Fig. 4).

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5.

CONCLUSION

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In summary, nanocrystalline SnO samples have been prepared by ball-milling of

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micron-SnO under nitrogen protection for 0.5, 2, or 8 h at 25℃. A cation deficiency

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was found in the nano-SnO samples, as detected by XPS. These off-stoichiometric

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samples were further annealed at either 350, 450, or 550℃ under dynamic vacuum

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conditions. Some oxygen atoms were thereby removed and vacancies were left in the

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lattice, as verified by EPR. These gave rise to weak ferromagnetism, and sample S450

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with both 6.5% Sn and at least 7.5% O vacancies showed the maximum saturation

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magnetization, reaching almost 1.7 × 10−3 emu/g at room temperature. A first-

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principles method was used to calculate the ferromagnetism of the samples. Once there

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is a tin vacancy in the lattice, oxygen vacancies will gather around it. An L-shaped

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defect model was established, comprising one Sn vacancy and two O vacancies, which

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imparted a magnetic moment of 0.55 μB. DFT calculations revealed that the magnetic 14 ACS Paragon Plus Environment

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moments are mainly provided by spin-polarized electrons in the p orbital of a two-

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coordinated Sn atom at the corner of each L-shaped defect.

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Supporting Information

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Magnetic hysteresis loops of nano-SnO powder samples produced by ball-

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milling of micron-size SnO for 2 and 8 h, respectively, (a) and (b), and then after

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annealing at 350, 450, or 550℃ (S1). Conductivity calculated from the impedance

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spectrum of the nano-SnO powder produced by ball-milling of micron-size SnO for

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0.5 h (S2).

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ACKNOWLEDGMENTS

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The National Basic Research Program of China provided funding through Grant

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no. 2016YFA0201604. Part of the numerical calculations was carried out in the High

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Performance Computing Center (HPCC) of Nanjing University. We thank Prof. Weiyi

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Zhang for his helpful discussions.

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TOC Graphic

2 3



Figure 1. XRD patterns of S25, S350, S450, and S550. 79x64mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Figure 2. XPS patterns: high-resolution O1s and Sn3d scans (a) and survey scans (b) for m-SnO, S25, S350, S450, and S550. 159x69mm (300 x 300 DPI)



Figure 3. EPR results for samples m-SnO, S25, S350, S450, and S550. 79x56mm (300 x 300 DPI)


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Figure 4. Magnetic hysteresis loops for samples m-SnO, S25, S350, S450, and S550. 79x62mm (300 x 300 DPI)



Figure 5. Diagrams of defect models. 79x48mm (300 x 300 DPI)


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Figure 6. Spin-polarized density of states of a (3 × 3 × 2) supercell containing an L-shaped defect (a), and distribution map of spin density in three dimensions (b). Different colors denote majority and minority spins. 159x63mm (300 x 300 DPI)



Figure 7. Expanded supercell and two types of antiferromagnetic ordering: (a) intraplanar and (b) interlaminar antiferromagnetism. The opposite spin is marked with different color. 79x57mm (300 x 300 DPI)


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A Study on the Weak Ferromagnetism of Nanocrystalline Stannous

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