Crystal Facet Effects on Nanomagnetism of Co3O4

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 19235−19247

Crystal Facet Effects on Nanomagnetism of Co3O4 Wenxian Li,*,†,∥,⊥ Yan Wang,† Xiang Yuan Cui,*,‡ Shangjia Yu,† Ying Li,*,†,∥ Yemin Hu,† Mingyuan Zhu,† Rongkun Zheng,§ and Simon P. Ringer‡

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†

Institute of Materials, School of Materials Science and Engineering, Shanghai University, 149 Yanchang Road, Shanghai 200072, China ‡ Australian Centre for Microscopy and Microanalysis, and School of Aerospace, Mechanical and Mechatronic Engineering, and § School of Physics, The University of Sydney, Sydney, New South Wales 2006, Australia ∥ Institute for Sustainable Energy, Shanghai University, Shanghai 200444, China ⊥ Shanghai Key Laboratory of High Temperature Superconductors, Shanghai 200444, China S Supporting Information *

ABSTRACT: The magnetic performance of nanomaterials depends on size, shape, and surface of the nanocrystals. Here, the exposed crystal planes of Co3O4 nanocrystals were analyzed to research the dependence of magnetic properties on the configuration environment of the ions exposed on different surfaces. The Co3O4 nanocrystals with exposed (1 0 0), (1 1 0), (1 1 1), and (1 1 2) planes were synthesized using a hydrothermal method in the shapes of nanocube, nanorod, hexagonal nanoplatelet, and nanolaminar, respectively. Ferromagnetic performance was detected in the (1 0 0) and (1 1 1) plane-exposed samples. First-principles calculation results indicate that unlike the nonmagnetic nature in the bulk, the Co3+ ions exposed on or close to the surface possess sizable magnetic moments because of the variation of coordination numbers and lattice distortion, which is responsible for the ferromagnetic-like behavior. The (1 1 0)exposed sample keeps the natural antiferromagnetic behavior of bulk Co3O4 because either the surface Co3+ ions have no magnetic moments or their moments are in antiferromagnetic coupling. The (1 1 2)-exposed sample also displays antiferromagnetism because the interaction distances between the magnetized Co3+ ions are too long to form effective ferromagnetic coupling. KEYWORDS: crystal facet effect, nanomagnetism, configuration environment, exposed crystal plane, density functional theory calculations

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INTRODUCTION

on the generation of exchange-bias performance in CoO/Al− ZnO multilayer structure, 15 La 0.7Sr0.3 MnO3 /YMnO3 bilayers, 7,8,16 FeO/CoFe2O4 nanocrystals,17 and Co/CoO core−shell nanowires.6 According to the heterogeneous effects, the SiO2 coating layer can modify the performance of SiO2coated Fe3O4,18 and polystyrene-coated cobalt oxide nanowires show obvious ferromagnetic properties compared to uncoated cobalt oxide nanowires, which are antiferromagnetic.19 For the pristine compounds, the intrinsic magnetisms vary with the nanocrystal geometrical parameter inducing finite-size effect, shape effect, surface effect, and the interparticle interaction effect. The particle size can change the magnetic behaviors of nanocrystals in comparison with their corresponding bulk counterpart. Size dependence of the nanomagnetic behavior was found in Fe3O4,5 Cr2O3,20 Co3O4,21 NiO,22,23 and CoO.24,25 The phenomenon is obvious in ferromagnetic and

Nanomagnetism denotes the variation of the magnetic performance for crystals decreasing into a submicron level.1,2 The configuration environments of atoms or ions are modified because of the large specific surface area in nanocrystals, which induces electron redistribution. The intrinsic magnetism may be covered up or replaced by the other kinds of coupling states due to different coupling states of spins. Novel magnetic behaviors, such as superparamagnetism,3−5 exchange-bias,6−8 asperomagnetism,9 and spin glass,10 were induced through the modified coupling modes of the magnetic moments in crystals, on surfaces and interfaces. To explain the versatile magnetic performance of nanocrystals, the origin of nanomagnetism was elucidated in terms of the exotic moment coupling compared with the primitive magnetic order. Exogenous origins include doping effect and heterogeneous effect. Doping techniques were employed to generate diluted magnetic semiconducting properties in diamagnetic ZnO nanocrystals to generate room-temperature magnetic behaviors.11−14 Heterogeneous structures show practical potentials © 2018 American Chemical Society

Received: March 9, 2018 Accepted: April 30, 2018 Published: April 30, 2018 19235

DOI: 10.1021/acsami.8b03934 ACS Appl. Mater. Interfaces 2018, 10, 19235−19247

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ACS Applied Materials & Interfaces

Figure 1. Microstructures and phase compositions of precursors: (a,b) microstructure observation of Co3O4-100 precursors with different magnifications. (c) Indexed XRD pattern of Co3O4-110 precursors indicates all diffraction peaks coming from the Fd3̅m space group of Co3O4. (d,e) Microstructure observation of Co3O4-110 precursors with different magnifications. (f) Indexed XRD pattern of Co3O4-110 precursors indicates the mixture of Co(CO3)0.5(OH)·0.11H2O and a small amount of Co2(OH)2CO3. (g,h) Microstructure observation of Co3O4-111 precursors with different magnifications. (i) Indexed XRD pattern of Co3O4-111 precursors indicates the mixture of β-Co(OH)2 and a small amount of Co3O4. (j,k) Microstructure observation of Co3O4-112 precursors with different magnifications. (l) Indexed XRD pattern of Co3O4-112 precursors indicates the mixture of Co2(OH)2CO3 and a small amount of Co3O4.

antiferromagnetic materials because the equilibrium domain sizes of bulk materials may be larger than those of the nanocrystal sizes. Then, the domain configuration and domain microstructure are tuned to the new equilibrium states to balance the system. Zeng et al. found the size and orientation dependence of the room-temperature ferromagnetic performance of antiferromagnetic Co3O4 nanowires.26 Karthik et al. demonstrated the possible asperomagnetism and/or spin glass behavior of the NiO nanoparticles with a size of 16 nm.9 The ferromagnetic Fe3O4 nanoparticles show superparamagnetic properties when the particle size is less than ∼30 nm.4,27,28 A high specific surface area is one of the most unique characteristics of nanocrystals, which induces massive relaxed atoms on the exposed surface. Then, the particle surface plays the dominant role for the magnetic performance in nanocrystals with small enough sizes. The moments in the nanoparticle

surface layer can pattern in different ways compared with those in the core because of the missing rigorous confinement from the surrounding moments. Ohnishi et al. theoretically concluded that the surface-layer magnetic moment was found to increase by 0.73 μB to 2.98 μB/atom from the center layer.29 Islam et al. calculated the moments of Mn4+ on different exposed crystal planes in α-MnO2 based on density functional theory (DFT). The moments of (1 0 0) and (1 1 0) are 3.70 μB, which is a little bit higher than that in the bulk materials, about 3.12 μB, because of their similar atom arrangements with bulk materials and low coordination numbers, whereas the moments on the (1 1 1) and (1 1 2) planes increase to 4.31 μB. The moments on (2 1 1) are decreased because the electrons in the MnO4 tetrahedron are in low spin states. It should be noted that the surface effect depends on the shape of nanocrystals because the particle shape determines the relative number of 19236


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Figure 2. Microstructures and phase compositions of Co3O4 nanoparticles: (a) SEM image of Co3O4-100. (b) TEM image of Co3O4-100. The insets show the high-resolution image and FFT pattern, which indicate that the exposed facets are the (1 0 0) planes. (c) Indexed XRD pattern with refinement results of Co3O4-100 (refined with Rietica, weighted profile R-factor, Rwp: 8.61). (d) SEM image of Co3O4-110. (e) TEM image of Co3O4-110. The insets show the high-resolution image and FFT pattern, which indicate that the dominant exposed facets are the (1 1 0) planes. (f) Indexed XRD pattern with refinement results of Co3O4-110 (refined with Rietica, weighted profile R-factor, Rwp: 7.09). (g) SEM image of Co3O4111. (h) TEM image of Co3O4-111. The insets show the high-resolution image and FFT pattern, which indicate that the dominant exposed facets are the (1 1 1) planes. (i) Indexed XRD pattern with refinement results of Co3O4-111 (refined with Rietica, weighted profile R-factor, Rwp: 9.52). (j) SEM image of Co3O4-112. (k) TEM image of Co3O4-112. The insets show the high-resolution image and FFT pattern, which indicate that the dominant exposed facets are (1 1 2) planes. (l) Indexed XRD pattern with refinement results of Co3O4-112 (refined with Rietica, weighted profile Rfactor, Rwp: 8.00).

surface atoms.30 Jamet et al. demonstrated the influences of size, surface, and shape of single cobalt and iron nanoclusters containing almost 1000 atoms on the three-dimensional switching field distribution and simulated the anisotropy constant variation.31 Liu et al. prepared two batches of αFe2O3, that is, quasi-cubic bound by the (0 1 2), (1 0 2)̅ , and (1 1̅ 2) facets and bipyramid bound by the {0 1 2} facets.32 The quasi-cubic nanoparticles show defect ferromagnetism without Morin transformation at 240 K, whereas the hexagonal bipyramid counterparts display spin-canted ferromagnetism above the Morin transition temperature. Lv et al. synthesized highly symmetric dodecahedral single-crystalline α-Fe2O3

particles enclosed by twelve (1 0 1) planes.33 Although the dodecahedral nanocrystals expose different planes compared with the hexagonal bipyramid synthesized by Liu et al.,32 they show similar magnetic behaviors. Wu et al. synthesized α-Fe2O3 nanocrystals in cube shapes exposing the (1 0 4) and (1̅ 1̅ 0) planes and orthorhombic shapes exposing the (0 1 2) planes.34 The nanocubes show ferromagnetism in the whole measurement region with a blocking temperature around 200 K. The orthorhombic nanoparticles show a Morin transformation at 270 K, which is 30 K higher than that of the hexagonal bipyramid synthesized by Liu et al.32 It should be noted that the orthorhombic nanoparticles have similar exposed planes 19237


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∼200 nm, as shown in Figure 1a,b. The indexed XRD pattern reveals that the precipitates are Co3O4 with a spinel structure. The Co3O4-110 precursor is a kind of nanorod, as indicated by the SEM images shown in Figure 1d,e. The nanorods are a mixture of Co(CO3)0.5(OH)·0.11H2O and Co2(OH)2CO3, as indicated by the indexed XRD pattern shown in Figure 1f. The precursor of Co3O4-111 is hexagonal nanoplatelets which are composed of main phase Co(OH)2 and a small amount of Co3O4. The Co3O4-112 precursor shows a nanolaminar containing Co2(OH)2CO3 and a small amount of Co3O4, as shown in Figure 1j−l. The calcinated precursors keep their shapes and sizes, as indicated by the SEM and transmission electron microscopy (TEM) images shown in Figure 2, and the specific surface areas are shown in Figure S1. The size of Co3O4-100 is about 200 nm (Figure 2a). The distances between the atomic layers on the exposed surface are inconsistent with the lattice spacing of {0 2 2}, as shown by the high-resolution TEM (HRTEM) image (inset of Figure 2b). It is concluded that the cube exposes the {1 0 0} crystal planes combing the indexed fast Fourier transformation (FFT) pattern (inset of Figure 2b) and the HRTEM image. The refined XRD pattern of the final products after being calcined at 500 °C for 3 h, as shown in Figure 2c, does not show obvious difference compared to that of the precursor, which may be attributed to the high crystallization of the precursor as deposited in the hydrothermal process. All diffraction peaks can be indexed by the normal spinel Co3O4 phase (JCPDS card no. 42-1467, a = 0.808 nm), without detectable impurities. The crystal structures were refined by the Rietveld’s profile technique in the cubic structure with the Fd3̅m space group. The unit cell parameters (a = b = c) along with the agreement factors (Rp and Rwp) were calculated and are represented in Table S1. All unit cell parameters are in good agreement with the normal spinel Co3O4 structure. Co3O4-110, Co3O4-111, and Co3O4-112 underwent phase transition during sintering to release volatile components forming Co3O4. Crystal collapse can be observed clearly in the SEM and TEM images. Figure 2d presents the nanowires of Co3O4-110 with a length ranging from 50 to 200 nm and a width about 10 nm. In Figure 2e, reconstructed nanorods were observed because of the calcination of the precursor, which is a common phenomenon in a similar process of transformation from the precursor to Co3O4. The HRTEM image shown in Figure 2e demonstrates the (1 1̅ 3) and (0 0 2) crystal planes with 0.242 and 0.406 nm lattice spacings, respectively, growing along the [1 1 0] direction. The indexed FFT pattern also supports that the nanorods have the {1 1 0} planes exposed on the side walls. Co(CO3)0.5(OH)0.11·H2O nanorods underwent phase transition into Co3O4 after calcining at 300 °C in air for 3 h, as indicated by the refined XRD pattern shown in Figure 2f. Figure 2g,h displays the SEM and TEM images of hexagonal nanoplatelets. The shape remains unchanged compared to the precursor. However, collapsed parts are distributed on platelets. The HRTEM image shown in Figure 2h exhibits the (2 2̅ 0), (2 0 2̅), and (0 2̅ 2) crystal planes with a d-spacing of 0.286 nm and an interfacial angle of 60°, which clearly shows the nanoplatelets with the exposed {1 1 1} facets. The FFT pattern (inset of Figure 2h) also confirmed the conclusion. The refined XRD pattern also reveals its Co3O4 structure, as shown in Figure 2i. Figure 2j,k presents the SEM and TEM images of nanolaminars. The HRTEM images of the selected parts of the sample illustrate that the (1 1 1̅), (1 3̅ 1), and (2 2̅ 0) planes, which possess 0.467, 0.244, and 0.286 nm, respectively, grow

compared with the quasi-cubes synthesized by Liu et al.32 The different magnetic performance is attributed to the (1 1̅ 2) plane exposed in the quasi-cube. The finite size effect, shape effect, and surface effect were evidenced in different systems. However, the origin of such a magnetic transition cannot be displayed at the atomic level because of the unknown electron spin state and coupling mode motivated by the diverse nanostructures. Li et al. tuned the magnetism of α-MnO2 nanowires by inducing different exposed planes. 35 Although the intrinsic magnetisms are both antiferromagnetic for (1 1 0) and (2 1 0) plane-exposed nanowires, an obvious ferromagnetic behavior can be detected for the sample with (1 1 0)-exposed planes. Furthermore, firstprinciples calculations confirm the intrinsic ferromagnetic coupling of the Mn4+ ions on the (1 1 0) plane-exposed surface. In this work, Co3O4 with a spinel crystal structure was chosen as the research object for its bivalence states of Co ions compared with sole valence of Fe and Mn ions in α-Fe2O3 and α-MnO2. Co3O4 is indexed as a normal spinel crystal structure based on a cubic close packing array of oxide ions with occupation of tetrahedral 8a sites by Co2+ and octahedral 16d sites by Co3+.36 Co3+ ions have zero permanent magnetic moment because of the splitting of the 3d levels instead of complete filling of t2g levels by the octahedral field. Each Co2+ ion possesses a magnetic moment of 3.02 μB, and each Co2+ ion is surrounded by four nearest neighbors with opposite spins. Hence, bulk Co3O4 behaves as an antiferromagnet with a Néel temperature (T N ) of 40 K. 37 The nanocrystallization technology expends the antiferromagnetism of Co3O4 into novel ways. Takada et al. dispersed 3 nm Co3O4 nanocrystals into a SiO2 matrix, and the system shows superparamagnetism with a tunable blocking temperature from 3.4 to 5.2 K while the frequency of the ac susceptibility varied from 0.01 to 997 Hz.38 Yin et al. also found that ∼7.5 nm Co3O4 nanoparticles dispersed on a graphene substrate show ferromagnetism or superparamagnetism for the missing coercivity.39 Makhlouf demonstrated that 20 nm-sized Co3O4 particles exhibited a phase transition at TN ≈ 25 K, which may be ascribed to a finite size effect.40 Salabaş et al. prepared Co3O4 nanowires with a diameter of about 8 nm and observed exchange-bias and training effect, which indicated the presence of an exchange interaction between the antiferromagnetic core and the surface spins.41 Here, a novel strategy is employed to tune the magnetic behavior of Co3O4 based on the crystal facet engineering.42,43 Nanocubes enclosed by the (1 0 0) (defined as Co3O4-100) crystal planes as well as nanorods, hexagonal nanoplatelets, and nanolaminars with predominantly exposed (1 1 0) (defined as Co3O4-110), (1 1 1) (defined as Co3O4-111), and (1 1 2) (defined as Co3O4-112) planes, respectively, were synthesized by a hydrothermal method to study their magnetic properties. Different magnetic behaviors were found in Co3O4 nanocrystals with various exposed crystal planes. DFT calculations were employed to study the magnetic ground states for the different exposed surfaces. The study advances the understanding of the origination of nanomagnetism as well as the modulation of the magnetic behavior of nanomaterials.

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RESULTS Microstructure Evolution. Figure 1 displays the SEM images and X-ray diffraction (XRD) patterns of the precipitates after hydrothermal reaction using different approaches. The precursor of Co3O4-100 is nanocubes with a side length of 19238


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Figure 3. Magnetization vs temperature plots for four Co3O4 nanocrystals: (a) Co3O4-100, (b) Co3O4-110, (c) Co3O4-111, and (d) Co3O4-112. Antiferromagnetic transition can be observed at 37 K. The insets display the fitting results of ZFC curves with Curie−Weiss law, which indicate the antiferromagnetic nature of all samples.

Figure 4. Magnetization vs applied field plots for four Co3O4 nanocrystals: (a) Co3O4-100, (b) Co3O4-110, (c) Co3O4-111, and (d) Co3O4-112. Co3O4-110 and Co3O4-112 keep antiferromagnetic behavior, whereas the other samples show ferromagnetic-like behaviors. The insets display the enlarged parts around the origin of coordinates.

Magnetic Performance. Co3O4 has been reported as an antiferromagnetic substance with a Néel temperature of TN ≈ 40 K.37 The magnetization versus temperature plots for four Co3O4 nanocrystals are shown in Figure 3. Both zero-fieldcooled (ZFC) and field-cooled (FC) magnetizations were measured from 5 to 350 K under 100 Oe field. Peaks located at around 37 K can be found for both ZFC and FC measurements in all samples, which are responsible for antiferromagnetic transition and known as TN. ZFC and FC curves bifurcate

along the [1 1 2] zone axis. Although holes and cracks are observed in the nanolaminars, the main exposed facet is the (1 1 2) plane, as indicated by the HRTEM image and indexed FFT pattern (insets of Figure 2k). Figure 2l shows the indexed XRD pattern with a typical Co3O4 structure. Compared with the XRD pattern of precursors, the peaks of Co3O4-110, Co3O4-111, and Co3O4-112 become sharp and smooth. The refinement results are displayed in Table S1. 19239


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Figure 5. DFT simulated results of magnetic configuration of various Co3O4 surfaces: (a,b) oblique view and side view of (1 0 0) crystal planeexposed Co3O4. Magnetic moments are induced on the Co3+ ions exposed on the surface. (c,d) Oblique view and side view of (1 1 0) crystal planeexposed Co3O4. The top surface contains Co3+ ions with zero magnetic moments and antiferromagnetically coupled Co2+ ions. (e) Side view of (1 1 1) crystal plane-exposed Co3O4. Intensive lattice distortion was observed, and the Co3+ ions own magnetic moments with same spin orientations. (f) Oblique view of (1 1 2) crystal plane-exposed Co3O4. Intensive lattice deformation was observed in the sublayers close to the exposed surface. More Co3+ ions show magnetic moments compared with the other exposed crystal planes. The spin-density isosurface is represented in yellow (spin-up) and blue (spin-down) with an isovalue of 0.01 electrons/Å.

below TN, indicating the antiferromagnetic coupling modes under FC process. The similar ZFC and FC behaviors of different nanoshapes and nanosizes can be attributed to their natural antiferromagnetism. A linear correlation can be observed between the hightemperature ZFC susceptibility for all samples. The susceptibility obeys the Curie−Weiss law, and the curves can be fitted with the equation 1/χ(T) = (T − θ)/C, where θ and C represent the Curie−Weiss temperature and Curie−Weiss constant, respectively. The fitted result is presented in the insets of Figure 3. The θ values are all negative, which indicates the antiferromagnetic behavior of the Co3O4 nanocrystals, with specific values of −132.88 K for (1 0 0) nanocubes, −117.13 K for (1 1 0) nanorods, −118.37 K for (1 1 1) nanoplatelets, and −181.27 K for (1 1 2) nanolaminars, whereas the C values are 1.02 × 10−2, 1.10 × 10−2, 1.23 × 10−2, and 1.40 × 10−2 emu K g−1 Oe−1, respectively. The antiferromagnetism of Co3O4 originates from the two antiparallel sublattices of Co2+. A mean field constant, ϵ, can be introduced to describe interactions within a sublattice, |θ|/TN = (μ + ϵ)/(μ − ϵ), with μ being the magnetic moment of Co2+. The interaction intensity is increasing with the value of |θ|/TN. The lower θ value of Co3O4-112 implies a stronger antiferromagnetic interaction in Co3O4 sublattices. The TN values of four samples are similar, which can be understood that the major coupling of the inside atoms is antiferromagnetic just like behaviors in the bulk Co3O4. Furthermore, the distinct surface magnetic ground states of different planes show considerable influence on the Curie−Weiss temperatures. The surface ground states generated by the first-principles calculation are shown in Figure S2, and the Co3+ close to or on different exposed planes possesses variable magnetic moments and spin directions. Because of the large specific surface areas of nanoparticles, the induced magnetic moments exhibit profound influence on the magnetic behaviors. The magnetizations as a function of applied field (M−H curves) was measured between ±20 kOe at 5 K of four samples, as shown in Figure S3. Figure 4 exhibits the M−H curves in the range of ±10 kOe. For Co3O4-100, hysteresis loops after ZFC

and FC processes are symmetric about the coordinate origin. A high remnant magnetism of about 0.067 emu g−1 and a large coercivity of about 1556 Oe were observed, indicating a mixed state of a component from the antiferromagnetic core of Co3O4 combined with stable net surface spins. For Co3O4-110 and Co3O4-112, both the ZFC and FC curves have a virtually linear shape in the applied field range. Such a shape of the curves is expected from an antiferromagnetic system,44 where the antiferromagnetic ordered core is the dominant contribution and the scarce net magnetic spins on the surface play a very weak role. As for Co3O4-111, obvious hysteresis effects were found in both ZFC and FC curves, indicating the existence of ferromagnetic coupling with a relatively weak remnant magnetism of 0.0058 emu g−1 and coercivity of about 61.5 Oe. Unsaturated hysteresis loops at ±20 kOe in Co3O4-100 are a common phenomenon in nanocrystalline alloys, compounds, and metal oxides.45,46 An open hysteresis loop can be clearly seen in the FC curve, indicating a loss of magnetization during one hysteresis cycle, which is attributed to the coupling relaxation,46,47 as shown in the insets of Figure S3.

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DISCUSSION

DFT Simulation. As a normal spinel crystal structure, Co3O4 has two different valence Co ions, with Co2+ (Co 3d7) occupying the tetrahedral 8a sites and Co3+ (Co 3d6) occupying the octahedral 16d sites.36 The octahedral crystal field splits the Co 3d orbitals into three t2g levels and two eg levels, as shown in Figure S4, where the energy of t2g levels is lower than that of eg levels. Electron exchange causes a further split of each t2g or eg level into majority spins (α) and minority spins (β).48 Therefore, the Co3+ ions in the octahedral coordination will completely fill three t2g levels according to Pauli exclusion principle and Hund’s rule, resulting in no unpaired electrons so that the Co3+ ions show no permanent magnetic moment. As for Co2+ ions in tetrahedral coordination, eg levels are lower in energy than t2g levels, but t2g(α) levels are lower in energy than eg(β) levels. Seven 3d electrons of Co2+ will fill completely in eg(α), t2g(α), and eg(β) levels in a proper order with no electron filling in t2g(β) levels, in which the three unpaired electrons in 19240


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Figure 6. O 1s photoelectron core-level spectrum of (a) Co3O4-100, (b) Co3O4-110, (c) Co3O4-111, and (d) Co3O4-112. p1 indicates the presence of surface water or O2 introduced during the experiment. p2 is attributed to the oxygen on or near the exposed surface, and p3 comes from the lattice oxygen in the bulk structure of spinel Co3O4.

t2g(α) orbitals with the same spinning directions are responsible for the antiferromagnetic coupling of Co3O4.49 Co3+ ions are nonmagnetic (i.e., carry zero magnetic moment) in octahedral coordination in bulk Co3O4, so it is very important to induce magnetic moment on the Co3+ ions on the exposed surface or near the surface region of the damaged lattice structure for the modification of Co3O4 nanomagnetism. DFT calculations within the generalized gradient approximation50 augmented with an on-site Coulomb repulsion U term in the 3d shell of the cobalt ions were performed to determine the magnetic coupling states of the Co ions on different exposed crystal planes. In the present study, the U repulsion value is chosen to be 5.5 eV for all Co ions.51 For bulk Co3O4, the calculated ground state is antiferromagnetic with a contribution of tetrahedral high spin state Co2+ (S = 3/ 2) of about 2.86 μB, in fine agreement with the results of Chen et al.52 and comparable with the experimental value 3.2 μB.37 The corresponding ferromagnetic state is higher in energy by 0.19 eV per unit cell. Note that the calculated atomic magnetic moment values only count for the muffin-tin spheres, whereas the contribution from the interstitial region is neglected. For both states, the octahedral Co3+ ion is indeed in a low spin state with a net moment of zero. The crystallographic structures of different exposed crystal planes are compared with one possible surface ion configuration in Figure S5. As Co3+ is not a natural magnetic ion in octahedron in the bulk, the magnetic moments induced on Co3+ on or near the exposed surface become crucial for the nanomagnetism modulation for the destructed symmetry of the lattice structure on the surface. Four supercell slab models were built to simulate the ground-state magnetic structures of different exposed surfaces, namely, (1 0 0), (1 1 0), (1 1 1), and (1 1 2). Each slab model contains 112 atoms and a vacuum region of 15 Å in thickness. For all of these surfaces models, there is no inversion symmetry, meaning that each involves two inequivalent surfaces, called A and B.

Figure 5a,b displays the calculated ground magnetic structure of (1 0 0)-exposed crystal plane with two different view orientations, which are occupied by Co2+ ions. All Co2+ ions in the bulk region are still in antiferromagnetic coupling, and the Co3+ ions are nonmagnetic therein. The magnetic moments of the outmost layer of Co2+ ions are varied because of the deformed lattice and missed coordinated ions. On (1 0 0)-A, the sublayer Co3+ ions show a magnetic moment of 1.74 μB in the same spin orientation. Furthermore, the outmost layer O2− ions also possess a sizable magnetic moment of 0.2 μB in the same direction of Co3+. In contrast, on (1 0 0)-B, the moment of the exposed Co3+ ions is only 0.15 μB. The induced magnetic moments of Co3+ and O2− are responsible for the net magnetic moment of 7.72 μB/cell for (1 0 0)-exposed Co3O4. The high saturated magnetization and coercivity of Co3O4-100 are attributed to the strong ferromagnetic coupling of the moments on exposed surfaces. For the (1 1 0) surfaces, the (1 1 0)-A surface layer involves both Co2+ and Co3+ exposed ions, whereas the (1 1 0)-B surface layer contains only Co3+ ions. Co2+ ions keep the same antiferromagnetic states as in the bulk with a slight intensity variation. Interestingly, whereas on (1 1 0)-A, Co3+ ions are nonmagnetic, on (1 1 0)-B, Co3+ ions are antiferromagnetically coupled, each with a moment of 1.69 μB. Small magnetic moments were observed on O2− ions with opposite spin, namely, 0.24 μB on (1 1 0)-A and 0.29 μB on (1 1 0)-B. However, the induced magnetic moments on either surface show opposite orientations, leading to an antiferromagnetic coupling with a net magnetic moment of zero for the supercell. These results agree with the magnetic behavior of Co3O4-110 as quite pure antiferromagnetism. Because of the low symmetry involved, for both (1 1 1) and (1 1 2) surfaces, only rather thin models were employed in this work, yet both models can capture the essential magnetic structure, where in the bulk, the Co2+ ions are antiferromagneti19241


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Figure 7. Co 2p photoelectron core-level spectrum of (a) Co3O4-100, (b) Co3O4-110, (c) Co3O4-111, and (d) Co3O4-112. p1 and p5 are attributed to Co3+ in the bulk structure of spinel Co3O4. p2 and p6 are attributed to Co2+ in the bulk structure. p3 and p7 are attributed to Co3+ on or near the exposed surface. p4 and p8 are attributed to Co2+ on or near the exposed surface.

cally coupled and the Co3+ ions are nonmagnetic (with zero moment). The exposed (1 1 1)-A crystal plane contains mixed Co2+ and Co3+ ions, as shown in Figure 5e. Intensive lattice distortion can be observed in the several sublayers close to this surface. The magnetic moments of Co2+ ions also vary with the position in the sublayers. Significantly, strong magnetic moments form on the Co3+ ions on the (1 1 1)-A surface, 1.02 μB, which align in the same direction. The O atoms located on the fourth sublayer possess size moment, 0.24 μB. In contrast, the (1 1 1)-B surface terminated with an O layer is structurally robust against relaxation. The moment of outmost O atoms on the (1 1 1)-B surface is very small, only 0.06 μB. For the (1 1 1) surface model, the net moment is 2.97 μB/cell. Thus, the ferromagnetic behavior observed in Figure 4c originates from Co3+−O−Co3+ on the (1 1 1)-A surface. While the M−H curve does not show high saturated magnetization and coercivity, it may be attributed to the weak coupling due to the large distance between Co3+ ions. Moreover, one may expect that a thicker slab model may lead to smaller net magnetization because of the effect of overlapping bands. The simulated results of the (1 1 2)-exposed plane reveal even more intensive lattice deformation than that in the (1 1 1)-exposed surface, and consequently the magnetic configuration becomes quite complex on the surface and in the sublayers. The magnetic moments of Co2+ and Co3+ ions vary significantly with the lattice distortion and the position in the supercell. The magnetic moments on sites A, B, C, and D as labeled in Figure 5f are 1.16, 2.20, 2.09, and 2.15 μB, respectively. It should be noted that the spin direction of moment on site A is opposite to those on sites B and C. Furthermore, not all Co3+ ions own magnetic moments in the sublayers, even though some ions are almost exposed to the vacuum. The total net magnetic moment is 1.36 μB per unit cell, which is lower than those of (1 0 0)- and (1 1 1)-exposed

cases. However, the residual magnetization and coercivity of Co3O4-112 are quite low with typical antiferromagnetism, as shown in Figure 4d. The difference between the simulation outcomes and the experimental results may be attributed to the long distance of the uncoupled Co3+ arrays weakening the possible ferromagnetic interaction. Configuration Environments of Co2+ and Co3+. Co3O4 contains mixed valence states of Co2+ and Co3+ because of the spinel crystal structure. Surface-sensitive X-ray photoelectron spectroscopy (XPS) was applied to examine the valence state of each element, especially Co ions in Co3O4. Survey patterns are plotted in Figure S6, and no impurity peak was found except C 1s, which is employed to correct the binding energy (BE) with respect to the standard peak of C 1s at 284.8 eV. Figure 6 displays the O 1s photoelectron core-level spectrum of four samples which expose (1 0 0), (1 1 0), (1 1 1), and (1 1 2). All spectra exhibit a small peak located at a BE of about 533.4 eV and a shoulder at a BE of about 531.5 eV with a taller but narrower peak at a BE of about 529.6 eV. The first peak (labeled as peak 1 in Figure 6) indicates the presence of surface water or O2 introduced during the experiment.53 Langell et al. studied the nature of oxygen in and at the surface of spinel oxides.54 They found that the lattice oxygen in the bulk structure just shows one type of nature in the XPS pattern, which is consistent with the peak centered at around 529.6 eV (labeled as peak 3 in Figure 6), whereas the state located at 531.5 eV is attributed to the intrinsic nature of the oxygen on the exposed spinel surface. This state is labeled as peak 2 in Figure 6. Judging from the high integrated areas of peak 2 of the four samples, it is concluded that the surface states account for a substantial proportion. Figure 7 demonstrates the Co 2p XPS patterns, which all consist of two shake-up satellite peaks and two main peaks resulted from spin−orbit splitting of the Co 2p photoelectron lines.55 The high-resolution scans of Co 2p1/2 and Co 2p3/2 19242


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Figure 8. Microstructure, phase composition, and magnetic behaviors of 111-Oct: (a,b) SEM images of the precursor of 111-Oct. (c) Indexed XRD pattern of the precursor of 111-Oct. (d) SEM image of 111-Oct. (e) TEM image of 111-Oct. The insets show the high-resolution image and FFT pattern, which indicate that the dominant exposed facets are the (1 1 1) planes. (f) Indexed XRD pattern of 111-Oct. (g) Magnetization vs temperature plots for 111-Oct. (h) Magnetization vs applied field plots for 111-Oct in the range of ±10 kOe. The inset magnifies the M−H curves near 0 Oe. (i) Magnetization vs applied field plots for 111-Oct in the range of ±20 kOe. The inset magnifies the M−H curves near 20 kOe.

were fitted with four Gaussian−Lorentz peaks as marked in the spectra, where p1−p4 are responsible for the observed 2p1/2 peak and p5−p8 for the 2p3/2 peak. All odd peaks are produced by Co3+, whereas even peaks are produced by Co2+. The ratio of Co3+ and Co2+ peaks is 2:1 in two different states, which corresponds with normal spinel crystal structure as mentioned above. In both 2p1/2 and 2p3/2 peaks, higher BE peaks such as p1, p2, p5, and p6 and lower BE peaks p3, p4, p7, and p8 constitute the bulk and surface plasmon loss peaks.56 The ratios of surface and bulk are about 1.23:1, 1.5:1, 1.38:1, and 1.94:1 for Co3O4-100, Co3O4-110, Co3O4-111, and Co3O4112, respectively, in the detectable depth of the samples. The results indicate that Co ions in the high-index facet-exposed samples show increased surface proportions because of the intensive lattice distortion, whereas the first-principles results indicate that the surface proportion is one of the factors to determine the magnetic moments of Co2+, Co3+, and O2−. It should be noted that the specific surface area depends on the shape, size, and porosity of nanoparticles rather than the X-ray penetration depth as in XPS measurements. The sample with a high specific surface area has a chance to show high surface proportion. Weak Shape and Size Dependence of Magnetism of (1 1 1)- and (1 1 2)-Exposed Nanocrystals. Octahedron Co3O4 nanocrystals with exposed (1 1 1) planes and hexagon Co3O4 nanocrystals with exposed (1 1 2) planes, named 111Oct and 112-Hex, respectively, were synthesized to confirm the simulation results and eliminate the debate about the influence

of size and shape. Figure 8 displays the morphologies and phase compositions of the precursors and final products of 111-Oct. The precursor of 111-Oct shows an octahedron shape with an edge length of about 100 nm and is composed of Co3O4, as indicated by Figure 8a−c. The final product of 111-Oct keeps the shape of the precursor, as indicated by the SEM and TEM images shown in Figure 8d,e. The HRTEM image shows the (2 2̅ 0), (2 0 2̅), and (0 2̅ 2) planes (the inset of Figure 8e). The FFT pattern confirms that the exposed surface planes are (1 1 1) for 111-Oct, which is composed of pure Co3O4, as indicated by the XRD pattern shown in Figure 8f. The ZFC and FC magnetizations under 100 Oe field indicate the antiferromagnetic nature of 111-Oct, as shown in Figure 8g. The TN is found at ∼37 K, and the Curie−Weiss temperature is −122.43 K, as indicated by the inset of Figure 8g. The M−H curves after ZFC and FC processes under 100 Oe magnetic field show a behavior quite similar to those of Co3O4-111, on which the magnetization deviates from a linear behavior. Figure 8h shows the M− H curves in the range of ±10 kOe, and its inset shows a coercivity field of 46.2 Oe and a residual magnetization of 4.7 × 10−3 emu g−1. Figure 8i shows the M−H curves under the whole measurement field, ±20 kOe, and its inset indicates the open ending of the M−H curves at 20 kOe. The phenomenon is attributed to the ferromagnetic coupling of Co3+ on the exposed (1 1 1) planes of the octahedral crystals. The precursor of 112-Hex is in the shape of a hexagon and composed of β-Co(OH)2, as shown in Figure 9a−c. The edge length is about 2 μm. The thermolysis of β-Co(OH)2 keeps the 19243


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Figure 9. Microstructure, phase composition, and magnetic behaviors of 112-Hex: (a,b) SEM images of the precursor of 112-Hex. (c) Indexed XRD pattern of the precursor of 112-Hex. (d) SEM image of 112-Hex. (e) TEM image of 112-Hex. The insets show the high-resolution image and FFT pattern, which indicate that the dominant exposed facets are the (1 1 2) planes. (f) Indexed XRD pattern of 112-Hex. (g) Magnetization vs temperature plots for 112-Hex. (h) Magnetization vs applied field plots for 112-Hex in the range of ±10 kOe. The inset magnifies the M−H curves near 0 Oe. (i) Magnetization vs applied field plots for 112-Hex in the range of ±20 kOe. The inset magnifies the M−H curves near 20 kOe.

induces net magnetic moments in the Co3O4 nanocrystals, whereas the (1 1 0) crystal plane-exposed nanorod keeps its natural antiferromagnetic behavior because the magnetic moments are zero for the Co3+ ions on or close to the exposed surface. It should be noted that the Co3+ ions exposed on the (1 1 2) surface have induced magnetic moments as indicated by the DFT calculations. However, the (1 1 2)-exposed nanolaminars display antiferromagnetism because of the long interaction distance between the magnetized Co3+ ions.

hexagon shape of the nanocrystals and produces pure Co3O4, as shown in Figure 9d,e. The main exposed crystal plane of the hexagonal platelet is (1 1 2), as indicated by the HRTEM image and FFT pattern of 112-Hex shown in the insets of Figure 9e. The Curie−Weiss temperature is −144.57 K, as shown in Figure 9f, which indicates the antiferromagnetic coupling behavior of the nanocrystals. Furthermore, the M−H curves after ZFC and FC processes under 100 Oe magnetic field show linear behaviors, as shown in Figure 9h,i. The magnetic coupling behavior is similar to those of Co3O4-112. The comparison of magnetic behaviors of 111-Oct and 112-Hex with those of Co3O4-111 and Co3O4-112 indicates that the nanomagnetic performance has weak dependence on the shape and size of the nanoparticles, whereas the exposed crystal planes play a dominant role to generate different magnetic performances of nanocrystals.

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METHODS

Synthesis of Co3O4-100 Nanocubes. In a typical procedure,57 0.02 mol of Co(NO3)2·6H2O and 0.01 mol of NaOH were dissolved in 40 mL of deionized water under stirring. The mixed reactants were transferred into a 50 mL Teflon-lined autoclave sealed by the stainlesssteel jar and heated at 180 °C for 5 h. After cooling to room temperature, the products were collected by centrifugation, washed with ethanol and deionized water several times, and subsequently dried at 60 °C in vacuum. The final products were calcined at 500 °C for 3 h. Synthesis of Co3O4-110 Nanorods. In a typical synthesis,58 10 mL of aqueous ammonia and 25 mL of ethylene glycol were mixed to form a homogeneous solution. An aqueous sodium carbonate solution (1.5 mL, 1 mol/L) was then added and stirred for a few minutes under magnetic stirring. Afterward, 5 mL of 1 mol/L aqueous cobalt nitrate solution was added to the mixture by continuous stirring for 20 min. The resulting solution was transferred into a Teflon-lined stainlesssteel autoclave with a volume of 50 mL, which was then heated to 170 °C and maintained at the temperature for 17 h. The cooled suspension was centrifuged, and the precipitate was rinsed with deionized water

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CONCLUSIONS In summary, the magnetic behavior of Co3O4 nanocrystals with different exposed crystal planes was systematically discussed based on the measurements and analysis of the phase composition, microstructure, ion coordination environment, magnetic performance, and first-principles DFT simulation. The nanomagnetism of Co3O4 shows strong dependence on the exposed crystal plane, that is, (1 0 0), (1 1 0), (1 1 1), and (1 1 2). Most Co3+ ions on or close to the exposed (1 0 0) and (1 1 1) surface show induced magnetic moments because of the missing or deformed configuration environments, which 19244



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and ethanol several times and then dried at 60 °C in vacuum to get Co(CO3)0.5(OH)0.11·H2O nanorods. The Co3O4 nanorods were obtained after calcining at 300 °C in air for 3 h. Synthesis of Co3O4-111 Hexagonal Nanoplatelets. In a typical process,59 1.2 g of Co(NO3)2·6H2O was dissolved in a mixed solution with 5 mL of deionized water and 5 mL of ethanol. Then, 1 g of polyvinylpyrrolidone was added as a surfactant and stirred for 30 min, following which 25 mL of 0.4 mol/L NaOH aqueous solution was added drop by drop, which took 90 min accompanied by the color change from light red to dark green. The resulting suspension was transferred into a 50 mL Teflon-lined autoclave. The autoclave was heated at 120 °C for 10 h. After cooling down, the precipitate was centrifuged and washed with ethanol and deionized water several times to obtain β-Co(OH)2. The final Co3O4 nanocrystals were obtained by annealing the β-Co(OH)2 precursor at 450 °C for 2 h in air. Synthesis of Co3O4-112 Nanolaminars. {1 1 2} facet-exposed Co3O4 nanolaminars were obtained by calcining the Co2(OH)2CO3 precursor and were produced by a simple hydrothermal method.60 Briefly, 2 mmol of Co(NO3)2·6H2O, 8 mmol of urea (CO(NH2)2), and 0.4 g of cetyltrimethylammonium bromide as a soft template were dissolved in 40 mL of deionized water. The mixture was stirred for 30 min and transferred into a 50 mL Teflon-lined autoclave, which was then heated and maintained at 140 °C for 12 h. After cooling to room temperature, the suspension was centrifuged and rinsed with deionized water and ethanol several times. The final mesoporous Co3O4 nanolaminars were obtained by calcining the Co2(OH)2CO3 precursor at 450 °C for 2 h in air. Synthesis of 111-Oct Octahedrons. In a typical process, 0.01 mol of Co(NO3)2·6H2O and 0.01 mol of CoCl2·6H2O were dissolved in 30 mL of deionized water. After stirring for several minutes, 10 mL of 1 mol/L NaOH aqueous solution was added into the solution, as mentioned above, drop by drop. The resulting solution was transferred into a 50 mL Teflon-lined autoclave sealed by the stainless-steel jar and heated at 180 °C for 5 h. After cooling down, the products were centrifuged, washed with ethanol and deionized water several times, and subsequently dried at 60 °C in vacuum. The final products were calcined at 400 °C for 3 h. Synthesis of 112-Hex Hexagonal Nanoplatelets. Co3O4 hexagonal nanoplatelets with exposed {1 1 2} facets were produced by calcining the β-Co(OH)2 precursor and were obtained via a homogeneous precipitation method.61 Briefly, 5 mmol of CoCl2·6H2O and 60 mmol of hexamethylenetetramine were dissolved in 200 cm3 of a 9:1 mixture of deionized water and ethanol. The solution was heated to 90 °C with magnetic stirring for 1 h. The resulting green suspension was centrifuged, washed with ethanol and deionized water several times, and then dried in air at room temperature to obtain β-Co(OH)2 precursors. The precursors were calcined at 400 °C for 3 h to obtain Co3O4 nanoplatelets. Characterizations. All samples were examined by XRD (D/Max, Cu Kα, λ = 0.154187 nm) in conjunction with Rietveld refinement (Rietica) to identify the phase compositions and crystal structures. The morphologies and sizes of the products were observed by field emission gun SEM (JSM-7500F) and TEM (JEOL-2100) with HRTEM to further analyze the microstructures. XPS (EscaLab 250IXL, Al Kα) was used to determine the chemical compositions and the configuration environments of Co and O in the samples. Magnetic properties were measured using a commercial vibrating sample magnetometer model physical properties measurement system (Quantum Design, 14 T) in applied magnetic fields up to 20 kOe. The specific surface areas were collected by N2 adsorption isotherm using an Autosorb-iQ2 analyzer at 77 K. First-Principles Simulation. Extensive spin-polarized DFT calculations were performed using the VASP code.62 A plane wave basis set cutoff energy of 500 eV was used. The Monkhorst−Pack grids of (8 × 8 × 8) were used for the 56-atom unit cell. For the supercells, the grids were folded to obtain the same or similar sampling of the reciprocal space. The energy convergence criterion between two electronic steps was 10−4 eV. The convergence criteria for the forces on the atoms were less than 0.01 eV/Å.

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b03934. Comparison of the specific surface areas, ground magnetic states, M−H loops in the whole measurement field, splitting of Co 3d orbitals due to octahedral and tetrahedral crystal fields as well as electron exchange, crystallographic structures of different exposed crystal planes of Co3O4, XPS survey patterns of Co3O4-100, Co3O4-110, Co3O4-111, and Co3O4-112 samples, and crystal lattice parameters (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.L.). *E-mail: [email protected] (X.Y.C.). *E-mail: [email protected] (Y.L.). ORCID

Yan Wang: 0000-0001-9853-5721 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (grant no. 51572166) and the Shanghai Key Laboratory of High Temperature Superconductors (no. 14DZ2260700). The authors thank the Analysis and Research Center of Shanghai University for their technical support. W.L. acknowledges research support from the Program for Professors with Special Appointments (Eastern Scholar: TP2014041) at Shanghai Institutions of Higher Learning. This research was also undertaken with the assistance of resources from the National Computational Infrastructure (NCI), which is supported by the Australian Government under the NCRIS program.

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REFERENCES

(1) Bader, S. D. Colloquium: Opportunities in Nanomagnetism. Rev. Mod. Phys. 2006, 78, 1−15. (2) Tian, Y.; Bakaul, S. R.; Wu, T. Oxide Nanowires for Spintronics: Materials and Devices. Nanoscale 2012, 4, 1529−1540. (3) Xie, J.; Yan, C.; Yan, Y.; Chen, L.; Song, L.; Zang, F.; An, Y.; Teng, G.; Gu, N.; Zhang, Y. Multi-modal Mn-Zn Ferrite Nanocrystals for Magnetically-induced Cancer Targeted Hyperthermia: A Comparison of Passive and Active Targeting Effects. Nanoscale 2016, 8, 16902−16915. (4) Lu, N.; Zhang, P.; Zhang, Q.; Qiao, R.; He, Q.; Li, H.-B.; Wang, Y.; Guo, J.; Zhang, D.; Duan, Z.; Li, Z.; Wang, M.; Yang, S.; Yan, M.; Arenholz, E.; Zhou, S.; Yang, W.; Gu, L.; Nan, C.-W.; Wu, J.; Tokura, Y.; Yu, P. Electric-field Control of Tri-state Phase Transformation with a Selective Dual-ion Switch. Nature 2017, 546, 124−128. (5) Yang, P.; Li, H.; Zhang, S.; Chen, L.; Zhou, H.; Tang, R.; Zhou, T.; Bao, F.; Zhang, Q.; He, L.; Zhang, X. Gram-scale Synthesis of Superparamagnetic Fe3O4 Nanocrystal Clusters with Long-term Charge Stability for Highly Stable Magnetically Responsive Photonic Crystals. Nanoscale 2016, 8, 19036−19042. (6) Maurer, T.; Zighem, F.; Ott, F.; Chaboussant, G.; André, G.; Soumare, Y.; Piquemal, J.-Y.; Viau, G.; Gatel, C. Exchange Bias in Co/ CoO Core-shell Nanowires: Role of Antiferromagnetic Superparamagnetic Fluctuations. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, 064427. 19245


Research Article

ACS Applied Materials & Interfaces (7) Tian, Y. F.; Ding, J. F.; Lin, W. N.; Chen, Z. H.; David, A.; He, M.; Hu, W. J.; Chen, L.; Wu, T. Anomalous Exchange Bias at Collinear/Noncollinear Spin Interface. Sci. Rep. 2013, 3, 1094. (8) Ding, J. F.; Lebedev, O. I.; Turner, S.; Tian, Y. F.; Hu, W. J.; Seo, J. W.; Panagopoulos, C.; Prellier, W.; Van Tendeloo, G.; Wu, T. Interfacial Spin Glass State and Exchange Bias in Manganite Bilayers with Competing Magnetic Orders. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 054428. (9) Karthik, K.; Selvan, G. K.; Kanagaraj, M.; Arumugam, S.; Jaya, N. V. Particle Size Effect on the Magnetic Properties of NiO Nanoparticles Prepared by a Precipitation Method. J. Alloy. Comp. 2011, 509, 181−184. (10) Li, W.; Zeng, R.; Sun, Z.; Tian, D.; Dou, S. Uncoupled Surface Spin Induced Exchange Bias in α-MnO2 Nanowires. Sci. Rep. 2014, 4, 6641. (11) Andriotis, A. N.; Menon, M. Universal Features Underlying the Magnetism in Diluted Magnetic Semiconductors. J. Phys. Condens. Matter 2018, 30, 135803. (12) Duc-The, N.; Le Thanh, C.; Nguyen Huu, C.; Cao Thai, S.; Pham Thanh, H.; Nguyen Duc, D. Local Structure and Chemistry of C-Doped ZnO@C Core-Shell Nanostructures with Room-Temperature Ferromagnetism. Adv. Funct. Mater. 2018, 28, 1704567. (13) Zhu, M.; Zhang, Z.; Zhong, M.; Tariq, M.; Li, Y.; Li, W.; Jin, H.; Skotnicova, K.; Li, Y. Oxygen Vacancy Induced Ferromagnetism in Cu-doped ZnO. Ceram. Int. 2017, 43, 3166−3170. (14) Tariq, M.; Li, Y.; Li, W.; Zhang, Z.; Hu, Y.; Zhu, M.; Jin, H.; Li, Y.; Skotnicova, K. Ferromagnetic Coupling of Fe3+-VO-Fe3+ Polarons in Fe-doped ZnO. Ceram. Int. 2018, 44, 71−75. (15) Charilaou, M.; Hellman, F. Mean-field Simulation of Metal Oxide Antiferromagnetic Films and Multilayers. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 184433. (16) Zandalazini, C.; Esquinazi, P.; Bridoux, G.; Barzola-Quiquia, J.; Ohldag, H.; Arenholz, E. Uncompensated Magnetization and Exchange-bias Field in La0.7Sr0.3MnO3/YMnO3 bilayers: The Influence of the Ferromagnetic Layer. J. Magn. Magn. Mater. 2011, 323, 2892− 2898. (17) Sytnyk, M.; Kirchschlager, R.; Bodnarchuk, M. I.; Primetzhofer, D.; Kriegner, D.; Enser, H.; Stangl, J.; Bauer, P.; Voith, M.; Hassel, A. W.; Krumeich, F.; Ludwig, F.; Meingast, A.; Kothleitner, G.; Kovalenko, M. V.; Heiss, W. Tuning the Magnetic Properties of Metal Oxide Nanocrystal Heterostructures by Cation Exchange. Nano Lett. 2013, 13, 586−593. (18) Dong, Y.; Wen, B.; Chen, Y.; Cao, P.; Zhang, C. Autoclave-free Facile Approach to the Synthesis of Highly Tunable Nanocrystal Clusters for Magnetic Responsive Photonic Crystals. RSC Adv. 2016, 6, 64434−64440. (19) Keng, P. Y.; Kim, B. Y.; Shim, I.-B.; Sahoo, R.; Veneman, P. E.; Armstrong, N. R.; Yoo, H.; Pemberton, J. E.; Bull, M. M.; Griebel, J. J. Colloidal Polymerization of Polymer-coated Ferromagnetic Nanoparticles into Cobalt Oxide Nanowires. ACS Nano 2009, 3, 3143− 3157. (20) Tobia, D.; Winkler, E. L.; Zysler, R. D.; Granada, M.; Troiani, H. E. Superparamagnetism in AFM Cr2O3 Nanoparticles. J. Alloy. Comp. 2010, 495, 520−523. (21) Dutta, P.; Seehra, M. S.; Thota, S.; Kumar, J. A Comparative Study of the Magnetic Properties of Bulk and Nanocrystalline Co3O4. J. Phys. Condens. Matter 2008, 20, 015218. (22) Yi, J. B.; Ding, J.; Feng, Y. P.; Peng, G. W.; Chow, G. M.; Kawazoe, Y.; Liu, B. H.; Yin, J. H.; Thongmee, S. Size-dependent Magnetism and Spin-glass Behavior of Amorphous NiO Bulk, Clusters, and Nanocrystals: Experiments and First-principles Calculations. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 224402. (23) Kodama, R. H.; Makhlouf, S. A.; Berkowitz, A. E. Finite Size Effects in Antiferromagnetic NiO Nanoparticles. Phys. Rev. Lett. 1997, 79, 1393. (24) Ambrose, T.; Chien, C. L. Finite-Size Effects and Uncompensated Magnetization in Thin Antiferromagnetic CoO Layers. Phys. Rev. Lett. 1996, 76, 1743.

(25) Tang, Y. J.; Smith, D. J.; Zink, B. L.; Hellman, F.; Berkowitz, A. E. Finite Size Effects on the Moment and Ordering Temperature in Antiferromagnetic CoO Layers. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 67, 054408. (26) Zeng, R.; Wang, J. Q.; Chen, Z. X.; Li, W. X.; Dou, S. X. The Effects of Size and Orientation on Magnetic Properties and Exchange Bias in Co3O4 Mesoporous Nanowires. J. Appl. Phys. 2011, 109, 07B520. (27) Chen, J. S.; Zhang, Y.; Lou, X. W. D. One-Pot Synthesis of Uniform Fe3O4 Nanospheres with Carbon Matrix Support for Improved Lithium Storage Capabilities. ACS Appl. Mater. Interfaces 2011, 3, 3276−3279. (28) Wang, H.; Shen, J.; Li, Y.; Wei, Z.; Cao, G.; Gai, Z.; Hong, K.; Banerjee, P.; Zhou, S. Porous Carbon Protected Magnetite and Silver Hybrid Nanoparticles: Morphological Control, Recyclable Catalysts, and Multicolor Cell Imaging. ACS Appl. Mater. Interfaces 2013, 5, 9446−9453. (29) Ohnishi, S.; Freeman, A. J.; Weinert, M. Surface Magnetism of Fe(001). Phys. Rev. B: Condens. Matter Mater. Phys. 1983, 28, 6741− 6748. (30) Tompsett, D. A.; Parker, S. C.; Islam, M. S. Surface Properties of α-MnO2: Relevance to Catalytic and Supercapacitor Behaviour. J. Mater. Chem. A 2014, 2, 15509−15518. (31) Jamet, M.; Wernsdorfer, W.; Thirion, C.; Dupuis, V.; Mélinon, P.; Pérez, A.; Mailly, D. Magnetic Anisotropy in Single Clusters. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 69, 024401. (32) Liu, R.; Jiang, Y.; Fan, H.; Lu, Q.; Du, W.; Gao, F. Metal Ions Induce Growth and Magnetism Alternation of α-Fe2O3 Crystals Bound by High-Index Facets. Chem.Eur. J. 2012, 18, 8957−8963. (33) Lv, B.; Liu, Z.; Tian, H.; Xu, Y.; Wu, D.; Sun, Y. H. SingleCrystalline Dodecahedral and Octodecahedral α-Fe2O3 Particles Synthesized by a Fluoride Anion-Assisted Hydrothermal Method. Adv. Funct. Mater. 2010, 20, 3987−3996. (34) Wu, W.; Yang, S.; Pan, J.; Sun, L.; Zhou, J.; Dai, Z.; Xiao, X.; Zhang, H.; Jiang, C. Metal Ion-mediated Synthesis and Shapedependent Magnetic Properties of Single-crystalline α-Fe2O3 Nanoparticles. CrystEngComm 2014, 16, 5566−5572. (35) Li, W.; Cui, X.; Zeng, R.; Du, G.; Sun, Z.; Zheng, R.; Ringer, S. P.; Dou, S. X. Performance Modulation of α-MnO2 Nanowires by Crystal Facet Engineering. Sci. Rep. 2015, 5, 8987. (36) Shi, R.; Chen, G.; Ma, W.; Zhang, D.; Qiu, G.; Liu, X. Shapecontrolled Synthesis and Characterization of Cobalt Oxides Hollow Spheres and Octahedra. Dalton Trans. 2012, 41, 5981−5987. (37) Roth, W. L. The Magnetic Structure of Co3O4. J. Phys. Chem. Solids 1964, 25, 1−10. (38) Takada, S.; Fujii, M.; Kohiki, S.; Babasaki, T.; Deguchi, H.; Mitome, M.; Oku, M. Intraparticle Magnetic Properties of Co3O4 Nanocrystals. Nano Lett. 2001, 1, 379−382. (39) Yin, K.; Ji, J.; Shen, Y.; Xiong, Y.; Bi, H.; Sun, J.; Xu, T.; Zhu, Z.; Sun, L. Magnetic Properties of Co3O4 Nanoparticles on Graphene Substrate. J. Alloy. Comp. 2017, 720, 345−351. (40) Makhlouf, S. A. Magnetic Properties of Co3O4 Nanoparticles. J. Magn. Magn. Mater. 2002, 246, 184−190. (41) Salabaş, E. L.; Rumplecker, A.; Kleitz, F.; Radu, F.; Schüth, F. Exchange Anisotropy in Nanocasted Co3O4 nanowires. Nano Lett. 2006, 6, 2977−2981. (42) Xie, X.; Li, Y.; Liu, Z.-Q.; Haruta, M.; Shen, W. Lowtemperature Oxidation of CO Catalysed by Co3O4 Nanorods. Nature 2009, 458, 746−749. (43) Yang, H. G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Anatase TiO2 Single Crystals with a Large Percentage of Reactive Facets. Nature 2008, 453, 638−641. (44) Benitez, M. J.; Petracic, O.; Salabas, E. L.; Radu, F.; Tüysüz, H.; Schüth, F.; Zabel, H. Evidence for Core-shell Magnetic Behavior in Antiferromagnetic Co3O4 Nanowires. Phys. Rev. Lett. 2008, 101, 097206. (45) Giri, S.; Patra, M.; Majumdar, S. Exchange Bias Effect in Alloys and Compounds. J. Phys. Condens. Matter 2011, 23, 073201. 19246


Research Article

ACS Applied Materials & Interfaces (46) Benitez, M. J.; Petracic, O.; Salabas, E. L.; Radu, F.; Tüysüz, H.; Schüth, F.; Zabel, H. Evidence for Core-Shell Magnetic Behavior in Antiferromagnetic Co3O4 Nanowires. Phys. Rev. Lett. 2008, 101, 097206. (47) Salabaş, E. L.; Rumplecker, A.; Kleitz, F.; Radu, F.; Schüth, F. Exchange Anisotropy in Nanocasted Co3O4 Nanowires. Nano Lett. 2006, 6, 2977−2981. (48) Jeng, S.-P.; Henrich, V. E. Direct Observation of Defect-induced Surface Covalency in Ionic CoO (1 0 0). Solid State Commun. 1990, 75, 1013−1017. (49) Cui, X. Y.; Delley, B.; Freeman, A. J.; Stampfl, C. Magnetic Metastability in Tetrahedrally Bonded Magnetic III-Nitride Semiconductors. Phys. Rev. Lett. 2006, 97, 016402. (50) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (51) Chen, J.; Selloni, A. Electronic States and Magnetic Structure at the Co3O4(110) Surface: A First-principles Study. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 085306. (52) Chen, J.; Wu, X.; Selloni, A. Electronic Structure and Bonding Properties of Cobalt Oxide in the Spinel Structure. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 245204. (53) Hagelin-Weaver, H. A. E.; Hoflund, G. B.; Minahan, D. A.; Salaita, G. N. Electron Energy Loss Spectroscopic Investigation of Co Metal, CoO, and Co3O4 Before and After Ar+ Bombardment. Appl. Surf. Sci. 2004, 235, 420−448. (54) Langell, M. A.; Kim, J. G.; Pugmire, D. L.; McCarroll, W. Nature of Oxygen at Rocksalt and Spinel Oxide Surfaces. J. Vac. Sci. Technol., A 2001, 19, 1977−1982. (55) Frost, D. C.; McDowell, C. A.; Woolsey, I. S. Evidence for Multiplet Splitting of 2p Photoelectron Lines of Transition Metal Complexes. Chem. Phys. Lett. 1972, 17, 320−323. (56) Grosvenor, A. P.; Wik, S. D.; Cavell, R. G.; Mar, A. Examination of the Bonding in Binary Transition-metal Monophosphides MP (M = Cr, Mn, Fe, Co) by X-ray Photoelectron Spectroscopy. Inorg. Chem. 2005, 44, 8988−8998. (57) Xiao, X.; Liu, X.; Zhao, H.; Chen, D.; Liu, F.; Xiang, J.; Hu, Z.; Li, Y. Facile Shape Control of Co3O4 and the Effect of the Crystal Plane on Electrochemical Performance. Adv. Mater. 2012, 24, 5762− 5766. (58) Wang, Y.; Zhong, Z.; Chen, Y.; Ng, C. T.; Lin, J. Controllable Synthesis of Co3O4 from Nanosize to Microsize with Large-scale Exposure of Active Crystal Planes and Their Excellent Rate Capability in Supercapacitors Based on the Crystal Plane Effect. Nano Res. 2011, 4, 695−704. (59) Su, D.; Dou, S.; Wang, G. Single Crystalline Co 3 O 4 Nanocrystals Exposed with Different Crystal Planes for Li-O2 Batteries. Sci. Rep. 2014, 4, 5767. (60) Cui, L.; Li, J.; Zhang, X.-G. Preparation and Properties of Co3O4 Nanorods as Supercapacitor Material. J. Appl. Electrochem. 2009, 39, 1871−1876. (61) Liu, Z.; Ma, R.; Osada, M.; Takada, K.; Sasaki, T. Selective and Controlled Synthesis of α-and β-cobalt Hydroxides in Highly Developed Hexagonal Platelets. J. Am. Chem. Soc. 2005, 127, 13869−13874. (62) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab initio Total-energy Calculations Using a Plane-wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186.

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