Crystal Facet Effects on Nanomagnetism of Co3O4 - ACS Publications

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Crystal Facet Effects on Nanomagnetism of Co3O4 Wenxian Li, Yan Wang, Xiang Y. Cui, Shangjia Yu, Ying Li, Yemin Hu, Mingyuan Zhu, Rongkun Zheng, and Simon P. Ringer ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03934 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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ACS Applied Materials & Interfaces

Crystal Facet Effects on Nanomagnetism of Co3O4 Wenxian Li1,4,5*, Yan Wang1, Xiang Yuan Cui2**, Shangjia Yu1, Ying Li1,4***, Yemin Hu1, Mingyuan Zhu1, Rongkun Zheng3 and Simon P. Ringer2

1

Institute of Materials, School of Materials Science and Engineering, Shanghai University, 149 Yanchang Road, Shanghai, 200072, China

2

Australian Centre for Microscopy and Microanalysis, and School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, NSW 2006, Australia 3

School of Physics, The University of Sydney, Sydney, NSW 2006, Australia

4

Istitute for Sustainable Energy, Shanghai University, Shanghai 200444, China

5

Shanghai Key Laboratory of High Temperature Superconductors, Shanghai 200444, China

*E-mail: [email protected] (W. X. Li). **E-mail: [email protected] (X. Y. Cui) ***E-mail: [email protected] (Y. Li).

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 exposed (1 0 0), (1 1 0), (1 1 1), and (1 1 2) were synthesized using hydrothermal method in 1

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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 due to 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

Introduction Nanomagnetism denotes the variation of the magnetic performance for crystals decreasing into submicron level 1,2. The configuration environments of atoms or ions are modified due to 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

3,4,5

, exchange-bias

6,7,8

, 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 were elucidated in terms of the exotic moment coupling compared with the primitive magnetic order. 2


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Exogenous origins include doping effect and heterogeneous effect. Doping technique were employed to generate diluted magnetic semiconducting properties in diamagnetic ZnO nanocrystals to generate room temperature magnetic behaviors

11,12,13,14

. Heterogeneous structures show practical potentials on the

generation of exchange-bias performance in CoO/Al-ZnO multilayer structure bilayers

7,8,16

, FeO/CoFe2O4 nanocrystals

15

, La0.7Sr0.3MnO3/YMnO3

17

, and Co/CoO core-shell nanowires 6. Similar with the

heterogeneous effects, the SiO2 coating layer can modify the performance of SiO2 coated Fe3O4

18

,

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, as well as the interparticle interaction effect. The particle size can change the magnetic behaviors of nanocrystal in comparison with corresponding bulk counterpart. Size dependence of the nanomagnetic behavior were found in Fe3O4 5, Cr2O3 20, Co3O4 21, NiO 22, 23

, and CoO

24 25

. The phenomenon is obvious in ferromagnetic and antiferromagnetic materials because

the equilibrium domain sizes of bulk materials may be larger than 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. High specific surface area is one of the most unique characteristics of nanocrystal, which induces massive relaxed atoms on the exposed surface. Then the particle surface becomes the dominant role for the magnetic performance in the nanocrystal with small enough sizes. The moments in the nanoparticle surface layer can pattern in different ways compared with those in the core due to the missing rigorous confinement from the surrounding moments. Ohnishi et al. theoretically concluded that the surface-layer magnetic moment was found to 3



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increase by 0.73 to 2.98 /atom from the center layer 29. Islam et al. calculated the moments of Mn4+ on different exposed crystal planes in α-MnO2 based on the density functional theory (DFT). The moments of (1 0 0) and (1 1 0) are 3.70 , which is a little bit higher than that in the bulk materials, about 3.12 , due to their similar atom arrangements with bulk materials and low coordination numbers. While the moments on (1 1 1) and (1 1 2) planes increase to 4.31 . The moments on (2 1 1) is decreased because the electrons in MnO4 tetrahedron are in the 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 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 constants variation 31. Liu et al. prepared two batches of α-Fe2O3, i.e. quasi-cubic bound by (0 1 2), (1 0 2), and (1 1 2) facets and bipyramid bound by {0 1 2} facets

32

. The quasi-cubic nanoparticles

show defect ferromagnetism without Morin transformation at 240 K, while the hexagonal bipyramid counterparts display spin-canted ferromagnetism above 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 the shapes of cube exposing (1 0 4) and (1 1 0) planes and thorhombic shapes exposing (0 1 2) planes

34

. The nanocubes show ferromagnetism in the whole measurement region with a block

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 compared with the quasi-cubes synthesized by Liu et al. 32. The different magnetic performance is attributed to the (1 1 2) plane exposed in 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 due to the unknown electron spin 4


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state and coupling mode motivated by the diverse nanostructures. Li et al. tuned the magnetism of α-MnO2 35

nanowires by inducing different exposed planes

. Although the intrinsic magnetisms are both

antiferromagnetic for (1 1 0) and (2 1 0) plane exposed nanowires, obvious ferromagnetic behavior can be detected for the sample with (1 1 0) exposed planes. Furthermore, first principles calculations confirm the intrinsic ferromagnetic coupling of the Mn4+ ions on the (1 1 0) plane exposed surface.

In this work, Co3O4 with spinel crystal structure was chosen as the research object for its bi-valence states of Co ions compared with sole valence of Fe and Mn ions in α-Fe2O3 and α-MnO2. Co3O4 is indexed as 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 levels by the octahedral field. Each Co2+ ion possesses a magnetic moment of 3.02 , and each Co2+ ion is surrounded by four nearest neighbors with opposite spins. Hence, bulk Co3O4 behaves as an antiferromagnet with a Néel temperature (TN) of 40 K 37

. The nanocrystallization technology expends the antiferromagnetism of Co3O4 into novel ways. Takada et

al. dispersed 3 nm Co3O4 nanocrystals into SiO2 matrix and the system shows superparamagnetism with 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 the ~7.5 nm Co3O4 nanoparticles dispersed on graphene substrate

show ferromagnetism, or superparamagnetism for the missing coercivity 39. Makhlouf demonstrated 20 nm size Co3O4 particles exhibited a phase transition at TN ≈ 25 K which may be ascribed to a finite size effect 40. Salabas et al. prepared Co3O4 nanowires with 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.

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Here, a novel strategy is employed to tune the magnetic behavior of Co3O4 based on the crystal facet engineering 42, 43. Nanocubes enclosed by (1 0 0) (defined as Co3O4-100) crystal planes as well as nanorods, hexagonal nanoplatelets and nanolaminars 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 hydrothermal method to studied their magnetic properties. Different magnetic behaviors were found in Co3O4 nanocrystals exposed with various 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 modulating the magnetic behavior of nanomaterials.

Results Microstructure evolution: Figure 1 displays the SEM images and XRD patterns of the precipitates after hydrothermal reaction using different approaches. The precursor of Co3O4-100 is nanocubes with a side length of ~200 nm as shown in Figure 1a and 1b. The indexed XRD pattern reveals that the precipitates are Co3O4 with spinel structure. Co3O4-110 precursor is a kind of nanorods as indicated by the SEM images shown in Figure 1d and 1e. The nanorods are 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 comprised of main phase Co(OH)2 and a small amount of Co3O4. Co3O4-112 precursor shows as 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 TEM images as 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 image (inset of Figure 2b). It is concluded that the 6


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cube exposes {100} crystal planes combing the indexed fast Fourier transformation (FFT) pattern (inset of Figure 2b) and the high resolution TEM image. The refined XRD pattern of the final products after being calcined at 500 oC for 3 h, as shown in Figure 2c, do not show obvious difference between that of the precursor, which may be attributed to the high crystallization of the precursor as deposited in the hydrothermal process. All the 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 Rietveld’s profile technique in the cubic structure with Fd-3m space group. The unit cell parameters (a = b = c) along with the agreement factors (Rp, Rwp) were calculated and represented in Table s1. All unit cell parameters are in good agreement with the normal spinel Co3O4 structure.

7



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 space group of Co3O4. (d,e) Microstructure observation indicates all diffraction peaks coming from 3 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) 8


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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.

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 the exposed facets are (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)

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TEM image of Co3O4-110. The insets show the high-resolution image and FFT pattern, which indicate the dominant exposed facets are (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 Co3O4-111. (h) TEM image of Co3O4-111. The insets show the high-resolution image and FFT pattern, which indicate the dominant exposed facets are (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 the dominant exposed facets are (1 1 2) planes. (l) Indexed XRD pattern with refinement results of Co3O4-112 (refined with Rietica, weighted profile R-factor, Rwp: 8.00).

Co3O4-110, Co3O4-111, and Co3O4-112 underwent phase transition during the 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 length ranging from 50 to 200 nm and width about 10 nm. In Figure 2e, reconstructed nanorods were observed due to the calcination of precursor, which is a common phenomenon in a similar process of transformation from precursor to Co3O4. HRTEM image showed in Figure 2e demonstrates (1 1 3) and (0 0 2) crystal planes with 0.242 and 0.406 nm lattice spacings, respectively, growing along [1 1 0] direction. The indexed FFT pattern also supports the nanorods have {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 oC in air for 3 h as indicated by the refined XRD pattern as shown in Figure 2f. Figures 2g and 2h display the SEM and TEM images of hexagonal nanoplatelets. The shape remains unchanged compared to the precursor. But collapsed parts are distributed on platelets. The HRTEM image showed in Figure 2h exhibits (2 2 0), (2 0 2) and (0 2 2) crystal planes with 0.286 nm d-spacing and interfacial angle of 60°, which make clear that the nanoplatelets are exposed with {1 1 1} facets. The FFT 10


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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 and 2k present the SEM and TEM images of nanolaminars. The HRTEM images of selected parts of the sample illustrate (1 1 1), (1 3 1), and (2 2 0) planes which possess 0.467, 0.244, and 0.286 nm respectively, grow along [1 1 2] zone axis. Although holes and cracks are observed in the nanolaminars, the main exposed facet is (1 1 2) plane as indicated by the HRTEM and indexed FFT pattern (inset of Figure 2k). Figure 2l shows the indexed XRD pattern with 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.

Magnetic performance: Co3O4 has been reported as an antiferromagnetic substance with the Néel temperature TN ≈40 K

37

. The magnetization versus temperature plots for four Co3O4 nanocrystals are

showed in Figure 3. Both zero-field-cooled (ZFC) and field-cooled (FC) magnetization were measured from 5 K to 350 K under 100 Oe field. Peaks located around 37 K can be found for both ZFC and FC measurements in all the samples, which are responsible for antiferromagnetic transition and known as TN. ZFC and FC curves bifurcate 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 high temperature 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)

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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, while C values are 1.02×10-2 emu K g−1 Oe−1, 1.10×10-2 emu K g−1 Oe−1, 1.23×10-2 emu K g−1 Oe−1 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, ||/ = + ⁄ − , with the magnetic moment of Co2+. The interaction intensity is increasing with the value of ||/ . 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 principle calculation are shown in Figure s2 and the Co3+ close to or on different exposed planes possess variable magnetic moments and spin directions. Due to the big specific surface areas of nanoparticles, the induced magnetic moments exhibit profound influence on the magnetic behaviors.

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Figure 3: Magnetization versus 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.

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Figure 4: Magnetization versus 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 keeps antiferromagnetic behavior. While the other samples show ferromagnetic like behaviors. The insets display the enlarged parts around origin of coordinates.

The magnetizations as a function of applied field (M-H curves) were measured between ±20 kOe at 5 K of four samples, as shown in Figure s3. Figure 4 exhibited 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 about 0.067 emu g−1 and a large coercivity 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 are a virtually linear shape between the applied field range. Such a shape of the curves is expected from an antiferromagnetic system

14


44

, where the

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antiferromagnetic ordered core is the dominant contribution and the scarce net magnetic spins on the surface paly 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 relatively weak remnant magnetism of 0.0058 emu g−1 and coercivity about 61.5 Oe. Unsaturated hysteresis loops at ±20 kOe in Co3O4-100 is a common phenomenon in nanocrystalline alloys, compounds and metal oxides

45 46

. An open hysteresis loop can be

clearly seen in 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.

Discussion Density functional theory (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 showed in Figure s4, where the energy of t2g levels are lower than 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 no unpaired electrons so that the Co3+ ions show no permanent magnetic moment. As for Co2+ ions in tetrahedral coordination, eg levels in energy are lower than t2g levels, but t2g(α) levels in energy are lower in eg(β) levels. Seven 3d electrons of Co2+ will fill completely in eg(α), t2g(α) and

eg(β) levels in proper order with no electron filling in t2g(β) levels, which causes t2g levels in tetrahedral coordination exist unpaired electrons to form magnetic moment

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 strucrure

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for the modification of Co3O4 nanomagnetism.

DFT calculations within the generalized gradient approximation (GGA)

50

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 5.5 eV for all Co ions 51. For bulk Co3O4, the calculated ground state is antiferromagnetic with the contribution of tetrahedral high spin state Co2+ (S=3/2) of about 2.86 , in fine agreement with the results of Chen et al.

52

and comparable with the experimental value 3.2

37

. The corresponding ferromagnetic

state is higher in energy by 0.19 eV per unit cell. Note 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 ions configuration in Figure s5. As the 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 surface. Four supercell slab moldels 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 vacume region of 15 Å in thickness. For all these surfaces models, there is no inversion symmetry, meaning each involves two inequivalent surfaces, called A and B.

Figure 5a, b display 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 non-magnetic therein. The magnetic moments of outmost layer of Co2+ ions are varied due to the deformed lattice and missed coordinated ions. On (1 0 0)-A, the

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sublayer Co3+ ions show magnetic moment of 1.74 in the same spin orientation. Furthermore, the outmost layer O2- ions also possess sizable magnetic moment of 0.2 in the same direction of Co3+. In contast, on (1 0 0)-B, the moment of the exposed Co3+ ions is only 0.15 . The induced magnetic moments of Co3+ and O2- are responsible for the net magnetic moment of 7.72 /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 (1 1 0) surfaces, (1 1 0)-A surface layer involves both Co2+ and Co3+ exposed ions, while (1 1 0)-B surface layer contains only Co3+ ions. Co2+ ions keep the same antiferromagnetic states as in the bulk with slightly intensity variation. Interestingly, while on (1 1 0)-A, Co3+ ions are non-magentic, on (1 1 0)-B, Co3+ ions are antiferromagnetically coupled, each with a moment of 1.69 . Small magnetic moments were observed on O2- ions with opposite spin, namely 0.24 on (1 1 0)-A and 0.29 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.

Due to 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+ ion are antiferromagetic coupled and the Co3+ ions are non-magnetic (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 (1 1 1)-A surface, 1.02 , which align in the same direction. The O atoms locating on the fourth sublayer

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possess size moment, 0.24 . In contast, the (1 1 1)-B surface terminated with O layer is structurally robust againt relaxtion. The moment of outmost O atoms on (1 1 1)-B surface is very small, only 0.06 . For (1 1 1) surface model, the net moment is 2.97 per cell. Thus, the ferromagnetic behavior observed in Figure 4c originates from the 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 thicker slab model may lead to smaller net magnetization due to the effect of overlapping bands.

The simulated results of (1 1 2) exposed plane reveal even more intensive lattice deformation than that in (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 , respectively. It should be noted that the spin direction of moment on site A is opposite with those on sites B and C. Furthermore, not all the Co3+ ions own magnetic moments in the sublayers, even some ions are almost exposed to the vacuum. The total net magnetic moment is 1.36 per unit cell, which is lower than those of (1 0 0) and (1 1 1) exposed cases. While the residual magnetization and coercivity of Co3O4-112 are quite low with typical antiferromagnetic 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.

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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 plane exposed Co3O4. Magnetic moments are induced on the Co3+ ions exposed on surface. (c,d) Oblique view and side view of (1 1 0) crystal plane exposed 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 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 iso-value of 0.01.

Configuration environments of Co2+ and Co3+: Co3O4 contains mixed valence states of Co2+ and Co3+ due to the spinel crystal structure. The surface sensitive X-ray photoelectron spectroscopy (XPS) was applied to examine the valence state of each element especially Co ions in Co3O4. Survey patterns were 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 standard peak of C 1s at 284.8 eV. Figure 6 displays O 1s photoelectron core level

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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, 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 peak1 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 in consistent with the peak centered at around 529.6 eV (labeled as peak 3 in Figure 6). While 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 the peak 2 of the four samples, it is concluded that the surface states account for a substantial proportion.

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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.

Figure 7 demonstrate the Co 2p XPS patterns which all are 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 were fitted with four Gaussian-Lorentz peaks as marked in spectra, where p1-p4 are responsible for observed 2p1/2 peak, p5-p8 for the 2p3/2 peak, respectively. All odd peaks are produced by Co3+ while even peaks by Co2+. The ratio of Co3+ and Co2+ peaks are 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 binding energy peaks such as p1and p2, p5 and p6, and lower binding energy peaks p3 and p4, p7 and p8 constitute the bulk and surface plasmon loss peaks 56. The ratio 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 Co3O4-112, respectively, in the detectable depth of the samples. The results indicate that Co ions in the high-index facets exposed samples show increased surface proportions due to the intensive lattice distortion. While the first principle 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 high specific surface area have a chance to show high surface proportion.

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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 the Co3+ in the bulk structure of spinel Co3O4. p2 and p6 are attributed to the Co2+ in the bulk structure. p3 and p7 are attributed to the Co3+ on or near the exposed surface. p4 and p8 are attributed to the Co2+ on or near the exposed surface.

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 as 111-Oct 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 octahedron shape with edge length of about 100 nm and is composed of Co3O4 as indicated by Figure 8a-c. The final product of 111-Otc keeps the shape of the precursor as indicated by the SEM and TEM images as shown by 22


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0), (2 0 2 ), and (0 2 2) planes in Figures 8d and 8e. The high-resolution image of TEM image show (2 2

the inset of Figure 8e. The FFT pattern confirms the exposed surface planes are (1 1 1) for 111-Otc, which is composed with pure Co3O4 as indicated by the XRD pattern shown in Figure 8f. The ZFC and FC magnetization under 100 Oe field indicate the antiferromagnetic nature of 111-Otc 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 quite similar behavior with those of Co3O4-111, on which the magnetization deviates from linear behavior. Figure 8h shows the M-H curves in the range of ±10 kOe and the inset shows the coercivity field of 46.2 Oe and 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 the inset indicates the open ending of the M-H curves at 20 kOe. The phenomenon is attributed to the ferromagnetic coupling of the Co3+ on the exposed (1 1 1) planes of the octahedral crystals.

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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 the dominant exposed facets are (1 1 1) planes. (f) Indexed XRD pattern of 111-Oct. (g) Magnetization versus temperature plots for 111-Oct. (h) Magnetization versus applied field plots for 111-Oct in the range of ±10 kOe. The inset magnifies the M-H curves near 0 Oe. (i) Magnetization versus applied field plots for 111-Oct in the range of ±20 kOe. The inset magnifies the M-H curves near 20 kOe.

The precursor of 112-Hex is in shape of hexagon and composed of β-Co(OH)2 as shown in Figures 9a-c. The edge length is about 2 µm. The thermolysis of β-Co(OH)2 keeps the hexagon shape of the nanocrystals and produces pure Co3O4 as shown in Figure 9d-e. The main exposed crystal plane of the hexagonal platelet

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is (1 1 2) as indicated by the high-resolution TEM image and FFT pattern of 112-Hex as shown in the insets of Figure 9d. 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 Figures 9h and 9i. The magnetic coupling behavior is similar with those of Co3O4-112. The comparison of magnetic behaviors of 111-Oct and 112-Hex with Co3O4-111 and Co3O4-112 indicates that the nanomagnetic performance has weak dependence on the shape and size of the nanoparticles. While the exposed crystal planes play a dominant role to generate different magnetic performance of nanocrystals.

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.

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(e) TEM image of 112-Hex. The insets show the high-resolution image and FFT pattern, which indicate the dominant exposed facets are (1 1 2) planes. (f) Indexed XRD pattern of 112-Hex. (g) Magnetization versus temperature plots for 112-Hex. (h) Magnetization versus applied field plots for 112-Hex in the range of ±10 kOe. The inset magnifies the M-H curves near 0 Oe. (i) Magnetization versus applied field plots for 112-Hex in the range of ±20 kOe. The inset magnifies the M-H curves near 20 kOe.

Conclusion In summary, the magnetic behavior of Co3O4 nanocrystal with different exposed crystal planes were 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 show strong dependence on the exposed crystal plane, i.e. (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 due to the missing or deformed configuration environments, which induces net magnetic moments in the Co3O4 nanocrystals. While 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 (1 1 2) surface have induced magnetic moments as indicated by the DFT calculations. However, the (1 1 2) exposed nanolaminars display antiferromagnetism due to the long interaction distance between the magnetized Co3+ ions.

Methods Synthesis of Co3O4-100 nanocubes: In a typical procedure

57

, 0.02 mol Co(NO3)2·6H2O and 0.01 mol

NaOH were dissolved in 40 mL deionized water under stirring. The mixed reactants were transferred into a

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50 mL Teflon-lined autoclave sealed by the stainless-steel jar and heated at 180 oC for 5 h. After cooling to room temperature, the products were collected by centrifugation and washed with ethanol and deionized water several times, subsequently dried at 60 oC in vacuum. The final products were calcined at 500 oC for 3 h.

Synthesis of Co3O4-110 nanorods: In a typical synthesis

58

, 10 mL of aqueous ammonia and 25 mL

ethylene glycol (EG) were mixed to form a homogeneous solution. 1.5 mL of 1 mol/L aqueous sodium carbonate solution was then added and stirred for a few minutes under magnetic stirring. Afterwards, 5 mL of 1 mol/L aqueous cobalt nitrate solution was added to the mixture by continuous stirring for 20 mins. The resulting solution was transferred into a Teflon-lined stainless-steel autoclave with a volume of 50 mL which was then heated to 170 oC and maintained at the temperature for 17 h. The cooled suspension was centrifuged, and the precipitate was rinsed with deionized water and ethanol several times, then dried at 60 o

C in vacuum to get Co(CO3)0.5(OH)0.11·H2O nanorods. The Co3O4 nanorods were obtained after calcining at

300 oC in air for 3 h.

Synthesis of Co3O4-111 hexagonal nanoplatelets: In a typical process

59

, 1.2 g of Co(NO3)2·6H2O were

dissolved in a mixed solution with 5 mL deionized water and 5 mL ethanol. Then 1 g of polyvinylpyrrolidone (PVP) were added as a surfactant and stirred for 30 mins. Following 25 mL of 0.4 mol/L NaOH aqueous solution was added drop by drop, taking 90 mins accompany with the color evolution from light red to dark green. The resulting suspension was transferred into a 50 mL Teflon-lined autoclave. The autoclave was heated at 120 oC 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 β-Co(OH)2 precursor at 450 oC for 2 h in air.

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Synthesis of Co3O4-112 nanolaminars: {1 1 2} facets exposed Co3O4 nanolaminars were obtained by calcining the Co2(OH)2CO3 precursor which were produced by a simple hydrothermal method 60. Briefly, 2 mmol Co(NO3)2·6H2O, 8 mmol urea (CO(NH2)2), and 0.4 g of cetyltrimethylammonium bromide (CTAB) as a soft template were dissolved in 40 mL of deionized water. The mixture was stirred for 30 minutes and transferred into a 50 mL Teflon-lined autoclave which was then heated and maintained at 140 oC for 12 h. After cooling to the room temperature, the suspension was centrifuged and rinsed with deionized water and ethanol several times respectively. The final mesoporous Co3O4 nanolaminars was obtained by calcining the Co2(OH)2CO3 precursor at 450 oC for 2 h in air.

Synthesis of 111-Oct octahedrons: In a typical process ， 0.01 mol Co(NO3)2·6H2O and 0.01 mol CoCl2·6H2O were dissolved in 30 mL deionized water. After stirring 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 oC for 5 h. After cooling down, the products were centrifuged and washed with ethanol and deionized water several times, subsequently dried at 60 oC in vacuum. The final products were calcined at 400 oC for 3 h.

Synthesis of 112-Hex hexagonal nanoplatelets: Co3O4 hexagonal nanoplatelets exposed {1 1 2} facets were produced by calcining the β-Co(OH)2 precursor which were obtained via a homogeneous precipitation method

61

. Briefly, 5 mmol CoCl2·6H2O and 60 mmol hexamethylenetetramine (HMT) were dissolved in

200 cm3 of a 9:1 mixture of deionized water and ethanol. The solution was heated to 90 oC with magnetic stirring for 1 h. The resulting green suspension was centrifuged and washed with ethanol and deionized

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water several times and then dried in air at room temperature to obtain β-Co(OH)2 precursors. The precursors were calcined at 400 oC for 3 h to obtain Co3O4 nanoplatelets.

Characterizations: All samples were examined by X-ray diffraction (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 scanning electron microscopy (FEG-SEM: JSM-7500F) and transmission electron microscopy (TEM: JEOL-2100) with high resolution TEM (HRTEM) to further analyze the microstructures. X-ray photoelectron spectroscopy (XPS: EscaLab 250-IXL, 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 (VSM) model physical properties measurement system (PPMS: Quantum Design, 14 T) in applied magnetic fields up to 20 kOe. The specific surface areas were collected by N2 adsorption isotherm using a Autosorb-iQ2 analyzer at 77 K.

First principles simulation: Extensive spin-polarized DFT calculations were performed using the VASP code

62

. The plane wave basis set cutoff energy 500 eV was used. The Monkhorst-Pack grids of (8x8x8)

were used for the 56-atom unit cell. For the supercells, the grids have been 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 are less than 0.01 eV/Å.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.xxxxxxxx.

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Comparison of the specific surface areas, the ground magnetic states, the 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, the crystallographic structures of different exposed crystal planes of Co3O4, and the XPS survey patterns of Co3O4-100, Co3O4-110, Co3O4-111, and Co3O4-112 samples, and the crystal lattice parameters.

Notes The authors declare no competing financial interest.

Acknowlegement This work is ﬁnancially supported by 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. Wenxian Li 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.

Supporting Information Available Supporting Information is available from the Internet at http://pubs.acs.org. or from the author.

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Crystal Facet Effects on Nanomagnetism of Co3O4 - ACS Publications

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