Yin_et_al-2010-Angewandte_Chemie_(International_ed._in_English)

Nanoparticles DOI:10.1002/anie.201002557 Low-Symmetry Iron Oxide Nanocrystals Bound by High-Index Facets**

Jingzhou Yin,Zhinan Yu,Feng Gao,*Jianjun Wang,Huan Pang,and Qingyi Lu*

Single crystals have a basic property of anisotropy and exhibit different physical and chemical properties on various facets or in diverse directions.[1–3]Generally,the properties of nano-crystals can be finely tuned,spanning a range of applications, by their shape to determine surface atomic arrangement and coordination.[4–7]The surface properties of materials highly depend on the shape of the nanocrystals and have great influence on the activity of nanocrystals in chemical reac-tions.[1]Thus,unprecedented research efforts have been focused on the controllable preparation of micro-and nano-crystals with various geometries and exposed surfaces.To date,most of the synthesized nanocrystals are enclosed by low-index{111}and{100}surfaces.Examples include tetra-hedral,[8]octahedral,[9]decahedral,[10]and icosahedral[11]nano-crystals bound by{111}surfaces and nanocubes[12]enclosed by {100}surfaces to minimize surface energy.[2]Compared to the low-index facets,high-index facets usually have high surface energy and grow faster than the other facets,which makes them ultimately disappear during crystal growth.[3]However, also because of the high surface energy and the high density of atomic steps,ledges,and kinks of high-index facets,these facets can endow nanocrystals with high activity,thus promoting their potential applications as highly efficient catalysts and in special optical,electrical,and magnetic devices.[1,2]Accordingly,the synthesis of nanocrystals with exposed high-energy facets has become an important and challenging task.To date,there are just a few reports on the preparation of nanocrystals with exposed high-index facets. For example,Sun and co-workers first reported the synthesis of tetrahexahedral platinum nanocrystals in2007.[1]Then the groups of Han and Kuang reported the syntheses of gold nanocrystals with high-index facets(such as{110}).[2,3]In2009, Fang and co-workers reported the high-yield synthesis of elongated tetrahexahedral gold nanocrystals enclosed by24 {037}facets.[13]The compounds in these reports about exposed high-index facets are metal elements with simple cubic crystal systems,and there are few reports on the synthesis of binary compounds with complex crystal systems,except the synthesis of GeO2and TiO2with high-energy facets.[14–16]However, compared with metal elements,binary compounds and compounds that do not crystallize in the cubic crystal system are more complex and have wider applications.The preparation of these kinds of compounds with high-index surfaces exposed would bring materials with high and special activities,thus facilitating their potential applications and expanding their application ranges.

Herein,we report for the first time two kinds of iron oxide crystals in the hexagonal crystal system:tetrakaidecahedra and oblique parallelepipeds with high-index facets exposed. The tetrakaidecahedral form has a three-fold axis bound by {012},{102},and{001}facets,while the oblique parallelepiped form looks like a cube but with one angle that is approx-imately858bound by{012},{01à4},and{à210}facets.Owing to the fact that iron oxide belongs to the hexagonal system, but not to the cubic system,these exposed high-index facets are very special,and the two kinds of brand-new polyhedra have never been reported before.Magnetic studies uncovered that these two forms of iron oxide have distinct differences. The tetrakaidecahedral iron oxide nanocrystals might be spin-canted ferromagnetically controlled at room temperature, and the ferromagnetism disappears at temperatures lower than T m.The oblique parallelepiped nanocrystals might have coexistent spin-canted and defect ferromagnetism at room temperature and be defect ferromagnetically controlled at low temperature.

These two kinds of nanocrystals were obtained separately on a large scale through a simple reaction assisted by viscous macromolecules to adjust the reaction and growth rates.For the synthesis of tetrakaidecahedral iron oxide nanocrystals,a mixture of K3[Fe(CN)6](0.16g),N2H4solution(80%, 0.8mL),and 2.8g Là1sodium carboxymethyl cellulose (CMC,300–800MPa S,10mL)solution was kept at1608C under solvothermal conditions for6h.Powder X-ray diffrac-tion(XRD)confirms that the obtained product collected from the supernatant solution has hexagonal iron oxide structure(JCPDS84-0311)with high purity and crystallinity (Figure1a).The M?ssbauer spectrum(Figure S1a in the Supporting Information)of the sample at room temperature shows a single sextet and thus provides clear evidence for the presence of a-Fe2O3rather than g-Fe2O3or Fe3O4.[17]Fig-

[*]J.Z.Yin,Prof.F.Gao,J.J.Wang

Department of Materials Science and Engineering

Nanjing University,Nanjing210093(China)

E-mail:fgao@a58f944eb9d528ea80c7798b

J.Z.Yin,Z.N.Yu,H.Pang,Prof.Q.Y.Lu

State Key Laboratory of Coordination Chemistry

Coordination Chemistry Institute

Nanjing National Laboratory of Microstructures

Nanjing University,Nanjing210093(China)

E-mail:qylu@a58f944eb9d528ea80c7798b

J.Z.Yin

School of Chemistry and Chemical Engineering

Huaiyin Normal University,Huai’an223300(China)

[**]This work was supported by the National Natural Science Foundation of China(Grant Nos.50772047,20671049,and

20721002),the National Basic Research Program of China(Grant No.2007CB925102),and the Program for New Century Excellent

Talents in

University.

Supporting information for this article is available on the WWW

under a58f944eb9d528ea80c7798b/10.1002/anie.201002557.

Communications

6328 2010Wiley-VCH Verlag GmbH&Co.KGaA,Weinheim Angew.Chem.Int.Ed.2010,49,6328–6332

ure 2a shows a typical large-area scanning electron micro-scope (SEM)image of the sample,indicating the presence of homogeneous,well-shaped nanocrystals with sizes ranging from 200to 400nm.As shown in high-magnification SEM images (Figure 2b,c),the as-prepared nanocrystals are well-shaped polyhedra comprising two top surfaces and twelve side surfaces and are,thus,tetrakaidecahedral.The two top surfaces are triangles with side lengths less than 100nm.The cross-section in the middle of the crystal is an inequi-lateral hexagon with a trigonal axis.In other words,the tetrakaidecahedral crystals are bound by two top triangles,six side triangles,and six side trapezoids.This tetrakaidecahe-dron does not have a six-fold axis and is a shape rarely observed for nanocrystals.In Figure 2d,a geometrical model of an ideal tetrakaidecahedron enclosed by these crystal planes has been presented from side and top views,and it is in agreement with the as-prepared nanocrystals.The two top surfaces could be indexed to (001)and (00à1),respectively.The exposed side planes are (012),(102),(1à12),(0à12),(à102),(à112),and (01à2),(10à2),(1à1à2),(0à1à2),(à10à2),(à11à2).These results could be further confirmed by high-resolution transmission electron microscopy (HRTEM)images projected from the [241]direction.TEM images (Figure 2e,f)of the obtained sample display poly-hedron-like structures with uniform size around 400nm.The HRTEM image (Figure 2g)shows two groups of facets perpendicular to each other;their crystal plane spacings are

2.5and 2.7 ,which could be indexed to be (à210)and

(01à4),respectively.Thus,this HRTEM image of the tetrakaidecahedron nanocrystals can be indexed to the [241]zone axis of a single iron oxide crystal.Similar results could be obtained from the SAED pattern shown in the inset of Figure 2g.These results not only suggest that the nanocrystals are single crystals,rather than multiply twinned crystals,but they also are in good agreement with the ideal tetrakaideca-hedron model enclosed by {012}trapezoid-series facets,{102}triangle-series facets,and {001}top-series facets.Facets belonging to the same family usually have the same growth ratio;however,in the special tetrakaidecahedron,the facets in same family have been found to have two different growth ratios,which might open a door in crystal growth design.Also,

it can be found that some of the surfaces of iron

oxide crystals look rough.This phenomenon

might be caused by the secondary growth of

crystals in the basic solution.[18]

For the synthesis of the other kind of iron

oxide nanocrystals,a mixture of K 3[Fe(CN)6]

(0.16g),N 2H 4solution (80%,3mL),and

2.8g L à1CMC solution (10mL)was kept at

1608C under solvothermal conditions for 6h.

The product collected from the supernatant

solution is also confirmed to be hexagonal iron

oxide by XRD (Figure 1b)and M?ssbauer spec-

troscopy (Figure S1b in the Supporting Informa-

tion).SEM investigations reveal that the majority

of the sample is composed of quasi-cubic nano-

crystals with an average edge length of 300nm.

Figure 3a shows a representative SEM image of

the as-prepared product,indicating that the

obtained sample has quasi-cubic shape.From the

high-magnification SEM images shown in Fig-

ure 3b,c it could be seen that these quasi-cubes

seem to be oblique parallelepipeds with one

dihedral angel near 908(ca.858),which means

that the quasi-cubic crystal might not be enclosed

by {001}as usual.According to the standard data

of the hexagonal iron oxide crystal structure,(012)and (01à4)are perpendicular to (à210),and the dihedral angel between (012)and (01à4)is 858,which is in quite good agreement with the above oblique parallelepiped.Thus,these nanocrystals might be bound by {012},{01à4},and {à

210}.Figure 2.a–c)SEM images of the tetrakaidecahedral iron oxide nanocrystals.Scale bars:a)2m m,b)1m m,c)500nm.d)Side-view (left)and top-view (right)geo-metrical models of the tetrakaidecahedral iron oxide nanocrystals bound by {012},{102},and {001}facets.e,f)TEM images of the tetrakaidecahedral iron oxide nanocrystals.g)HRTEM image and SAED pattern (inset)of the tetrakaidecahedral iron oxide nanocrystals projected from the [241]direction.

6329Angew.Chem.Int.Ed.2010,49,6328–6332 2010Wiley-VCH Verlag GmbH &Co.KGaA,Weinheim a58f944eb9d528ea80c7798b

Figure 3d presents a geometrical model of an ideal oblique

parallelepiped enclosed by these facets in two different side

views,which are in agreement with the as-prepared nano-

crystals.Figure 3e–g show the sample s TEM images and the

HRTEM image and its FFT transformation.The TEM images also confirm that the obtained sample has oblique parallele-piped shape with edge length of about 300nm.The HRTEM image of an oblique parallelepiped nanocrystal shows two groups of facets which are at 858and have crystal plane spacings of 3.7and 2.7 ,corresponding to be (012)and (01à4)planes,respectively.Thus,the HRTEM image of the oblique parallelepiped nanocrystal is projected from the [100]zone axis of a single crystal of hexagonal iron oxide.Similar results could be obtained from its FFT transformation.These results suggest that the nanocrystals are single crystals bound by {012},{01à4},and {à210},in a good agreement with SEM results and the ideal oblique parallelepiped model.In our experiments,the tetrakaidecahedral and the oblique parallelepiped nanocrystals could be synthesized separately,both in high yields.These two kinds of crystals could be obtained on a large scale by a very simple reaction in the presence of sodium carboxymethyl cellulose and the addition of N 2H 4.Without the addition of N 2H 4and CMC,when aqueous K 3[Fe(CN)6]was treated under hydrothermal conditions at 1608C for 6h,a -Fe 2O 3micropine dendrites

were obtained with a size of several micrometers (Figure S2in the Supporting Information),which is the result of the weak dissociation of [Fe(CN)6]3àions under hydrothermal condi-tions and the fast growth along six crystallographically

equivalent directions of a -Fe 2O 3.A detailed

study of the dendritic micropine Fe 2O 3was also

reported.[19]The morphology of a -Fe 2O 3can be

dramatically changed in the presence of CMC.

When K 3[Fe(CN)6]in aqueous CMC was hydro-

thermal treated at 1608C for 6h but without the

addition of N 2H 4,the obtained Fe 2O 3nanoparti-

cles are cubic-like with a size of about 80nm,

much smaller than the size of the micropine

dendrites.As known,the CMC molecules have

many carboxymethyl side groups,which prevents

CMC backbones from getting close to each

other.[20]So the solution can be divided into

numerous “channels”by the CMC molecules in

the reaction system to confine the growth of a -

Fe 2O 3nanoparticles,thus leading to the formation

of Fe 2O 3nanoparticles with much smaller size.

With the addition of N 2H 4,the basic environment

would make the Fe 2O 3nanoparticles grow faster.

The different adsorption properties of the differ-

ent planes of Fe 2O 3would lead to the formation of

Fe 2O 3polyhedra with different exposed surfa-ces.[21]For the synthesis of tetrakaidecahedral iron oxide nanocrystals,0.8mL N 2H 4solution (80%)was used,while for the synthesis of the other kind of iron oxide nanocrystals,3mL N 2H 4solution (80%)was used.The amount of N 2H 4determines the final form of the crystals.As the amount of

N 2H 4is gradually changed from 0.8to 3mL,the

morphology of the iron oxide nanocrystals trans-

forms from pure terakaidecahedron to pure oblique paral-lelepiped.Figure S3a–c in the Supporting Information shows the SEM images of the products with different amounts of N 2H 4.With 1.0mL N 2H 4,the morphology of the products is almost tetrakaidecahedral,and very few oblique parallele-piped particles can be seen in Figure S3a.As the amount of N 2H 4increases,the percentage of tetrakaidecahedral Fe 2O 3particles decreases,while that of the oblique parallelepiped particles increases (Figure S3b,c).The amount of N 2H 4can influence the pH value of the solution.For comparison,we also used NH 3·H 2O as additive rather than N 2H 4,but only particles with spherical morphology can be obtained (see SEM images in Figure S4in the Supporting Information).From these results,the addition of N 2H 4and CMC is the key factor in the formation of the terakaidecahedral and oblique parallelepiped iron oxide nanocrystals with exposed high-indexed surfaces.

Hexagonal iron oxide is an important magnetic material,and it would be of great interest to investigate the magnetic properties of two a -Fe 2O 3nanocrystals with different shapes and different exposed high-index facets.a -Fe 2O 3is antiferro-magnetic below T N (955K);[22]intrinsic (spin-canted)ferro-magnetism and defect ferromagnetism occur in a -Fe 2O 3,and it shows weak ferromagnetic properties at room temper-

ature.[23]The magnetic phase transition from the spin-canted ferromagnetic phase to the antiferromagnetically ordered state has been reported to be at approximately 260K,which

leads to a sharp decrease in magnetization,called the Morin

transition temperature (T m ).[24]Unlike intrinsic

spin-canted Figure 3.a–c)SEM images of the oblique parallelepiped iron oxide nanocrystals.Scale bars:a)5m m,b)500nm,c)400nm.d)Geometrical model of the oblique parallelepiped iron oxide nanocrystal bound by {012},{01à4},and {à210}facets.R =Rotation.e,f)TEM images of the oblique parallelepiped iron oxide nanocrystal.g)HRTEM image and FFT transformation pattern (inset)of the oblique parallele-piped iron oxide nanocrystals projected from the [100]direction.

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a58f944eb9d528ea80c7798b 2010Wiley-VCH Verlag GmbH &Co.KGaA,Weinheim Angew.Chem.Int.Ed.2010,49,6328–6332

ferromagnetism,the defect ferromagnetism is sensitive to structure and is altered by stress or heating and suppresses the Morin transition.[23]Figure 4a–c shows the magnetization-temperature curves (field-cooled under 100Oe (FC)and zero-field-cooled (ZFC))of tetrakaidecahedral iron oxide nanocrystals and the corresponding hysteresis loops at 300and 50K.The M –T curves display a decrease at a temper-ature of approximately 230K,corresponding to Morin transition.This value is lower than the T m of the bulk iron oxide samples because of the size and shape dependence of T m .[19]The noticeable hysteresis at 300K clearly shows that tetrakaidecahedral iron oxide nanocrystals are in a weak ferromagnetic state with a coercive field of approximately 300Oe.At the lower temperature of 50K the hysteresis loop disappears and the sample reveals an antiferromagnetic state,which corresponds to the Morin transition.This result indicates that the tetrakaidecahedral iron oxide nanocrystals might be spin-canted ferromagnetically controlled,and the

ferromagnetism disappears at temperatures lower than T m .However,the magnetic behaviors of oblique parallelepiped nanocrystals are different.As shown in Figure 4d–f,the

magnetization-temperature curves just show a slight decrease

near 230K,corresponding to T m .The sample shows another decrease at temperature of approximately 120K.With

further decrease of the temperature,the oblique parallele-

piped nanocrystals still show weak ferromagnetic properties,which is different from the bulk iron oxide materials and the tetrakaidecahedral iron oxide nanocrystals.Correspondingly,the noticeable hysteresis at 300K clearly shows that oblique parallelepiped nanocrystals are in a weak ferromagnetic state with a coercive field of about 400Oe.The hysteresis loop measured at 50K shrinks but still exists,unlike that of tetrakaidecahedra.This finding means that the oblique parallelepiped sample is weakly ferromagnetic at low temper-

ature,in agreement with the M –T curves.These results show

that although the transition from spin-canted ferromagnetism

to antiferromagnetism is also observed in oblique parallele-

piped nanocrystals,it is not the controlled phase transition in

the sample.The oblique parallelepiped nanocrystals still show

weak ferromagnetism under T m ,which means a long-range

magnetic ordering still exists at low temperature.Similar

phenomena have been recently observed in a -Fe 2O 3nano-

tubes,for which the origin of the magnetic phase transition

was attributed to the defects in nanotubes coming from the

curl of layers.[25]In our case,the oblique parallelepiped

nanocrystals are surrounded by high-indexed crystal faces

having high surface energy and might show defect-controlled

ferromagnetism because of the structure-sensitivity of defect

ferromagnetism.[23]Although to reach a clear conclusion

requires further investigations,the oblique parallelepiped

nanocrystals might show coexistent spin-canted and defect

ferromagnetism at room temperature and be defect ferro-

magnetically controlled at low temperature.

In summary,unusual tetrakaidecahedral and oblique

parallelepiped iron oxide nanocrystals with exposed high-

index facets have been successfully synthesized in high yields

with the assistance of the viscous macromolecule sodium

carboxymethyl cellulose.The tetrakaidecahedral crystals

have a three-fold axis bound by two top surfaces ((001)and (00à1))and twelve side surfaces,including six triangles ((102),(0à12),(à112),(10à2),(0à1à2),(à11à2))and six trapezoids ((012),(1à12),(à102),(0à12),(1à1à2),(à10à2)).The oblique parallelepiped crystals have two-fold

axes enclosed by {012},{01à4},and {à210}facets.Magnetic

measurements confirm that these two kinds of nanocrystals

display shape-dependent magnetic behaviors.The tetrakai-

decahedral iron oxide nanocrystals might be spin-canted ferromagnetically controlled at room temperature,and the ferromagnetism disappears at temperatures lower than T m .The oblique parallelepiped nanocrystals might show coex-istent spin-canted and defect ferromagnetism at room tem-perature and be defect ferromagnetically controlled at low temperature.The proposed new and simple method could not only be developed for the syntheses of nanocrystals with various high-index facets exposed but also be beneficial to the exploration of materials with new properties.

Received:April 29,2010

Published online:July 26,2010

.Keywords:carbohydrates ·high-index facets ·iron oxide ·magnetic properties ·nanoparticles

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