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Phase-Selective Growth of Assembled FeSe2 Nanorods fromOrganometallic Polymers and Their Surface MagnetismJuan Xu,† Kwonho Jang,† Jeho Lee,† Hae Jin Kim,‡ Jaehong Jeong,§ Je-Geun Park,*,§ and Seung Uk Son*,†

†Department of Chemistry and Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Korea‡Korea Basic Science Institute, Daejeon 350-333, Korea§Department of Physics & Astronomy, Seoul National University, Seoul 151-747, Korea

bS Supporting Information

During the last two decades, well-defined metal, metal oxide,and metal chalcogenide nanocrystalline materials have been

extensively prepared via diverse wet-chemical synthetic approa-ches.1 As a synthetic example, hydrothermal or solvothermaltreatment of metal ions and additives resulted in diverse metal-containing nanocrystals.2 For more efficient size- or shape-controlled synthesis of materials, the colloidal approach has beenapplied using organic surfactants with long alkyl or alkenylchains.3 Usually, metal ions form the conventional coordinationcomplexes through interaction with polar groups of organicsurfactants such as oleylamine, oleic acid, and trioctylphosphine.Thermal decomposition of these complexes with/without oxideor chalcogenide sources induces formation of the kineticallycontrolled metal, metal oxide, and metal chalcogenide nano-crystals. One can easily speculate that in the colloidal synthesis,the chemical properties of the precursor complexes are pivotaltoward obtaining the shape-controlled synthesis of nanocrystals.In this regard, the organometallic compounds and the relatedchemistry have been successfully applied in the colloidal synth-esis of nanocrystals.4 For example, recently our research groupapplied the square planar rhodium(I) carbonyl complexes andtheir related unique d8-d8 interaction chemistry to obtain theunprecedented two-dimensional (2D) shape of cubic-phaserhodium nanocrystals at low temperature.4c,d In addition toshape-controlled synthesis, organometallic compounds and re-lated chemistry can be applied to the phase-selective growth ofnanocrystals.5 In practice, designed organometallic or coordina-tion complexes have been continuously applied toward thephase-selective synthesis of high quality thin films by chemicalvapor deposition.6

Metal chalcogenides are important semiconducting materials.7 Asignificant number of high-quality metal chalcogenide nanocrystals

have been prepared employing colloidal synthetic approach.7 Inparticular, most studies have focused on II�VI semiconductornanocrystals.7 However, greater effort is needed for otherpotential semiconducting metal chalcogenide nanocrystals, espe-cially thematerials having phase-selectivity problems in the synthesis.

Recently, semiconducting and magnetic composites8 or hy-brid multifunctional materials9 have been designed and preparedfor specific applications including biomedical imaging. As asynthetic example, these multifunctional composites could beprepared by grafting both the semiconducting and magneticnanomaterials onto solid supports.8 In addition, dumbbell typehybrid nanomaterials have been prepared.9 The core�shell typematerials also can serve as efficient structural candidates formultifunctionalities. However, core�shell semiconducting-magnetic materials were relatively less explored due to thelimited synthetic routes for these materials.

In this work, we report the phase-selective growth of assembledsemiconducting FeSe2 nanorods by an organometallic approach andtheir magnetic properties through surface oxidation.

In the literature, electrodeposition, chemical vapor deposition,and hydrothermal or solvothermal synthetic approaches havebeen applied to prepare iron selenide materials.10 Usually, ironselenides have two stoichiometric phases, FeSe and FeSe2.

10

Both phases have Fe2þ ions, and their phase-selective synthesesare challenging.10 Iron diselenide (FeSe2) is p-type semiconduc-tor material with a 1.0 eV band gap and has a potential as anelectrode material in tandem photovoltaic devices.11 For syn-thesis of colloidal FeSe2 nanomaterials, we started from the

Received: May 2, 2011Revised: May 24, 2011

ABSTRACT: By anorganometallic approach based on 1,2,3-triselena[3]ferrocenophane,assembled FeSe2 nanorods were phase-selectively prepared. Abstraction of thecentral selenium from 1,2,3-triselena[3]ferrocenophane by reaction with triphenyl-phosphite resulted in organometallic polymers through Se�Se bond formation.Successive heating of the polymers in the presence of oleylamine resulted information of the assembled FeSe2 nanorods, which were characterized by scanningelectron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, powder X-ray diffraction, and X-rayphotoelectron spectroscopy analysis. Interestingly, the obtained FeSe2 nanorods underwent air oxidation readily to form amagneticcoating on the surface. These observations are suggestive toward preparation of magnetically coated core/shell type semiconductornanomaterials.

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organometallic compound containing the iron and seleniumatoms, 1,2,3-triselena[3]-ferrocenophane.12 The overall syn-thetic scheme is displayed in Scheme 1.

1,2,3-Triselena[3]ferrocenophane was synthesized by reac-tion of 1,10-dilithioferrocene with selenium powder.12 It isnoteworthy that although selenium powder can be consideredas a selenide source for iron selenides, actually it has very poorsolubility in alkylamines.13 The central selenium in 1,2,3-triselena[3]-ferrocenophane has a high reactivity toward phos-phines. Abstraction of the central selenium by reaction withphosphines results in the formation of insoluble organometallicpolymers through the Se�Se bond formation as shown inScheme 1.14 In this work, to induce formation of the polymericmaterials, we utilized the relatively stable triphenylphosphite inthe presence of oleylamine.15 At 70 �C, the orange-coloredparticles were formed and dispersed throughout the oleylaminesolution. Generation of triphenylphosphite selenide was con-firmed by proton (1H)-nuclear magnetic resonance spectroscopy(NMR) and high-resolution (HR) mass spectroscopy. The re-sultant organometallic polymers were isolated by centrifugationand investigated by scanning electron microscopy (SEM), solid-phase NMR spectroscopy, X-ray photoelectron spectroscopy(XPS), elemental analysis (EA), and thermogravimetric analysis

(TGA). The SEM image of the obtained organometallic poly-mers reveals irregularly shaped particles with an average size of0.67 μm (Figure 1a). The solid-phase 1H- and 13C NMR spectrashowed broad peaks at 4.35 and 77.1 ppm, respectively, support-ing the existence of the ferrocene moieties (Figure 1b,c). Con-sidering the absence of the peaks from the phenyl rings, thetriphenylphosphite used was not present in the organometallicpolymers. FromXPS analysis of the organometallic polymers, theSe 3d peaks appeared at 55.6 eV as a broad single peak and theFe 2p3/2 and 2p1/2 peaks at 708.0 and 720.8 eV, respectively(Figure S1 in the Supporting Information). Elementary analysis(observed values; C, 35.19%; H, 2.32%) confirmed stoichio-metric chemical composition of the materials (calculated valuesfor [-FcSeSe-]n; C, 35.13%; H, 2.36%). TGA analysis showed theorganometallic polymers start to decompose at ∼165 �C.(Figure S2 in the Supporting Information)

The dispersed organometallic polymers in oleylamine (insetin Figure 1a) were gradually heated to 200 �C. During this step,the orange color from the dispersed particles gradually changedinto dark black (inset in Figure 2a). At 200 �C, the reactionmixture was stirred for an additional 1 h. After the reactionmixture was cooled to room temperature, addition of excessmethanol resulted in formation of precipitates which wereretrieved by centrifugation and washed several times withmethanol. The obtained materials were investigated by SEMand transmission electron microscopy (TEM).

As shown in Figures 2 and S3 in the Supporting Information,the obtained materials possess a spherical shape with an averagediameter of 0.72 μm. Careful analysis of the spheres by TEMshowed that the materials consist of the assembled nanorodswith 40 nm average thickness. In contrast, direct thermolysis of

Scheme 1. Organometallic Approach for Phase-SelectiveSynthesis of Assembled FeSe2 Nanorods

Figure 1. SEM image and photograph (a), solid-phase 1H (b), and 13CNMR spectra (c) of the organometallic polymers.

Figure 2. SEM image and photograph (a), TEM (b), HR-TEM (c)images, PXRD pattern (d), and EDS spectrum (e) of assembled FeSe2nanorods. The peak marked with an asterisk in EDS spectrum corre-sponds to copper from grid. Also, see Figures S3 and S5 in theSupporting Information for magnified SEM and HR-TEM images.

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1,2,3-triselena[3]ferrocenophane in oleylamine without use oftriphenylphosphite resulted in a messy mixture of materials(Figure S4 in the Supporting Information). HR-TEM analysisrevealed that the nanorods grew in the direction of the (110)crystalline plane (Figure 2c). The powder X-ray diffraction(PXRD) pattern in Figure 2d showed that the assemblednanorods are orthorhombic-phased FeSe2. All peaks matchedwell with those in the known PXRD pattern (JCPDS No. 21-0432). The energy dispersive X-ray absorption (EDS) spectros-copy supports the 1:2 stoichiometric ratio of iron to selenium(Figure 2e).

During XPS investigation of the chemical surroundings of ironand selenium on the materials, we found that the materials easilyundergo the oxidation in air. In particular, washing the excesssurfactant resulted in the clear appearance of new XPS peaks ofFe and Se within a week (Figure 3a,b). The major respective Fe2p3/2 and 2p1/2 peaks at 706.8 and 719.7 eV correspond to thosefrom conventional FeSe2 nanomaterials.16 The minor peaks at711.2 and 725.2 eV imply the existence of higher oxidation stateof the iron species.16,17 Similarly, in addition to a major Se 3dpeak at 54.2 eV, a minor and broad peak at 58.4 eV wasobserved.16 In several reports on the FeSe2 thin film fabrication,similar surface oxidations were observed.16 Figures 3c,d and S5 inthe Supporting Information show the HR-TEM images on thesurface of the oxidized FeSe2 nanorods which were exposed to airfor a week, displaying an amorphous coating with a ∼2.1 nmthickness. Even after exposure to air for three months, thisthickness was maintained in HR-TEM analysis. In the PXRDpattern, the peaks from iron oxides or other new species were notdetected, supporting the amorphous character of the oxidizedspecies. The bulk orthorhombic marcasite-type FeSe2 is usually

known to have nonmagnetic properties.10a,18However, it can bespeculated that oxidation of the surface of FeSe2 nanorodscan generate magnetic species on the surface.18,19 Thus, usingthe superconducting quantum interference device (SQUID)analysis, the possible magnetic properties of the materials wereinvestigated.

As expected, the surface-oxidized FeSe2 nanomaterials weremagnetically active. As shown in temperature-dependent hyster-esis curves of nanomaterials in Figure 3e, due to reduced thermalfluctuation of magnetic dipoles, the remanent magnetization(Mr) and coercivity (Hc) sharply increased at low temperaturefrom 7.0 � 10�6 emu/g and 0.70131 Oe (at 300 K) to 6.7 �10�2 emu/g and 1846 Oe (at 4 K), respectively. In addition,temperature-dependent magnetization of materials with field/zero-field (FC/ZFC) cooling process was observed with atransition temperature around 280 K (Figure 3f). As oxidationprogressed, the magnetism and transition temperature graduallyincreased (Figure S6 in the Supporting Information). Thissurface magnetic effect would be more enhanced in smaller-sizedFeSe2 nanomaterials.

In conclusion, this work shows successful application of orga-nometallic chemistry toward the phase-selective synthesis of FeSe2nanocrystals. The FeSe2 nanorods easily underwent air oxidationto form a magnetic coating on the surface. We believe that theseobservations can be applied toward preparation of the magneti-cally coated core/shell type semiconductor nanomaterials.

’ASSOCIATED CONTENT

bS Supporting Information. XPS spectra and TGA curve oforganometallic polymers, additional SEM and TEM images. Thisinformation is available free of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: sson@skku.edu, jgpark10@snu.ac.kr.

’ACKNOWLEDGMENT

This work was supported by Grants NRF-2009-0064488through the National Research Foundation of Korea funded bythe Ministry of Education, Science and Technology. K.H.J.acknowledges support by Grants NRF-2010-0029698 (PriorityResearch Centers Program) and R31-2008-10029 (WCUprogram). H.J.K. acknowledges support from Hydrogen EnergyR&D Center, a 21st century Frontier R&D Program. JGP issupported by the National Research Foundation of Korea(Grants KRF-2008-220-C00012, R17-2008-033-01000-0).

’REFERENCES

(1) (a) Klabunde, K. J., Ed. Nanoscale Materials in Chemistry; Wiley:New York, 2001. (b) Daniel, M. C.; Astruc, D. Chem. Rev. 2004,104, 293–346.

(2) Recent selected examples: (a) Sardar, K.; Lees, M. R.; Kashtiban,R. J.; Sloan, J.; Walton, R. I. Chem. Mater. 2011, 23, 48–56.(b) Kiatkittipong, K.; Ye, C.; Scott, J.; Amal, R. Cryst. Growth Des.2010, 10, 3618–3625. (c) Zhang, W.; Yanagisawa, K.; Kamiya, S.; Shou,T. Cryst. Growth Des. 2009, 9, 3765–3770. (d) Roy, P.; Srivastava, S. K.Cryst. Growth Des. 2006, 6, 1921–1926. (e) Fang, Y.-P.; Xu, A.-W.; Qin,A.-M.; Yu, R.-J. Cryst. Growth Des. 2005, 5, 1221–1225. (f) Jeon, S.;Braun, P. V. Chem. Mater. 2003, 15, 1256–1263.

Figure 3. XPS spectra (a, b), HR-TEM images (c, d), and SQUIDcharacterization (e, f) of FeOxSey/FeSe2 nanomaterials.

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(3) (a) Recent review:Talapin, D. V.; Lee, J.-S.; Kovalenko, M. V.;Shevchenko, E. V. Chem. Rev 2010, 110, 389–458. Selected examples:(b) Chockla, A. M.; Harris, J. T.; Korgel, B. A. Chem. Mater. 2011,23, 1964–1970. (c) Tian, L.; Tan, H. Y.; Vittal, J. J. Cryst. Growth Des.2008, 8, 734–738. (d) Lim, T. H.; Ingham, B.; Kamarudin, K. H.;Etchegoin, P. G.; Tilley, R. D. Cryst. Growth Des. 2010, 10, 3854–3858.(e) Taniguchi, T.; Watanabe, T.; Sakamoto, N.; Matsushita, N.;Yoshimura, M. Cryst. Growth Des. 2008, 8, 3725–3730.(4) (a) Kahn, M. L.; Glaria, A.; Pages, C.; Monge, M.; Macary, L. S.;

Maisonnat, A.; Chaudret, B. J. Mater. Chem. 2009, 19, 4044–4060.(b) Jang, K.; Kim, H. J.; Son, S. U. Chem. Mater. 2010, 22, 1273–1275.(c) Jang, K.; Jung, I. G.; Nam, H.-J.; Jung, D.-Y.; Son, S. U. J. Am. Chem.Soc. 2009, 131, 12046–12047. (d) Krumm, M.; Pueyo, C. L.; Polarz, S.Chem. Mater. 2010, 22, 5129–5136.(5) (a) Khanderi, J.; Schneider, J. J. Eur. J. Inorg. Chem.

2010, 4591–4594. (b) Maneeprakorn, W.; Malik, M. A.; O’brien, P.J. Mater. Chem. 2010, 20, 2329–2335.(6) Recent examples: (a)Meng, Q.;Witte, R. J.; Gong, Y.; Day, E. L.;

Chen, J.; May, P. S.; Berry, M. T. Chem. Mater. 2010, 22, 6056–6064.(b) Ramasamy, K.; Malik, M. A.; Helliwell, M.; Raftery, J.; O’Brien, P.Chem. Mater. 2011, 23, 1471–1481. (c) Dezelah, C. L.; Niinisto, J.;Arstila, K.; Niinisto, L.; Winter, C. H. Chem. Mater. 2006, 18, 471–475.(d) Chen, T.; Xu, C.; Baum, T. H.; Stauf, G. T.; Roeder, J. F.; Dipasquale,A. G.; Reingold, A. L. Chem. Mater. 2010, 22, 27–35. (e) Petrella, A. J.;Deng, H.; Roberts, N. K.; Lamb, R. N. Chem. Mater. 2002, 14, 4339–4342.(7) Selected examples: (a) Murray, C. B.; Norris, D. J.; Bawendi,

M. G. J. Am. Chem. Soc. 1993, 115, 8706–8715. (b) Colvin, V. L.;Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354–357.(8) (a) Jie, G.; Wang, L.; Zhang, S. Chem.—Eur. J. 2011,

17, 641–648. (b) Zhou, H.; Chen, J.; Sutter, E.; Feygenson,M.; Aronson,M. C.; Wong, S. S. Small 2010, 6, 412–420.(9) (a) Cozzoli, P. D.; Pellegrino, T.; Manna, L. Chem. Soc. Rev.

2006, 35, 1195–1208. (b) Selvan, S. T.; Patra, P. K.; Ang, C. Y.; Ying, J. Y.Angew. Chem., Int. Ed. 2007, 46, 2448–2452.(10) (a) Xie, Y.; Zhu, L.; Jiang, X.; Lu, J.; Zheng, X.; He, W.; Li, Y.

Chem. Mater. 2001, 13, 3927–3932. (b) Ge, J.; Cao, S.; Yuan, S.; Kang,B.; Zhang, J. J. Appl. Phys. 2010, 108, 053903–053905. (c) Ouertani, B.;Ouerfelli, J.; Saadoun, M.; Zribi, M.; Rabha, M. B.; Bessa€is, B.; Ezzaouia,H. Thin Solid Films 2006, 511�512, 457–462.(11) (a) Kwon, H. J.; Thanikaikarasan, S.; Mahalingam, T.; Park,

K. H.; Sanjeeviraja, C.; Kim, Y. D. J. Mater. Sci. Mater. Electron. 2008,19, 1086–1091. (b) Puthussery, J.; Seefeld, S.; Berry, N.; Gibbs, M.; Law,M. J. Am. Chem. Soc. 2011, 133, 716–719.(12) Bishop, J. J.; Davison, A.; Katcher, M. L.; Lichtenberg, D. W.;

Merrill, R. E.; Smart, J. C. J. Organomet. Chem. 1971, 27, 241–249.(13) Selenium powder is completely insoluble in oleyamine at

ambient temperature.(14) Reactions of the sulfur analogues have been known. Brandt,

P. F.; Rauchfuss, T. B. J. Am. Chem. Soc. 1992, 114, 1926–1927.(15) Triphenylphosphite is relatively electron-deficient, compared

with air-unstable trialkylphosphines. Tolman’s electronic parameters νcoin Ni(CO)3PR3 for P(Me)3 and P(OPh)3 are known as 2064 and,2085 cm�1, respectively. Spessard, G. O.; Miessler, G. L. OrganometallicChemistry; Oxford: New York, 2010.(16) Hamdadou, N.; Khelil, A.; Morsli, M.; Bern�ade, J. C. Vacuum

2005, 77, 151–156.(17) 2p3/2 XPS peaks of Fe(þ3) ion in ferric chloride were observed

in range of 711.0�711.5 eV. Moulder, J. F.; Stickle, W. F.; Sobol, P. E.;Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; PhysicalElectrics Inc.: Chanhassen, MN, 1995.(18) Fe(þ2) with octahedral geometry in marcasite FeSe2 has a low

spin electronic configuration with the filled t2g orbitals. Also see Garg,R.; Garg, V. K.; Nakamura, Y. Hyperfine Interact. 1991, 67, 447–451.(19) Campos, C. E. M.; Drago, V.; de Lima, J. C.; Grandi, T. A.;

Machado, K. D.; Silva, M. R. J. Mag. Mag. Mater. 2004, 269, 6–14.