Self-Generation of Tiered Surfactant Superstructures for One-Pot

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Langmuir 2004, 20, 9780-9790

Self-Generation of Tiered Surfactant Superstructures for One-Pot Synthesis of Co3O4 Nanocubes and Their Closeand Non-Close-Packed Organizations Rong Xu† and Hua Chun Zeng‡,* Institute of Chemical and Engineering Sciences, Block 28, Ayer Rajah Crescent, Singapore 139959, and Department of Chemical and Biomolecular Engineering, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 Received April 1, 2004 Self-generation of ionic organic capping from nonionic surfactant polyoxyethylene (20) sorbitan trioleate (Tween-85) has been realized for the controlled synthesis of single crystalline Co3O4 quantum dots (3.0-5.7 nm) in cubic morphology from related layered hydroxide precursors at 80-95 °C. With chemical modification of hydrophobic functional groups on the surface of Co3O4 nanocubes; furthermore, various nanocubecontaining micellar superstructures can be further assembled through hydrophobic interactions between Tween-85 molecules and the surface coating under “one-pot” conditions. In particular, square arrangements, spherical domains, and line-assemblies of the prepared Co3O4 nanocubes and their inter-transformations have been attained for the first time by manipulating intersurfactant-interactions. Hydrolysis of Tween-85 and the resultant tiered surfactant superstructures have been investigated with FTIR/UV-vis/EA/TGA/ DTA/XPS methods, and the capping species has been identified to be alkylated oleic carboxylate anions derived from Tween-85. Pronounced quantum confinement effects have been observed with the prepared Co3O4 nanocubes, and the optical band gap energies determined are 3.95 and 2.13 eV, respectively, for O2-f Co2+ and O2-f Co3+ charge-transfer processes.

Introduction Over the past decade, much attention has been paid to the controlled synthesis of nanostructured materials, owing to their shape/size-dependent optical, electronic, magnetic, and catalytic properties.1-14 Driven by the minimization of surface energy, nanoscale solids tend to either grow into larger particles or aggregate into large coalescences during their formation. To prevent these undesired processes, surface modifying/stabilizing com* To whom correspondence should be addressed. Tel: +65 874 2896. Fax: +65 779 1936. E-mail: [email protected]. † Institute of Chemical and Engineering Sciences. ‡ National University of Singapore. (1) (a) Rao, C. N. R.; Cheetham, A. K. J. Mater. Chem. 2001, 11, 2887. (b) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J.; Edwards, P. P. Chem. Eur. J. 2002, 8, 29. (c) Kovtyukhova, N. I.; Mallouk, T. E. Chem. Eur. J. 2002, 8, 4355. (d) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. Adv. Mater. 2003, 15, 353. (2) (a) Iijima, S. Nature 1991, 354, 56. (b) Ajayan, P. M.; Stephan, O.; Redlich, P.; Colliex, C. Nature 1995, 375, 564. (3) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (4) Lakshmi, B. B.; Dorhout, P. K.; Martin, C. R. Chem. Mater. 1997, 9, 857. (5) Huang, M. H.; Wu, Y. Y.; Feick, H.; Tran, N.; Weber, E.; Yang, P. D. Adv. Mater. 2001, 13, 113. (6) Li, Y. D.; Sui, M.; Ding, Y.; Zhang, G. H.; Zhuang, J.; Wang, C. Adv. Mater. 2000, 12, 818. (7) de la Fuente, J. M.; Barrientos, A. G.; Rojas, T. C.; Rojo, J.; Canada, J.; Fernandez, A.; Penades, S. Angew. Chem., Int. Ed. 2001, 40, 2258. (8) Black, C. T.; Murray, C. B.; Sandstrom, R. L.; Sun, S. H. Science 2000, 290, 1131. (9) Chen, S. W.; Sommers, J. M. J. Phys. Chem. B 2001, 105, 8816. (10) Canton, P.; Meneghini, C.; Riello, P.; Balerna, A.; Benedetti, A. J. Phys. Chem. B 2001, 105, 8088. (11) Vaucher, S.; Li, M.; Mann, S. Angew. Chem., Int. Ed. 2000, 39, 1793. (12) Mamedov, A. A.; Belov, A.; Giersig, M.; Mamedova, N. N.; Kotov, N. A. J. Am. Chem. Soc. 2001, 123, 7738. (13) Buhro, W. E.; Colvin, V. L. Nat. Mater. 2003, 2, 138. (14) Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Nat. Mater. 2003, 2, 382.

pounds, both inorganic and organic ones, have been commonly employed in the constrained preparation of nanomaterials.13-25 Ionic or nonionic organic surfactants, for example, are the most widely used surface-capping agents or soft colloidal templates to control the growth.22,23 In a broader sense, the thus-prepared nanostructures can also be viewed as inorganic-organic core/shell nanohybrids which may also have different chemophysical properties from their individual components. In many cases, furthermore, unwashable capping agents that are chemically adsorbed on active surface sites may introduce additional organization capacity for discrete nanounits (e.g., free-standing nanocrystallites) by organic interactions.26 As an important result, this type of intermolecular interactions via hydrophobic chains/headgroups has been utilized in mesoscale organization of nanobuilding blocks into two- and three-dimensional (2D & 3D) superlattices, including one-dimensional (1D) arrays, to optimize and/ (15) Puntes, V. F.; Zanchet, D.; Erdonmez, C. K.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 12874. (16) (a) Sun, S.; Murray, C. B. J. Appl. Phys. 1999, 85, 4325. (b) Sun, S.; Zeng, H.; Robinson, D. B.; Raoux, S.; Rice, P. M.; Wang, S. X.; Li, G. J. Am. Chem. Soc. 2004, 126, 273. (17) Wang, Z. L.; Dai, Z.; Sun, S. Adv. Mater. 2000, 12, 1944. (18) Petit, C.; Taleb, A.; Pileni, M. P. J. Phys. Chem. B 1999, 103, 1805. (19) Zeng, H.; Li, J.; Liu, J. P.; Wang, Z. L.; Sun, S. Nature 2002, 420, 395. (20) Redl, F. X.; Cho, K.-S.; Murray, C. B.; O’Brien, S. Nature 2003, 423, 968. (21) Shevchenko, E. V.; Talapin, D. V.; Schnablegger, H.; Kornowski, A.; Festin, O ¨ .; Svedlindh, P.; Haase, M.; Weller, H. J. Am. Chem. Soc. 2003, 125, 9090. (22) (a) Petit, C.; Taleb, A.; Pileni, P. Adv. Mater. 1998, 10, 259. (b) Motte, L.; Billoudet, F.; Pileni, P. J. Phys. Chem. 1995, 99, 16425. (23) (a) Pileni, P. Nat. Mater. 2003, 2, 145. (b) Pileni, M. P. J. Phys. Chem. B 2001, 105, 3358. (24) Xu, R.; Zeng, H. C. J. Phys. Chem. B 2003, 107, 926. (25) Feng, J.; Zeng, H. C. Chem. Mater. 2003, 14, 2829. (26) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978.

10.1021/la049164+ CCC: $27.50 © 2004 American Chemical Society Published on Web 09/17/2004

Self-Generation of Tiered Surfactant Superstructures Scheme 1

or to enhance the performance of materials.27-30 Nevertheless, little research has been devoted to the investigation of interfacial chemistry and physics between surfactants and resultant capped particles (nanobuilding blocks).31 In most cases, it is assumed that surfactants used in synthesis or self-assembly do not undergo further chemical reactions, though it is well conceived that this assumption may not be entirely valid. Thus, understanding formation and self-assembly mechanisms remains as an important goal for advanced design and related technological applications.23 In this article, we will investigate surfactant chemistry of polyoxyethylene (20) sorbitan trioleate (Tween-85) with respect to the synthesis and self-organizations of nanoscale cobalt spinel oxide, Co3O4. The compounds of the Tween series belong to nonionic type surfactants and they are commonly used as emulsifier to stabilize both water-inoil and oil-in-water emulsions.32 The structural image of Tween-85 is shown in Scheme 1. One important structural feature of Tween-85 is its dissociable ester segment (dashed line). By controlling reaction mediums (e.g., pH), in principle, ester hydrolysis may further occur, giving a new surfactant product (i.e., alkylated oleic acid, its anionic form is indicated as “A” in Scheme 1). Although Tween-85 is relatively less used, other members of the Tween family with different aliphatic headgroups,32 Tween-20 (monolaurate) and Tween-80 (monooleate), for example, are more frequently applied in colloidal chemistry as surface modifiers to tailor particle size and reduce agglomeration of gold, R-alumina, silica, titania, titanium-silicalite-1, barium titanate, yttria, and cobalt-nickel-copper composites.33-40 In most of these studies, however, the fundamental surfactant chemistry was rarely touched, although a hypothesis on physisorbed Tween-20 mono(27) (a) Wang, Z. L. Adv. Mater. 1998, 10, 13. (b) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153. (28) Ohara, P. C.; Heath, J. R.; Gelbart, W. M. Angew. Chem., Int. Ed. Engl. 1997, 36, 1078. (29) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Cleveland, C. C.; Luedtke, W. D.; Landman, U. Adv. Mater. 1996, 8, 429. (30) Kiely, C. J.; Fink, J.; Brust, M.; Bethell, D.; Schiffrin, D. J. Nature 1998, 396, 444. (31) Lorenz, J. K.; Ellis, A. B. J. Am. Chem. Soc. 1998, 120, 10970. (32) Moroi, Y. Micelles: Theoretical and Applied Aspects; Plenum Press: New York, 1992; p 12. (33) Zhang, R. B.; Gao, L. In High-Performance Ceramics 2001, Proceedings; Trans Tech Publications, Inc.: Zurich, 2002; Vol. 224-2, p 573. (34) (a) Sharma, P. K.; Varadan, V. V.; Varadan, V. K. J. Am. Ceram. Soc. 2002, 85, 2584. (b) Sharma, P. K.; Jilavi, M. H.; Varadan, V. K.; Schmidt, H. J. Phys. Chem. Solids 2002, 63, 171. (35) Boissiere, C.; Larbot, A.; van der Lee, A.; Kooyman, P. J.; Prouzet, E. Chem. Mater. 2000, 12, 2902. (36) Lu, S. W.; Lee, B. I.; Wang, Z. L.; Samuels, W. D. J. Cryst. Growth 2000, 219, 269. (37) Sharma, P. K.; Jilavi, M. H.; Schmidt, H.; Varadan, V. K. Int. J. Inorg. Mater. 2000, 2, 407. (38) Khomane, R. B.; Kulkarni, B. D.; Paraskar, A.; Sainkar, S. R. Mater. Chem. Phys. 2002, 76, 99. (39) Zhang, W. W.; Cao, Q. Q.; Xie, J. L.; Ren, X. M.; Lu, C. S.; Zhou, Y.; Yao, Y. G.; Meng, Q. J. J. Colloid Interface Sci. 2003, 257, 237. (40) Aslan, K.; Perez-Luna, V. H. Langmuir 2002, 18, 6059.

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layer had been recently proposed.40 It is believed that this Tween-20 surface layer prevents the aggregation of gold nanoparticles by means of steric interactions due to the presence of oligo(ethylene glycol) moieties on the headgroup of Tween-20, despite lack of direct experimental characterization.40 Spinel oxide, Co3O4, studied herein is an important functional material for a wide range of technological applications such as sensors, pigments, energy storage, magnetism, catalysis, and electrochemistry.41-43 In recent years, many efforts have been devoted to the synthesis of nanosized Co3O4 crystallites with well-defined morphology and/or monodispersivity.24,25,44-48 In particular, we had devised a nitrate-salt-mediated synthetic route of Co3O4 nanocubes and achieved size-controlled growth of Co3O4 nanocubes with an edge length in the range of 10-100 nm under ambient and aqueous conditions.24,25 Octylamine-covered Co3O4 nanoparticles at ca. 4.2 nm had also been prepared by thermal decomposition of cobalt cupferronates in the presence of n-octylamine and tri-noctylamine at 300 °C.46 In addition, self-assembly of these oxide nanoparticles in hexagonal arrays has been obtained by redispersing in toluene followed by subsequent depositing and drying. Nonetheless, no control over the geometrical shape of Co3O4 had been attained in that work, although the particle size was reduced. Similar to other previous investigations, little attention was paid to surfactant chemistry and synthesis/self-organization mechanisms.46 This contribution reports a self-generation of tiered surfactants for wet-synthesis of Co3O4 nanocubes and their close- and nonclose-packed organizations. Taking advantage of the unique structural feature of nonionic Tween85, ionic surfactant in the form of A (Scheme 1) can be derived in-situ during the synthesis. The self-generated anionic species forms a chemisorbed surface coating on the resulting Co3O4 nanocubes, preventing agglomeration. Furthermore, this newborn ionic surfactant and pristine nonionic Tween-85 work together to form various tiered surfactant structures for both synthesis and self-organizations of Co3O4 nanocubes, giving rise to an excellent dispersity of the solid nanocubes in the aqueous phase. Remarkably, structural anisotropy has been generated for these highly isotropic nanocubes with the assistance of the tiered surfactant structures. To the best of our knowledge, non-close-packed low-dimensional arrays, such as 2D square arrangement and 1D line assemblies for highly symmetrical Co3O4 nanocubes, and their reverse transformations have been achieved for the first time with the tiered surfactant superstructures derived from the single-molecular source Tween-85 under the current onepot conditions. Experimental Section I. Materials. Cobalt nitrate hexahydrate [Co(NO3)2‚6H2O, 99.0%] and ethanol (CH3CH2OH, 99.8%) were purchased from Merck. Sodium hydroxide (NaOH, >98%) and Tween-85 [polyoxyethylene(20) sorbitan trioleate] were purchased from Aldrich. (41) Ocana, M.; Gonzalez-Elipe, A. R. Colloid Surf., A 1999, 157, 315. (42) Sugimoto, T.; Matijevic´, E. J. Inorg. Nucl. Chem. 1979, 41, 165. (43) Matijevic´, E. Chem. Mater. 1993, 5, 412. (44) Yin, J. S.; Wang, Z. L. J. Mater. Res. 1998, 14, 503. (45) (a) Zeng, H. C.; Lim, Y. Y. J. Mater. Res. 2000, 15, 1250. (b) Zhong, Z.; Gates, B.; Xia, Y. Langmuir 2000, 16, 10369. (46) Thomas, P. J.; Saravanan, P.; Kulkarni, G. U.; Rao, C. N. R. Pramana 2002, 58, 371. (47) Jiang, Y.; Wu, Y.; Xie, B.; Xie, Y.; Qian, Y. T. Mater. Chem. Phys. 2002, 74, 234. (48) Liu, Y. K.; Wang, G. H.; Xu, C. K.; Wang, W. Z. Chem. Commun. 2002, 1486.

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Purified air [O2 ) 21 ( 1%, H2O < 2 vpm (volume per million), and hydrocarbons < 5 vpm] was purchased from Soxal. II. Synthesis of Co3O4 Nanocubes. Thirty milliliters (mL) of 0.3 M NaOH solution was mixed with 6.0 mL of Tween-85 in a two-necked round-bottom flask (measured pH ) 12.7). Upon stirring, a milky emulsion in light yellow appeared. To this mixture at room temperature, 6.0 mL of 1.0 M Co(NO3)2 solution was added under stirring within 1 min, which resulted in an instantaneous blue precipitate, after which, the pH was changed to 7.7 (the hydroxyl groups were consumed during the precipitation; without adding any acid). The flask was then transferred into an oil bath and the temperature of the reaction mixture was maintained at 80-95 °C. The reaction mixture was vigorously stirred and bubbled with purified air at 60 mL/min. A watercooled reflux condenser was mounted on top of the reaction flask. Upon heating (up to 72-79 h), the color of the precipitates slowly changed to green, light green, brown, and finally black, showing the following pH variations: 6.4 (5 min), 6.3 (40 min), and 6.0 (79 h). The final black reaction mixtures, named CoT85 (coexistence of organic-capped Co3O4 and Tween-85) hereafter, appeared to be viscous and slurrylike. III. Total and Partial Precipitations. The Co3O4 nanoparticles synthesized did not sedimentate even at a centrifugation speed of 20 000 rpm. This was attributed to the stabilization effect in the presence of highly concentrated surfactant molecules (Tween-85). To obtain the nanoparticles for further bulk analysis, approximately 40 mL of ethanol as a flocculant was added to CoT85, followed by vigorous stirring for 5 min. The Co3O4 nanoparticles in CoT85 were then obtained by centrifugation at 5000 rpm for 15 min. The resultant supernatant solution was clear and colorless, which indicated that all the particles were separated completely from the solution phase. The Co3O4 nanocube sample was then washed thoroughly with ethanol until the UV-visible spectrum of the supernatant solution showed no absorbance signal of Tween-85. The Co3O4 precipitate was then dried in an oven at 50 °C overnight. To investigate the chemical effect of flocculant on the studied surfactants and, thus, the variation in organization modes, a small amount of ethanol was added to the CoT85 mixture. In the actual experiment, approximately 10 g of CoT85 was further mixed with 15 mL of deionized water and stirred to obtain a homogeneous mixture. To this mixture, various amounts of ethanol, 2, 3, and 4 mL were added, respectively, and the mixture was then stirred for 5 min. After centrifugation at 5000 rpm for 30 min, some Co3O4 precipitates were observed at the bottom of the centrifuge tube for all three cases. The supernatant mixtures were then transferred into other clean sample bottles for TEM sample preparation. In particular, the supernatant from some cases (e.g., the experiment with 3 mL of ethanol) was left open in the bottle and heated at 60 °C for 1 h to evaporate the ethanol, after which 1 mL of Tween-85 was added to compensate for possible loss during this process. The bottle was then covered, and the sample mixture was gently stirred and kept at 90 °C for 28 h. IV. Characterization of Samples. A. Transmission Electron Microscopy (TEM). Morphology, nanocrystal size, and lattice fringes were measured by TEM (JEM-2010, JEOL) and high-resolution TEM (Tecnai-G2, FEI). The electron beam accelerating voltage of the microscope was set at 200 kV. The specimens were prepared by depositing a drop of a dilute solution of CoT85 (i.e., Co3O4 nanocrystals in the reaction mixtures were further diluted with deionized water) onto a copper grid of 200 mesh coated with carbon film. Co3O4 nanocrystals in the ethanoltreated and Tween-85-re-treated supernatants (see section III above) were also placed onto the copper grids from their respective suspensions without further dilution. All the copper grids were air-dried under ambient conditions before analysis. B. Powder X-Ray Diffraction (XRD). The crystallographic information of precipitated Co3O4 samples was obtained with the powder XRD method on a Bruker-AXS diffractometer with Cu KR radiation (λ ) 1.5406 Å) at a scanning rate of 1°/min. A small amount of polycrystalline silicon powder was added as an internal standard for the scanning-angle calibration. The average size of the nanocrystallites was also estimated by using the

Xu and Zeng Debye-Scherrer formula,49 where the strongest peak (311) was used to estimate the dimension and lattice expansion of the nanocubes. C. Thermogravimetric and Differential Thermal Analysis (TGA and DTA). The thermal behavior of the samples was analyzed in a TA instrument SDT-B960, using the TGA/DTA method. During the experiment, about 12 mg of the precipitated Co3O4 sample was heated at a rate of 10 °C/min (from room temperature to 1100 °C) with an air-flow at 100 mL/min. D. Fourier Transform Infrared (FTIR) Spectroscopy. Chemical bonding information on both the surfactant and Co3O4 nanocrystals was studied with Fourier transform infrared (FTIR, Bio-Rad) using the potassium bromide (KBr) pellet technique. Each FTIR spectrum was collected after 40 scans with a resolution of 4 cm-1 from 400 to 4000 cm-1. In making the KBr pellets, about 1 mg of precipitated Co3O4 powder was diluted with approximately 100 mg of KBr powder. For liquid Tween-85 and the solution mixture CoT85, around 2 µL of the samples measured by a micropipet was finely mixed with about 100 mg of KBr powder. E. X-Ray Photoelectron Spectroscopy (XPS). An X-ray photoelectron spectroscopy (XPS) investigation was conducted in an AXIS-Hsi spectrometer (Kratos Analytical) using a monochromated Al KR X-ray source (1486.6 eV). The precipitated Co3O4 sample was mounted onto the double-sided adhesive tape on the sample stud. One drop of CoT85, in a slurry form, was placed onto a glass substrate and allowed to air-dry under ambient conditions (to evaporate water) before the measurement. The XPS spectra of the studied elements were measured with the constant analyzer pass energy of 40.0 eV. All binding energies (BEs) were referred to the C 1s peak (BE ) 284.7 eV) arising from surface hydrocarbons (or adventitious hydrocarbon).50-52 Prior to peak deconvolution, X-ray satellites and inelastic background (Shirley-type) were subtracted. Full-width-at-halfmaximum (fwhm) is kept the same for chemical components within the same core level of an element. In addition, all component peaks are set to be Gaussian-type. The atomic ratio was estimated with Perkin-Elmer sensitivity factors, assuming the integrated baseline and homogeneous surface layer.53 F. Elemental Analysis (EA). The weight percentage of carbon in the as-prepared Co3O4 samples was measured in a EURO EA elemental analyzer. G. UV-Visible Absorption Spectroscopy. UV-visible absorption spectroscopy measurements for the Co3O4 nanocrystals were carried out on a Shimadzu UV-2550 spectrometer using ethanol as a reference solvent. The Co3O4 nanocrystals were dispersed into ethanol solvent and ultrasonicated before the measurement.

Results and Discussion I. Precursors and Crystallographic Properties of Co3O4 Nanocrystals. When Co2+ ions were added to the water/NaOH/Tween-85 mixture, a blue precipitate was formed instantly (pH was reduced to 7.7, see Experimental Section). The precipitate then changed to a green color within 5 min. In Figure 1, the XRD investigation indicates that there are three different layered compounds in this precipitate: CoxIIICo1-xII(OH)2(A)x‚nH2O, CoxIIICo1-xII(OH)2(NO3)x‚nH2O, and β-Co(OH)2. The first compound has a large inter-brucite-sheet distance, and the intercalated monovalent anions in a bilayer configuration were derived from hydrolysis of Tween-85 (A ) alkylated oleic anion, see FTIR investigation later in Figure 6). The observed basal distance increases slightly (49) Azarof, L. V.; Buerger, M. J. The Powder Method in X-ray Crystallography; McGraw-Hill: New York, 1958. (50) Barr, T. L. Modern ESCA; Chemical Rubber: Boca Raton, FL, 1994. (51) Barr, T. L.; Seal, S. J. Vac. Sci. Technol. A 1995, 13, 1239. (52) Alexander, M. R.; Payan, S.; Duc, T. M. Surf. Interface Anal. 1998, 26, 961. (53) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handboodk of X-ray Photoelectron Spectroscopy; Perkin-Elmer: Eden Prairie, MN, 1992.

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Figure 1. XRD patterns of precursor precipitates after various aging treatments [5 min (green color, see the photo in the right), 4, and 18 h at 80 °C] under air bubbling. Three solid phases are found: (i) CoxIIICo1-xII(OH)2(A)x‚nH2O [diffraction peaks without subscript, A ) CH3(CH2)7CHCHCH2CR2(CH2)5COO-, R ) CH2CHCHCH2(CH2)6CH3)], (ii) CoxIIICo1-xII(OH)2(NO3)x‚nH2O (indicated with “HT”), and (iii) (β-Co(OH)2 (indicated with “β”). The middle illustration shows the inter-brucite-like sheet distances of the three compounds and their intercalants (lines represent brucite-like sheets, ovals represent nitrate anions, and round-headed sticks represent the carboxylate anions, A).

(d003 ) 37.925-39.321 Å, Figure 1) with time, owing to increases in trivalent cation and anion populations upon the reactions.54 The second compound also has a wellknown hydrotalcite-like phase (inter-brucite-like sheet distance d003 ) 7.655 Å),24,54 while the third one has a brucite-like crystallographic structure (basal distance d001 ) 4.633 Å).24,54 The precipitated phases are rather stable in the water/Tween-85 mixture (test tube, Figure 1), although they can be centrifuged out of the liquid phase. The organic-inorganic nature of CoxIIICo1-xII(OH)2(A)x‚ nH2O is further demonstrated with our dissolution tests; this compound is dissolved completely in ethanol while the other two are basically insoluble in the same solvent (confirmed with XRD method, not shown). With air bubbling, the above Tween-85-wrapped layered crystallites were gradually converted into Co3O4 (i.e., asprepared CoT85), in which the Co3O4 nanocrystals were stably dispersed in an aqueous medium with the presence of Tween-85. Figure 2 shows an XRD pattern of the Co3O4 nanocubes obtained by a complete precipitation (with 40 mL of ethanol, see Experimental Section). The pattern (all indexed peaks) clearly indicates that the product is exclusively in the spinel phase of Co3O4 (cubic symmetry; space group: Fd3m, lattice constant ao ) 0.8084 nm; JCPDS file no. 43-1003), despite peak broadness due to small crystallite size. The estimated crystallite size based on the peak (311) is about 7.4 nm using the DebyeScherrer method,49 which is in broad agreement with our TEM observation (will be presented shortly). A small lattice expansion (e.g., along the [311] direction; +0.41%) was detected in Co3O4 nanocubes with an edge-length of 3.5 nm (Figure 3) with no appreciable lattice relaxation for greater crystallites. Since the conversion mechanism of the Co3O4 phase from its layered precursors had been explored in our previous work,24 the present research will be focused more on the effect of the surfactants, Tween85, as well as its derivatives under the synthetic conditions, and their morphological control and organization of the final spinel products.

II. Surfactant-Assisted Synthesis of Co3O4 Nanocubes. It has been well known that surfactant structures change with increasing concentration and are complicated,23 especially for a large complex molecule like Tween85. The concentration of Tween-85 in our starting reaction mixture (80 mM, see Experimental Section) far exceeded its critical micelle concentration [CMC(Tween-85) ) 0.01 mM in aqueous system].55 During the synthesis, therefore,

(54) (a) Cavani, F.; Trifiro, F.; Vaccari, A. Catal. Today 1991, 11, 173. (b) Rives, V.; Ulibarri, M. A. Coord. Chem. Rev. 1999, 181, 61.

(55) Surinenaite, B.; Bendikiene, V.; Juodka, B.; Bachmatova, I.; Marcinkevichiene, L. Biotechnol. Appl. Biochem. 2002, 36, 47.

Figure 2. XRD pattern of Co3O4 nanocubes prepared at 95 °C for 72 h; peaks with an asterisk belong to polycrystalline Si powder used as an internal standard for angle-calibration. The photo shows an as-prepared black suspension of Co3O4 nanocubes (CoT85) prepared at 80 °C for 79 h before adding ethanol.

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Figure 3. TEM images of hexagonal self-assembly of Co3O4 nanocubes synthesized at 80 °C for 79 h. The suspension used for TEM measurement (deposited onto a copper grid, see Experimental Section) was prepared by diluting about 1 g of reaction mixture (i.e., CoT85) with 3 mL of deionized water followed by ultrasonication for 15 min.

various types of micelles are expected to form for the normal micelles. Our synthesized products indeed show that these micellar “nanoreactors” are working in controlling the particle size of Co3O4. For example, at a lower temperature of 80 °C, the average particle size is around 3.5 nm (Figure 3), whereas at 95 °C, the average size is increased to 5.7 nm (Figure 4, a standard deviation of 1.1 nm, on the basis of statistic TEM image measurements: 100 counts, see Supporting Information for a large-area TEM image). Note that the Co3O4 particles were developed into nanocubes with clear facets of {100} surfaces in the latter case, as reported in Figure 5. The lattice fringes of d400 ()0.2021 nm) of Co3O4 can be clearly observed from the [001] direction, indicating the single-crystalline nature of the nanocubes. In all the cases, the resultant Co3O4 nanocubes were stably dispersed (i.e., CoT85), and no precipitates were seen in the product emulsions, which can be ascribed to a tiered surfactant structure formed (i.e., a bilayer shell), where the hydrophilic headgroups of Tween-85 are pointing toward the aqueous phase. It has been recognized that upon being placed on the carbon film (in TEM sample preparation), the original micellar organization of Co3O4 nanocubes can be preserved to a certain extent when the water solvent is gradually dried out. The nanocrystal shape effect on the selfassembly has been observed. As indicated in Figure 3, Co3O4 nanoparticles prepared at 80 °C typically selfassemble into hexagonally close-packed arrays upon deposition in our TEM sample preparation. Due to the increase in size and presence of flat cube surfaces at 95 °C, the observed hexagonal arrays are switched to square packing, as shown in Figure 4. In particular, the straight alignment of the nanocubes within a small domain size is generally observed. However, the overall arrangement of these nanocubes is rather random because of the short cube edge. Many curves composed of several to tens of nanocubes are also observed. The above morphological properties have never been observed for Co3O4 nanoparticles synthesized before.24,25,42,48 Compared to those obtained in our previous salt-mediated synthesis at the same reaction temperature (95°C, average edge length of 47 nm),24 the Co3O4 nanocubes synthesized in this work are much smaller despite a longer reaction time. Clearly, the surfactant Tween-85 is playing its vital role in controlling particle size and organizing the resultant nanocubes.

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The commonly observed interparticle space is only at 3.0 nm. However, the estimated length of Tween-85 from the hydrophilic head to hydrophobic tail is as large as 4.7 nm.56 In view of this significant space difference and the bulkiness of Tween-85, it is suggested that the capping agents on the surfaces of these Co3O4 nanocubes are unlikely to be the pristine Tween-85 molecules. The actual surface-stabilizing species will be further elucidated with our UV-vis, EA, TGA, FTIR, and XPS investigations shortly. On the other hand, considering a high concentration of Tween-85, it is likely that these nonionic surfactants would self-assemble into large lamellar micelles and envelope the chemically capped Co3O4 nanocubes after growth reactions since their 2D superlattice assemblies have been predominantly formed under our drying conditions (this will be further addressed shortly). A small percentage (less than 5% in overall nanocube population) of larger nanocubes (around 9-11 nm) was generated inside the lamellar micelles containing a large amount of the nanocubes in 2D arrays. The enlargement of nanocubes is attributed to agglomeration of the neighboring nanocubes on which less surfactant molecules were adsorbed (imperfect capping) noting that the edge lengths of the large nanocubes are generally about twice of that of the smaller nanocubes nearby. These large nanocubes were likely formed through a self-multiplication process. Such a growth mechanism has been described as a “cementing mechanism” or an “oriented attachment” and has been discussed in detail in several reports.57-62 In the current case, this mechanism is strongly supported by observations of diminishing intercube space and of various combination modes of primitive nanocubes (inset, Figure 4B). The formation and division of these lamellar micelles will be further addressed later. Besides the above ordered nanocubes formed within the lamellar micelles, the occasional observations of randomly arranged nanocubes in Figure 4C could be attributed to the presence of “single-cube” micelles in which only one Co3O4 nanocube is encapsulated. This is supported by the observation of a large interparticle space of 8.0-9.5 nm (about two times that of the surfactant shell thickness) and lack of definite patterns of intercube connectivity, i.e., the capped Co3O4 is further shielded with Tween-85 surfactants, resulting in random orientation of the nanocubes. The observed range of interparticle space reflects possibilities of titling and interpenetrating of the surfactants in the two neighboring micelles. III. Capping Species on Co3O4 Nanocube Surfaces. Both TGA/DTA and EA experiments indicate that the asprecipitated Co3O4 nanocubes are coated with a layer of organic capping agent even after thorough washing. As reported in Table 1, 22.3 wt% was lost over the temperature range of 200-400 °C. This weight loss is also accompanied by a strong exothermic peak at 271 °C, resulting from the combustion of organic compounds. An additional peak on the DrTGA curve at a lower temper(56) (a) Chem 3D Ultra simulation, with MM2 program. (b) The Cambridge Crystallography Database [calculated linear dimension for oleic acid (cis-form) is 1.8913-1.9620 nm]. Abrahamsson, S.; RyderstedtNahringbauer, I. Acta Crystallogr. 1962, 15, 1261]. (57) (a) Kolthoff, I. M.; Noponen, G. E. J. Am. Chem. Soc. 1938, 60, 499. (b) Kolthoff, I. M.; Eggertsen, F. T. J. Am. Chem. Soc. 1941, 63, 1412. (c) Kolthoff, I. M.; Bowers, R. C. J. Am. Chem. Soc. 1954, 76, 1510. (58) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969. (59) Chemseddine, A.; Moritz, T. Eur. J. Inorg. Chem. 1999, 235. (60) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem., Int. Ed. 2002, 41, 1188. (61) Sampanthar, J. T.; Zeng, H. C. J. Am. Chem. Soc. 2002, 124, 6668. (62) (a) Lou, X. W.; Zeng, H. C. J. Am. Chem. Soc. 2003, 125, 2697. (b) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2003, 125, 4430.

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Figure 4. TEM images of self-assemblies of Co3O4 nanocubes synthesized at 95 °C for 72 h. The suspension used for TEM measurement (deposited onto a copper grid, see Experimental Section) was prepared by diluting about 1 g of reaction mixture (i.e., CoT85) with 3 mL of deionized water followed by ultrasonication for 15 min. (A) Square and curved arrangements, (B) oriented attachment among the nanocubes (inset indicates different attachment modes in the circled areas), and (C) random arrangement.

Figure 5. High-resolution TEM image of a Co3O4 nanocube synthesized at 95 °C for 72 h (same as those in Figure 5). Table 1. Thermal Decomposition Data of Alkylated-Oleic-Anion-Capped Co3O4 Nanocubes temp (°C)

weight loss (%)

25-200 200-400

3.4 22.3

910-950

5.0

DrTGA (°C)

DTA (°C)

228, 271

271

928

928

thermal event dehydration organic anion decomposition and combustion decomposition of Co3O4 to CoO

ature of 228 °C is also observed, which can be associated to an adiabatic fragmentation of the organic species prior to the oxidative combustion.63 Our EA analysis also confirms the presence of carbon element in the sample (16.1 wt%). Nonetheless, the presence of free Tween-85 molecules in these organic-capped Co3O4 nanocubes was ruled out by UV-visible measurement (see Experimental (63) Xie, W.; Gao, Z.; Pan, W. P.; Hunter, D.; Singh, A.; Vaia, R. Chem. Mater. 2001, 13, 2979.

Section). Therefore, the organic capping matter must be present on the surface of Co3O4 nanocubes by chemisorption; it cannot be washed away by ethanol either. Indicated by a further weight loss (ca. 5%), the phase transformation of Co3O4 to CoO takes place at 928°C under air atmosphere; this temperature is markedly lower than those observed for the larger Co3O4 particles (e.g., 943 °C),25,64-66 as smaller size particles are generally more reactive in thermal decomposition. FTIR spectra A and B in Figure 6 show that Tween-85 remains largely intact in the bulk mixture of CoT85, as all its fingerprint absorptions can be clearly observed in the both spectra. For instance, the broad bands at 34353452 cm-1 and the peaks at 1639-1644 cm-1 correspond to O-H stretching and bending modes of Tween-85, respectively. The peaks at 2925 and 2855 cm-1 are due to asymmetric and symmetric C-H stretches of the hydrophobic chains.67 The sharp and symmetric peaks at 17361737 cm-1 are attributed to stretching mode of the ester carbonyl in the molecule.67 The broad peaks at 11031111 cm-1 can be assigned to C-O-C stretching mode.67,68 The extra absorption peaks are due to the presence of NO3- (from the starting reagent, 1384 cm-1) and the formation of Co3O4 spinel oxide (669 and 585 cm-1).64 It should be noted that Tween-85, as a nonionic surfactant, does not form irreversible binding with the metal oxide surface. In agreement with TGA/DTA/EA/ UV-vis results, direct capping of pristine Tween-85 on the as-synthesized Co3O4 nanocubes can also be ruled out unambiguously, on the basis of following IR observations (spectrum C, Figure 6): (i) the absence of strong C-O-C stretching near 1111 cm-1 [the weak bands left at 1098 and 1040 cm-1 correspond to the coupled stretch vibration, ν(C-C/C-O), (ii) a much weaker absorption band for the ester carbonyl group at 1734 cm-1, (iii) two new peaks at 1550 and 1411 cm-1 [typical for stretching vibrations of carboxylate group, -COO-, νas(OCO) and νs(OCO), respectively],69 (iv) strengthening of C-H stretching at 2923 (64) Xu, Z. P.; Zeng, H. C. J. Mater. Chem. 1998, 8, 2499. (65) Xu, Z. P.; Zeng, H. C. Chem. Mater. 1999, 11, 67. (66) Xu, Z. P.; Zeng, H. C. Chem. Mater. 2000, 12, 3459. (67) Dean, J. A. Analytical Chemistry Handbook; McGraw-Hill: New York, 1995; p 61. (68) Zalipsky, S.; Gilon, C.; Zilkha, A. Eur. Polym. J 1983, 19, 1177.

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Figure 6. FTIR spectra of (A) Tween-85, (B) reaction mixture CoT85 (prepared after 72 h at 95 °C), and (C) anionic-surfactantcapped Co3O4 nanocubes (i.e., the precipitated product from CoT85 suspension with ethanol).

and 2852 cm-1, (v) a significant red-shift of O-H band to 3380 cm-1, which can be attributed to surface hydroxylation of the nanocubes (Co-O-H), and (vi) a sharp increase in absorptions at 576 and 664 cm-1 for the spinel Co3O4, which indicates a drastic increase in weight ratio of the inorganic phase in this final product. All these observations collectively reveal that Tween-85 has been essentially removed and the surface-modifying species on Co3O4 nanocubes originated from the hydrophobic headgroup of Tween-85, i.e., alkylated oleate group [in the ionic form of CH3(CH2)7CHCHCH2CR2(CH2)5 COO-, R ) CH2CHCHCH2(CH2)6CH3]; the carboxylate anion binds to surface cobalt cations to produce a tight ligand shell (see Figure 8 shortly). In fact, it has been well documented that transition metal ions can promote the cleavage of a carbonyl ester.70,71 In the present case, cobalt cations of Co3O4 may serve as active catalytic sites for this cleavage, self-producing a layer of organic capping (i.e., the chemically adsorbed alkylated carboxylate anions; Co cations serve as Lewis sites for this carboxylate adsorption, Figure 8). As a further confirmation, the presence of unattended neutral -COOH (i.e., of the alkylated oleic acid) could also be ruled out, since there is no IR stretching absorption of carbonyl double bond in this functional group at around 1702 cm-1. In other words, this functional group forms a surface complex with cobalt cations once it is produced. Our XPS analysis further provides additional evidence on the above hydrolysis reaction and resulted surface modification. In Figure 7, C 1s spectra of the studied samples exhibit the C-OX (alcohol/ether/ester, e.g., (69) Arenas, J. F.; Marcos, J. I. Spectrochim. Acta, Part A 1979, 35, 355.

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C-O-C & C-OH in the present cases) species at a BE of 286.2-286.3 eV.50-52 On the basis of the peak area calculation, about 24.8% of carbon detected belongs to C-OX for the CoT85 sample, while there is a significant drop to 11.2% for the organic-capped Co3O4 nanocubes. In agreement with the FTIR results, this peak is primarily contributed by the carbons in the polyoxyethylene sorbitan groups (C-OH) of Tween-85 due to its significant presence in CoT85. In contrast, the same peak (286.2 eV) of the Co3O4 nanocubes may arise largely from the oxidized carbon species of adventitious carbons.50-52 The predominant carbon peaks at 284.7 eV are assigned to aliphatic hydrocarbon chains and several minor peaks at higher BEs are due to presences of oxygen-associated carbons, such as ester carbonyl (287.7 eV) and carboxylate end (288.9 eV) derived from the alkylated oleate group.50-52 Indeed, the ester carbonyl of Tween-85 at 287.7 eV is no longer observed in the Co3O4 nanocubes after removal of Tween-85. As anticipated, the peak at 288.9 eV is shifted to 288.2 eV due to stronger ionic interactions between carboxylate anion and trivalent cobalt cation on nanocube surfaces (Figure 8).72,73 The O 1s spectra also show the consistent results. The highest peak in CoT85 at BE of 532.4 eV is attributed to C-O-C and C-OH group of Tween-85.74 The contribution of NO3- anions to this peak should be negligible as the relative atomic concentration of N with respect to O is less than 2%. The peak at the same BE (532.4 eV) in the organic-capped Co3O4 nanocubes is much smaller and it can be assigned unambiguously to the carboxylate ends (derived from the alkylated oleate groups), which again confirms the absence of Tween-85 on the surface of Co3O4 nanocubes. The peaks at 529.8529.9, 531.3-531.4, and 533.5-533.7 eV in both spectra, on the other hand, can be assigned to oxygen species in Co3O4, Co-OH, and H2O molecules, respectively.75 The representative XPS spectra of Co 2p are also reported in Figure 7. BEs of Co 2p3/2 peaks at 780.0 eV are attributed to the surface phase of clean Co3O4,16,34,35,76 while the peaks at 781.9-782.2 eV primarily due to cobalt cations chelated/ adsorbed by the carboxylate anions (these peaks may also contain a small fraction of Co-OH species, since the samples were handled in laboratory air during XPS measurements).75 In agreement with the C 1s result for adsorbed carboxylate anions at 288.2 eV (vs 288.9 eV), a higher BE of the Co 2p3/2 is measured at 782.2 eV (vs 781.9 eV) can be ascribed to a higher degree of charge transfer from cobalt cations to carboxylate in the capped sample, owing to stronger adsorbate-substrate interactions after removal of Tween-85 envelope. Nonetheless, intensities of the shake-up satellite peaks in the samples are largely similar, noting that the main peaks of Co 2p1/2 branch are located at 795.1-795.2 eV and the spin-orbit splitting of 15.2 eV is also identical to that of the purephase Co3O4 in the literature.16,34,35,75 IV. Self-Organization Modes of Co3O4 Nanocubes. Colloidal chemistry involving surfactants is commonly investigated under the scope of very dilute concentrations of both reactants and surfactants since the reaction kinetics and the behavior of surfactant molecules are more predictable at low concentrations. However, such a system (70) Fanti, M.; Mancin, F.; Tecilla, P.; Tonellato, U. Langmuir 2000, 16, 10115. (71) Mancin, F.; Tecilla, P.; Tonellato, U. Langmuir 2000, 16, 227. (72) Burkstrand, J. M. J. Appl. Phys. 1981, 52, 4795. (73) DeKoven, B. M.; Hagans, P. L. Appl. Surf. Sci. 1986, 27, 199. (74) Beamson, G.; Briggs, D. High-Resolution XPS of Organic PolymerssThe Scienta ESCA300 Database; John Wiley & Sons: New York, 1992; App 3.1. (75) Xu, R.; Zeng, H. C. Chem. Mater. 2003, 15, 2040. (76) Sampanthar, J. T.; Zeng, H. C. Chem. Mater. 2001, 13, 4722.

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Figure 7. XPS investigation of C 1s, O 1s, and Co 2p for samples: (A) reaction mixture CoT85 (at 95 °C for 72 h), and (B) anionic-surfactant-capped Co3O4 nanocubes (i.e., the precipitated product from CoT85 suspension with ethanol).

Figure 8. Maximum linear dimension of an oleic acid molecule [CH3(CH2)7CHdCH(CH2)7COOH] which forms a backbone of the alkylated oleic carboxylate group [CH3(CH2)7CHdCHCH2CR2(CH2)5COO-, R ) CH2CHCHCH2(CH2)6CH3] and carboxylate adsorption modes to cobalt cations on the Co3O4 nanocube surface.

often suffers from low productivity. As we attempt to use high-concentration precursor solutions (e.g., [Co2+] ) 1.0 M) in all our syntheses,24,25 it becomes a challenging task to elucidate a formation mechanism of the Co3O4 nanocubes and their self-assembly.

With a combination of nonionic Tween-85 and the insitu generated adsorbed carboxylate anions, the growth of Co3O4 nanocubes can be well controlled in both size and dispersive organizations in which the as-formed nanoparticles are stable from aggregation. According to the weight-loss data (Table 1), it is calculated that every surface of the nanocubes is covered with around 57 of such organic molecules, or a total of around 8.6% of Tween85 molecules underwent the ester hydrolysis, noting that this process was reversible under the acidic conditions (Scheme 1, final pH ) 6.0-6.4, see Experimental Section). Considering the molecular dimension estimated in Figure 8,56 the surface of the nanocubes must have been covered with a rather compact monolayer of alkylated oleic anions, although possible formation of multilayers cannot be entirely ruled out at this stage. Thus, the commonly observed interparticle space of ca. 3 nm can be well explained by two partially penetrating layers of these highly branched and tangled organic molecules (i.e., alkylated oleic anions). As illustrated in Figure 9 for the tiered surfactant arrangements, it is believed that due to the interactions (i) between hydrophobic headgroups of alkylated oleic anions and those of Tween-85, and (ii) between the bulky hydrophilic ends of Tween-85 and water, the overall solid-

Figure 9. Reversible structural transforming processes of micelles upon the addition or removal of ethanol (cross-sectional views, only the boundary Tween-85 is drawn): lamellar micelles (top view) in which Co3O4 nanocubes self-assemble into a 2D sheet, a mixture of rodlike and spherical micelles, and individual “single-cube” micelles.

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Figure 10. Disassembly of Co3O4 nanocubes after partial precipitation by adding different amounts of ethanol to a mixture comprised of 10 g of CoT85 (at 95 °C for 72 h) and 15 mL of deionized water: 2 mL of ethanol (TEM images A and B); 3 mL of ethanol (TEM image C); and 4 mL of ethanol (TEM image D). Reassembly of Co3O4 nanocubes (TEM images E and F) was conducted by adding 1 mL of Tween-85 to the mixture prepared with addition of 3 mL ethanol (TEM image C, see Experimental Section).

liquid system is extremely stable and no nanocube sedimentation occurs even at a centrifugation speed of 20 000 rpm (see Experimental Section). In principle, furthermore, the Co3O4 nanocubes should exist favorably in well-dispersed oil-rich domains within the water medium because their surfaces are coated with an “oil” layer that favors hydrophobic interactions. It is well known that many nonionic surfactants can self-assemble into lamellar structures at high concentration.23,77,78 In the presence of lamellar Tween-85 bilayers in the reaction mixture, the surface-capped Co3O4

nanocubes could also be packed into the 2D arrays, as illustrated in Figure 9 (also refer to Figures 3 and 4 and the earlier discussion). The structural arrays of Tween85 molecules were further studied in our controlled precipitation experiments using ethanol as a flocculant (see Experimental Section), noting that the added ethanol would disturb the original balances between Tween-85 and water, as well as between Tween-85 and the adsorbed alkylated oleic anions. It should be mentioned that recent research in this area has focused on the self-assembly of non-close-packed superstructures by using surfactant-

(77) (a) Pileni, M. P. J. Phys. Chem. 1993, 97, 6961. (b) Moroi, Y. Micelles: Theoretical and Applied Aspects; Plenum Press: New York, 1992; p 45.

(78) Lindman, B.; Tiberg, F.; Piculell, L.; Olsson, U.; Alexandridis, P.; Wenneerstrom, H. In Micelles, Microemulsions, and Monolayers; Shah, D. O., Ed.; Marcel Dekker: New York, 1998; p 105.

Self-Generation of Tiered Surfactant Superstructures

coated nanoparticles with high shape anisotropy.23,79 Remarkably, for the first time, structurally isotropic Co3O4 nanocubes can be arranged into anisotropic nanostructured arrays with the tiered surfactant combination! As reported in Figure 10A, the Co3O4 nanocubes exhibit some discrete “curves” (i.e., non-close-packed one-dimensional arrays of Co3O4 nanocubes) upon an addition of 2 mL of ethanol. Once again, the interparticle distance within the curve is still kept at the constant value of 3 nm, while the distance between the two neighboring curves becomes much wider in a range of 8.0-9.5 nm (about two times of the surfactant shell thickness, Figure 9). Since there are basically no multiple stacks of nanocubes, these 1D arrays can be attributed to the formation of tubular channels wrapped with Tween-85 molecules, as depicted in Figure 9. Further branching of this tubular structure is still possible, which leads to some interconnectivity among the 1D “nanocube arcs” (Figure 10A). In addition to this, many nanocubes are self-arranged into circular domains (Figure 10B). Because the nanocubes within the circles are not particularly well organized and more cube deposition occurs along the circumferences, this type of arrangement indicates the coexistence of large spherical micelles formed by Tween-85 under the similar conditions. In agreement with this finding, the nanocube edges are arranged perpendicularly to the micellar circumferences, revealing a strong interaction between the hydrophobic headgroups of the anchored carboxylate anion (on Co3O4 cube surfaces) and Tween-85 surfactant (which forms the micellar walls with their hydrophilic headgroups pointing toward the water phase). When more ethanol was added (3 to 4 mL, Figure 10C and D), the above large micellar arrangements were further divided and the Co3O4 nanocubes become more and more chaotic, which can be attributed to the formation of “single-cube” micelles (Figure 9). On the basis of these observations, it can be concluded that the addition of ethanol favors the formation of smaller micelles. This close-packed to non-close-packed transformation is reversible. When ethanol was evaporated and more Tween-85 was added (see Experimental Section), the original 2D assemblies can be restored (Figure 10E). Many circular assemblies of nanocubes have also re-emerged in this case (Figure 10F). In most of the domains, the same constant interparticle space of 3 nm as those observed in Figures 3 and 4 is still well maintained. V. Optical Band Gap Determination. To investigate the dimensional and shape effects on the optical band gap (Eg), UV-visible absorbance spectra were further measured for the Co3O4 nanocubes, as shown in Figure 11a. The relationship between optical density and Eg can be related by the equation Rhν ) constant(hν - Eg)n.80 Assuming a direct transition with n equal to 1/2, Figure 11b plots the curves of (Rhν)2 versus hν. The value of hν extrapolated to R ) 0 gives an absorption band gap energy. Two curves are plotted in the wavelength ranges of 250350 and 400-600 nm, giving two Eg values for both samples. The determined Eg’s for the Co3O4 nanocubes (∼5.7 nm) prepared in the present work are 3.95 and 2.13 eV (optical band gap energy difference: ∆Eg ) Eg1 - Eg2 ) 1.82 eV), whereas for those with a larger size (∼47 nm, see Experimental Section), they are 3.15 and 1.77 eV (∆Eg ) Eg1 - Eg2 ) 1.38 eV), respectively. As has been investigated in the literature,81,82 the first band gap can be associated to O2- f Co2+ charge-transfer process (basic (79) (a) Co¨lfen, H.; Mann, S. Angew. Chem., Int. Ed. 2003, 42, 2350. (b) Ziherl, P.; Kamien, R. D. J. Phys. Chem. B 2001, 105, 10147. (80) Mott, N. F.; Davis, E. A. Electronic Processes in Noncrystalline Materials; Clarendon Press: Oxford, 1979; p 273.

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Figure 11. (a) UV-vis spectra, and (b) Plot of (REphoton)2 versus Ephoton for the direct transition. (A) large Co3O4 nanocubes with an edge length of 47 nm, and (B) small Co3O4 nanocubes with an edge length of 5.7 nm. Optical band gap energy of Co3O4 nanotubes was obtained by extrapolation to R ) 0.

optical band gap energy, or valence to conduction band excitation) while the second one to O2- f Co3+ charge transfer (Co3+ level is located below the conduction band).82 The multiple band gap energies for the Co3O4 particles may also suggest the possibility of degeneracy of the valence band.83,84 As for the film Co3O4 crystallites, the reported Eg’s have smaller values at 1.88-2.13 and 1.501.52 eV (∆Eg ) 0.38-0.61 eV), respectively.82,85,86 Compared to these bulk data, more importantly, the Co3O4 nanocubes synthesized with our two-tier surfactants have entered the quantum confinement regime, which allows one to have a fine-tuning on their material properties. In the nanoscale regime, our results are generally consistent with the reported data for nonfaceted Co3O4 nanoparticles (30 nm, 3.40 and 2.26 eV, ∆Eg ) 1.14 eV);84 these findings have confirmed that the optical band gap energies become larger as the crystallite size decreases. Although Eg1 increases monotonically with the dimensional reduction, however, the value of Eg2 apparently depends also on the shape of the nanoparticles. For example, our Eg2 values (1.77 and 2.13 eV) are lower than that reported for the nonfaceted Co3O4 nanoparticles (2.26 eV), resulting in a larger ∆Eg for the samples with the nanocube morphology. Since both a cubic symmetry and a small size are achieved, the prepared nanocubes represent an ideal model system for theoretical investigation (such as “particle-in-a-box” approximations) on quantum confinement effects. (81) Cheng, C.-S.; Serizawa, M.; Sakata, H.; Hirayama, T. Mater. Chem. Phys. 1998, 53, 225. (82) Barreca, D.; Massignan, C.; Daolio, S.; Fabrizio, M.; Piccirillo, C.; Armelao, L.; Tondello, E. Chem. Mater 2001, 13, 588. (83) Murad, W. A.; Alshamari, S. M.; Alkhateb, F. H.; Misho, R. H. Phys. Status Solidi A 1998, 106, K143. (84) Kumar, R. V.; Diamant, Y.; Gedanken, A. Chem. Mater. 2000, 12, 2301. (85) Cheng, C.-S.; Serizawa, M.; Alkhateb, F. H.; Misho, R. H. Mater. Chem. Phys. 1998, 53, 225. (86) Gulino, A.; Dapporto, P.; Rossi, P.; Fragala`, I. Chem. Mater. 2003, 15, 3748.

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Conclusion In summary, without preintroduction of complex surfactants, tiered surfactant combinations can be selfgenerated from a single-source surfactant Tween-85 during the synthesis of single-crystalline Co3O4 nanocubes. With this novel method, both the size and shape of Co3O4 can be controlled under “one-pot” conditions at relatively low reaction temperatures. Furthermore, various Co3O4 containing micellar superstructures can be organized through hydrophobic interactions between Tween-85 molecules and the anchored organic groups on the surfaces. In addition to the known hexagonal close-packed 2D arrays, Co3O4 nanocubes can also be arranged into squarely packed, spherically assembled, and linearly aligned arrays (non-close-packed superlattices) despite their highly symmetrical morphology. It has been found that these nanocube-superlattices are inter-transformable

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in their sedimentation on the solid supports. On the basis of our FTIR/UV-vis/EA/TGA/DTA/XPS investigations, it is confirmed that the organic coating (adsorbed alkylated oleic carboxylate anions) on the nanocubes was derived from Tween-85 through hydrolysis, and the constant intercube distance of 3 nm can be well explained as a partial overlapping of protruding hydrophobic headgroups. Strong quantum confinement effects have been observed for the Co3O4 nanocubes; the optical band-gap energies of 3.95 and 2.13 eV determined for the nanocubes (5.7 nm in the edge-length) correspond, respectively, to O2- f Co2+ and O2- f Co3+ charge-transfer processes. Supporting Information Available: Large-area TEM image. This material is available free of charge via the Internet at http://pubs.acs.org. LA049164+