Use of Block Copolymer Micelles on Formation of Hollow MoO3

State University of New York at Stony Brook, Stony Brook, New York 11794-2275. Received February 28, 2000. In Final Form: May 31, 2000. Triblock copol...
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Use of Block Copolymer Micelles on Formation of Hollow MoO3 Nanospheres† Tianbo Liu,‡ Yi Xie,‡,§ and Benjamin Chu*,‡,| Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794-3400, and Department of Materials Science and Engineering, State University of New York at Stony Brook, Stony Brook, New York 11794-2275 Received February 28, 2000. In Final Form: May 31, 2000 Triblock copolymer micelles have been used successfully as templates to form porous, highly uniform, and hollow MoO3 nanospheres, with their diameter being equivalent to the micellar core size. By use of a combination of static and dynamic light scattering, small-angle X-ray scattering, and wide-angle X-ray diffraction, the MoO3 nanoparticles were found to crystallize into an extremely ordered simple cubic structure. The MoO3 was synthesized from the precursor compound MoO2(OH)(OOH) and formed a “thin layer” possibly at the interface between the micellar core and shell. The formation of MoO3 also affected the triblock copolymer micelles. The remaining bare nanospheres tended to aggregate into large aggregates. In the gel region formed by the triblock copolymers, the above process was so slow that the nanospheres were able to form before the core deformation. These nanospheres could subsequently be packed into a very ordered structure. The proposed mechanism can partly be suggested by the observation that the size of MoO3 nanospheres could be correlated with micellar core size. Therefore, the size of the hollow MoO3 nanospheres, over a limited size range, can be accurately controlled by choosing a suitable micellar core.

* To whom correspondence should be addressed. † Part of the Special Issue “Colloid Science Matures, Four Colloid Scientists Turn 60 at the Millennium”. ‡ Department of Chemistry, State University of New York at Stony Brook. §Permanent address: Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, China. | Department of Materials Science and Engineering, State University of New York at Stony Brook.

catalysts, superparamagnetic materials, and biominerals. These materials are important because, in some instances, many of their physical and chemical properties show a particle size and modified-surface dependence.4 The modified materials on a nanometer scale are designed to achieve special structures and/or functions. However, two major limitations persist. One is that the preordered matrix structure often loses its form after the organic matrix has been removed. The other is that the matrixes created by common surfactants exhibit limited selfassembled structures. Block copolymers have been used extensively as matrix scaffolding materials because their nanoscopic structures can be manipulated. Commercially available amphiphilic EPE and EBE type triblock copolymers (where E, P, and B are poly(oxyethylene), poly(oxypropylene), and poly(oxybutylene), respectively) can self-assemble to form micellar structures in a selective solvent, which is a good solvent for one block and a nonsolvent or poor solvent for another block.9-11 The difference in solubility of different copolymer blocks leads to self-association. The less soluble block tends to shrink in solution and to aggregate together to form micellar cores while the soluble block forms a micellar shell or corona. At high polymer concentrations, the closed packing of micelles or the rearrangement of the hydrophilic and hydrophobic regions leads to the formation of gel-like ordered structures, e.g., body-centered cubic (bcc), face-centered cubic (fcc), hexagonal, and lamellar forms can be easily created. The phase behaviors of amphiphilic block copolymers in selective solvents have been widely reported in the literature.12-18 The sizes of

(1) Mann, S. Nature 1993, 365, 499 and the references therein. (2) Muthukumar, M.; Ober, C. K.; Thomas, E. L. Science 1997, 277, 1225. (3) Braun, P. V.; Osenar, P.; Stupp, S. I. Nature 1996, 380, 325. (4) Eftekharzadeh, S.; Stupp S. I. Chem. Mater. 1997, 9, 2059. (5) Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schuth, F.; Stucky, G. D. Nature 1994, 368, 317. (6) Zhou, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (7) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 1999, 11, 2813.

(8) Komarneni, S.; Parker, J. C.; Thomas, G. J. In Nanophase and Nanocomposite Materials; MRS Symposium proceeding; Materials Research Society: Pittsburgh, PA 1993; p 286. (9) Chu, B.; Zhou, Z. In Nonionic Surfactants: Polyoxyalkylene Block Copolymers; Nace, V. M., Ed.; Marcel Dekker: New York, 1996; Chapter 3. (10) Tuzar, K.; Kratochvil, P. In Surface and Colloid Science; Matijevic, E., Ed.; Plenum Press: New York,1993; Vol. 15. (11) Chu, B. Langmuir 1995, 11, 414. (12) Wanka, G.; Hoffmann, H.; Ulbright, W. Colloid Polym. Sci. 1990, 268, 101.

Introduction It is a pleasure to contribute to a Special Issue of Langmuir in honor of Professors Almgren, Holzwarth, Mackay, and Wyn Jones on the occasion of their 60th birthdays. In particular, Professor Ray A. Mackay was the first Ph.D. graduate of the State University of New York at Stony Brook, representing a symbol of maturity for colloid science and for Stony Brook. Yet, the science and the University are emerging into exciting new endeavors as presented in the article below. A promising route to the synthesis of nanostructured composites is to use the desired supramolecular preorganized microenvironment as a matrix to nucleate and grow nanostructures of inorganic materials.1,2 An important approach to forming organized matrixes is to use the self-assembled amphiphilic surfactants or block copolymers, which can create ordered structures in the nanometer scale. Recently, a great deal of research has dealt with the preparation and characterization of nanostructured inorganic/organic composites.3-8 The studied inorganic materials have been extended to semiconductors,

10.1021/la000282g CCC: $19.00 © 2000 American Chemical Society Published on Web 08/03/2000

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the unit cell of the ordered structures are usually on a nanometer scale. Therefore, such self-assembled matrixes can be used to synthesize inorganic compounds with nanoscale modifications. Transition-metal oxides such as Fe3O4 and TiO2 have been studied extensively with nanoscale modifications.1 Yang et al. presented a general and simple procedure to synthesize bulk mesoporous metal oxides (no Mo) with ordered large pores.7 Metal chlorides were used as precursors and mixed with block copolymer and ethanol. After completing the reactions and calcination, metal oxides with large pores could be obtained. The samples showed certain orders in packing with several scattering peaks that could be detected by small-angle X-ray diffraction. In another paper,19 we reported an interesting phenomenon: the crystallization of hollow MoO3 nanospheres by using a commercial triblock copolymer, E45B14E45, as a synthetic matrix template. Our approach was different from that of Yang et al. as our aim was to prepare thinlayered hollow nanospheres instead of bulk materials with large holes. Also, instead of using fast inorganic reactions and calcination to burn out the polymer matrix, we chose mild, slow reactions and used water to dissolve the polymer matrix. Under these conditions, we expected to obtain hollow MoO3 nanospheres with extremely uniform sizes. MoO3 was synthesized by the decomposition of the soluble precursor compound, MoO2(OH)(OOH). After the reaction and removal of all the copolymer chains and H2O, we obtained a special crystal, whose basic units were not molecules or atoms, but hollow nanospheres with a diameter of about 5 nm. Over 100 orders of scattering peaks could be detected in the entire small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXD) region. The 7th, 15th, 23rd, 28th, 31st, ..., scattering peaks were missing, indicating that the nanospheres were packed into a rare simple cubic (sc) structure. At the same time, the MoO3 crystalline structure had been distorted due to the formation of a hollow spherical structure. More importantly, these hollow MoO3 nanospheres were extremely homogeneous in size, as they could be packed into crystals with sizes approaching the micrometer scale. MoO3 is an electrochromic- and photochromic-sensitive material for optical device applications.20-22 The unique electronic and optical properties of MoO3 constitute the fundamental reasons behind their technological importance. Like most of the nanostructured materials, its optical and electronic properties are also size-dependent and are sensitive to its modification. Studies have also shown that MoO3 is a promising material for photoelectrochemical energy production, with the high surface area having higher photo efficiencies.23,24 Modified MoO3 nano(13) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145. (14) Mortensen, K.; Brown, W.; Norden, B. Phys. Rev. Lett. 1992, 68, 2340. (15) Mortensen, K.; Brown, W.; Jorgensen, E. Macromolecules 1994, 27, 5654. (16) Alexandridis, P.; Olsson, U.; Lindman, B. J. Phys. Chem. 1996, 100, 280. (17) Alexandridis, P.; Olsson, U.; Lindman, B. Langmuir 1996, 12, 1419. (18) Holmvist, P.; Alexandridis, P.; Lindman, B. Macromolecules 1997, 30, 6788. (19) Liu, T.; Xie, Y.; Liu, L.-Z.; Chu, B. Crystallization of Hollow MoO3 Nanospheres. To be sumbitted. (20) King, S. T. J. Catal. 1991, 131, 215. (21) Yao, J. N.; Hashimoto K.; Fujishima A. Nature 1991, 335, 624. (22) Svachula, J.; Tichy, J.; Machek, J. J. Catal. Lett. 1989, 3, 257. (23) Honma, I.; Zhou, H. S. Chem. Mater. 1998, 10, 103. (24) Janauer, G. G.; Dobley, A.; Guo, J.; Zavalij, P.; Whettingham, M. S. Chem. Mater. 1996, 8, 2096.

Liu et al.

particles could be a useful photocatalyst because of its extremely large surface areas. The advantage of using block copolymer gels as scaffolding matrixes is the rich variety of structures available in such gel-like materials. Block copolymer systems can provide very rich phase diagrams including bcc (bodycentered cubic), fcc (face-centered cubic), hexagonal, lamellar, and bicontinuous structures. Its easy accessibility permits us to choose a proper chain length, chain ratio, and chemical nature of the blocks (EP or EB) to suit the objective on hand. In this work, we have extended our study by using polymer gels at different polymer concentrations, to obtain some detailed information on the nanoparticle formation. Static light scattering (SLS) and dynamic light scattering (DLS) techniques were used to monitor the reaction process in solution and to elucidate the reaction mechanism. Such a synthetic pathway can no doubt extend the process for synthesis of other new novel materials. Experimental Section Sample Preparation. Commercial triblock copolymer B205000 (E45B14E45) was obtained as a gift from the Dow Chemical Co.. It was purified by hexane to remove the very hydrophobic impurities that were present in the samples during synthesis. The detailed procedure has been described elsewhere.25 Block copolymer samples were dissolved in cold water near 0 °C, at which temperature the B block was relatively soluble at dilute concentrations. To ensure complete dissolution and equilibration of the systems, the cold dilute solution was stirred for a long time (hours) to ensure its homogeneity. The samples in the gel region, where the copolymer concentrations were high, were centrifuged at a speed of 8000 rpm (≈7.0 × 103 g) at room temperature for at least 2 days to make sure that the components were thoroughly mixed. The gelation and the gel structures are usually temperature dependent, but thermoreversible. The polymer gels were put into sample cells and stayed at least 15 min at measuring temperature to make sure that the structures have reached equilibrium. Preparation of MoO3 Hollow Nanospheres. Pure metallic Mo powder was mixed with 30% H2O2 aqueous solution to form a yellow, water-soluble precursor compound MoO2(OH)(OOH).26 Then this solution was heated to drive off excess H2O2. The MoO2(OH)(OOH) solution and E45B14E45 copolymer powders were then mixed to form a homogeneous solution or by centrifugation to form a homogeneous gel at higher polymer concentrations. The as-formed “gel”, which was transparent and with a yellow color, was kept for at least 2 weeks in order to be sure that MoO2(OH)(OOH) had decomposed into MoO3 with the release of O2 gas. The MoO3/E45B14E45 gel was semitransparent and had a dark blue color. A large amount of distilled water was used to immerse the MoO3/E45B14E45 gel. A blue precipitate was obtained in the solution representing the typical color of Mo(V) and Mo(VI) mixture. E45B14E45 would have been dissolved in excess water. The blue precipitate was collected and dried at 100 °C to remove water. Elemental analysis showed that in the blue precipitate, the carbon content was less than 1.7 wt % and the H content was sufficiently low as to be not detectable, implying that most of the polymer matrix had been removed by water. Static Light Scattering (SLS) and Dynamic Light Scattering (DLS). SLS and DLS were used to characterize the formation and the structures of block copolymer micelles in aqueous solution. We used a standard laboratory-built light scattering spectrometer27 that was capable of both SLS and DLS measurements over an angular range of 15-140°. The spectrometer was equipped with a Coherent Radiation 200 mW diodepumped solid-state (DPSS) laser (model 532) operating at 532 nm and a Brookhaven Instruments (BI 9000) correlator. The (25) Liu, T.; Zhou, Z.; Wu, C.; Chu, B.; Schneider D. K.; Nace, V. M. J. Phys. Chem. B 1997, 101, 8808. (26) Yasuhiko, K. Bull. Chem. Soc. Jpn. 1981, 54, 293. (27) Chu, B.; Onclin, M.; Ford, J. R. J. Phys. Chem. 1984, 88, 6566.

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sample chamber was thermostated and could be controlled to within (0.02 °C. The basis of SLS is the Rayleigh-Gans-Debye equation, valid for small, interacting particles in the form:28

1 Hc ) + 2A2C R90 Mw

(1)

where H ≡ 4π2n02(dn/dc)2/NAλ4 is an optical parameter with n0 being the solvent refractive index, NA is Avogadro’s constant, λ is the laser wavelength (532 nm), Mw is the weight-average molecular weight, A2 is the second virial coefficient, and dn/dc is the refractive index increment. R90 is the excess Rayleigh ratio of the copolymer solution at θ ) 90°, with θ being the scattering angle and it is equal to RBz,90(I - I0)/IBz(n2/nBz2), where RBz,90 is the Rayleigh ratio of benzene at θ ) 90° and has a value of 2.0 × 10-5/cm at 532 nm,26 I, I0, and IBz are the scattered intensities of the solution, the solvent, and benzene, respectively, and nBz is the refractive index of benzene. Dynamic light scattering (DLS) measures the intensityintensity time correlation function G(2)(Γ) by means of a multichannel (BI-9000) digital correlator.

G(2)(Γ) ) A(1 + b|g(1)(τ)|2)

(2)

where A, b, and |g(1)(τ)| are, respectively, the background, a coherence factor, and the normalized electric field time correlation function. The field correlation function was analyzed by the constrained regularized CONTIN29 method, to yield information on the distribution G(Γ) of the characteristic line width (Γ) from

|g(1)(τ)| )

∫G(Γ)e

-Γτ

dΓ

(3)

The normalized distribution function of the characteristic line width, G(Γ), so obtained can be used to determine an average apparent diffusion coefficient and the apparent hydrodynamic radius Rh,app via the Stokes-Einstein equation

Dapp )

Γ kT ) 2 6πηR q h,app

(4)

where q ≡ [(4πn/λ) sin(θ/2)] is the magnitude of the scattering wave vector, k is the Boltzmann constant, and η is the viscosity of water at temperature T. From DLS measurements, we can obtain the particle-size distribution in solution from a plot of ΓG(Γ) versus Rh,app, with ΓiG(Γi) being proportional to the scattered intensity of all the i particles having an apparent hydrodynamic radius Rh,i. The subscript “app” is used to denote DLS measurements performed at finite concentrations when interparticle interactions have been neglected. Small-Angle X-ray Scattering (SAXS) and Wide-Angle X-ray Diffraction (WAXD). SAXS experiments were performed at the X3A2 SUNY Beam Line, National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL), using a laser-aided prealigned pinhole collimator.30 The incident beam wavelength was tuned at 0.128 nm. Fuji imaging plates were used as the detection system. The sample-to-detector distances were 1114 mm for SAXS and 130 mm for WAXD, respectively. A temperature-controlled sample holder was used to hold the sample. The temperature in the sample holder was adjusted by a water bath that could vary temperatures ranging from 5 to 90 °C and controlled to (0.1 °C. The MoO3 hollow nanoparticles were studied by SAXS and WAXD simultaneously at room temperature. A thin-layer of each sample was placed in a small sample cell in order to alleviate the strong absorption of Mo atoms. (28) Hiemenz, P. Z. Principle of Colloid and Interface Chemistry; Marcel Dekker, Inc.: New York, 1985. (29) Provencher, S. W. Makromol. Chem. 1979, 180, 201; Comput. Phys. Commun. 1982, 27, 213, 229. (30) Chu, B.; Harney, P. J.; Li, Y.; Linliu, K.; Yeh, F.; Hsiao, B. S. Rev. Sci. Instrum. 1994, 65, 597.

Figure 1. A tentative temperature-polymer concentration phase diagram of E45B14E45 triblock copolymer in aqueous solution. A solution region, a phase-separated region, and three gel-like regions (bcc packing, hexagonal packing, and amorphous gel) were detected.

Results and Discussions EBE Micelles in Dilute Solution. The characterization of the micelle formation of block copolymers in dilute solution by LLS has been reported before.25,31-36 Closeassociated spherical micelles were formed in solution with the hydrophobic B blocks forming the micellar core and the hydrophilic E blocks forming the micellar shell. For E45B14E45, its micellization showed only a weak temperature dependence. The critical micelle concentration (cmc) changed from 1.11 mg/mL at 20 °C to 0.18 mg/mL at 40 °C, while the association number (Nw) increased from 3 to 18, indicating a small, positive enthalpy change (∆H°) during micellization, and that micellization was an entropy-driven process. DLS measurements showed that the E45B14E45 micelles had an average hydrodynamic radius (Rh) of about 6.0 nm.35 Gelation of E45B14E45 in Aqueous Medium and General Phase Diagram. The boundary of the sol-gel transition was measured by visual observation. Figure 1 shows the binary temperature-polymer concentration phase diagram of E45B14E45 in water. The sol-gel transition occurred at around 26 wt % polymer concentration. Similar to the micellization process, it also showed nearly no concentration dependence in the range from 10 to 70 °C. On the contrary, both the sol-gel and the unimermicelle transition curves of EPE type block copolymers usually have a very obvious temperature dependence, e.g., F127 (E99P69E99).36 It is obvious that sol-gel transition and micellization are intercorrelated, reflecting the solubility change of the less-soluble block with changing temperature. At high temperatures, the gel-like structure collapsed into solution again. The gelation region diminished at high temperatures (>86 °C) and high polymer concentrations (>84 wt %). Inside the gel region, three different structures could be identified by SAXS measurements: body-centered cubic (31) Yang, Y.-W.; Yang, Z.; Zhou, Z.; Attwood, D.; Booth, C. Macromolecules 1996, 29, 670. (32) Zhou, Z.; Chu, B.; Nace, V. M. Langmuir 1996, 12, 5016. (33) Liu, T.; Zhou, Z.; Wu, C.; Chu, B.; Schneider, D. K., Nace, V. M. J. Phys. Chem. B. 1997, 101, 8074. (34) Liu, T.; Zhou, Z.; Wu, C.; Nace, V. M., Chu, B. J. Phys. Chem. B 1998, 102, 2875. (35) Liu, T.; Nace, V. M.; Chu, B. Langmuir 1999, 15, 3109. (36) Wu, C.; Liu, T.; Chu, B.; Schneider, D. K.; Graziano, V. Macromolecules 1997, 30, 4574.

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Liu et al. Table 1. Structure Parameters of E45B14E45/Water bcc Gel-like Structure at 25 ˚C and the Radii of MoO3 Hollow Nanospheres Calculated from SAXS Measurements E45B14E45/water gels calcd Rc q1 (nm-1) a (nm) Nw (nm) 30 wt % 40 wt % 45 wt % 50 wt % 60 wt %

Figure 2. SAXS profile of 45 wt % E45B14E45/water system at 25 °C. The five scattering peaks show a simple mathematical relation of 1:21/2:31/2:41/2:51/2, suggesting a body-centered cubic (bcc) ordered packing of spherical micelles.

(bcc), hexagonal, and amorphous. The bcc structure generally appeared at low copolymer concentrations next to the solution region, formed by the closed packing of spherical micelles.14 The hexagonal structure appeared at higher copolymer concentrations, formed by cylindrical micelles, while the amorphous phase appeared at higher temperatures. The amorphous phase could be understood as the intermediate region between an ordered gel-like structure and the solution region. The ordered micellar packing was destroyed with increasing temperature. It first became an amorphous gel-like structure. At even higher temperatures, the gel-like structure was melted into a mobile state. Body-Centered Cubic (bcc) Region. The bcc region of the E45B14E45/water system appeared between 26 and 61 wt % and 0 and 75 °C. Figure 2 shows a profile of SAXS measurement on 45 wt % E45B14E45 at 25 °C. The y axis is the logarithmic value of scattered intensity while the x axis denotes the magnitude of the scattering wave vector q (in unit of nm-1). Five scattering peaks can be observed in Figure 2, with their q values following a simple mathematical relation of 1:21/2:31/2:41/2:51/2, which is the typical Bragg scattering pattern for a body-centered cubic (bcc) structure. Although the seventh ordered peak was not detectable in order to rule out the simple cubic (sc) packing, we could make the judgment by noticing that the third peak was stronger than the second one, which was typical for bcc packing, but not for sc. The relevant domain-domain distance, d, can be calculated from the expression

d ) 2π/q

(5)

On the basis of known diffraction rules for the bcc structure (m is even), the five scattering peaks can be indexed to the Miller Index hkl ) 110, 200, 211, 220, and 310, respectively. The crystal lattice size (a) is calculated from the domain-domain distance (dhkl) related to the first scattering peak (hkl ) 110)

a ) xh2 + k2 + l2 dhkl ) x2 d110

(6)

The distance between two adjacent d110 layers was about 8.3 nm based on eq 5, and the distance between two adjacent micelles could be calculated to be about 10.6 nm. In dilute solution, the Rh of E45B14E45 micelles was 6.0

0.728 0.751 0.761 0.764 0.769

12.2 11.8 11.6 11.6 11.5

33 40 44 51 55

2.4 2.5 2.6 2.7 2.8

MoO3 nanospheres R (nm) 2.3 2.4 2.4 2.5 2.6

nm, corresponding to a 12.0 nm micelle-micelle contact distance. At high polymer concentrations, this distance was closer due to the overlap among micellar shells. By assuming that all E45B14E45 chains had formed micelles because of the low cmc value, the Nw of micelles in the bcc gel-like region could be estimated as36

Nw )

CFNAa3 2Mw

(8)

with C being the copolymer wt % concentration, a the size of cubic lattice, Mw the weight-average molecular weight of E45B14E45 block copolymer chains (5000 g/mol), and F the density of the gel-like system

F ) WBFB + WEFE + WwFw

(9)

where WB, FB, WE, FE, Ww, and Fw are the weight fraction and density of B block, E block, and water, respectively. By assuming that the micellar core consists of only B blocks and that all B blocks reside in the micellar core (strong segregation), we can estimate the size of the micellar core (Rc) by using Nw and the following relation

NwMw,B 4 πRc3 ) 3 NAFB

(10)

where Mw,B is the weight-average molecular weight of the B block. Table 1 summarizes the Nw, a, and Rc values of the E45B14E45 bcc structure at different concentrations and 25 °C. It should be noted that the above assumption was not exactly valid for these block copolymer micelles, because a small amount of water could still exist inside the micellar cores. There should be an intermediate region between each micellar core and shell, i.e., an interface with a finite thickness. However, we can still use the above expression to roughly estimate the Rc value. In Table 1, the q1 value increased with increasing E45B14E45 concentration, suggesting that the crystal lattice became smaller; i.e., the micelles packed more compactly at higher concentrations. At the same time, the Nw value increased with increasing copolymer concentration, from 33 at 30 wt % to 56 at 60 wt %. In dilute micellar solution, the Nw value should be fairly constant with polymer concentration at a fixed temperature (e.g., at 25 °C, Nw ) 7). However, in the gel-like region, the Nw value showed an obvious concentration dependence and it could become much higher than that in dilute solution. The q1 value remained roughly unchanged for the same sample by increasing temperature from 30 to 70 °C. With the assumption that the polymer bulk density remained relatively constant within this temperature range, we could conclude that the Nw of E45B14E45 micelles in the gel-like region did not change with temperature. This observation is consistent with our former conclusion on the gel structures of F127 triblock copolymer micelles in water and in 1X TBE buffer,36 but it is quite different

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Figure 3. Schematic drawings of (a) normal MoO3 octahedral units and (b) MoO3 hollow nanospheres.

from the Nw of polyoxyalkylene block copolymer micelles in dilute solution, which usually has a temperature dependence. Preparation of MoO3 Hollow Nanospheres with Accurate Size Control. The nanoscaled isolated hydrophilic and hydrophobic regions in micellar systems make them suitable templates for preparing materials with nanoscale modifications. From the discussion above, we have concluded that even for the same block copolymer, the Rc value of the micelles was different at different concentrations. Therefore, by choosing a suitable polymer concentration, we can obtain micelles with slightly different micellar core sizes; i.e., we can modify the dimension of the synthetic templates. Thus, we should be able to influence the size of the synthesized hollow MoO3 nanoparticles. With MoO2(OH)(OOH) being soluble in water, MoO2(OH)(OOH) should reside basically in the hydrophilic micellar shells. The first nucleation of MoO3 from the decomposition of MoO2(OH)(OOH) was generated in the transparent gel with little O2 bubbles forming in the gel. The resulting MoO3 could continue to form and grow with the first-generated MoO3 crystalline seeds. All reactions should take place in the hydrophilic region. Former studies showed that the existence of electrolytes might change the structure of polymer gels.37,38 However, in the current system, SAXS measurements proved that the existence of a small amount of MoO2(OH)(OOH) with a molar ratio of MoO2(OH)(OOH)/E45B14E45 ) 3.1:1 did not affect the gel structure.19 As shown before, in the presence of MoO2(OH)(OOH), the micelles still choose a bcc closed packing structure, and the peak position shifted only very little toward the larger qmax values, indicating a slightly smaller unit cell dimension with a ) 11.3 nm. It is noted that in the layer structure of molecular crystalline MoO3, the bonding between Mo-O atoms in the same or neighboring units which share the O atoms or edges should be much stronger than that between two (37) Jorgenson, E. B.; Hvidt, S.; Brown. W.; Schillen K. Macromolecules 1997, 30, 2355. (38) Ruppelt, D.; Kotz, J.; Jaeger, W.; Friberg, S. E.; Mackay, R. A. Langmuir 1997, 13, 3316.

layers, as shown schematically in Figure 3a. The strong bonding leads the retention of this atom grouping in two dimensions. However, even stoichiometry no longer permits a three-dimensional perovskite network. So in the limited space among the hydrophobic cores, it is proposed that the bonding of two layers might be separated, but the same layer should keep this strong bonding, as shown schematically in Figure 3b. We used a large amount of water to remove the polymer chains instead of using calcination to obtain the inorganic compound because of the special experimental condition: limited final products in a large volume. The concentration of MoO3 was quite low (