纳米材料粒径公式TiO2

2936Chem. Mater. 2003, 15, 2936-2941

Selective Distribution of Surface-Modified TiO 2Nanoparticles in Polystyrene-b-poly (Methyl

Methacrylate) Diblock Copolymer

Chin-Cheng Weng and Kung-Hwa Wei*

Department of Materials Science and Engineering, National Chiao Tung University,

Hsinchu, Taiwan 30049Republic of China

Received April 10, 2003

Ordered aggregates of surfactant-modified TiO 2nanoparticles in the selective block of lamellar assemblies of the diblock copolymer PS-b-PMMA have been prepared. The hydrophobic or hydrophilic nature of the tethered surfactant determines the location of TiO 2nanoparticles in the corresponding block, as confirmed by transmission electron microscopy, differential scanning calorimetry, and Fourier transform infrared spectroscopy. The modes of dispersion of TiO 2in the blocks depend on the type of bonding between the surfactant and TiO 2. Photoluminescence studies of these nanocomposites demonstrate that the location of TiO 2nanoparticles affect the block copolymer’sluminescence at different wavelengths.

Introduction

Owing to their optical and electrical properties, semiconductor nanoparticles or clusters are emerging materials and have the potential to be used in a wide range of applications. 1,2For semiconductor or metal oxide nanoparticles with sizes close to their Bohr radius (typicallybetween 1and 10nm), the size-dependent band gap results in tunable optical properties. 1Nano-particles that are not treated with a surfactant or bonded to polymer chains will, however, form large aggregates. Furthermore, optoelectronic devices require nanoparticles to form ordered, one-to three-dimensional structures. 3

Block copolymers (BCPs)are a versatile platform material because they can self-assemble into various periodic structures for proper compositions and under adequate conditions, owing to the microphase separation between dissimilar blocks. 4,5A diblock copolymer, the simplest case, self-assembles into various equilibrium morphologies, such as alternating layers, complex to-pologically connected cubic structures, cylinders on hexagonal lattices, and spheres on a body-centered lattice. Self-assembly of BCPs can therefore serve as templates for the spatial arrangement of nanoparticles in thin films or in bulk samples and can provide an effective means to manipulate their positions.

In recent years, much effort has been directed toward the synthesis of semiconductor or metal oxide nanopar-ticles within block copolymer matrix materials. 6-16For

*To whom correspondence should be addressed. Tel:886-35-731871. Fax:886-35-724727. E-mail:[email protected].(1)Henglein, A. Chem. Rev . 1989, 89, 1861.

(2)Wang, Y.; Herron, N. J. Phys. Chem. 1991, 95, 525.

(3)Murry, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270. (4)Bates, F. S. Science 1991, 251, 898. (5)Thomas, E. L. Science 1999, 286, 1307.

(6)Sankaran, V.; Cummins, C. C.; Schrock, R. R.; Cohen, R. E.; Silbey, R. J. J. Am. Chem. Soc. 1990, 112, 6858.

(7)Cummins, C. C.; Schrock, R. R.; Cohen, R. E. Chem. Mater. 1992, 4, 27.

instance, BCPs/semiconductornanoparticle nanocom-posites have been synthesized for applications involving photonic band gap devices. 17,18Studies using ZnS, 7,10,11PbS, 6,8,9and CdS 7,12-14within BCPs and CdS in salt-induced BCPs micelles 15,16have also been reported. Among these studies, the common approach has been to synthesize the nanocrystal clusters within microphase-separated diblock copolymer films by attaching metal complexes to the functionalized block of the copolymer before microdomain formation. Then, the composite block copolymers are treated with hydrogen sulfur gases for obtaining nanoparticles in situ. Although the func-tional groups in the monomer that are used to bind the metals can be designed appropriately for one block of the copolymer, variations within the nanocrystal cannot be easily controlled within the microdomains of the block copolymers. Furthermore, these functionalized block copolymers are not suitable for use as large area templates, as opposed to the more readily available block copolymers such as polystyrene-b-poly (methylmethacrylate) (PS-b-PMMA)or polystyrene-b-poly (eth-ylene oxide) (PS-b-PEO).In the present study we have adopted an approach of synthesizing nanoparticles with modified surfactants. The surfactant can be either hydrophilic or hydrophobic, with one of its ends tethered

(8)Kane, R. S.; Cohen, R. E.; Silbey, R. Chem. Mater . 1996, 8, 1919. (9)Tassoni, R.; Schrock, R. R. Chem. Mater . 1994, 6, 744.

(10)Yue, J.; Sankaran, V.; Cohen, R. E.; Schrock, R. R. J. Am. Chem. Soc. 1993, 115, 4409.

(11)Sankaran, V.; Yue, J.; Cohen, R. E. Chem. Mater . 1993, 5, 1133. (12)Moffitt, M.; Eisenberg, A. Chem. Mater . 1995, 7, 1178

(13)Moffitt, M.; McMahon, L.; Pessel, V.; Eisenberg, A. Chem. Mater . 1995, 7, 1185.

(14)Moffitt, M.; Vali, H.; Eisenberg, A. Chem. Mater . 1998, 10, 1021.

(15)Zhao, H.; Douglas, E. P.; Harrison, B. S.; Schanze, K. S. Langmuir 2001, 17, 8428.

(16)Zhao, H.; Douglas, E. P. Chem. Mater . 2002, 14, 1418.

(17)Fink, Y.; Urbas, A. M.; Bawendi, M. G.; Joannopoulos, J. D.; Thomas, E. L. J. Lightwave Technol. 1999, 17, 1963.

(18)Edrington, A. C.; Urbas, A. M.; DeRege, P.; Chen, C. X.; Swager, T. M.; Hadjichristidis, N.; Xenidou, M.; Fetters, L. J.; Joannopoulos, J. D.; Fink, Y.; Thomas, E. L. Adv. Mater. 2001, 13, 421.

10.1021/cm0300617CCC:$25.002003American Chemical Society

Published on Web 06/17/2003

Surface-Modified TiO 2Nanoparticles in BCPs Chem. Mater., Vol. 15, No. 15, 20032937

Scheme 1. Synthesis of TiO 2Nanoparticles by Ionic or Nonionic

Surfactants

Table 1. Compositions of TiO 2Colloidal Solutions THF (mL)

TiO 2-TMAC in THF TiO 2-TMS in THF TiO 2-H +in THF

555

TMAC (g)0.085

0.016TMS (g)

H 2O (g)0.050.050.05

HCl (g)(36%)0.0260.0260.026

TTIP/IPA(mL)

0.50.50.5

to a nanoparticle by an ionic bond or a covalent bond. The selective dispersion of these nanoparticles in one block of the diblock copolymer through either van der Waals or polar interactions between the particular block and the surfactants without altering the chemical structure of the diblock copolymer is desired. This selectivity is important in designing the optical proper-ties of nanoparticles -block copolymer hybrid systems. For instance, in a diblock copolymer, nanoparticles can be placed into blocks with the higher refractive index for enlarging the differences between the refractive indices of the two blocks for photonic crystal applica-tion. 17,18

Here, we report on dispersing surfactant-modified TiO 2nanoparticles into either block of a PS-b-PMMA diblock copolymer with an ordered lamellar phase. TiO 2was first synthesized in tetrahydrofuran (THF)instead of in a water or alcohol phase. 19-21Cetyl trimethylam-monium chloride (TMAC)amphiphilics or 3-(methacry-loyloxypropyl)-trimethoxy silane (TMS)surfactant was used to modify the TiO 2nanoparticles. To our knowl-edge, this presents a new approach to selectively disperse quantum-confined nanoparticles in a PS-b -PMMA diblock copolymer with an ordered lamellar phase.

Experimental Section

Materials. Polystyene-block -polymethyl methacrylate (PS-b-PMMA) diblock copolymer was purchased from Polymer

(19)Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. J. Phys. Chem . 1988, 92, 5196.

(20)Joselevich, E.; Willner, I. J. Phys. Chem . 1994, 98, 7628. (21)Serpone, N.; Lawless, D.; Khairutdinov, R. J. Phys. Chem . 1995, 99, 16646. Source, Inc. The polydispersity index, M w /M n , was 1.12, and the number-average molecular weights (M n ) of the PS and PMMA blocks were 85000and 91000g/mol,respectively, as determined by SEC. Cetyl trimethylammonium chloride (TMAC)was obtained from Taiwan Surfactant, Inc. 3-(Meth-acryloyloxypropyl)-trimethoxy silane (TMS,97%)was obtained from Lancaster. Titanium tetra (isopropoxide)(TTIP,98%,Acros USA), isopropyl alcohol (IPA,99.9%,TEDIA), THF (99.0%,Pharmco USA), and HCl (36%,Acros) were obtained from commercial sources.

Synthesis of TiO 2Nanoparticles. Reagent-grade chemi-cals and solvents were used without further purification. The precursor TTIP was diluted to 0.1M with IPA (TTIP/IPA).The TiO 2-TMAC and TiO 2-H +colloidal solutions were prepared by mixing TMAC, HCl, and deionized water in THF for 30min. Afterward, TTIP/IPAwas dropped in slowly with rapid stir-ring. The compositions of these TiO 2colloidal solutions are given in Table 1. The TiO 2-H +colloidal solution was then put in a vacuum oven at 65°Cfor solvent removal. Subsequently, a light yellow powder was obtained. TiO 2-TMS colloidal solution was first prepared with HCl, deionized water, and TTIP/IPA.TMS was added to the solution after 2h. The synthesis of surfactant-modified TiO 2nanoparticles is shown in Scheme 1.

Preparation of TiO 2/PS-b-PMMANanocomposites. A 0.1-g aliquot of PS-b-PMMA was added to 5mL of TiO 2-TMAC, TiO 2-H +, or TiO 2-TMS colloidal solutions. After the mixture was stirred for 3h, it was transferred to a Petri dish and organic solvent was removed at 65°Cfor 12h.

Characterization. Transmission electron microscopy (TEM)studies were carried out on a JEOL 2000FX electron micro-scope operating at 200keV. The samples for TEM studies were prepared by directly dispersing the TiO 2solution on a carbon film supported on a holey copper grid. Ultrathin sections of TiO 2/PS-b-PMMAfor TEM studies were deposited on copper grids after microtoning with a Leica ultracut Uct. The mor-phologies of all the bulk films were obtained by TEM after cutting a roughly 100-nm thin film. Energy-dispersive X-ray scattering (EDS)spectra were taken on a Link ISIS (OXFORD)

2938Chem. Mater., Vol. 15, No. 15, 2003Figure 1. UV -vis absorbance spectra of TiO 2colloidal solutions.

detector connected to the electron microscope. UV -vis absorp-tion spectra were obtained on an Aglient 8453UV -vis spectroscopy system by scanning between 190and 1000nm. The nanosized TiO 2colloidal solutions were diluted to 6×10-5M for the UV -vis experiment. From the spectral absorption edge (λos ), the diameters of the TiO 2nanoparticles were calculated by using absorption onset data. 19The X-ray dif-fraction study was carried out with a MAC Science MXP 18X-ray diffractometer (50kV, 40mA) with copper target and Ni filter at a scanning rate of 4°/min.The glass transition temperatures (T g ) of the bulk films were obtained from a Dupont DSC 2910at a heating rate of 20°C/min.Fourier transform infrared spectroscopy (FTIR)spectra of the samples were obtained using a Nicolet PROTE ÄGE Ä-460. The photolu-minescence of TiO 2/PS-b-PMMAwas observed under excitation of the sample by UV light at 260nm, in air, with a Hitachi F4500fluorescence spectrophotometer at room temperature. The 29Si solid-state NMR spectrum was recorded with a Bruker DMX-600NMR spectrometer.

Results and Discussion

Figure 1shows the UV -vis spectra of TiO 2-TMAC, TiO 2-H +, and TiO 2-TMS solutions. The concentration of TiO 2nanoparticles in both colloidal solutions is about 6×10-5M. The shifts in the UV -vis absorption onset of these TiO 2colloidal solutions suggest that the TiO 2nanoparticles have quantum-confined properties. 22,23The reference absorption and the corresponding band gap energy are λref ) 385nm and Eg ref ) 3.2eV (weassume an anatase crystal shape for TiO 2in the calculation). From the onset absorption wavelength (λos ), the radii of the TiO 2particles were calculated using eq 1.

Eg ≈2π211.8e 2

Eg os -Eg ref ) ∆2R

2×µ- R

(1)

Here, R is the radius of the particle, µis the reduced

mass of the exciton (i.e., µ-1) m /-1-1/

h +m /e , where m e is the effective mass of the electron and m /h is the effective mass of the hole), and is the dielectric constant of the material. Here, we use mean values of µ) 1.63m e and ) 184for the calculation.

(22)Brus, L. J. Phys. Chem . 1986, 90, 2555.

(23)Liu, Y.; Claus, O. J. Am. Chem. Soc. 1997, 119, 5273.

Weng and Wei

Figure 2. 29

Si NMR spectrum of the TiO 2-TMS colloidal

solution.

Figure 3. Transmission electron microscopy image and electron diffraction pattern of TiO +2nanoparticles from TiO 2-H colloidal solution.

Table 2. Onset of UV -Vis Absorbance and Calculated

Radii of TiO 2Nanoparticles

absorbance onset wavelength (nm)

radius (nm)

TiO 2-TMAC 3590.96TiO 2-TMS 3641.07TiO 2-H +

376

1.59

Table 2shows the calculated radii of TiO 2particles in the colloidal solutions. The growth of TiO 2particles by the sol -gel reaction is catalyzed by acid, and, therefore, the TiO 2-H +colloidal solution has the largest TiO 2radius (1.59nm) among all colloidal solutions. The sizes of TiO 2in TiO 2-TMAC and TiO 2-TMS are similar (0.96and 1.07nm), but the stabilities of the solutions are different. After being stirred for 36h, the TiO -H +2-TMAC and TiO 2colloidal solutions became muddy after an initially being transparent yellow-brown. The TiO 2-TMS solution, however, remained transparent after 36h stirring, and became light yellow and trans-parent after 45days. This phenomenon indicates that the covalently bonded surfactant (TMS)effectively prevents TiO 2particles from aggregating. Figure 2shows the 29Si NMR spectrum of TiO 2-TMS. The spectrum shows a T 3peak, resulting from the bonding structure of [Si(OSi)3R],at -67ppm. 24-26There is no presence of a T 1peak, which would originate from a (RSi(OSi)(OR′) 2) structure, at -45ppm or a T 2peak, from a (RSi(OSi)2(OR′) 1) structure, at -57ppm. This indicates that most of the Si -OH groups of TMS react

(24)Isdoa, K.; Kuroda, K. Chem. Mater . 2000, 12, 1702.

(25)Delattre, L.; Babonneau, F. Chem. Mater. 1997, 9, 2385.

(26)Leu, C. M.; Wu, Z. W.; Wei, K. H. Chem. Mater . 2002, 14,

3016.

Surface-Modified TiO 2Nanoparticles in BCPs Figure 4. X-ray diffraction curve of TiO 2-H +

nanoparticles.

Figure 5. Transmission electron microscopy images of (a)PS-b-PMMA, (b)TiO 2-TMAC/PS-b-PMMA,and (d)TiO 2-TMAC/PS-b-PMMA stained with RuO 4. (c)Shows an energy-dispersive X-ray diffraction pattern of the dark particles in

(b).

to give Si -O -M (M) Si or Ti). The splitting of the T 3peak (-67.0and -67.7ppm) implies that TMS is at-tached to +TiO 2by a covalent bond. The TEM image of TiO 2-H in Figure 3indicates that the particle size is about 3nm, which is also the value estimated by eq 1. Figure 4shows the X-ray diffraction curve of TiO 2-H +nanoparticles, and the TiO 2nanoparticles are deter-mined to have an anatase phase, which has partial crystallinity. The diffraction pattern of TiO 2particles on a carbon grid is shown in the bottom-right corner of Figure 3and indicates that the TiO 2particles are partially crystalline.

Morphology and Photoluminescence of TiO 2/PS-b-PMMA. Figure 5(a)shows the lamellar morphology of PS-b-PMMA after staining with RuO 4. The periodic lamellar thickness of PS-b-PMMA is about 50nm. The dark region is the PS domain, owing to staining, and the PS volume fraction of PS-b-PMMA is 0.55, which

Chem. Mater., Vol. 15, No. 15, 20032939

Figure 6. Differential scanning calorimetry curves of PS-b-PMMA, TiO 2-TMS/PS-b-PMMA,and TiO 2-TMAC/PS-b-PM-

MA.

Figure 7. Fourier transform infrared spectra of PS-b-PMMA and TiO 2/PS-b-PMMAnanocomposites.

falls into the ordered lamellar phase region 4(aPS volume fraction between 0.34and 0.62). The TiO 2-TMAC/PS-b-PMMAmorphology is shown in Figure 5(b).In Figure 5(b),the presence of TiO 2in the dark spots is confirmed by EDS (Figure5(c));the size of TiO 2ag-gregates (darkspots) is about 15-20nm. The Ti band peak indicates the existence of TiO 2at the PS domains, whereas the presence of Cu peaks is caused by the Cu grid used in the sample preparation. In Figure 5(d),the gray phase is the PS domain, which is a result of staining with RuO 4, while the light phase is the PMMA domain. Dark TiO 2nanoparticles are found to disperse in the gray domain (PSdomain) in the lamellar PS-b-PMMA. That the TiO 2-TMAC nanoparticles can be dispersed in the PS domain corresponds to the fact that both the cetyl trimethylammonium chloride (TMAC),containing 10methylene units, and the polystyrene domain are hydrophobic and miscible. The presence of TiO 2-TMAC in the PS domain is further supported by differential scanning calorimetry (DSC)results. Figure 6reveals that the glass transition temperature (T g ) of the PS domain in TiO 2-TMAC/PS-b-PMMAincreased by 9°C,as compared to that of neat PS-b-PMMA (104°Cvs 95°C).This increase might be attributed to TiO 2aggregates, which hinder the molecular movement

of

2940Chem. Mater., Vol. 15, No. 15, 2003Weng and Wei

Figure 8. Transmission electron microscopy image of TiO 2-

TMS/PS-b-PMMA.

the PS domain, indicating that TiO 2-TMAC aggregates are located at the PS domain. Because the heat capacity of glass transition of PMMA is much smaller than that of PS (0.03W/gvs 0.065W/g),the Tg of PMMA is indetectable in this case. 27Therefore, the presence of TiO 2in the PMMA phase of PS-b-PMMA can only be confirmed by other means. The Fourier transform infrared (FTIR)spectra of TiO 2/PS-b-PMMAnanocom-posites are shown in Figure 7. The peaks at 1741cm -1and 1726cm -1result from the carbonyl groups of the PMMA domain in neat PS-b-PMMA. The carbonyl band of TiO 2-TMS/PS-b-PMMAshifts to lower wavenumbers (from1726to 1714cm -1) as compared to that of PS-b-PMMA. This indicates the possibility that TiO 2is present in the PMMA domain because hydrogen bond-ing between the remainder of the dangling -OH groups on the surface of TiO 2and the carbonyl groups of the PMMA domains causes the carbonyl band to shift to smaller wavenumbers. Figure 8shows a transmission

electron microscopy image of TiO 2-TMS/PS-b-PMMA.That the TiO 2nanoparticles are dispersed rather uni-formly in the PMMA phase is consistent with the fact that TMS contains methacrylate structures. The differ-ence in the modes of dispersion of TiO 2in PS and in PMMA domains can be manifested by the bonding difference between the surfactants and TiO 2. In the TiO 2-TMAC/PS-b-PMMAcase, the polar-ionic bondings between TiO 2surfaces and TMAC are weak and hence allow TiO 2nanoparticles to rearrange to form aggre-gates during the solvent removal process. Whereas in the TiO 2-TMS/PS-b-PMMAcase, TMS is bonded to TiO 2surfaces covalently, and this type of bonding is well maintained during the solvent removal process. The co-valently tethered TMS prevents TiO 2from aggregating, resulting in a better dispersion. A schematic drawing of the formation of these two types of dispersion of TiO 2in the PS and PMMA block is presented in Figure 9. Figure 10shows the photoluminescence of the TiO 2/PS-b-PMMA nanocomposites as excited by 260nm UV light. A mild 410nm luminescence peak, caused by the band-to-band transition, 24is displayed by the TiO 2nanoparticles modified by TMS. For neat PS-b-PMMA, the 320nm luminescence peak is resulted from the PS domain. In the case of TiO 2-TMAC/PS-b-PMMA,only a broad and weak 326nm luminescence peak appeared. Whereas in the case of TiO 2-TMS/PS-b-PMMA,there are two luminescence peaks (323and 400nm) present. The stark difference in the two cases can be interpreted by the morphological evidences as discussed in the previous paragraph. When TiO 2-TMAC forms aggre-gates in the PS domain, a large portion of the excitation light is absorbed by the PS domain, which also lumi-nesces at shorter wavelength, resulting in a small portion of excitation light reaching TiO 2

aggregates.

Figure 9. Schematic drawing of different dispersion modes by ionic-polar and covalent bondings between TiO 2and surfactants in PS-b-PMMA.

Surface-Modified TiO 2Nanoparticles in BCPs Figure 10. Photoluminescence of TiO 2-TMS, PS-b-PMMA, and TiO 2/PS-b-PMMAnanocomposites.

This results in nonluminescence by TiO 2-TMAC in the PS domains. On the other hand, because TiO 2-TMS

(27)Guegan, P.; Cernohous, J. J.; Khandpur, A. K.; Hoye, T. R.; Macosko, C. W. Macromolecules 1996, 29,

4605.

Chem. Mater., Vol. 15, No. 15, 20032941

nanoparticles dispersed more uniformly in the PMMA domain, both TiO 2-TMS nanoparticles and the PS domain can luminescence independently. This lumines-cence phenomenon is consistent with our previous argument on the distribution of TiO 2nanoparticles in different blocks.

Conclusion

The dispersion of TiO 2nanoparticles can be controlled in one of the two blocks of lamellar PS-b-PMMA by using hydrophobic or hydrophilic surfactants, as re-vealed by transmission electron microscopy, differential scanning calorimetry, and Fourier transform infrared spectroscopy. The modes of dispersion of TiO 2nanopar-ticles in different blocks are determined by the type of bondings between the surfactant and the nanoparticles. The photoluminescence of the TiO 2/PS-b-PMMAnano-composites depends on the location of the TiO 2nano-particles.

Acknowledgment. We appreciate the financial sup-port of the National Science Council, Taiwan, through project NSC 91-2120-M-009-001.

CM0300617