Figure 3-1(a) shows a TEM micrograph of a PS-b-P4VP / SVP252 thin film after staining with RuO4. The micellar structure was constituted by the P4VP and PS blocks in the matrix, due to the selectivity of toluene solvent during the spin-coating process.
The size of the P4VP sphere is about 65 nm and the interdomain distance between P4VP spheres is about 125 nm. Figure 3-1 (b) shows a transmission electron microscopy image of a Ti(OH)22+/SVP252 thin film at a molar ratio (P) of Ti to P4VP equal to 1.
The dark region, which has high electron density, indicates that Ti(OH)22+ ions have been incorporated into the P4VP core of the micelles due to ionic-polar interactions. The distance between the nearest two cores (Ti(OH)22+/P4VP) and the cores sizes are similar for the different P ratios, implying that the concentration of Ti(OH)22+ does not affect the micelle size. Figure 3-1(c) shows the topology of a SVP252 monolayer thin film (the thickness of the thin film is only slightly larger than the size of the P4VP spheres (85nm
vs. 65nm)). The morphology of the Ti(OH)22+/SVP229 thin films is similar to that of Ti(OH)22+/SVP252, but with a difference; the distance between two micelles is 160 nm and the film thickness is about 60 nm. Figure 3-2 (a) shows the AFM topology in height images of TiO seeds remaining on a silicon substrate after O2 2 plasma treatment. In the image, the ordered TiO2 seeds are 5 nm in height and 50 nm in width. For comparison, a monolayer thin film of SVP252 on a Si wafer was treated with O2 plasma and no remaining material could be detected. The average distance between two seeds is about 120 nm. Figure 3-2 (b) displays an SEM image of a TiO2/SVP252 thin film. The composition of the remaining TiO2 seeds with short-range order was confirmed by EDS spectra. The ratio of the elemental percentage in the upper left-hand corner indicates that TiO particles are present. 2
As a control experiment, Figures 3-3 (a), (b), and (c) show SEM images of a pure Si wafer, without TiO2 seeds, immersed in 10 M Ti precursor solution for 1, 6 and 12 hrs, -4 respectively. The images reveal that heterogeneous nucleation will occur on a blank Si wafer,[32-33] but roughly12hrs are needed to form structured TiO2 needles on Si
substrates with Si-O-Ti bonds. The morphology of the TiO2 films on Si substrates are similar to those observed by Yang et al.[32]. Table 3-1 provides the sample name, reaction conditions of films deposited from Ti precursor solutions onto various substrates for 1 and 6hrs, respectively. Figure 3-4(a) shows an SEM image of a 10-4M Ti precursor solution deposited with TiO2-seeded substrates for 1 hr (252L1). Figure 3-4 (b) displays an SEM image of TiO2 short needles, 40~50 nm in length, after 6 hrs of growth (252L6).
Figure 3-4 (c) shows a cross-sectional profile of sample 252L6. The distance between the nearest two bunches of needle-like TiO is about 120 nm, similar to that for the TiO2 2
seeds. When the precursor concentration is increased, the morphology becomes different. During the initial reaction stage, they are not easily distinguished. Figure 3-4(d) shows an SEM image of 252H1, which shows small islands of TiO , similar to the 2
case of 252L1. Figure 3-4 (e) is an image revealing the long-needle structure. The length of the needles is about 130~150 nm. The tip of the needle for 252H6 is sharper than that for 252L6. From the cross-sectional profile image presented in Figure 3-4(f), the tip size of the needle is about 3 nm. The needle length can be controlled by
changing the growth parameters, such as reaction times or the reaction concentration.
When the reaction time is fixed at short times such as in our cases, the growth of TiO2
needles is controlled by the reaction kinetics, which affects the needles morphology.
When using the PS-P4VP block copolymer templates at the same molecular weights such as in sample 252L6 and 252H6, the TiO2 seeds density at per unit area are the same.
Hence, as the concentration of Ti precursors increase, the reaction speed will increase accordingly, which in turn results in larger size of TiO2 needles. This is the reason why the needle length and width of TiO2 needles in the case of 252H6 is larger than that in the 252L6 case. Figures 3-5 (a) and (b) show an SEM image of TiO2 seeds generated from a larger molecular weight PS-b-P4VP (SVP229) template; the seeds grow larger and needle structures grow as the reaction time is increased to 6 hrs (229H6). Figure 3-5(c) show a cross-sectional profile of sample 229H6. As compared with Figure 3-4(f), the distance between the two needle bunches is larger (160 nm vs. 120 nm). In the architecture of PS-b-P4VP micelle, the P4VP blocks form the core while the PS blocks constitute the corona. When the molecular weight of P4VP block differs only slightly in the diblock copolymers and the molecular weight of the PS block is much larger than that of P4VP block in the diblock copolymers, the distance between the P4VP cores is
actually controlled by the size of or the molecular weight of the PS block. In the present study, the molecular weights of the P4VP blocks in SVP252 and SVP229 are 32,700 g/mol and 29,400 g/mol, respectively, indicating that the domain size of the P4VP core in the thin films is roughly the same in two cases. Whereas, the molecular weights of the PS block in SVP252 and SVP229 are 92,700 g/mol and 365,300 g/mol, respectively.
The distance between two P4VP cores in SVP229 is larger than that in SVP252. The structural variation in PS-b-P4VP is therefore used to vary the distance between the nucleation sites of TiO . 2 This suggests that a variable density of TiO2 nanostructures can be fabricated on a substrate using PS-b-P4VP block copolymer templates with different molecular weights.
Figure 3-6 shows diffraction peaks that can be indexed as the tetragonal rutile phase (JCPDS card File No. 75-1757). Figure 73- (a) shows typical TEM images of the TiO2
nanostructure and Figure 3-7(b) shows a selective area electronic diffraction (SAED) pattern. Three typical diffraction spots are indexed as the 111, 110 and 220 planes by the ratio of 1/dhkl. From this TEM image, the needle-like TiO2 nanostructure is observed to be more than 100 nm in length, terminating with a sharp pinnacle. A HRTEM image is shown in Figure 3-7(c). It reveals that TiO2 has a rutile crystal structure. The lattice spacing is about 3.2 Å between adjacent lattice planes of the TiO2
needles, corresponding to the distance between (110) crystal planes of the rutile phase.
The rutile and anatase crystals were form at the condition near thermodynamic
equilibrium of Ti(OH)22+/rutile and Ti(OH)22+/anatase, respectively.[29-30] At low pH values or acidic conditions, the chemical potential of Ti(OH)22+ (μi) is slightly larger than that of the rutile phase (μr) and less than that of the anatase phase(μa). As a result, the rutile crystal are grown in the solution at pH values between 0.5 and 1.8. As the pH value increases, the anatase crystal will form in the solution instead by the fact that the μa
value is slight lower than μ . (μ μi r < i < μa → μr < μa < μi , as pH value increasing).
3-4 Conclusion
By using TiO2 seeds prepared from a PS-b-P4VP diblock copolymer template, we have been able to fabricate arrayed, needle-like rutile TiO2 nanostructures with variable spatial positions and densities. The distance between two TiO needle bunches (120 nm and 2
160 nm) can be controlled using block copolymer templates with different molecular weights.
3-5 References
[1] Joannopoulos, J. D.; Villeneuve, P. R.; Fan, S. Nature, 1997, 386, 143.
[2] Sunada, K.; Kikuchi, Y.; Hashimoto, K.; Fujishima, A. Environ. Sci. Technol. 1998, 32, 726.
[3] Cai, R.; Kubota, Y.; Shuin, T.; Hashimoto, K.; Fujishima, A. Cancer Res. 1992, 52, 2346.
[14] Forster, S.; Antonietti, M. Advanced Materials 1998, 10, 195.
[15] Lazzari, M.; Lopez-Quintela, M. A. Advanced Materials 2003, 19, 1583.
[16] Park, C.; Yoon, J.; Thomas, E. L. Polymer 2003, 44, 6725.
[17] Tokuhisa, H.; Hammond, P. T. Langmuir, 2004, 20, 1436.
[18] Reiter, G.; Castelein, G.; Sommer, J.-U.; Rollele, A.; Thurn-Albrecht, T. Phys. Rev.
Lett. 2001, 87, 226101.
[19] Park, M.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Science 1997, 276, 1401.
[20] Shin, K.; Leach, K. A.; Goldbach, J. T.; Kim, D. H.; Jho, J. Y.; Tuominen, M.;
Hawker, C. J.; Russell, T. P. Nano Letters 2002, 2, 933.
[21] Cheng, J. Y.; Ross, C. A.; Chan, V. Z.-H.; Thomas, E. L.; Lammertink, R. G. H.;
Vancso, G. J. Advanced Materials 2001, 13, 1174.
[22] Lopes, W. A.; Jaeger, H. M. Nature, 2001, 414, 735.
[23] Haupt, M.; Miller, S.; Glass, R.; Arnold, M.; Sauer, R.; Thonke, K.; Moller, M.;
Spatz, J. P. Advanced Materials 2003, 15, 829.
[24] Spatz, J. P.; Mossmer, S.; Hartmann, C.; Moller, M.; Herzog, T.; Krieger, M.; Boyen, H. G.; Ziemann, P.; Kabius, B. Langmuir 2000, 16, 407.
[25] Abes, J. I.; Cohen, R. E.; Ross, C. A. Chem. Mater. 2003, 12, 1125.
[26] Sohn, B. H.; Choi, J. M.; Yoo, S.; Yun, S. H.; Zin, W. C.; Jung, J. C.; Kanehara, M.;
Hirata, T.; Teranishi, T. J. Am. Chem. Soc. 2003, 125, 6368.
[27] (a) Yeh, S. W.; Wei, K. H.; Sun, Y. S.; Jeng, U. S.; Liang, K. S. Macromolecules
[31] Sathyamoorthy, S.; Moggridge, G. D.; Hounslow, M. J. Cryst. Growth & Des. 2001, 1, 123.
[32] Yang, H. G.; Zeng, H. C. J. Phys. Chem. B 2003, 107,12244.
[33] Kandori, K.; Kon-no, K.; Kitahara, A. J. Colloid Interface Sci. 1988, 122, 78.
nanostructures with ordered patterns.
Scheme 3-1. Synthesis of needle-like TiO2
Dip in Ti
Table 3-1. Reaction compositions of TiO2 nanostructures deposited from Ti precursor solutions for 1 and 6 hrs with ordered TiO seeds on the Si substrate. 2
1
1. Two kinds of block copolymers (SVP252 and SVP229) were used as templates in this study.
2. R is defined as molar ratio of urea to Ti. The value is fixed at 200 in this study.
[Chem. Mater. 2004, 16, 4080-4086]
Sample name
PS-b-P4VP Ti concentration (M)(with urea
Reaction time(hr)
2R=200)
252L1 SVP252 0.0001 1
252H1 SVP252 0.0005 1
229H1 SVP229 0.0005 1
252L6 SVP252 0.0001 6
252H6 SVP252 0.0005 6
229H6 SVP229 0.0005 6
a
100 nm b
66nm
122nm
100 nm c
2
1
0
0 1 2 μm
Figure 3-1. (a) Transmission electron microscopy image of SVP252 stained with RuO4 , (b) transmission electron microscopy image and (c) AFM topology in height images of a Ti(OH)22+/SVP252 (P=1) thin film.
[Chem. Mater. 2004, 16, 4080-4086]
2
1
0
a
0 1 2 μm
Element
Figure 3-2. (a) AFM topology and line-section analysis of ordered TiO2 seeds from a TiO2/SVP252 (P=1) thin film and (b) SEM image of TiO2 seeds from TiO2/SVP252 (after O2 plasma treating).
[Chem. Mater. 2004, 16, 4080-4086]
Atomic%
b
O K 10.27 Si K 85.17 Ti K 4.56
a
b
c
Figure 3-3. FE-SEM micrograph of TiO deposited on a Si wafer without TiO2 2 seeds in Ti precursor solution for (a) 1, (b) 6 and (c) 12 hrs.
[Chem. Mater. 2004, 16, 4080-4086]
a d
b e
c f
100 nm 100nm
Figure 3-4. FE-SEM micrograph of TiO seeds from TiO2 2/SVP252 reacted in 0.0001M Ti precursor solution for (a) 1 hr (252L1) and (b) 6 hrs (252L6); and (c) a
cross-sectional profile of a 252L6 TiO2 needle film; reacted in 0.0005M Ti precursor solution for (d) 1hr (252H1) and (e) 6 hrs (252H6); and (f) a cross-sectional profile of a 252H6 TiO needle film. 2
[Chem. Mater. 2004, 16, 4080-4086]
a
b
c
150 nm
Figure 3-5. FE-SEM micrograph of TiO seeds from TiO2 2/SVP229 reacted in 0.0005M Ti precursor solution for (a) 1 hr (229H1) and (b) 6 hrs(229H6); and (c) a
cross-sectional profile of a 229H6 TiO needle film. 2
[Chem. Mater. 2004, 16, 4080-4086]
20 30 40 50 60 229H6
252H6
252L6
Rutile
Intensity(a. u.) 110 101 200 111 211
210 220
2θ
Figure 3-6. X-ray diffraction curves of 252L6, 252H6 and 229H6 TiO2 needle-like nanostructures.
[Chem. Mater. 2004, 16, 4080-4086]
20 nm a
111
110 220 b
4nm [110]
3.26 Å c
Figure 3-7. (a) TEM image, (b) electron diffraction pattern and (c) HRTEM lattice image of the 252H6 TiO2 needle-like nanostructure.
[Chem. Mater. 2004, 16, 4080-4086]