Although the shape and crystal phase of the TiO2 nanoneedles can be controlled by changing the values of the pHi, the ratio R, and the Ti precursor concentration, [7,8] the individually distinct single-crystalline TiO2 nanoneedles formed only under a strict set of conditions. When we changed the value of pHi from 1.0 to 1.4 and R was 200, the initial growth of TiO2 was in the form of wide plates and multiple needles formed in the confining cavities. At a value of pHi slightly higher than 1.0, the conditions in solution
are closer to the thermodynamic equilibrium between Ti(OH)22+/rutile or Ti(OH)22+/anatase.[6a] This situation causes too many TiO2 nuclei to form simultaneously within the cavities. Concurrently, the value of pH of the solution increases as the urea decomposes, which causes the nuclei to form rapidly. The TiO2
underlayer in rutile phase facilitates the growth of vertical TiO2 needles in the
nanocavities across the photoresist layer homogeneously. Without this underlayer, the vertical growth of TiO needles was more difficult. The effect of crystal phase of TiO2 2
underlayer on the crystal phase of grown TiO2 needles depends on the pH value of the solution. For instant, as the value of the pH increases, the crystal phase of the TiOi 2
nanoneedles changes from rutile to anatase.
It is well known that the surfaces of rutile and anatase have different wettabilities that depend upon the crystal plane.[9,10] The (00l) planes of rutile and anatase, which are perpendicular to the c axis, are comparatively inert in the absence of more-reactive bridging site oxygen atoms. Other crystal planes that possess bridging site oxygen atoms and are parallel to the c axis are relatively hydrophilic. Crystal growth perpendicular to the c axis is suppressed when co-existing species, such as urea and ammonium ions, become adsorbed selectively onto the more-hydrophilic surfaces that exist parallel to the c axis of the crystallites. This phenomenon results in the preferred growth toward needle-like TiO2 structures, rather than disk-like structures. For instance, Figure 4-8 shows that the orientation of an anatase TiO2 needle is along [002] direction with the spacing between adjacent lattice planes to be ca. 4.7 Å as determined by HRTEM/SAED experiments. On the other hand, increasing the molar ratio of urea to titanium should result in TiO2 nanoneedles having higher aspect ratios. Indeed, when the ratio R is over 300, the value of pH rises quickly because of decomposition of urea;
this phenomenon leads to increase precipitation in the aqueous solution and a decrease in deposition because the aqueous solutions quickly become supersaturated with TiO . In 2
our study, we found that the best conditions for obtaining long, single TiO2 nanoneedles were a pHi of 1.0–1.2 and a ratio R of 150–300.
4-4 Conclusions
In summary, we have fabricated arrays of single, aligned TiO2 nanoneedles within nanocavities by using a solution crystal growth process under an applied electric field.
The values of pHi and the ratio R both affect the morphology of the TiO2 nanoneedles.
When the pHi was >1.2, the nuclei formed too quickly and we did not fabricate any single needles; when the ratio R was larger than 300, needles did not form within the
nano-cavities. We believe that this new class of aligned TiO2 nanostructures will find a wide range of future applications.
4-5 References
[1] (a) O’Regan B. C.; Lenzmann F. J. Phys. Chem. B 2004, 108, 4342. (b) Jeon S.;
Braun, P. V. Chem. Mater. 2003, 15, 1256. (c) Adachi, M.; Okada, I.;
Ngamsinlapasathian, S.; Murata, Y.; Yoshikawa, S. Electrochemistry 2002, 70, 449.
[2] Li, D.; Xia, Y. N. Nano Lett. 2003, 3, 555.
Phys. Lett. 2001, 78, 1125.
[4] Miao, Z.; Xu, D.; Ouyang, J.; Guo, G.; Zhao, X.; Tang, Y. Nano Lett. 2002, 2, 717.
[5] (a) Weng, C. C.; Wei, K. H. Chem. Mater. 2003, 15, 2936. (b) Weng, C. C.; Hsu, K. F.;
Wei, K. H. Chem. Mater. 2004, 16, 4080.
[6] (a) Yamabi, S.; Imai, H. Chem. Mater. 2002, 14, 609. (b) Yamabi, S.; Imai, H. Chem.
Lett. 2001, 30, 220. (c) Sathyamoorthy, S.; Moggridge, G. D.; Hounslow, M. J. Cryst.
Growth Des. 2001, 1, 123. (d) Yang, H. G.; Zeng, H. C. J. Phys. Chem. B 2003, 107, 12244. (e) Kandori, K.; Kon-no, K.; Kitahara, A. J. Colloid Interface Sci. 1988, 122, 78.
[7] Yamabi, S.; Imai, H. Thin Solid Films 2003, 434, 86.
[8] Goh, G. K. L.; Donthu, S. K.; Pallathadka, P. K. Chem. Mater. 2004, 16, 2857.
[9] (a) Oskam, G.; Nellore, A.; Penn, R. L., Searson, P. C. J. Phys. Chem. 2003, 107, 1734.
(b) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.;
Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431.
[10] (a) Wang, R.; Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. J. Phys. Chem.
B 1999, 103, 2188. (b) Watanabe, T.; Nakajima, A.; Wang, R.; Minabe, M.; Koizumi, S.; Fujishima, A.; Hashimoto, K. Thin Solid Films 1999, 351, 260.
Scheme 4-1. Graphical representations of (a) the synthesis of aligned single TiO2
nanoneedles and (b) the growth of nuclei within nanocavities.
(a)
Photoresist E-beam writer
Si TiO2 underlayer
Ti(OH)22+
Electric field
+
-Growth
Electric field
+
-Growth
(b)
Crystal growth in solution
Nuclei
Table 4-1. The number of TiO nanoneedles within a single nanocavity. 2 Hole size
E fi
lectric
eld 30 nm 40 nm 50 nm 80 nm 100 nm
0 V/cm – – S, S, D D M M Flower-like Flower-like
250 V/cm – S S, D M Flower-like
500 V/cm S S S, D M Flower-like
625 V/cm S S S, D M Flower-like
750 V/cm S S S, D M Flower-like
1. The symbols S, D, and M refer to single, double, and multiple nanoneedles, respectively, present within a cavity. “Flower-like” refers to particles containing branches of nanoneedles.
20 30 40 50 60 70
Intensity (a. u.)
2θ 110
101
200
111210
211
220 002
Rutile
TiO2 underlayer
TiO2 powder
Figure 4-1. Wide-angle X-ray diffraction pattern of the TiO underlayer and powder. 2
(a)
100 nm
50 nm (b)
Figure 4-2. SEM images of a nano-patterned array of 50-nm cavities. (a) Plan view.
(b) Cross-sectional image.
(a)
200nm (c)
150 nm (b)
150 nm (d)
100 nm
Figure 4-3. SEM images (plan views, tilted 15°) of arrays of TiO2 nanoneedles grown from nanocavities sized at (a) 100, (b) 50, and (c) 30 nm. The
concentration of the Ti precursor solution was 5 × 10–4 M, the ratio R was 200, the value of the initial pH was 1.0, and the applied electric field was 625 V/cm. (d) Cross-sectional image of the TiO2 nanoneedles grown from the 30-nm-sized nanocavities.
(a)
10 nm
2nm
111 110 220
3.2 Å [110] (b)
Figure 4-4. (a) An HRTEM image of a TiO2 nanoneedle. The spacing between adjacent lattice planes is ca. 3.2 Å. (b) An SAED pattern indicating that the TiO2
particle possesses a rutile crystal phase.
20 30 40 50 60
pHi=1.4 pHi=1.2
A: anatase R: rutile
R R R
R
A A A
R R R
A
A
R A R
RA R
Intensity (a. u.)
2θ A R
pHi=1
Figure 4-5. Wide-angle X-ray diffraction patterns of TiO2 nanoneedles obtained from aqueous solutions possessing different values of pH . i
100 nm (a)
100 nm (b)
100 nm (c)
Figure 4-6. SEM images (plan views, tilted 15°) of arrays of TiO2 nanoneedles grown from nanocavities sized at (a) 100, (b) 50, and (c) 30 nm. The
concentration of the Ti precursor solution was 5 × 10–4 M, the ratio R was 200, the value of the initial pH was 1.4, and the applied electric field was 625 V/cm.
100 nm (a)
100 nm (b)
100 nm (c)
Figure 4-7. SEM images of TiO2 nanoneedles grown when the ratio R was (a) 300, (b) 400, and (c) 500. The concentration of the Ti precursor solution was 5 × 10–4 M, the value of the initial pH was 1.0, and the applied electric field was 625 V/cm.
(a)
2 nm
4.7 Ǻ (b)
004 2-24 2-20
(c)
Figure 4-8 (a) An HRTEM image of an anatase TiO2 needle. (b) The spacing between adjacent (002) lattice planes is ca. 4.7 Å as shown in the enlarged section. (c) An SAED pattern indicating that the TiO2 needle possesses an anatase crystal phase.
Chap 5 Conclusions
The dispersion of TiO2 nanoparticles can be controlled in one of the two blocks of lamellar PS-b-PMMA by using hydrophobic or hydrophilic surfactants, as revealed by transmission electron microscopy, differential scanning calorimetry and
Fourier-transform infrared spectroscopy. The modes of dispersion of TiO2 nanoparticles in different blocks are determined by the type of bondings between the surfactant and the nanoparticles. The photoluminescence of the TiO2/PS-b-PMMA nanocomposites depends on the location of the TiO nanoparticles. 2
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 TiO2 needle bunches (120 nm and 160 nm) can be controlled using block copolymer templates with different molecular weights.
We have fabricated arrays of single, aligned TiO2 nanoneedles within nanocavities by using a solution crystal growth process under an applied electric field. The values of pHi and the ratio R both affect the morphology of the TiO nanoneedles. When the pH2 i
was >1.2, the nuclei formed too quickly and we did not fabricate any single needles;
when the ratio R was larger than 300, needles did not form within the nanocavities. We believe that this new class of aligned TiO2 nanostructures will find a wide range of future applications.