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J. Phys. Chem. C 2007, 111, 3934-3937
Ordered Nanogroove Arrays on n-TiO2 with a Variation of the Groove Depth, Formed by Self-Organized Photoetching Shuji Nakanishi,*,†,‡ Takatoshi Tanaka,† Yuichiro Saji,† Etsushi Tsuji,† Satoshi Fukushima,† Kazuhiro Fukami,† Tomoyuki Nagai,† Ryuhei Nakamura,† Akihito Imanishi,† and Yoshihiro Nakato§ DiVision of Chemistry, Graduate School of Engineering Science, Osaka UniVersity, Toyonaka, Osaka 560-8531, Japan, PRESTO, Japan Science, and Technology Agency, Osaka 560-8531, Japan, and CREST, Japan Science, and Technology Agency, Osaka 560-8531, Japan ReceiVed: NoVember 14, 2006; In Final Form: January 17, 2007
Photoelectrochemical etching of n-TiO2 (rutile) single crystals produced ordered arrays of nanogrooves with the spacing of about 300 nm over a macroscopically wide area of 0.5 × 0.5 cm at the surface without any use of imprints or templates. Moreover, the photoetching by irradiation with patterned light intensities gave nanogrooves with a desired depth at a desired location, interesting from the point of view of nanoscience and nanotechnology. A plausible model is proposed for the formation of ordered arrays of nanogrooves in terms of self-organized photoetching in which autocatalytic accelerated photoetching at the bottom of the grooves, caused by an excess concentration of photogenerated holes due to a high potential gradient, is coupled with its retardation by a downward shift in the flat-band potential of n-TiO2 at the bottom of the grooves, induced by an increase in the solution pH due to accelerated photoreactions.
Introduction Solid surfaces with ordered nanostructures, such as dots, holes, and grooves (or ridges), have provided unique microelectronic, optical, magnetic, and micromechanical properties, as summarized in a recent review.1 Focused photon-, electron-, ion-, and molecular-beam lithography have been widely used for creating desired pattern relief at the surface. However, these techniques are now facing serious problems, such as the difficulty in mass production and cost increases by use of expensive specialized apparatus. In the hope of overcoming the problems belonging to the conventional techniques, a selforganization method has recently attracted a lot of attention. In fact, a variety of surface nanorelief structures have successfully been formed at solid surfaces by using etching in vacuum,2,3 buckling,4,5 and periodic perecipitation,6 and used in practical applications.7,8 Electrochemical (photo-)etching is also known to be effective to produce ordered nanogrooves or holes at metal9,10 and semiconductor11-17 surfaces. However, for most examples, the ordered structures were produced only in small (microscopic) areas and therefore the ordering on a macroscopically large scale was achieved with the assist of externally introduced imprints or templates.11-13 In addition, the ordered structures reported included a lot of defects or irregularity11-17 and the extent of regularity was not high enough. We also previously reported that the electrochemical photoetching of n-TiO2 produced nanoholes and grooves.18-20 However, the ordering of the nanoholes and grooves formed on n-TiO2 were also limited to small areas. The issues to be tackled next are to improve the regularity and to put the nanorelief with * Corresponding author. E-mail:
[email protected]. † Osaka University. ‡ PRESTO, Japan Science, and Technology Agency. § CREST, Japan Science, and Technology Agency.
Figure 1. Schematic illustration of the experimental setup.
desired sizes on desired locations. Here we report that the ordered arrays of nanogrooves with high regularity are produced spontaneously over a macroscopically wide area of 0.5 cm × 0.5 cm by electrochemical photoetching of TiO2 (rutile) by adjusting the experimental conditions. We also succeeded in producing the nanogrooves with a desired depth at a desired location by the photoetching with patterned irradiation. Experimental Single-crystal TiO2 (rutile) wafers of 99.99% in purity, 0.5 cm × 0.5 cm × 1.0 mm in size, and having surfaces cut parallel to the (110) face were obtained from Furuuchi Chemical Co., Ltd. The wafers were slightly reduced by heating at 550 °C for 3 h and cooled down under a hydrogen atmosphere to get n-type semiconductivity.19 Ohmic contact on the n-TiO2 wafers for preparing n-TiO2 electrodes was obtained with indium-gallium alloy. Specimens of 0.2 to 1.0 Ω cm were used for experiments. Figure 1 shows a schematic illustration of the experimental setup for the photoelectrochemical etching. The n-TiO2 electrode was immersed in 0.05 M H2SO4 (pH ) 1.3). The illumination was performed by a 365-nm band (about 8 nm in the halfband width and 250 nm in the penetration depth) from a 300-W highpressure mercury lamp, chosen with glass filters. The electrode potential was regulated with a commercial potentiostat and a
10.1021/jp067549q CCC: $37.00 © 2007 American Chemical Society Published on Web 02/21/2007
Ordered Nanogroove Arrays on n-TiO2
J. Phys. Chem. C, Vol. 111, No. 10, 2007 3935
Figure 2. The jpc vs U dependence for a (110)-cut n-TiO2 (rutile) electrode in 0.05 M H2SO4.
Figure 4. (a) Schematic drawing of illumination with a varied intensity. (b) Schematic illustration of expected groove array with varied groove depth. (c) SEM images for various local positions of the TiO2 surface after photoetching with Qp ) 50 C cm-2. The numbers 1, 2, and 3 added on the SEM images mean that they were obtained at the positions marked by the same number on (a).
Figure 3. SEM images of a part of the n-TiO2 surface of 0.5 cm × 0.5 cm in size after the photoetching with Qp ) 50 C cm-2.
potential programmer. A Pt plate was used as the counter electrode (CE) and a Ag/AgCl/sat. KCl electrode was used as the reference electrode (RE). The photocurrent density (jpc) vs applied potential (U) was measured using the same measuring apparatus and electrodes as the photoetching experiments. The morphology of the electrode surface was inspected with a high-resolution scanning electron microscope (SEM, Hitachi S-5000) and an atomic force microscope (AFM, Digital Instruments Nanoscope IIIa). Results and Discussion Figure 2 shows the jpc versus U characteristic for an n-TiO2 electrode in 0.05 M H2SO4. An anodic photocurrent flowed under anodic bias (i.e., at a positive potential U), and the photoetching reaction proceeded at the electrode surface slightly (with a quantum efficiency of about 1%19) and competitively with oxygen photoevolution reaction. Interestingly, the photoetching produced a remarkably ordered array of nanogrooves with the spacing of about 300 nm. The direction of the grooves was parallel to the c-axis of rutile TiO2 and independent from the direction of the gravity. Figure 3 displays SEM images of a part of the n-TiO2 surface photoetched at U ) 2.0 V versus Ag/AgCl/sat. KCl under continuous illumination at a constant intensity yielding j ) 15 mA cm-2, the total charge (Qp) passing across the electrode surface being kept 50 C cm-2. (Note that both the jpc and Qp values in the present work are the sum of the corresponding values consumed for the photoetching as well as the oxygen photoevolution.) The same SEM image was obtained at any part of the surface, indicating that the highly ordered nanostructrure was formed over a macroscopically wide area of 0.5 cm × 0.5 cm. It should be noted that the regularity of the ordered nanogrooves shown in Figure 3 is much improved compared with those reported previously.19 Our recent experiments have revealed that the period of time of the reduction of TiO2 at elevated temperatures under a hydrogen atmosphere is critical for the high regularity of the nanogrooves over a macroscopically large size. In the previous study,19 the TiO2 wafers were
reduced for only 30 min, whereas in the present work the reduction time was extended to 3 h, which caused a drastic improvement in the regularity over a macroscopic area. This is probably because the increase in the reduction time led to a highly homogeneous reduced (semiconducting) layer at the TiO2 surface. It was also found that it was important to carry out the cooling of the TiO2 wafers after the above-mentioned thermal reduction under a hydrogen atmosphere, not under an atmosphere of nitrogen or air. The present experiments also showed that the nanogrooves became deeper with the increase in the UV-light intensity when the Qp was fixed. Importantly, this fact implies that the irradiation of the UV light with a spatial intensity gradient leads to formation of nanogrooves with a variation of the depth. Figure 4 shows a representative result of experiments done by this strategy. As shown in Figure 4(a), the UV-light incident onto the center of an n-TiO2 electrode was attenuated by inserting a photomask. The photomask was placed about 2 mm above of the TiO2 substrate so that the UV-light reaches to the region covered by the photomask due to its diffraction and the UVlight intensity at the TiO2 surface has a spatial gradient. Figure 4(c) displays SEM images of the TiO2 surface after the photoetching with varied illumination intensities. The numbers added to the images represent that the images are obtained at the positions with the same numbers in Figure 4(a). Figure 4(c) clearly shows that the groove depth decreases with the decreasing illumination intensity, as illustrated in Figure 4(b). Figure 5 shows a time sequence of the formation of an ordered array of nanogrooves, as represented as a function of the Qp. The n-TiO2 surface before the photoetching was flat on a scale of a few nanometers (Figure 5(a)). After the photoetching with Qp ) 1 C cm-2, fine grooves were formed parallel to the c-axis of TiO2 (rutile) (Figure 5(b)). With increasing Qp, the grooves became more and more regular (Figures 5(c) and (d)), and finally a highly ordered array of nanogrooves was formed at Qp ) 50 C cm-2 (Figure 5(e)). How is such a highly ordered array of nanogrooves formed? It should be noted first that the formation of the ordered array cannot be explained by light interference effects because the shape and size of the formed nanopatterns do not depend on the wavelength of illuminated light, which was controlled by use of monochromator, but strongly depend on the crystal faces and axes of TiO2. The formation can also not be explained by
3936 J. Phys. Chem. C, Vol. 111, No. 10, 2007
Nakanishi et al. potential distribution (or the band bending) in the space charge layer of n-TiO2. The key consequence is that the potential gradient in the TiO2 increases near the bottom of the surface hollows, which accelerates the migration of photogenerated holes to the bottom of the surface hollows. The acceleration of the migration of the holes, in turn, induces enhancement of the photoetching reaction, which makes the surface hollows deeper. Namely, the autocatalytic photoetching occurs at the bottom of the surface hollows. There is, however, another process that retards the photoetching reaction. The photogenerated holes migrating to the bottom of the surface hollows produce a high concentration of H+ ions in the electrolyte near the bottom through the oxygen photoevolution reaction
2 H2O + 4 h+ f 4 H+ + O2
Figure 5. The AFM (a-d) and SEM (e) images of the n-TiO2 surface after the photoetching with the varied Qp in a range from 0 to 50 C cm-2.
where h+ refers to a photogenerated hole. Thus, the concentration of H+ ions increases in the electrolyte near the bottom of the surface hollows, which leads to a decrease in the pH and hence a downward shift in the flat-band potential (Ufb) of n-TiO2 there, because the Ufb is known27 to shift in proportion to the solution pH at a ratio of -0.059 V/pH at room temperature. The downward shift in the Ufb causes a decrease in the potential gradient in the space charge layer of n-TiO2 near the bottom of the surface hollows and thus a decrease in the migration of photogenerated holes there, hence leading to the retardation of the photoetching reaction. A combination of this retardation process with the aforementioned autocatalytic photoetching leads to the self-organized production of nanogroove arrays with spatial periodicity. To show that the above argument can really explain the formation of nanogroove arrays, we consider the qualitative mechanism in a formulated way on the basis of MullinsSekerka instability.28 For simplicity, we consider here the onedimensional reaction field. The potential (φ) in the space charge layer of n-TiO2 can be represented as a function of the coordinate (x) along the n-TiO2 surface and the coordinate (y) in the direction toward the interior of n-TiO2, as shown in Figure 6(a). The φ is determined by Poisson’s equation
∆φ(x, y) ) -F/s0
Figure 6. Schematic models explaining the formation of an ordered nanogroove array. (a) The band bending near the n-TiO2 surface; x, the coordinate along the n-TiO2 surface; y, that toward the n-TiO2 interior; φ, the electric potential; and y ) aq sin qx, a curve representing fluctuation-induced surface hollows. (b) The distributions of φ in the n-TiO2 crystal. Dashed curves in n-TiO2: equi-potential surfaces. x: valence-band hole.
anisotropic etching such as observed for alkali-etching of Si, which does not show any regularity in size.21 A possible answer will be found in the mechanism of dynamic self-organization via nonequilibrium kinetics of the surface instability induced by autocatalytic photoetching, similar to the theory for formation of porous semiconductors.22-26 The n-TiO2 electrode under the anodic bias has upward band bending near the surface (see Figure 6(a)), and by the band bending, photogenerated holes in the valence band migrate to the surface and cause the photoetching. Let us assume the n-TiO2 surface is initially flat and introduce a small sine curve perturbation on its shape, as schematically shown in Figures 6(a) and (b). The perturbation induces the modulation of the
(1)
(2)
where F refers to the density of the space charge, s is the dielectric constant of TiO2, and 0 is the permittivity of vacuum. As the first step, we introduce a small sine curve perturbation to the surface shape
yq ) aq sin qx
(3)
where aq and q are the amplitude and the wavenumber of the perturbation, respectively. The later self-organized photoetching will select a sine wave with a specific wavenumber q. Namely, q is regarded as a parameter. Hereafter, we consider how the photoetching depends on q. The production of surface hollows induces the modulation in the φ distribution in the space charge layer of n-TiO2. For simplicity, let us assume that the photoetching rate (R) is in proportion to the flux of photogenerated holes to the TiO2 surface, which in turn is in proportion to the gradient of φ in the space charge layer
R ) k1 ∂φ/∂y where k1 is a proportionality constant (k1 > 0).
(4)
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J. Phys. Chem. C, Vol. 111, No. 10, 2007 3937
The oxygen photoevolution rate can also be expressed in a similar way to eq 4 with another proportionality constant. This reaction produces H+ ions (eq 1) and causes a pH decrease in the electrolyte at the TiO2 surface, which modulates the Ufb (or φ) at the TiO2 surface. Because the reaction rate is highest near the bottom of the grooves, the local pH near the bottom is lower than the other parts. Thus, we simply assume that the pH decrease at the TiO2 surface is in proportion to the curvature (κ) of the surface hollows. Because φ increases with the pH decrease and κ is given by the negative second derivative of eq 3 (i.e., aqq2 sin qx) the φ at the TiO2 surface is given by
φ|y ) a sin qx ) φ0(1 + k2κ) ) φ0(1 + k2aqq2 sin qx) (5) where φ0 is the φ for the flat surface and k2 a proportionality constant (k2 > 0). We can thus obtain the analytical solution for Poisson equation (eq 2) under the boundary condition of eq 5 as follows
φ(x, y) ) φ0 + yR0/k1 - y2F/2s0 + (k2q2φ0 - R0/k1) e-qyaq sin qx (6) where R0 is the photoetching rate at the flat surface and independent of x. Thus, the photoetching rate, R(x) at a position, x, is calculated from eq 4 as follows:
R(x) ) R0 + (-k1k2φ0q3 + R0q - k1F/s0) aq sin qx
(7)
Equation (7) takes the maximum at q ) qmax
qmax ) (R0/3k1k2φ0)1/2
(8)
This implies that the surface fluctuation (surface hollows) with the wave number of qmax is most enhanced and selected in the course of the photoetching, or in other words, a nanogroove array with the wave number of qmax (or with the spacing of 1/qmax) is produced after prolonged photoetching. The mechanism argued above falls into the category of selforganized pattern formation induced by Mullins-Sekerka instability.28 Note that the ordering of nanogrooves in the present work is much higher than that of patterns formed by photoetching in other semiconductors.11-17 The high anisotropy of the mobility of photogenerated holes in rutile-TiO2 (i.e., the high mobility along the c-axis relative to the other directions)29 might be responsible for this fact as another factor. In conclusion, the ordered nanogroove arrays with a gradient in the groove depth are formed in a single step over a macroscopically wide area of 0.5 cm × 0.5 cm of n-TiO2 by self-organized photoetching under patterned irradiation. The present work has opened a novel effective route for formation
of designed patterned nanorelief structures at semiconductor surfaces. We believe that the nanorelief structures will provide various practical applications in the fields of optics, electronics, nanochemistry, and biosensors. Acknowledgment. The present work is supported by the Grant-in-Aid for Scientific Research (KAKENHI) in Priority Area “Molecular Nano Dynamics” from Ministry of Education, Culture, Sports, Science and Technology, and by the Kurata Memorial Hitachi Science and Technology Foundation. References and Notes (1) Geissler, M.; Xia, Y. AdV. Mater. 2004, 16, 1249. (2) Perez, J. Z.; Tomas, C. M.; Sanjose, V. M.; Munera, C.; Ocal, C.; Laugt, M. J. Appl. Phys. 2005, 98, 034311. (3) Sugawara, A.; Hembree, G. G.; Sheinfein, M. R. J. Appl. Phys. 1997, 82, 5662. (4) Bowden, N.; Brittain, S.; Evans, A. G.; Hutchinson, J. W.; Whitesides, G. M. Nature 1998, 393, 146. (5) Ohzono, T.; Matsushita, S. I.; Shimomura, M. Soft Mater. 2005, 1, 227. (6) Grzybowski, B. A.; Bishop, K. J. M.; Campbell, C. J.; Fialkowski, M.; Smoukov, S. Soft Matter 2005, 1, 114. (7) Smoukov, S. K.; Bitner, A.; Campbell, C. J.; Grzybowska, K. K.; Grzybowski, B. A. J. Am. Chem. Soc. 2005, 127, 17803. (8) Teranishi, T.; Sugawara, A.; Shimizu, T.; Miyake, M. J. Am. Chem. Soc. 2002, 124, 4210. (9) Masuda, H.; Fukuda, K. Science 1995, 268, 1466. (10) Yuzhakov, V. V.; Chang, H. C.; Miller, A. E. Phys. ReV. B. 1997, 56, 12608. (11) Fo¨ll, H.; Langa, S.; Carstensen, J.; Christophersen, M.; Tiginyanu, I. M. AdV. Mater. 2003, 15 183. (12) Cullis, A. G.; Canham, L. T.; Calcott, P. D. J. J. Appl. Phys. 1997, 82, 909. (13) Tenne, R.; Hodes, G. Appl. Phys. Lett. 1980, 37, 428. (14) Mu¨ller, N.; Tenne, R. Appl. Phys. Lett. 1981, 39, 283. (15) Erne´, B. H.; Vanmaekelbergh, D.; Kelly, J. J. AdV. Mater 1995, 7, 739. (16) Schmuki, P.; Fraser, J.; Vitus, C. M.; Graham, M. J.; Isaacs, H. S. J. Electrochem. Soc. 1996, 143, 3316. (17) Sugiura, T.; Itoh, S.; Ooi, T.; Yoshida, T.; Kuroda, J.; Minoura, H. J. Electroanal. Chem. 1999, 473, 204. (18) Nakato, Y.; Akanuma, H.; Shimizu, J.; Magari, Y. J. Electroanal. Chem. 1995, 396, 35. (19) Tsujiko, A.; Kisumi, T.; Magari, Y.; Murakoshi, K.; Nakato, Y. J. Phys. Chem. B. 2000, 104, 4873. (20) Kisumi, T.; Tsujiko, T.; Murakoshi, K.; Nakato, Y. J. Electroanal. Chem. 2003, 545, 99. (21) Bressers, P. M. M. C.; Kelly, J. J.; Gardeniers, J. G. E.; Glwenspoek, M. J. Electrochem. Soc. 1996, 143, 1744. (22) Chazalviel, J. N.; Wehrspohn, R. B.; Ozanam, F. Mater. Sci. Eng., B. 2000, 69, 1. (23) Kang, Y.; Jorne´, J. J. Electrochem. Soc. 1993, 140, 2258. (24) Valance, A. Phys. ReV. B. 1995, 52 8323. (25) Valance, A. Phys. ReV. B. 1997, 55, 9706. (26) Wehrspohn, R. B.; Ozanam, F.; Chazalviel, J. N. J. Electrochem. Soc. 1999, 146, 3309. (27) Morrison, S. R.; Electrochemistry at Semiconductor and Oxidized Metal Electrodes; Plenum: NewYork, 1980. (28) Mullins, W. W.; Sekerka, T. F. J. Appl. Phys. 1963, 34, 323. (29) Byl, O.; Yates, J. T. J. Phys. Chem. B. 2006, 110, 22967.
