Abstract - In this paper, we report the simulation of electrical field distribution and growth of gradient arrayed TiO2 nanotubes by electrochemical anodization. A rectangle electrochemical electrode configuration, in which the end of cathode (Pt wire) is perpendicularly pointed to anode surface (Ti foil), is applied in NH4F glycol electrolyte. Electric field distribution on the surface of the titanium foil shows exponentially decreasing behavior as increasing the distance from the end of cathode. The relationship between nanotube diameter and applied voltage has been analyzed according to the images of Scanning Electron Microscopy (SEM) and simulated electrical distribution. These gradient TiO2 nanotube arrays might be a promising nano-architecture for increasing photon-to-electrical conversion efficiency of dye sensitized solar cells (DSSC).

Keywords – simulation, electrical field distribution, electrochemical anodization, gradient TiO2 nanotubes arrays.

I. INTRODUCTION

Highly ordered TiO2 nanotube arrays have been intensively investigated since the discovery reported by Gong et al. [1]. It has been widely applied in the field of photocatalysis [2], gas sensors [3], biomedical devices [4], electrochromic material [5], photo-electrochemistry [6], and DSSCs [7]. To meet the requirements of the applications, as well as to improve existing performance (such as photo-to-current conversion rate for DSSCs), different structures have been developed to increase the absorption of solar light photons. Doohun Kim et al. managed to optimize experimental conditions and achieved non-thickness-limited mesoporous TiO2 network [8]. Albu et al. developed bamboo-shaped nanotube by alternatingly growing tubes and compact layers [9]. Chen et al. elevated the effect of electrolyte temperature on the morphology of TiO2 nanotubes and uniform U-shaped nanotubes were fabricated [10]. It shows that the morphology is sensitive to the heating rate of electrolyte as well as electric field distribution [11].

Recently, Li et al. studied gradient TiO2 nanotube arrays via asymmetric anodization [12]. They used the equivalent circuit

Ying Zhao, Kang Du, Guohua Liu, Nils Hoivik and Kaiying Wang are with the Department of Micro and Nano Systems Technology, Vestfold University College, Horten, 3184, Norway; Haisheng San is with MEMS Researching Centre, Xiamen University, China, (* corresponding author to provide phone: +47 3303 7721; fax: +47 3303 1103; e-mail: Kaiying.Wang@hive.no).

model for their etching configuration to explain the formation mechanism of the gradient structure. The influence of electrolyte conductivity on the gradient phenomenon was discussed. Obviously, the effect of electrical distribution is an important parameter to understand the gradient growth of TiO2 nanotube on Ti foils and thus is desirable to be investigated furthermore. Although the formation of gradient nanotubes have been reported in the references [13, 14], the effect of local electric field distribution on anode is less studied.

In this paper, we report the nano-structure with gradient diameters and lengths, which are induced from a perpendicular positioning configuration of anode and cathode. To find the relationship between nanotube diameters and anodization voltage, a set of experiments are performed on parallel-configuration electrodes under different applied voltages for calibration of the continuously gradient anodization. This result is further used to verify the diameter distribution on the gradient-arrayed nanotubes. Meanwhile, we perform mathematical simulation of electrical field strength to reveal the actual electric field distribution on anode surface, which appears to be exponential in terms of the distance from the cathode end. At the end, a new concept of DSSC based on gradient-arrayed TiO2 nanotubes is proposed to build up the DSSC, which is expected to be of higher efficiency.

II. EXPERIMENTAL DATA

A. Material Preparation Prior to anodization process, the high-purity titanium foil

(Purity of 99.7%, size of 20 mm × 10 mm × 0.5 mm) were dip into acetone and cleaned with ultra-sonication for degreasing the surface. Afterwards, we applied de-ionized water and nitrogen to wash and dry the surface. When the Ti metal was ready, the anodic oxidation process was conducted at 50V in electrolytes with composition of NH4F (0.5wt%)/ glycol/ H2O (3vol%). A platinum wire (Purity of 99.9%, diameter of 0.25mm and available length of 20 mm) was used as cathode in this process. After 4-hour anodization, the annealing treatment was eventually carried out at 450°C for 3h in air atmosphere for crystal structure formation of anodic TiO2.

B. Experimental Setup In order to have gradient-size nanotubes, a gradient

electrical distribution has to be applied on the anode (Ti foil). Instead of traditional two-parallel-electrode setup, we alternatively place a platinum wire perpendicularly against a Ti foil, as illustrated in Fig. 1.

After electrochemical anodization, the surface morphology of anodized titanium was characterized by using SEM, where

Study of Electrical Field Distribution and Growth of Gradient-arrayed TiO2 Nanotubes by Electrochemical Anodization

Ying Zhao, Kang Du, Guohua Liu, Haisheng San, Nils Hoivik and Kaiying Wang*

Proceedings of the 13thIEEE International Conference on NanotechnologyBeijing, China, August 5-8, 2013

Crown 401

the diameter and length of the nanotubes were obtained from top-view and cross-section images. In order to get better resolution under SEM, the samples were sputtered with a 5

nm-thick Pt layer on the surface before characterization. Since TiO2 membrane is semiconductor, this additional layer

of Pt increases the conductivity of the sample, thus increase the signal magnification of SEM imaging.

Figure 1. Experimental setup: with anode (Ti foil) and cathode (Pt wire)

perpendicular with each other. The distance between Ti foil and the end point of Pt wire is 5mm.

III. RESULTS AND DISCUSSIONS

A. Simulation of electrical field distribution Local electric field distribution is an important factor on

determining the morphology of TiO2 film [15]. In order to investigate the effect of different positioning methods on anode surface electric intensity, electric field of the anodization system was simulated by using electric currents module of commercial available COMSOL software.

The electric potential is governed by the following continuity equation [16]: σ V 1

where Je, σ, V and Q stand for external generated current density, electric conductivity, electric potential and quantity of electric charge, respectively.

The boundary condition between electrodes and electrolyte is [16]: 0 2

where J1,2 represents current density in domain 1 or 2, and n2 is the outward normal from domain 2.

The relationship between electric field strength on anode front surface with the distance measuring from the top end of anode are displayed in Fig.2 a-b (cross-section) and d (top view) for three different geometrical arrangements of the anode and cathode. The simulation of fig 2a is corresponding to the blue curve in fig 2d, which reads 1V/mm of electric intensity on the top side and 20V/mm on the bottom side. The slightly increased value on the top end is attributed to the

corner effect, where the current tends to focus on the sharp corner.

To exclude the corner effect that happens on the bottom end of the anode, an additional simulation (fig 2b) that the cathode is pointed to the middle of the anode is shown by the red line in Fig.2 d. As we can see, the blue and red curves overlap when x < 18 mm (with x standing for the distance from top end of the anode), indicating the corner effect happens within 2 mm at the bottom end.

As a comparison, the same simulation work has been done on a pair of parallel positioned electrodes (with anodization voltage being 60 V, and electrodes distance being 5 mm), shown in green in Fig.2 c and d (green line). The rapid increases on the two ends are also due to the corner effects as mentioned previously.

(a) (b) (c)

(d)

Figure 2. Comsol simulation of electric field distribution. (a)(b)(c) Electric field distribution in the whole anodization system corresponding to three

different electrode configurations; (d) Electric field strength distribution on anode front surface for three different conditions shown in previous diagrams

(a)(b)(c).

B. Electrochemical Anodization It has been reported that both diameter and length of the

nanotubes made by anodization method are proportional to the anodization voltage [17-21]. Firstly, we constructed the relationship of individual diameter of TiO2 nanotube vs

402

applied voltage, which the electrochemical anodization have been performed under different voltage with parallel electrode configuration in the identical NH4F glycol electrolyte [18]. Based on the top-view SEM images of TiO2 nanotube samples anodized at different voltages (20, 40, 60, 80, 100 V), the nanotube inner diameter are observed to increase linearly with increasing anodization voltages, as given in Fig. 3. The black line is the added linear trend line of our experimental curve, which shows a slope of 2.0 mm/V.

Figure 3 Diameter of the prepared nanotubes defined by applied voltages

Secondly, according to the simulation graph shown in Fig. 2, electric field strength of each local position on the anode surface is assumed to be linearly dependent on the diameter of TiO2 nanotubes. Fig. 4 shows the top-view and cross-section SEM images of nanotubes at different regions under the condition of perpendicular electrodes configuration. Fig. 4a-b give the average diameter and length of the nanotubes on position 1 (refer to position mark in Fig. 1, also corresponds to a coordinate x = 2 mm in Fig. 2a), which reads 119 nm and 440 nm, respectively. Fig.4 c-d give the same parameters of nanotubes from position 2 (x = 18 mm), in which the corresponding values of the nanotubes are 148 nm and 1.8 µm.

Figure 4 SEM images of the gradient nanotube structures. (a)(b) give the average diameter and length of the nanotubes from position 1, and (c)(d)

shows the same parameters from position 2.

Comparing with the calibration data in Fig. 3, the voltage on surface of the anode are calculated from electric field distribution simulated in Fig.2, by the following equation: · 3

where d is estimated from the contour line. We assume d1 equals to the arc length of one quarter circle with radius of 18 mm, which is approximately 28.3 mm, and d2 is equivalent to the arc length of half circle with radius of 5 mm, which equals 7.9 mm. The applied voltages are show as follows: · 2 / · 28.26 60 4 · 10 / · 7.85 80 5

When comparing these values to the tendency function given in Fig.3 (y = 2,0798x - 16,912), the values of the tube diameters are 107 nm and 149 nm. Considering the standard deviation and standard errors of the experimental results, it is reasonable to say that the results are consistent with the graph in Fig. 3.

C. Discussion of potential gradient-array TiO2-based DSSCs

Among the applications of TiO2 nanotubes mentioned, the DSSC is a promising future energy generating technique which is more cost effective and environmental friendly as compare to conventional silicon based solar cells [22]. However, the relatively low overall efficiency is a strong drawback of this new generation solar cell.

Until now, lots of efforts have been done to further improve the overall efficiency of DSSCs. Roy et al. reviewed and summarized several different types of dyes with their maximum achieved solar cell efficiencies, among which are the most efficient Ru-complex dye with efficiency of 11.18%, and metal-free dye with efficiency of 9.1% [23]. Jennings et al. proved that DSSCs reach its maximum internal conversion efficiency when using tubes of a length of approximately 20μm [24]. Other advanced geometries, though basing on different working principles, have also helped to increase the overall efficiency. For example, bamboo tubes allowed more optimized volume filling because of higher surface-to-volume ratio than normal 1D straight tubes [9]. According to the latest report, a porphyrin-sensitized solar cell with cobalt (II/III)–based redox electrolyte has exceeded 12 percent efficiency [25].

Figure 5 DSSC using gradient TiO2 nanotube structures, to use the incoming light to the largest extend.

y = 2.079x - 16.91

0

50

100

150

200

250

0 20 40 60 80 100

Nan

otub

es D

iam

eter

(nm

)

Anodization Voltage (V)

e-

I- I3-

a c

b d

403

Here we propose the concept of improving DSSC overall efficiency by applying the gradient arrayed TiO2 nanotubes. As far as we know, nanotubes take the place of conventional nanoparticles owe to the 1D orientation path, as well as their large surface-to-volume ratio which allows dye molecules to be adsorbed to the largest extend. However, the available light absorption area is limited. Due to the well and compactly ordered structures, only a small portion on the top side of the nanotubes can be illuminated (indicated by red color), while the rest is left in the shadow of the nanotubes in front. In comparison, when applying nanotubes of gradient-arrayed length, light is able to cover larger region of the tube length, implying that more dye molecules can absorb illumination and thus eject electrons more effectively.

IV. CONCLUSION Gradient nanotube structures with both tube diameter and tube length gradually changing along the sample have been prepared by electrochemical anodozation. Simulation work was conducted to find the electric field distribution inside the perpendicular positioning electrodes configuration. The gradient diameters phenomenon was verified by the linear relationship between the anodiztion voltage and tube diameter graph. The gradient length formation still needs to be studied furthermore. At the end, we put forward a concept of applying these gradient length nanotubes for DSSC application. Higher efficiency is expected from the DSSCs with configuration shown in Fig.7.

ACKNOWLEDGMENT This work was partially implemented in “Department of

Micro/Nano Systems Technology-IMST, Vestfold University College” and “MEMS Researching Centre, Xiamen University”. The author YZ acknowledges financial support from Quota Scheme at the Vestfold University College, Oslofjord Fund and NorFab in Norway.

REFERENCES [1] D. Gong, C. A. Grimes, and O. K. Varghese, “Titanium oxide nanotube

arrays prepared by anodic oxidation,” Journal of Materials Research, vol. 16, pp. 3331-3334, 2001.

[2] Y. Xie, and D. Fu, “Photoelectrocatalysis reactivity of independent titania nanotubes,” Journal of Applied Electrochemistry, vol. 40, pp. 1281-1291, 2010.

[3] Y. Li, X. Yu, and Q. Yang, “Fabrication of TiO2 Nanotube Thin Films and Their Gas Sensing Properties,” Journal of Sensors, vol. 2009, 2009.

[4] D. V. Portan, A. A. Kroustalli, D. D. Deligianni, and G. C. Papanicolaou, “On the biocompatibility between TiO2 nanotubes layer and human osteoblasts,” Journal of Biomedical Materials Research Part A, vol. 100A, pp. 2546-2553, 2012.

[5] Y. Song, Z. Gao, J. Wang, X. Xia, and R. Lynch, “Multistage Coloring Electrochromic Device Based on TiO2 Nanotube Arrays Modified with WO3 Nanoparticles,” Advanced Functional Materials, vol. 21, pp. 1941-1946, 2011.

[6] R. Asmatulu, A. Karthikeyan, D. C. Bell, S. Ramanathan, and M. J. Aziz, “Synthesis and variable temperature electrical conductivity studies of highly ordered TiO2 nanotubes,” Journal of materials science, vol. 44, pp. 4613-4616, 2009.

[7] Y. Alivov, and Z. Y. Fan, “Dye-sensitized solar cells using TiO2 nanoparticles transformed from nanotube arrays,” Journal of materials science, vol. 45, pp. 2902-2906, 2010.

[8] D. Kim, K. Lee, P. Roy, B. I. Birajdar, E. Spiecker, and P. Schmuki, “Formation of a Non-Thickness-Limited Titanium Dioxide Mesosponge and its Use in Dye-Sensitized Solar Cells,” Angewandte Chemie, vol. 121, pp. 9490-9493, 2009.

[9] S. P. Albu, D. Kim, and P. Schmuki, “Growth of Aligned TiO2 Bamboo-Type Nanotubes and Highly Ordered Nanolace,” Angewandte Chemie International Edition, vol. 47, pp. 1916-1919, 2008.

[10] J. Chen, J. Lin, X. Chen, “Self-assembled TiO2 nanotube arrays with U-shaped profile by controlling anodization temperature,” J. Nanomaterials, vol. 2010, pp. 1-4, 2010.

[11] S. P. Albu, A. Ghicov, S. Aldabergenova, P. Drechsel, D. L. Clere, G.E. Thompson, J. M. Macak, and P. Schmuki, “Formation of Double-Walled TiO2 Nanotubes and Robust Anatase Membranes,” Advanced Materials, vol. 20, pp. 4135-4139, 2008.

[12] H. Li, J. Zhang, J. Cheng, Z. Chen, F. Liang, C. K. Tsang, H. Cheng, L. Zheng, S. Lee, and Y. Y. Li, “Gradient TiO2 Nanotube Arrays via Asymmetric Anodization,” ECS Journal of Solid State Science and Technology, vol. 1, pp. M6-M9, 2012.

[13] J. W. Cheng, C. K. Tsang, F. Liang, H. Cheng, and Y. Y. Li, “Gradient TiO2 nanotube arrays,” physica status solidi (c), vol. 8, pp. 1812-1814, 2011.

[14] J. Ni, C. J. Frandsen, L. Chen, Y. Zhang, J. Khamwannah, G. He, T. Tang, and S. Jin, “Fabrication of Gradient TiO2 Nanotubes on Ti Foil by Anodization,” Advanced Engineering Materials, pp. 1-5, 2013.

[15] G. K. Mor, O. K. Varghese, M. Paulose, K. Shankar, C. A. Grimes, “A review on highly ordered, vertically oriented TiO2 nanotube arrays: Fabrication, material properties, and solar energy applications,” Solar Energy Materials and Solar Cells, vol. 90, pp. 2011-2075, 2006.

[16] D. Yang, Y. Wang, G. Ren, S. Feng, Y. Chen, and W. Wang, “Formation Mechanism Study of TiO2 Film Comprising Nanotubes and Nanoparticles,” Chinese Journal of Chemical Physics, vol. 25, 2011.

[17] G. Liu, K. Wang, N. Hoivik, and H. Jakobsen, “Progress on free-standing and flow-through TiO2 nanotube membranes,” Solar Energy Materials and Solar Cells, vol. 98, pp. 24-38, March 2012.

[18] G. Liu, N. Hoivik, X. Wang, S. Lu, K. Wang, and H. Jakobsen, “Photoconductive, free-standing crystallized TiO2 nanotube membranes,” Electrochimica Acta, vol. 93, pp. 80-86, March 2013.

[19] G. Liu, N. Hoivik, and K. Wang, “Small diameter TiO2 nanotubes with enhanced photoresponsivity,” Electrochemistry Communications, vol. 28, pp. 107-110, March 2013.

[20] J. M. Macak, H. Tsuchiya, and P. Schmuki, “High-aspect-ratio TiO2 nanotubes by anodization of titanium,” Angewandte Chemie-International Edition, vol. 44, pp. 2100-2102, 2005.

[21] S. Bauer, S. Kleber, and P. Schmuki, “TiO2 nanotubes: Tailoring the geometry in H3PO4/HF electrolytes,” Electrochemistry Communications, vol. 8, pp. 1321-1325, Aug. 2006.

[22] M. Grätzel, “Photoelectrochemical cells,” Nature, vol. 414, pp. 338-344, 2001.

[23] P. Roy, et al., "TiO2 nanotubes and their application in dye-sensitized solar cells," Nanoscale, vol. 2, pp. 45-59, 2010.

[24] J. R. Jennings, A. Ghicov, L. M. Peter, P. Schmuki, and A. B. Walker, “Dye-Sensitized Solar Cells Based on Oriented TiO2 Nanotube Arrays: Transport, Trapping, and Transfer of Electrons,” Journal of the American Chemical Society, vol. 130, pp. 13364-13372, 2008.

[25] A. Yella, H. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Nazeeruddin, E. W. Diau, C. Yeh, S. M. Zakeeruddin, and M. Gratzel, “Porphyrin-sensitized solar cells with cobalt (II/III)-based redox electrolyte exceed 12 percent efficiency,” Science, vol. 334, pp. 629-34, 2011.

404