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Double-layer TiO2 nanotube arrays by two-step anodization: Used in back and front-side illuminated dye-sensitized solar cells
Materials Science in Semiconductor Processing 39 (2015) 255–264
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Double-layer TiO2 nanotube arrays by two-step anodization: Used in back and front-side illuminated dye-sensitized solar cells Fatemeh Mohammadpour, Mahmood Moradi n Department of Physics, College of Science, Shiraz University, Shiraz 71454, Iran
a r t i c l e i n f o
Keywords: TiO2 nanotube Anodization Double layer Dye-sensitized solar cells
abstract Double-layer TiO2 nanotube arrays were fabricated by a two-step anodization process on Ti foils. The first TiO2 nanotube layer was annealed after anodization and then exposed to the second anodization to grow the second TiO2 nanotube layer beneath it. The crystallized upper layer acts as a protective layer against chemical etching of the lower layer tubes top by electrolyte that leads to growing of thick layers with open-top-tubes beneath the upper one. The effect of different anodization parameters on the final geometry of the nanotubes, grown beneath the protective layer was investigated. The upper TiO2 layer was detached as an intact membrane at the end of the two-step anodization process and used in front-side illuminated dye-sensitized solar cells (DSSCs) with high conversion efficiency up to 8.66%. Also, the remained TiO2 nanotubes on the substrate with different diameters were used in the back-side illuminted DSSCs. & 2015 Elsevier Ltd. All rights reserved.
1. Introduction Over the past decade TiO2 nanotubes have been widely used as photoanode in dye-sensitized solar cells (DSSCs) [1–4]. Highly ordered geometry [5,6], promising optical and electronic properties [7–9] and one dimensional direction of electron movements [10,11] make them a good candidate for this purpose. A low cost and simple method for fabrication of nanoporous or nanotube structures is electrochemical anodization that can be applied on the metals, like: Al [12–14], Nb [15], Zr [16], Hf [17], W [18], Ta [19] and Ti [20–23]. It is important to control the morphology of the nanotube structures, since it is very effective on the performance of the devices. In the case of anodic TiO2 nanotubes, it is possible to control the geometry of the final structure by changing the anodization parameters,
n
Corresponding author. Tel.: þ98 7136137013; fax: þ98 7136460839. E-mail address: [email protected] (M. Moradi).
http://dx.doi.org/10.1016/j.mssp.2015.04.048 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
such as anodization voltage [24,25], water content [26,27] and fluoride concentration [28,29] of the electrolyte. It is well known that an optimized TiO2 nanotube length for photoanode in DSSCs is in the range of 10– 20 mm [30–32]. Fabrication of this thick nanotube array needs to a long anodization time. But chemical etching of the tubes top against electrolyte in a long time not only leads to creation of an undesired nanograss appearance on the surface of the tubular array but also dissolution of the tubes from top, results in fabrication of short tubes [32]. Some groups have tried to omit this effect by a second anodization method [33], but it was not applicable in long anodization times [34]. Another group has tried to prevent chemical etching of the tubes top by use of a thin TiO2 layer with rutile phase as protective layer [35]. Sonication [36], supercritical drying [37] and polishing of the surface [38] are the other methods for removing this defect. In the present work, we introduce a simple method to prevent chemical etching of the tubes top against electrolyte to have long and open-top TiO2 nanotube arrays. We
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fabricate a double-layer TiO2 nanotube array by a two-step anodization method. The first layer is annealed after firststep of anodization to be crystallized and the second layer grows beneath it by second-step of anodization. The crystallized upper layer has two advantages: (1) It acts as a protective layer against chemical etching of the tubes top of the lower TiO2 nanotubes by electrolyte and (2) after detaching from sublayer at the end of second anodization, the produced membrane is used in high efficient front-side illuminated DSSCs. Moreover, this is the first time that the effect of different anodization parameters such as voltage, water content, fluoride concentration and temperature of the electrolyte on the final geometry of the grown tubes beneath a thick upper-layer, of about 10 mm, has been investigated. Among the produced sublayers, three TiO2 nanotube arrays with different tubes diameter and the same length of 20 mm are selected to investigate the effect of TiO2 nanotubes diameter on the efficiency of back-side illuminated DSSCs. Dye-loading and transport time of electrons as effective parameters on the solar light conversion efficiency of the solar cells are considered. 2. Experimental Titanium foils (0.1 mm thick, 99.9% purity, Advent, England) were cleaned in acetone, ethanol and DI water by sonication for 10 min, subsequently and then dried in nitrogen stream. First, a common anodization was carried out in a two electrode cell at different voltages from 20 to 80 V with graphite as counterelectrode and titanium foil as working electrode. The electrolyte was ethylene glycol with 3 vol% DI water and 0.15 M NH4F. In order to fabricate long, open-top nanotube arrays without any nanograss appearance, double-layer TiO2 nanotube arrays were fabricated. At first, anodization was performed at 60 V for 20 min in the above mentioned electrolyte to grow TiO2 nanotube array with length of about 10 mm on Ti foil. This layer was annealed at 350 1C for 1 h to be crystallized. Afterward, the second layer of TiO2 nanotubes was fabricated beneath the first one by second anodization at different voltages from 20 to 80 V for 40 min. Since we have the fastest growth rate of anodization at 80 V, this voltage was selected in the rest of the experiment. Then, the effect of different anodization parameters, such as: water and fluoride concentration of the electrolyte and temperature, on the geometry of grown TiO2 nanotubes beneath protective layer (upper-layer) was investigated. Water content and fluoride concentration of the electrolyte was varied accordingly from 1 to 9 vol% and from 0.05 to 0.25 M, respectively. Also, the electrolyte temperature was varied from 5 to 35 1C. The upper-layer was detached at the end of anodization by shaking the sample in 0.07 M HF solution for some minutes. The detached upper-layer was an intact membrane. This membrane with length of 20 mm (for this purpose the first anodization was done for 40 min) was transferred on coated FTO glass with 2 mm nanoparticle paste (Ti-Nanoxide HT, Solaronix) and after crystallization at 500 1C with heating and cooling rate of 30 1C min 1 used in frontside illuminated DSSCs [1].
Also, the remained TiO2 nanotube layers on Ti foil were used to make back-side illuminated DSSCs. To investigate the effect of TiO2 nanotubes diameter on the efficiency of back-side illuminated DSSCs, three different lower-layers consisted of TiO2 nanotube arrays with diameters of 60 (80 V, 3 vol% DI water, 0.1 M NH4F, 20 1C), 100 (60 V, 3 vol% DI water, 0.15 M NH4F, 20 1C) and 140 nm (80 V, 3 vol% DI water, 0.15 M NH4F, 25 1C) and length of 20 mm were selected. To crystallize the nanotube arrays, they were annealed at 500 1C with heating and cooling rate of 30 1C min 1 for 1 h in air. For sensitization, the crystallized photoanodes were immersed in 300 mM dye solution (D719, Everlight, Taiwan) at 40 1C for 24 h. The as-prepared photoanodes were sandwiched with a Pt coated FTO (7 Ω cm 1) glass as a counter electrode by using a hot-melt spacer (25 mm, Surlyn, Dupont). The electrolyte (Io-li-tec, ES-0004) was introduced in the interspace of the electrodes. Morphological characterization of the TiO2 nanotubes was investigated by using a field-emission scanning electron microscope (FE-SEM, Hitachi s-4160). X-ray diffraction analysis (XRD) was performed with an X'pert Philips MPD with a Panalytical X'celerator detector using graphite monochromized Cu Kα radiation (wavelength 1.54056 Å\ t\5pt'). The current voltage characteristics were measured under simulated AM 1.5 illumination provided by a solar simulator (300 W Xe with an optical filter, Solarlight) while an external bias was applied to the cell. The generated photocurrent was measured with a Keithley model 2420 digital source meter. IPCE (Incident photon to current conversion efficiency) measurement was performed with a 150 W Xe arc lamp (LOT-Oriel Instrument) with an Oriel Cornerstone 7400 1/ 8 m monochromator. The light intensity was measured with an optical power meter. Intensity modulated photocurrent spectroscopy (IMPS) measurements were carried out using modulated light (10% modulation depth) from a high power green LED (λ ¼ 530 nm). The modulation frequency was controlled by a frequency response analyzer (FRA, Zahner). The light intensity incident on the cell was measured using a calibrated Si photodiode. Dye loading of the photoanodes was measured by immersing the dye-sensitized nanotube layers in 5 ml of 10 mM NaOH for 30 min. Then the absorption of the solutions was measured by an UV–vis Spectrophotometer (Lambda XLSþ, Perkin-Elmer).
3. Results and discussion In principle, TiO2 nanotubes growth is based on three key processes [39]: (i) Field-assisted oxidation at metal/oxide interface that is formulated as follows: Tiþ2H2O-TiO2 þ4H þ þ4e (ii) Field-assisted
dissolution
(1) at
oxide/electrolyte
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interface at the tubes bottom. (iii) Chemical etching of the fabricated tubes at the tubes top. The chemical reactions of (ii) and (iii) are formulated as: TiO2 þ6F þ 4H þ -TiF26 þ2H2O
(2)
The competition between chemical dissolution of the tubes bottom and tubes top determines the final length of the tubes. Hard etching conditions (high anodization voltage, high concentration of fluoride ions in the electrolyte and high temperature) usually cause to faster chemical etching rate of the tubes top in comparison to the tubes bottom. Thereby, short tubes with a nanograss appearance on the tubes top achieve at the end of an anodization process. To show the role of a protective layer against chemical etching of the electrolyte, first a usual anodization is done
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on Ti foils at different voltages from 20 to 80 V. Fig. 1(a)–(d) shows top-view FE-SEM images of produced TiO2 nanotubes at different voltages. It can be seen from these images that TiO2 nanotubes diameter is increasing with increment of the applied voltage. A nanograss appearance is created at the tubes top in higher anodization voltages such as 60 and 80 V, Fig. 1(c) and (d). It seems that at higher anodization voltages, faster movement of ions such as F leads to the more chemical etching of the tubes top which results in a nanograss appearance on the tubes top. The curves of anodization current density versus time of the samples at different voltages are shown in Fig. 1(e). As can be seen from this figure, the amount of current density at 80 V is much higher than its value in the other voltages. However at the beginning of anodization the current density increases in all of the voltages, not only the maximum of currents are bigger at higher voltages but also the decline rate is faster, too. At the beginning of anodization, the horizontal
Fig. 1. Top-view FE-SEM images of TiO2 nanotubes grown on Ti foil at first anodization with voltages of 20 V (a), 40 V (b), 60 V (c) and 80 V (d) and corresponding anodization current density versus time curves (e). The diameter and length of the tubes versus anodization voltage (f).
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component of the electric field results in the horizontal growth of the nanopores [40]. As the nanopores wall contact to each other, the horizontal growth of the nanopores stops and the current density decrease sharply and reach to steady state current density that lead to vertical growth of the nanotubes [40]. Creation of tubes with smaller diameter at lower voltages takes more time and thus causes to current density reach to steady state condition after a longer time. Fig. 1(e) shows higher steady state current densities at higher voltages that anticipate faster growth rate of the tubes. Fig. 1 (f) illustrates an almost linear relationship between diameter and length of the tubes with respect to the applied voltages. The diameter and length of the tubes changes from 28 nm and 2 mm at 20 V to 105 nm and 37 mm at 80 V, respectively. 3.1. Fabrication of protective layer on the Ti surface by a two-step anodization method A double-layer TiO2 nanotube array is fabricated by a two-step anodization process. First, the Ti foils are anodized at 60 V for 20 min. As the crystallized TiO2 nanotube layer shows more resistance against chemical etching of the electrolyte [41], the fabricated TiO2 nanotube layer is annealed at 350 1C for 1 h to be crystallized. Then, the second anodization is carried out to grow the second TiO2 nanotube layer beneath the first one. The crystallized upper-layer acts as a protective layer against chemical etching of the lower-layer tubes top by the electrolyte. The growth stages of the lower-layer TiO2 nanotubes can be explained as follows: A compact oxide layer is generated beneath the first TiO2 nanotube layer by oxidation of Ti substrate. The compact
oxide layer acts as an interlayer between the upper TiO2 nanotube array and the lower one. After applying the second anodization voltage, random seeding sites are produced in the intertube layer [42] with more chance at the tubes bottom of the upper-layer due to stronger electric field at these sites. Prolonging the anodization leads to growing the tubes from the seeding sites beneath the first crystallized TiO2 nanotubes layer. In fact, the existence of the protective layer decrease dissolution rate of the lowerlayer tubes top. This approach is schematically shown in Fig. 2. The upper TiO2 layer is detached at the end of anodization by shaking the sample in 0.07 M HF solution for some minutes. The interface between crystalline and amorphous TiO2 nanotube layers can be easily etched by HF solution. The detached membrane will be used in frontside illuminated DSSCs and the remained TiO2 nanotubes on the Ti foils will be used in back-side illuminated DSSCs. The second anodization is performed at different voltages from 20 to 80 V at the same electrolyte on the pre-anodized Ti foils. Fig. 3(a)–(d) shows FE-SEM images of grown TiO2 nanotubes beneath protective layer at different applied voltages. As can be seen from this figure, these grown nanotubes have more regularity in comparison to the previous one and no nanograss appearance can be seen on their surfaces. It can be concluded that the protective layer is considerably effective against chemical etching of the tubes top. The trend of anodization current density versus time in Fig. 3(e) is similar to the mild anodization. The current density decreases from the first maximum sharply
Fig. 2. Schematic of fabrication of two-layer TiO2 nanotube arrays by two-step anodization and using in front and back-side illuminated DSSCs.
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Fig. 3. Top-view FE-SEM images of TiO2 nanotubes grown on Ti foil at second anodization (lower-layer) with voltages of 20 V (a), 40 V (b), 60 V (c) and 80 V (d) and corresponding anodization current density versus time curves. (e). The diameter and length of the tubes versus anodization voltage (f).
to reach a minimum value and after that increases to reach a stable value. The existence of the crystallized upper-layer gives more time to the tubes to grow and reconstruct themselves and thereby they grow with more ordering at this condition. Fig. 3(f) shows that the diameter and length of the tubes increase with increment of the applied voltage, as it is expected. Also, we have the fastest growth rate of the tubes at 80 V. A long, open-top nanotube array about 30 mm without any nanograss on its surface is achieved at this condition. The tube diameter changes from 38 nm at 20 V to 135 nm at 80 V.
From now, the second anodization is done at 80 V and different anodization parameters will be changed. 3.1.1. Effect of the water content of the electrolyte on the geometry of the grown TiO2 nanotubes by second anodization To investigate the effect of water content of the electrolyte on the geometry of the grown nanotubes beneath protective layer, four different volume percentages of water are considered: 1, 3, 6 and 9 vol%. As can be seen from FE-SEM images in Fig. 4(a) and (b), there is a transition from nanoporous to nanotube
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Fig. 4. Top-view FE-SEM images of TiO2 nanotubes grown by second anodization and beneath protective layer (upper-layer) at different water contents of 1 (a), 3 (b), 6 (c) and 9 vol% (d) and the corresponding anodization current density versus time curves (e). The diameter and length of the tubes versus water content of the electrolyte (f).
structure with increasing the water content of the electrolyte from 1 to 3 vol%. It is known that there is a fluoride rich layer at the cell boundaries that is made by plastic flow mechanism [43] during the growth of the oxide. Chemical dissolution of this layer at the cell boundaries leads to formation of individual tubes [40]. The main solvents of this layer are water and fluoride ions. So, when the water content of the electrolyte is very low (1 vol% at this work), we have a porous structure rather than a tubular structure. Fig. 4(f) shows that the length of the anodized layer increases as the water content of the electrolyte reaches from 1 to 3 vol% and then decreases with more increase in water content and it reaches to very thin layer about 3 mm at 9 vol% of H2O. Lower growth rate at higher concentration of water can be ascribed to decreasing IR drop (voltage drop) within the electrolyte [44]. Also, it can be seen from Fig. 4(a)–(d) that the tubes diameter is increasing from 80 nm at 1 vol% H2O to 150 nm at 9 vol% H2O. The anodization current density versus time curves in Fig. 4(e) show that the current density decreases from the first maximum to reach a minimum value and then increases again to reach a second maximum. After that the current declines to reach a steady state condition. However this trend is the same in all cases, but the time interval of the minimum status varies from 30 s for 1 vol% H2O, 150 s for 3 vol% H2O, 250 s for 6 vol% H2O to 400 s for 9 vol% H2O. This trend can be described as follows: At the beginning of anodization the current density increases by applying
potential and after the growth of an oxide layer, it decreases and reach to a minimum value. Taking more time at this stage with increasing the volume percent of H2O, demonstrates growing of tubes with bigger diameter. After creation of the pores (horizontal growth), the current density increases again and reach a maximum value. Increasing the water content of the electrolyte leads to less viscosity of the electrolyte and according to Stokes–Einstein relation results in more diffusion coefficient of OH and F and finally higher dissolution rate of the oxide [45]. This effect along with the decreasing IR drop within the electrolyte leads to more dissolution of the oxide in horizontal direction to have bigger diameter at higher amount of water content. 3.1.2. Effect of fluoride concentration of the electrolyte on the geometry of the grown TiO2 nanotubes by second anodization The effect of fluoride concentration of the electrolyte on the morphology of grown TiO2 nanotubes beneath protective layer is investigated by changing this parameter from 0.05 to 0.25 M. As can be seen from FE-SEM images in Fig. 5(a)–(e), at low concentration of fluoride ions we have a porous structure rather than tubular structure. As it is discussed in the previous section there is a fluoride rich layer at the cell boundaries that is soluble with F and water. At low concentration of fluoride and water this layer does not solve and a porous structure can be seen on the surface. It is clear from Fig. 5(a)–(e) that increasing of the F concentration leads to TiO2 nanotubes with thinner
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walls that can be ascribed to the more etching of the inner side of the tubes wall by fluoride ions. Also, it can be observed in the insets of from Fig. 5(d) and (e) that the existence of the protective layer is not effective against chemical etching of the tubes top at F concentrations higher than 0.15 M and nanograsses are created on the surface. As the Fluoride concentration increases, the length of the TiO2 nanotube arrays increases to 30 mm at 0.15 M fluoride concentration and after that fast rate of chemical etching of the tubes top leads to decrement of the tubes length. However the higher concentration of F ions leads to the increase of dissolution rate at the bottom of the tubes, but passes through a certain fluoride concentration (0.15 M) results in higher speed of etching of the tubes top in comparison to dissolution rate of the tubes bottom and thereby the tubes length decreases. The current density versus time curves in Fig. 5(f) demonstrate that the current density increases by increase of the F ions at the first times of anodization. But after passing a short time of anodization of about 800 s, the
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current decreases sharply and reach to a low stable state current density. This trend confirms that the increase of F ions is not effectine in growing long tubes. 3.1.3. Effect of the electrolyte temperature on the geometry of the grown TiO2 nanotubes by second anodization Fig. 6(a)–(d) shows cross-sectional FE-SEM images of grown TiO2 nanotubes at different electrolyte temperature from 5 to 35 1C and the insets show top-view of the grown tubes by second anodization. These images show that the diameter and the length of the tubes increase with increasing temperature. As can be seen from Fig. 6(f), the increasing of the electrolyte temperature from 5 to 35 1C leads to increment of the diameter of the tubes from 70 to 150 nm and increment of the length of the tubes from 5 to 50 mm. The fast growth of the tubes in 35 1C can be ascribed to the faster ions flow at high temperatures that leads to a faster anodization process and higher dissolution rate of the tubes bottom with respect to the etching of the tubes top. High stable state current density at 35 1C confirms the fast growth of the tubes at this condition,
Fig. 5. Top-view FE-SEM images of TiO2 nanotubes grown by second anodization and beneath protective layer (upper-layer) at different fluoride concentrations of the electrolyte: 0.05 (a), 0.10 (b), 0.15 (c), 0.20 (d) and 0.25 M (e), The insets in (d,e) are low magnification top-view of the tubes and the scale bar is 6 mm.The corresponding anodization current density versus time curves (f). The diameter and length of the tubes versus fluoride concentration of the electrolyte (g).
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Fig. 6. Cross-sectional FE-SEM images of TiO2 nanotubes grown by second anodization and beneath protective layer at 80 V and different temperatures of 5 (a), 15 (b), 25 (c) and 35 1C (d). The insets show top-view FE-SEM images. Corresponding anodization current density versus time curves (e). The diameter and length of the tubes versus electrolyte temperatures from 5 to 35 1C (f).
Fig. 6(e). Top-view FE-SEM images of the grown tubes at 35 1C, inset in Fig. 6(d), show that the tubes top are bundled but no evident of chemical etching of the tubes top resulted in creation of nanograsses cannot be seen. It can be concluded that the faster movement of the F ions in higher temperatures is effective in growing the long tubes but higher concentration of the F ions is destructive.
3.2. Fabrication of dye-sensitized solar cells The detached protective layer with length of 20 mm was transferred on FTO coated glass with 2 mm nanoparticle paste by a doctor-blade method and was used in front-side illuminated DSSCs. Also, in order to investigate the effect of diameter size of TiO2 nanotubes on the efficiency of
back-side illuminated DSSCs, three sublayers consisted of TiO2 nanotube arrays with diameter of 60, 100 and 140 nm and length of about 20 mm were selected. All the photoanodes were annealed at 500 1C for 1 h with heating and cooling rate of 30 1C min 1 to be crystallized and then assembled to solar cells. XRD patterns of these photoanodes, Fig. 7(a) and (b), show a pure anatase phase in all cases, independent of TiO2 nanotubes substrates. However the sharper peaks in the XRD pattern of the transferred TiO2 nanotubes on FTO glass indicate higher crystallinity of the tubes at this configuration. Fig. 7(c) and (d) demonstrates J–V curves of the DSSCs and their corresponding photovoltaic characteristics under simulated AM 1.5 (100 mW cm 2). Clearly, the cell efficiency is markedly affected by the illumination side, so that, a significant enhancement of the photovoltaic characteristics is seen when a front-side illumination is
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Fig. 7. XRD patterns of (a) annealed TiO2 nanotubes on Ti foil with different diameters of 60, 100, 140 nm and (b) annealed TiO2 nanotubes on FTO glass coated by 2 mm nanoparticle paste at 500 1C with heating and cooling rate of 30 1C min 1. (c) J–V curves and (d) summary of photovoltaic characteristics of different DSSCs under simulated AM 1.5. (e) The IPCE measurement of front-side illuminated DSSC. (f) Transport time of electrons measured by IMPS on the different DSSCs.
occurred. Particularly, the solar light conversion efficiency is 8.66% in front-side illumination that is improved 64% from back-side illumination. The high current density of 18.58 mA cm 2 is well in line with IPCE measurement, Fig. 7(e). As can be seen from this figure, in some regions, about of 90% of incident photons to the cell convert to
electricity at this configuration. The first reason for this improvement can be ascribed to the absorbance of sunlight by electrolyte or its reflecting by Pt-coated counterelectrode in back-side illuminated DSSCs. Also, it is clear from this figure that DSSC consisted of TiO2 nanotube array with diameter of 60 nm shows the
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highest conversion efficiency up to 5.27% among back-side illuminated DSSCs and the conversion efficiency is decreasing with the increment of diameter. An effective factor that strongly affects on the photovoltaic performance of the DSSCs is the dye-loading. The larger surface area of the photoanode chemisorbs the larger amount of dye molecules that leads to higher light harvesting efficiency and thus improvement of the cell efficiency. The summarized data in Fig. 7(d) confirm this relationship between the dye-loading of the structure and solar light conversion efficiency. As the transferred TiO2 nanotubes on FTO glass have the highest surface area due to the existence of TiO2 nanoparticles beneath the tubes, this configuration has the highest solar light conversion efficiency and among the tubes on the Ti foils, surface area is increasing with decrement of tubes diameter and thus the tubes array with the smallest diameter of 60 nm has the highest efficiency. To better understand the effect of the illumination side and tubes diameter on the transport time of the electrons and thus the performance of DSSCs, the IMPS measurements are performed, Fig. 7(f). Data in Fig. 7(f) show that a front-side illumination configuration strongly improves the transport time of the electrons and transferred tube on FTO glass show faster electron transport properties in comparison to the tubes on the Ti foil. 4. Conclusions In the present work, double-layer TiO2 nanotube arrays have been fabricated by a two-step anodization method. The upper-layer with length of about 10 mm has been fabricated by anodization of Ti foil at 60 V in an ethylene glycol electrolyte consisted of 0.15 M NH4F and 3 vol% DI water at room temperature. This layer has been annealed at 350 1C for 1 h to be crystallized and then exposed to second anodization for 40 min at different conditions. At this way, the second layer TiO2 nanotube arrays have been fabricated beneath the first one with variety of lengths and diameters. The longest open-top TiO2 nanotube array with length of about 50 mm has been achieved with second anodization at 80 V in the above mentioned electrolyte and electrolyte temperature of 35 1C. The upper-layer has been detached at the end of anodization by a chemical method. The detached membrane has been used in front-side illuminated DSSCs with a high efficiency up to 8.66%. Also, the remained TiO2 nanotubes on the Ti foils with different diameters have been used in back-side illuminated DSSCs. It has been shown that the smallest diameter leads to the highest efficiency at this case. References [1] F. Mohammadpour, M. Moradi, K. Lee, G. Cha, S. So, A. Kahnt, D. M. Guldi, M. Altomare, P. Schmuki, J. Chem Commun. 51 (2015) 1631–1634. [2] F. Mohammadpour, M. Moradi, G. Cha, S. So, K. Lee, M. Altomare, P. Schmuki, J. ChemElectroChem 2 (2015) 204–207. [3] X. Liu, J. Lin, Y.H. Tsang, X. Chen, P. Hing, H. Huang, J. Alloy. Compd. 607 (2014) 50–53.
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Double-layer TiO2 nanotube arrays by two-step anodization: Used in back and front-side illuminated dye-sensitized solar cells Fatemeh Mohammadpour, Mahmood Moradi n Department of Physics, College of Science, Shiraz University, Shiraz 71454, Iran
a r t i c l e i n f o
Keywords: TiO2 nanotube Anodization Double layer Dye-sensitized solar cells
abstract Double-layer TiO2 nanotube arrays were fabricated by a two-step anodization process on Ti foils. The first TiO2 nanotube layer was annealed after anodization and then exposed to the second anodization to grow the second TiO2 nanotube layer beneath it. The crystallized upper layer acts as a protective layer against chemical etching of the lower layer tubes top by electrolyte that leads to growing of thick layers with open-top-tubes beneath the upper one. The effect of different anodization parameters on the final geometry of the nanotubes, grown beneath the protective layer was investigated. The upper TiO2 layer was detached as an intact membrane at the end of the two-step anodization process and used in front-side illuminated dye-sensitized solar cells (DSSCs) with high conversion efficiency up to 8.66%. Also, the remained TiO2 nanotubes on the substrate with different diameters were used in the back-side illuminted DSSCs. & 2015 Elsevier Ltd. All rights reserved.
1. Introduction Over the past decade TiO2 nanotubes have been widely used as photoanode in dye-sensitized solar cells (DSSCs) [1–4]. Highly ordered geometry [5,6], promising optical and electronic properties [7–9] and one dimensional direction of electron movements [10,11] make them a good candidate for this purpose. A low cost and simple method for fabrication of nanoporous or nanotube structures is electrochemical anodization that can be applied on the metals, like: Al [12–14], Nb [15], Zr [16], Hf [17], W [18], Ta [19] and Ti [20–23]. It is important to control the morphology of the nanotube structures, since it is very effective on the performance of the devices. In the case of anodic TiO2 nanotubes, it is possible to control the geometry of the final structure by changing the anodization parameters,
n
Corresponding author. Tel.: þ98 7136137013; fax: þ98 7136460839. E-mail address: [email protected] (M. Moradi).
http://dx.doi.org/10.1016/j.mssp.2015.04.048 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
such as anodization voltage [24,25], water content [26,27] and fluoride concentration [28,29] of the electrolyte. It is well known that an optimized TiO2 nanotube length for photoanode in DSSCs is in the range of 10– 20 mm [30–32]. Fabrication of this thick nanotube array needs to a long anodization time. But chemical etching of the tubes top against electrolyte in a long time not only leads to creation of an undesired nanograss appearance on the surface of the tubular array but also dissolution of the tubes from top, results in fabrication of short tubes [32]. Some groups have tried to omit this effect by a second anodization method [33], but it was not applicable in long anodization times [34]. Another group has tried to prevent chemical etching of the tubes top by use of a thin TiO2 layer with rutile phase as protective layer [35]. Sonication [36], supercritical drying [37] and polishing of the surface [38] are the other methods for removing this defect. In the present work, we introduce a simple method to prevent chemical etching of the tubes top against electrolyte to have long and open-top TiO2 nanotube arrays. We
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fabricate a double-layer TiO2 nanotube array by a two-step anodization method. The first layer is annealed after firststep of anodization to be crystallized and the second layer grows beneath it by second-step of anodization. The crystallized upper layer has two advantages: (1) It acts as a protective layer against chemical etching of the tubes top of the lower TiO2 nanotubes by electrolyte and (2) after detaching from sublayer at the end of second anodization, the produced membrane is used in high efficient front-side illuminated DSSCs. Moreover, this is the first time that the effect of different anodization parameters such as voltage, water content, fluoride concentration and temperature of the electrolyte on the final geometry of the grown tubes beneath a thick upper-layer, of about 10 mm, has been investigated. Among the produced sublayers, three TiO2 nanotube arrays with different tubes diameter and the same length of 20 mm are selected to investigate the effect of TiO2 nanotubes diameter on the efficiency of back-side illuminated DSSCs. Dye-loading and transport time of electrons as effective parameters on the solar light conversion efficiency of the solar cells are considered. 2. Experimental Titanium foils (0.1 mm thick, 99.9% purity, Advent, England) were cleaned in acetone, ethanol and DI water by sonication for 10 min, subsequently and then dried in nitrogen stream. First, a common anodization was carried out in a two electrode cell at different voltages from 20 to 80 V with graphite as counterelectrode and titanium foil as working electrode. The electrolyte was ethylene glycol with 3 vol% DI water and 0.15 M NH4F. In order to fabricate long, open-top nanotube arrays without any nanograss appearance, double-layer TiO2 nanotube arrays were fabricated. At first, anodization was performed at 60 V for 20 min in the above mentioned electrolyte to grow TiO2 nanotube array with length of about 10 mm on Ti foil. This layer was annealed at 350 1C for 1 h to be crystallized. Afterward, the second layer of TiO2 nanotubes was fabricated beneath the first one by second anodization at different voltages from 20 to 80 V for 40 min. Since we have the fastest growth rate of anodization at 80 V, this voltage was selected in the rest of the experiment. Then, the effect of different anodization parameters, such as: water and fluoride concentration of the electrolyte and temperature, on the geometry of grown TiO2 nanotubes beneath protective layer (upper-layer) was investigated. Water content and fluoride concentration of the electrolyte was varied accordingly from 1 to 9 vol% and from 0.05 to 0.25 M, respectively. Also, the electrolyte temperature was varied from 5 to 35 1C. The upper-layer was detached at the end of anodization by shaking the sample in 0.07 M HF solution for some minutes. The detached upper-layer was an intact membrane. This membrane with length of 20 mm (for this purpose the first anodization was done for 40 min) was transferred on coated FTO glass with 2 mm nanoparticle paste (Ti-Nanoxide HT, Solaronix) and after crystallization at 500 1C with heating and cooling rate of 30 1C min 1 used in frontside illuminated DSSCs [1].
Also, the remained TiO2 nanotube layers on Ti foil were used to make back-side illuminated DSSCs. To investigate the effect of TiO2 nanotubes diameter on the efficiency of back-side illuminated DSSCs, three different lower-layers consisted of TiO2 nanotube arrays with diameters of 60 (80 V, 3 vol% DI water, 0.1 M NH4F, 20 1C), 100 (60 V, 3 vol% DI water, 0.15 M NH4F, 20 1C) and 140 nm (80 V, 3 vol% DI water, 0.15 M NH4F, 25 1C) and length of 20 mm were selected. To crystallize the nanotube arrays, they were annealed at 500 1C with heating and cooling rate of 30 1C min 1 for 1 h in air. For sensitization, the crystallized photoanodes were immersed in 300 mM dye solution (D719, Everlight, Taiwan) at 40 1C for 24 h. The as-prepared photoanodes were sandwiched with a Pt coated FTO (7 Ω cm 1) glass as a counter electrode by using a hot-melt spacer (25 mm, Surlyn, Dupont). The electrolyte (Io-li-tec, ES-0004) was introduced in the interspace of the electrodes. Morphological characterization of the TiO2 nanotubes was investigated by using a field-emission scanning electron microscope (FE-SEM, Hitachi s-4160). X-ray diffraction analysis (XRD) was performed with an X'pert Philips MPD with a Panalytical X'celerator detector using graphite monochromized Cu Kα radiation (wavelength 1.54056 Å\ t\5pt'). The current voltage characteristics were measured under simulated AM 1.5 illumination provided by a solar simulator (300 W Xe with an optical filter, Solarlight) while an external bias was applied to the cell. The generated photocurrent was measured with a Keithley model 2420 digital source meter. IPCE (Incident photon to current conversion efficiency) measurement was performed with a 150 W Xe arc lamp (LOT-Oriel Instrument) with an Oriel Cornerstone 7400 1/ 8 m monochromator. The light intensity was measured with an optical power meter. Intensity modulated photocurrent spectroscopy (IMPS) measurements were carried out using modulated light (10% modulation depth) from a high power green LED (λ ¼ 530 nm). The modulation frequency was controlled by a frequency response analyzer (FRA, Zahner). The light intensity incident on the cell was measured using a calibrated Si photodiode. Dye loading of the photoanodes was measured by immersing the dye-sensitized nanotube layers in 5 ml of 10 mM NaOH for 30 min. Then the absorption of the solutions was measured by an UV–vis Spectrophotometer (Lambda XLSþ, Perkin-Elmer).
3. Results and discussion In principle, TiO2 nanotubes growth is based on three key processes [39]: (i) Field-assisted oxidation at metal/oxide interface that is formulated as follows: Tiþ2H2O-TiO2 þ4H þ þ4e (ii) Field-assisted
dissolution
(1) at
oxide/electrolyte
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interface at the tubes bottom. (iii) Chemical etching of the fabricated tubes at the tubes top. The chemical reactions of (ii) and (iii) are formulated as: TiO2 þ6F þ 4H þ -TiF26 þ2H2O
(2)
The competition between chemical dissolution of the tubes bottom and tubes top determines the final length of the tubes. Hard etching conditions (high anodization voltage, high concentration of fluoride ions in the electrolyte and high temperature) usually cause to faster chemical etching rate of the tubes top in comparison to the tubes bottom. Thereby, short tubes with a nanograss appearance on the tubes top achieve at the end of an anodization process. To show the role of a protective layer against chemical etching of the electrolyte, first a usual anodization is done
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on Ti foils at different voltages from 20 to 80 V. Fig. 1(a)–(d) shows top-view FE-SEM images of produced TiO2 nanotubes at different voltages. It can be seen from these images that TiO2 nanotubes diameter is increasing with increment of the applied voltage. A nanograss appearance is created at the tubes top in higher anodization voltages such as 60 and 80 V, Fig. 1(c) and (d). It seems that at higher anodization voltages, faster movement of ions such as F leads to the more chemical etching of the tubes top which results in a nanograss appearance on the tubes top. The curves of anodization current density versus time of the samples at different voltages are shown in Fig. 1(e). As can be seen from this figure, the amount of current density at 80 V is much higher than its value in the other voltages. However at the beginning of anodization the current density increases in all of the voltages, not only the maximum of currents are bigger at higher voltages but also the decline rate is faster, too. At the beginning of anodization, the horizontal
Fig. 1. Top-view FE-SEM images of TiO2 nanotubes grown on Ti foil at first anodization with voltages of 20 V (a), 40 V (b), 60 V (c) and 80 V (d) and corresponding anodization current density versus time curves (e). The diameter and length of the tubes versus anodization voltage (f).
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component of the electric field results in the horizontal growth of the nanopores [40]. As the nanopores wall contact to each other, the horizontal growth of the nanopores stops and the current density decrease sharply and reach to steady state current density that lead to vertical growth of the nanotubes [40]. Creation of tubes with smaller diameter at lower voltages takes more time and thus causes to current density reach to steady state condition after a longer time. Fig. 1(e) shows higher steady state current densities at higher voltages that anticipate faster growth rate of the tubes. Fig. 1 (f) illustrates an almost linear relationship between diameter and length of the tubes with respect to the applied voltages. The diameter and length of the tubes changes from 28 nm and 2 mm at 20 V to 105 nm and 37 mm at 80 V, respectively. 3.1. Fabrication of protective layer on the Ti surface by a two-step anodization method A double-layer TiO2 nanotube array is fabricated by a two-step anodization process. First, the Ti foils are anodized at 60 V for 20 min. As the crystallized TiO2 nanotube layer shows more resistance against chemical etching of the electrolyte [41], the fabricated TiO2 nanotube layer is annealed at 350 1C for 1 h to be crystallized. Then, the second anodization is carried out to grow the second TiO2 nanotube layer beneath the first one. The crystallized upper-layer acts as a protective layer against chemical etching of the lower-layer tubes top by the electrolyte. The growth stages of the lower-layer TiO2 nanotubes can be explained as follows: A compact oxide layer is generated beneath the first TiO2 nanotube layer by oxidation of Ti substrate. The compact
oxide layer acts as an interlayer between the upper TiO2 nanotube array and the lower one. After applying the second anodization voltage, random seeding sites are produced in the intertube layer [42] with more chance at the tubes bottom of the upper-layer due to stronger electric field at these sites. Prolonging the anodization leads to growing the tubes from the seeding sites beneath the first crystallized TiO2 nanotubes layer. In fact, the existence of the protective layer decrease dissolution rate of the lowerlayer tubes top. This approach is schematically shown in Fig. 2. The upper TiO2 layer is detached at the end of anodization by shaking the sample in 0.07 M HF solution for some minutes. The interface between crystalline and amorphous TiO2 nanotube layers can be easily etched by HF solution. The detached membrane will be used in frontside illuminated DSSCs and the remained TiO2 nanotubes on the Ti foils will be used in back-side illuminated DSSCs. The second anodization is performed at different voltages from 20 to 80 V at the same electrolyte on the pre-anodized Ti foils. Fig. 3(a)–(d) shows FE-SEM images of grown TiO2 nanotubes beneath protective layer at different applied voltages. As can be seen from this figure, these grown nanotubes have more regularity in comparison to the previous one and no nanograss appearance can be seen on their surfaces. It can be concluded that the protective layer is considerably effective against chemical etching of the tubes top. The trend of anodization current density versus time in Fig. 3(e) is similar to the mild anodization. The current density decreases from the first maximum sharply
Fig. 2. Schematic of fabrication of two-layer TiO2 nanotube arrays by two-step anodization and using in front and back-side illuminated DSSCs.
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Fig. 3. Top-view FE-SEM images of TiO2 nanotubes grown on Ti foil at second anodization (lower-layer) with voltages of 20 V (a), 40 V (b), 60 V (c) and 80 V (d) and corresponding anodization current density versus time curves. (e). The diameter and length of the tubes versus anodization voltage (f).
to reach a minimum value and after that increases to reach a stable value. The existence of the crystallized upper-layer gives more time to the tubes to grow and reconstruct themselves and thereby they grow with more ordering at this condition. Fig. 3(f) shows that the diameter and length of the tubes increase with increment of the applied voltage, as it is expected. Also, we have the fastest growth rate of the tubes at 80 V. A long, open-top nanotube array about 30 mm without any nanograss on its surface is achieved at this condition. The tube diameter changes from 38 nm at 20 V to 135 nm at 80 V.
From now, the second anodization is done at 80 V and different anodization parameters will be changed. 3.1.1. Effect of the water content of the electrolyte on the geometry of the grown TiO2 nanotubes by second anodization To investigate the effect of water content of the electrolyte on the geometry of the grown nanotubes beneath protective layer, four different volume percentages of water are considered: 1, 3, 6 and 9 vol%. As can be seen from FE-SEM images in Fig. 4(a) and (b), there is a transition from nanoporous to nanotube
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Fig. 4. Top-view FE-SEM images of TiO2 nanotubes grown by second anodization and beneath protective layer (upper-layer) at different water contents of 1 (a), 3 (b), 6 (c) and 9 vol% (d) and the corresponding anodization current density versus time curves (e). The diameter and length of the tubes versus water content of the electrolyte (f).
structure with increasing the water content of the electrolyte from 1 to 3 vol%. It is known that there is a fluoride rich layer at the cell boundaries that is made by plastic flow mechanism [43] during the growth of the oxide. Chemical dissolution of this layer at the cell boundaries leads to formation of individual tubes [40]. The main solvents of this layer are water and fluoride ions. So, when the water content of the electrolyte is very low (1 vol% at this work), we have a porous structure rather than a tubular structure. Fig. 4(f) shows that the length of the anodized layer increases as the water content of the electrolyte reaches from 1 to 3 vol% and then decreases with more increase in water content and it reaches to very thin layer about 3 mm at 9 vol% of H2O. Lower growth rate at higher concentration of water can be ascribed to decreasing IR drop (voltage drop) within the electrolyte [44]. Also, it can be seen from Fig. 4(a)–(d) that the tubes diameter is increasing from 80 nm at 1 vol% H2O to 150 nm at 9 vol% H2O. The anodization current density versus time curves in Fig. 4(e) show that the current density decreases from the first maximum to reach a minimum value and then increases again to reach a second maximum. After that the current declines to reach a steady state condition. However this trend is the same in all cases, but the time interval of the minimum status varies from 30 s for 1 vol% H2O, 150 s for 3 vol% H2O, 250 s for 6 vol% H2O to 400 s for 9 vol% H2O. This trend can be described as follows: At the beginning of anodization the current density increases by applying
potential and after the growth of an oxide layer, it decreases and reach to a minimum value. Taking more time at this stage with increasing the volume percent of H2O, demonstrates growing of tubes with bigger diameter. After creation of the pores (horizontal growth), the current density increases again and reach a maximum value. Increasing the water content of the electrolyte leads to less viscosity of the electrolyte and according to Stokes–Einstein relation results in more diffusion coefficient of OH and F and finally higher dissolution rate of the oxide [45]. This effect along with the decreasing IR drop within the electrolyte leads to more dissolution of the oxide in horizontal direction to have bigger diameter at higher amount of water content. 3.1.2. Effect of fluoride concentration of the electrolyte on the geometry of the grown TiO2 nanotubes by second anodization The effect of fluoride concentration of the electrolyte on the morphology of grown TiO2 nanotubes beneath protective layer is investigated by changing this parameter from 0.05 to 0.25 M. As can be seen from FE-SEM images in Fig. 5(a)–(e), at low concentration of fluoride ions we have a porous structure rather than tubular structure. As it is discussed in the previous section there is a fluoride rich layer at the cell boundaries that is soluble with F and water. At low concentration of fluoride and water this layer does not solve and a porous structure can be seen on the surface. It is clear from Fig. 5(a)–(e) that increasing of the F concentration leads to TiO2 nanotubes with thinner
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walls that can be ascribed to the more etching of the inner side of the tubes wall by fluoride ions. Also, it can be observed in the insets of from Fig. 5(d) and (e) that the existence of the protective layer is not effective against chemical etching of the tubes top at F concentrations higher than 0.15 M and nanograsses are created on the surface. As the Fluoride concentration increases, the length of the TiO2 nanotube arrays increases to 30 mm at 0.15 M fluoride concentration and after that fast rate of chemical etching of the tubes top leads to decrement of the tubes length. However the higher concentration of F ions leads to the increase of dissolution rate at the bottom of the tubes, but passes through a certain fluoride concentration (0.15 M) results in higher speed of etching of the tubes top in comparison to dissolution rate of the tubes bottom and thereby the tubes length decreases. The current density versus time curves in Fig. 5(f) demonstrate that the current density increases by increase of the F ions at the first times of anodization. But after passing a short time of anodization of about 800 s, the
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current decreases sharply and reach to a low stable state current density. This trend confirms that the increase of F ions is not effectine in growing long tubes. 3.1.3. Effect of the electrolyte temperature on the geometry of the grown TiO2 nanotubes by second anodization Fig. 6(a)–(d) shows cross-sectional FE-SEM images of grown TiO2 nanotubes at different electrolyte temperature from 5 to 35 1C and the insets show top-view of the grown tubes by second anodization. These images show that the diameter and the length of the tubes increase with increasing temperature. As can be seen from Fig. 6(f), the increasing of the electrolyte temperature from 5 to 35 1C leads to increment of the diameter of the tubes from 70 to 150 nm and increment of the length of the tubes from 5 to 50 mm. The fast growth of the tubes in 35 1C can be ascribed to the faster ions flow at high temperatures that leads to a faster anodization process and higher dissolution rate of the tubes bottom with respect to the etching of the tubes top. High stable state current density at 35 1C confirms the fast growth of the tubes at this condition,
Fig. 5. Top-view FE-SEM images of TiO2 nanotubes grown by second anodization and beneath protective layer (upper-layer) at different fluoride concentrations of the electrolyte: 0.05 (a), 0.10 (b), 0.15 (c), 0.20 (d) and 0.25 M (e), The insets in (d,e) are low magnification top-view of the tubes and the scale bar is 6 mm.The corresponding anodization current density versus time curves (f). The diameter and length of the tubes versus fluoride concentration of the electrolyte (g).
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Fig. 6. Cross-sectional FE-SEM images of TiO2 nanotubes grown by second anodization and beneath protective layer at 80 V and different temperatures of 5 (a), 15 (b), 25 (c) and 35 1C (d). The insets show top-view FE-SEM images. Corresponding anodization current density versus time curves (e). The diameter and length of the tubes versus electrolyte temperatures from 5 to 35 1C (f).
Fig. 6(e). Top-view FE-SEM images of the grown tubes at 35 1C, inset in Fig. 6(d), show that the tubes top are bundled but no evident of chemical etching of the tubes top resulted in creation of nanograsses cannot be seen. It can be concluded that the faster movement of the F ions in higher temperatures is effective in growing the long tubes but higher concentration of the F ions is destructive.
3.2. Fabrication of dye-sensitized solar cells The detached protective layer with length of 20 mm was transferred on FTO coated glass with 2 mm nanoparticle paste by a doctor-blade method and was used in front-side illuminated DSSCs. Also, in order to investigate the effect of diameter size of TiO2 nanotubes on the efficiency of
back-side illuminated DSSCs, three sublayers consisted of TiO2 nanotube arrays with diameter of 60, 100 and 140 nm and length of about 20 mm were selected. All the photoanodes were annealed at 500 1C for 1 h with heating and cooling rate of 30 1C min 1 to be crystallized and then assembled to solar cells. XRD patterns of these photoanodes, Fig. 7(a) and (b), show a pure anatase phase in all cases, independent of TiO2 nanotubes substrates. However the sharper peaks in the XRD pattern of the transferred TiO2 nanotubes on FTO glass indicate higher crystallinity of the tubes at this configuration. Fig. 7(c) and (d) demonstrates J–V curves of the DSSCs and their corresponding photovoltaic characteristics under simulated AM 1.5 (100 mW cm 2). Clearly, the cell efficiency is markedly affected by the illumination side, so that, a significant enhancement of the photovoltaic characteristics is seen when a front-side illumination is
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Fig. 7. XRD patterns of (a) annealed TiO2 nanotubes on Ti foil with different diameters of 60, 100, 140 nm and (b) annealed TiO2 nanotubes on FTO glass coated by 2 mm nanoparticle paste at 500 1C with heating and cooling rate of 30 1C min 1. (c) J–V curves and (d) summary of photovoltaic characteristics of different DSSCs under simulated AM 1.5. (e) The IPCE measurement of front-side illuminated DSSC. (f) Transport time of electrons measured by IMPS on the different DSSCs.
occurred. Particularly, the solar light conversion efficiency is 8.66% in front-side illumination that is improved 64% from back-side illumination. The high current density of 18.58 mA cm 2 is well in line with IPCE measurement, Fig. 7(e). As can be seen from this figure, in some regions, about of 90% of incident photons to the cell convert to
electricity at this configuration. The first reason for this improvement can be ascribed to the absorbance of sunlight by electrolyte or its reflecting by Pt-coated counterelectrode in back-side illuminated DSSCs. Also, it is clear from this figure that DSSC consisted of TiO2 nanotube array with diameter of 60 nm shows the
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highest conversion efficiency up to 5.27% among back-side illuminated DSSCs and the conversion efficiency is decreasing with the increment of diameter. An effective factor that strongly affects on the photovoltaic performance of the DSSCs is the dye-loading. The larger surface area of the photoanode chemisorbs the larger amount of dye molecules that leads to higher light harvesting efficiency and thus improvement of the cell efficiency. The summarized data in Fig. 7(d) confirm this relationship between the dye-loading of the structure and solar light conversion efficiency. As the transferred TiO2 nanotubes on FTO glass have the highest surface area due to the existence of TiO2 nanoparticles beneath the tubes, this configuration has the highest solar light conversion efficiency and among the tubes on the Ti foils, surface area is increasing with decrement of tubes diameter and thus the tubes array with the smallest diameter of 60 nm has the highest efficiency. To better understand the effect of the illumination side and tubes diameter on the transport time of the electrons and thus the performance of DSSCs, the IMPS measurements are performed, Fig. 7(f). Data in Fig. 7(f) show that a front-side illumination configuration strongly improves the transport time of the electrons and transferred tube on FTO glass show faster electron transport properties in comparison to the tubes on the Ti foil. 4. Conclusions In the present work, double-layer TiO2 nanotube arrays have been fabricated by a two-step anodization method. The upper-layer with length of about 10 mm has been fabricated by anodization of Ti foil at 60 V in an ethylene glycol electrolyte consisted of 0.15 M NH4F and 3 vol% DI water at room temperature. This layer has been annealed at 350 1C for 1 h to be crystallized and then exposed to second anodization for 40 min at different conditions. At this way, the second layer TiO2 nanotube arrays have been fabricated beneath the first one with variety of lengths and diameters. The longest open-top TiO2 nanotube array with length of about 50 mm has been achieved with second anodization at 80 V in the above mentioned electrolyte and electrolyte temperature of 35 1C. The upper-layer has been detached at the end of anodization by a chemical method. The detached membrane has been used in front-side illuminated DSSCs with a high efficiency up to 8.66%. Also, the remained TiO2 nanotubes on the Ti foils with different diameters have been used in back-side illuminated DSSCs. It has been shown that the smallest diameter leads to the highest efficiency at this case. References [1] F. Mohammadpour, M. Moradi, K. Lee, G. Cha, S. So, A. Kahnt, D. M. Guldi, M. Altomare, P. Schmuki, J. Chem Commun. 51 (2015) 1631–1634. [2] F. Mohammadpour, M. Moradi, G. Cha, S. So, K. Lee, M. Altomare, P. Schmuki, J. ChemElectroChem 2 (2015) 204–207. [3] X. Liu, J. Lin, Y.H. Tsang, X. Chen, P. Hing, H. Huang, J. Alloy. Compd. 607 (2014) 50–53.
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