TiO2 Wedgy Nanotubes Array Flims for Photovoltaic Enhancement

Applied Surface Science 257 (2011) 5059–5063

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TiO2 Wedgy Nanotubes Array Flims for Photovoltaic Enhancement Hao Pan a , Jieshu Qian b , Ang Yu a , Meigui Xu a , Luo Tu a , Qingli Chai a , Xingfu Zhou a,∗ a b

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, PR China Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6

a r t i c l e

i n f o

Article history: Received 17 November 2010 Received in revised form 4 January 2011 Accepted 4 January 2011 Available online 11 January 2011 Keywords: TiO2 wedgy nanotubes Dye-sensitized solar cell Light-scattering

a b s t r a c t In this study, TiO2 wedgy nanotubes with rectangular cross-sections were fabricated on transparent conductive substrates by using TiO2 nanorods as the precursor via the anisotropic etching route. TiO2 nanotubes with V-shaped hollow structure and the special crystal plane exposed on the tube wall possess nature of high surface area for more dye molecules absorption, and the strong light scattering effects and dual-channel for effective electron transport of the TiO2 V-shaped nanotubes based dye-sensitized solar cell exhibit a remarkable photovoltaic enhancement compared with the TiO2 nanorods. The photoanode based on our V-shaped TiO2 nanotubes with a length of 1.5 ␮m show a 123% increase of the dye loading and a 182% improvement in the overall conversion efficiency when compared with 4 ␮m rutile TiO2 nanorods photoanode. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Dye-sensitized solar cells (DSSCs) have attracted significant attention due to their low cost compared to commercial solar cells and their high efficiency since the seminal work of Grätzel and coworkers [1]. Semiconductor metal oxides such as TiO2 [2] and ZnO [3,4], which are frequently used as the photoanode materials in DSSCs, are of great interesting in the past several years. TiO2 has higher affinity to organic species than ZnO, thus, the performance of DSSCs made from TiO2 are better than that from ZnO [5,6]. As a result, a lot of efforts have been made to build titania with different nanostructures in order to improve its performance as DSSCs’ photoanode material. Nowadays, people have been able to prepare nanoscaled TiO2 such as nanowires (NWs) [7], nanorods (NRs) [8,9] and nanotubes (NTs) [10,11]. Various routes including the sol–gel process [12], hydrothermal method [13,14] and oxidation process [15] have been used to fabricate titania nanostructures. Among those nanoscaled TiO2 with different structures, titania NTs are promising for improving the performance of DSSCs because they can provide high specific surface area and dual-channel for electrons transportation from internal surface to electrode [11,16], namely, the electrons from the photo-excited dye which cover both the interior and exterior walls of the NT transfer to substrate by these two interior and exterior channels. Hydrothermal preparation of high-purity TiO2 NTs in concentrated aqueous NaOH solution suggests that the nanotubes are formed by scrolling of an exfoliated lamellar titanate nanosheet with a spiral cross sec-

tion [13]. The electrochemical anodic oxidation method is able to fabricate vertically oriented and highly ordered TiO2 NT arrays on titanium foil suitable for electrode material of DSSCs [10,17]. TiO2 NT arrays have been shown to be able to grow on any Ti substrate independent of its geometry via electrochemical anodization [18]. Liu et al. [19] used the electrochemical anodization method to synthesize TiO2 NT arrays around a titanium wire firmly, the obtained assembled dye-sensitized photovoltaic wires showed a relative good performance. However, the TiO2 NTs reported so far are in cylindrical shape, the as-made TiO2 NT arrays prepared by electrochemical anodization are amorphous and normally needs post-treatment of calcination to induce TiO2 crystallization [17,20]. Study indicates that rutile TiO2 has better chemical stability and higher refractive index by comparison with anatase TiO2 in DSSCs [5,21]. In this study, we prepared crystalline rutile TiO2 wedgy NTs with rectangular cross-sections on transparent conductive fluorine-doped tin oxide (FTO, 15  per square, Nippon Sheet Glass, Japan) substrates by hydrothermal process similar to our previous report [22]. Compared to traditional TiO2 nanoparticle films, TiO2 NT films have a tapered inner tubular and crystalline one-dimension structures, which enhance light scattering and charge-collection efficiencies [23]. Moreover, our results indicates that rutile TiO2 wedgy NT has better photovoltaic performance than NR based DSSC. 2. Experimental 2.1. Sample preparation

∗ Corresponding author. E-mail address: [email protected] (X. F. Zhou). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.01.021

All chemicals are analytical-grade regents without further purification. There are two steps in a typical synthesis. Step one:

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Fig. 1. SEM images and XRD patterns of TiO2 NR and wedgy NT films grown on FTO substrates. Top view (A) and crosssectional view (B) of NRs, (C) top view of NTs, inserts in (A) and (C) are magnified images. (D) XRD patterns of NRs (a) and NTs (b).

Growth of TiO2 NRs on FTO substrate via hydrothermal method which is similar to that as reported in Ref. [5]. 1 mL of tetrabutyl titanate was added into 40 mL 6 M hydrochloric acid solution and stirred magnetically for about 5 min. After staying at the room temperature for 2 min, the mixed solution was transferred to an 80 mL Teflon-lined autoclave with a piece of FTO substrate on the bottom, the FTO substrate has been ultrasonically cleaned with anhydrous ethanol and deionized water, respectively. The autoclave was heated at 403–453 K (exact temperature) for 10–20 h (exact time) in an electric oven. After synthesis, the autoclave was cooled to room temperature in air. In this step, we obtained TiO2 NR film on FTO substrate. Step two: Preparation of TiO2 wedgy NTs by chemical etching. The as-prepared TiO2 NR film were immersioned into a mixture of 5–20 mL deionized water and 5–20 mL hydrochloric acid (37 wt%), the solution was transferred into eight 80 mL Teflon-lined autoclave and maintained at 403–453 K for 10 h in an electric oven. The obtained TiO2 wedgy NTs were washed with anhydrous ethanol three times and dried at 353 K in air for 30 min. 2.2. Characterization The crystal structure of the product was investigated by powder X-ray diffraction (XRD) measurements on a Bruker-D8Advance Xray diffractometer, with graphite monochromatized high-intensity ˚ at 40 kV and 40 mA. The XRD patCu K␣ radiation ( = 1.5418 A) terns were recorded at angle of 2 value from 10◦ to 80◦ with a scanning step of 0.05◦ at a counting time of 0.2 s per step. The morphology of the samples were observed by scanning electron

microscopy (SEM, FEI, Quanta200), Field Emission scanning electron microscopy (FESEM, HITACHI S-4800), and high-resolution transmission electron microscopy on a JEOL JEM-2010UHR instrument at an acceleration voltage of 200 kV, the sample of TiO2 NTs was separated from the FTO substrate by ultrasonic treatment and pasted on a copper film. 2.3. DSSCs assembly For comparison, we analyzed the I–V measurements of DSSCs whose photo-anodes were composed of TiO2 NR and NT films, respectively. The TiO2 NR and NT films were firstly treated with 40 mM TiCl4 aqueous solution and annealed in air at 723 K for 30 min, then they were stained by 0.3 mM ethanolic solution of dye N719 (N719 = cis-bis(isothiocyanato)bis(2,2 -bipyrridyl-4-4 dicarboxylato)-ruthenium(II)bis-tetrabutylammonium) overnight at room temperature to complete the dye adsorption. After staining, the TiO2 NR and NT films were rinsed with absolute ethanol to remove the physically adsorbed dye molecules. The commercially available platinum-coated FTO substrates were used as counter electrode. The electrolyte solution contains 0.6 M 1-butyl-3-methyl imidazolium iodide, 0.03 M I2 , 0.10 M guanidinium thiocyanate and 0.5 M 4-tertbutylpyridine in a mixture of acetonitrile and valeronitrile (85:15 v/v), the electrolyte solution was introduced into the space between the electrodes by capillary force. The photocurrent–voltage experiments were performed under AM 1.5G solar simulator as light source with an illumination intensity of 100 mW/cm2 . The active area of the cell was typically 0.2 cm2 . The

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Fig. 2. TEM images of TiO2 wedgy NT films (A), single wedgy NT (B), UV–vis absorption spectra of sensitized wedgy NT (a) and NR (b) films, (C) and J–V characteristics of DSSCs based on NT (a) and NR (b) films (D).

current density–voltage (I–V) response of the devices was recorded using an electrochemical analyzer (CHI 660C Instruments). 3. Results and discussions After the first step of hydrothermal treatment, we obtained TiO2 NR film. Fig. 1A is a typical SEM image of the top view of the TiO2 NR film, which shows uniformly oriented rod-like structure, we can see that the NRs cover the entire surface of the substrate. The sideview in Fig. 1B shows that the densely packing NRs can reach about 4 ␮m in height and are vertical growing from the FTO substrate. The second hydrothermal treatment steps etched the TiO2 NR from the top, leaving wedgy TiO2 nanotubes, as shown in Fig. 1C, the FESEM image clearly shows that all the NRs are converted into tubular structures with square cross section. Fig. 1D presents the XRD patterns of the TiO2 NR and NT films, confirming that both the NRs and NTs possess the tetragonal rutile crystalline conformation (JCPDS ˚ From the XRD patterns, no. 88–1175, a = b = 4.517 A˚ and c = 2.940 A). we can see that the (0 0 2) diffraction peak of the NTs was significantly lower than that of NRs while the (1 0 1) diffraction peak was increased when NRs were converted to NTs. The [0 0 1] direction is the preferential growth direction of the rutile TiO2 NRs [8,21], the formation mechanism of rutile TiO2 wedgy NTs was proposed as the HCl preferentially etched the TiO2 NRs in the [0 0 1] growth direction which parallels to the NTs [22].

TEM images of the TiO2 NTs are given in Fig. 2. We know that the hollow part of TiO2 NT is a V-shape and the length of NT film has shortened down to ∼1.5 ␮m from the carefully observation of Fig. 2A. Fig. 2B indicates that the width of the nanotube is about 200 nm with wall thickness of 20–40 nm, the maximum and minimum inner width of V-shaped NT are ∼180 nm and ∼60 nm, respectively. After both the NRs and NTs are stained by dye molecules, we measured the UV–vis absorption spectra of them, the results are shown in Fig. 2C. We observe that there is an obvious increase of absorbance in the wavelength range from 400 to 800 nm when the NRs are converted to NTs. The increase of light absorbance at 400–700 nm comes from the increase amount of dye that NTs can absorb, because NTs have both interior and exterior wall for dye molecules absorption. To show the performance of DSSCs based on these materials, we present the I–V characteristics of NRs and NTs based DSSCs in Fig. 2D. The experiments were carried under the same condition of AM 1.5 (100 mW/cm2 ) illumination using a 450 W Xenon lamp. DSSCs with an active area of 0.2 cm2 were tested. Under the same condition of 100 mW/cm2 illumination, the open circuit voltages and the short circuit currents of the DSSCs with different photo-anodes have a significant difference. Table 1 summarized the characterized results of DSSCs based on NT and NR films. For the TiO2 wedgy NTs based DSSC, Jsc and Voc are found to be 7.37 mA/cm2 and 0.73 V, respectively, and the  is 2.62%, while the TiO2 NRs

Table 1 Characterized results of DSSCs based on NT and NR films. Samples

Jsc (mA/cm2 )

Voc (mV)

FF (%)

 (%)

Thickness (␮m)

Absorbed dye (×10−8 mol/cm2 )a

NTs NRs

7.37 4.08

730 670

49 34

2.62 0.93

1.5 4

2.9 1.3

a

A 0.1 M NaOH solution in water and ethanol (50:50, v/v) and dye-adsorbed films with a dimension of 1 cm2 were used for estimating the adsorbed dye concentration.

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Fig. 3. Schematic of dye-sensitized NTs solar cell and the transfer-path of light in TiO2 wedgy NT.

based DSSC, Jsc and Voc are 4.08 mA/cm2 and 0.67 V, respectively, and the  is 0.93%. The dye loading amounts of TiO2 NR and NT films are 1.32 × 10−8 mol/cm2 and 2.94 × 10−8 mol/cm2 , respectively. The photoanode based on our TiO2 V-shaped TiO2 nanotubes with a length of 1.5 ␮m show a 123% increase of the dye loading and a 182% improvement in the overall conversion efficiency when compared with 4 ␮m rutile TiO2 nanorods photoanode. The higher performance of TiO2 wedgy NTs based DSSC can be attributed to the higher specific surface area of nanotube and the special (1 0 0) crystal plane exposed on the tube wall for dye absorbtion and the effective light scattering effects and the dual-channel for effective electron transport, which will be discussed in details as follows. Regarding to the photovoltaic enhancement of TiO2 wedgy NT array films, the higher specific surface area of TiO2 wedgy NTs for dye absorbtion is the main factor. Furthermore, the special V-shape hollow structure of TiO2 NTs has a stronger light scattering effects, which is illustrated in Fig. 3 and it brings about light absorbance of NT film increased at 700–800 nm (Fig. 2C). Study show the V-shape hollow structured waveguides materials can stop or slow the light and trap the rainbow [24], if the V-shape hollow structure of TiO2 NTs is well-designed according to the theory of trapping the rainbow [25], we can trap the light more effectively and obtain a higher photovoltaic performance materials. The fact that NTs have dualchannel for electrons transportation also contributes to the increase of photovoltaic performance, all these characteristics shows that the NTs based DSSCs might show prominent performance than that of TiO2 NRs. The dual-channel of TiO2 wedgy NTs for effective electron transporting hold back the photogenerated carrier recombination, and the single crystal dual wall of TiO2 wedgy NTs provide the shorter electron transport length, thus also increases the open circuit voltage and the short circuit currents. However, the bad back-contact of the TiO2 films with the FTO glass bring the lower FF, and the  of the DSSCs with two different morphology photo-anodes are relatively low. As is known to all, electron transport in DSSCs are affected by the morphology, thickness, surface state of the TiO2 films [26]. Many literatures have demonstrated that under a certain thickness, the thicker TiO2 film bring the better performace of the DSSC [18,23,27]. In our work, the thickness of TiO2 films are only ∼4 ␮m (NRs) and ∼1.5 ␮m (NTs), but the shorter TiO2 wedgy NTs based DSSC show a better performance than NRs based DSSC, which sufficiently demonstrated that the photovoltaic property of TiO2 wedgy NT is a better candidate for the DSSC, and the higher efficiencies could be expected with a thicker and well-designed films.

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