Open-top TiO2 nanotube arrays with enhanced ...

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Open-top TiO2 nanotube arrays with enhanced photovoltaic and photochemical performances via a micromechanical cleavage approach† Yulong Liao,*a Dainan Zhang,b Qi Wang,a Tianlong Wen,a Lijun Jia,a Zhiyong Zhong,a Feiming Bai,a Longhuang Tang,a Wenxiu Quec and Huaiwu Zhang*a Anodic growth of TiO2 nanotube (NT) arrays has been proved to be very promising for energy conversion applications, e.g. in photovoltaic devices and fuel cells. However, disordered “nano-grass” layers were always found on the top of the anodic TiO2 NT arrays. In this paper, we demonstrate a novel and simple method using a micromechanical cleavage technique to peel off the disordered nanograss layer. Using

Received 17th April 2015 Accepted 29th May 2015

this method, 1 1.5 cm2 of uncapped TiO2 NT arrays with a high-aspect ratio can be easily obtained. The results further indicate that the treatment can improve the photovoltaic and photochemical performances. After the treatment, the conversion efficiency (h) of the dye sensitized solar cells (DSSCs)

DOI: 10.1039/c5ta02799c

increased by 29.3%. This work facilitates the growth and applications of high aspect-ratio anodic TiO2

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NT arrays in related devices and systems.

Introduction Since Grimes reported the preparation of TiO2 NT arrays by electrochemical anodic oxidation in 2001, their growth has attracted tremendous interest in recent years due to their unique properties as a result of their tailored geometries and multidimensional structures.1–4 Self-organized TiO2 NT arrays are found to be very promising for applications in the preparation of dye-sensitized solar cells (DSSCs),5,6 fuel cells,7,8 sensors,9–11 photocatalysts12–14 and so forth. Until now, three generations of anodic TiO2 NT arrays have been developed. In the rst generation, a dilute solution of the hydrouoric (HF) acid electrolyte was used and the thickness of TiO2 NT arrays was limited up to 0.5 mm due to the strong causticity of HF. With the replacement of HF acid with less aggressive uoride salts (like NH4F) and the replacement of water with organic electrolytes (like ethylene glycol), the growth of TiO2 NT arrays steps into the second and third generations, respectively.3,15 Well-aligned TiO2 NT arrays with a tunable diameter and length can now be grown in uoride electrolytes. Nevertheless, a typical anodic TiO2 NT lm grown by conventional methods is usually covered by a a

State Key Laboratory of Electronic Thin Film and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China. E-mail: [email protected]; [email protected]

b

Department of Electrical and Computer Engineering, University of Delaware, Newark, Delaware 19716, USA

c School of Electronic and Information Engineering, Xi'an Jiaotong University, Xi'an 710049, Shaanxi, China

† Electronic supplementary 10.1039/c5ta02799c

information

(ESI)

This journal is © The Royal Society of Chemistry 2015

available.

See

DOI:

grassed top layer, which is referred to as nanograss thereaer.16–18 Disordered nanograss cap layers are oen observed for high-aspect ratio TiO2 NTs, which require F rich electrolytes, a high applied voltage and/or long duration of anodization.19 The nanograss cap layer can signicantly reduce the efficiency of the charge transfer and/or transport. Well-aligned TiO2 NT arrays with wide open top are highly desirable for electron-transport applications. To obtain top-open TiO2 NT arrays, several approaches have been used, including ultrasonication,20,21 polishing,22 resistant layer techniques,23 double anodization or multiple-step anodization.24,25 Well aligned TiO2 NT arrays with open top could be obtained by the above approaches, and they have been used in photocatalysis and photovoltaic applications with improved performances. Nevertheless, these methods generally need cautious control of experimental conditions, and it cannot guarantee complete removal of the nanograss layer. For example, excessive ultrasonication power or time would affect the contact between the TiO2 NT arrays and the substrate, deteriorating the electron transport at the interface.1,15 It is still a challenge to prepare top-open TiO2 NT arrays with a high aspect-ratio using a facile and low cost method. Inspired by monolayer graphene fabrication using a micromechanical cleavage technique,26 we report a novel post-treatment method to exfoliate the disordered nanograss top-layer from anodic TiO2 NT arrays. The micromechanical cleavage technique is also called a “scotch tape technique”. Here, we will demonstrate how to use the simple scotch tape technique to peel off the nanograss layer and its effect on the efficiency of the anodic TiO2 NT array based DSSCs or the photocatalysis reaction.

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Results and discussion Fig. 1(a), (c) and (e) show the side and top views of the asgrown anodic TiO2 NT arrays on Ti substrates, respectively. Aer 3 h anodization, TiO2 NTs with a thickness of 20 mm are formed, which are well aligned in highly ordered arrays. The wall of the TiO2 NT is very smooth with outer and inner diameters of 110 nm and 90 nm, respectively (see Fig. S1 and S2 in the ESI†). Further, the diameter and length of anodic TiO2 NTs can be nely tailored by adjusting anodization protocols. As shown in our previous studies, we can precisely control the length of a tube from 10 to 100 mm,27 making it adaptable for broad applications such as DSSCs, sensors, batteries, photocatalysis. However, a disordered “nanograss” cap layer can be clearly observed on the top of the as-prepared anodic TiO2 NT arrays, as observed from the side in Fig. 1(a) and top in Fig. 1(c) and (e). These nano-grasses formed due to over etching of the electrolyte, especially for TiO2 NTs with a high aspect-ratio.15 Fig. 1(e) shows details of the NTs in the high magnication SEM image. As shown in Fig. 1(c), the nanograsses were randomly bundled together, burying the underneath TiO2 NT layer. Fig. 1(b), (d) and (f) show the morphologies of the anodic TiO2 NTs aer the “scotch tape technique” treatment which is schematically shown in Fig. 2. Nanograss layers have been successfully removed from the top of the arrays, as observed from the side in Fig. 1(b). The treated TiO2 NT arrays are still

Fig. 1 SEM images of the side view of anodic TiO2 NT arrays (a) before and (b) after peeling off the nanograss layer, and SEM images of the top view of the anodic TiO2 NT arrays (c and e) before and (d and f) after peeling off the nanograss layer, indicating that the disordered nanograss top layer was successfully removed.

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Schematic process of lifting off the disordered nanograss layer from the highly ordered TiO2 NT arrays by using the micromechanical cleavage technique (also called the “scotch tape technique”). Fig. 2

highly ordered and perpendicular to the Ti substrate. Further, no obvious change in the thickness of the TiO2 NT arrays was observed (see Fig. S3 in the ESI†), suggesting that the nanograsses have been attached to the scotch tape and removed from the NT arrays (see Fig. S4 in the ESI†). This was conrmed by SEM observations of the residual on the scotch tape (see Fig. 3), and we can only nd nanograsses le on the scotch tape aer peeling from the as-prepared TiO2 NT arrays lm. Fig. 1(d) and (f) show the top view of the uncapped TiO2 NT arrays, conrming complete removal of previously bundled nano-grasses. The diameter of the nanotube is now directly estimated to be 110 nm by observing the exposed NTs shown in Fig. 1(f), which is consistent with the TEM observations (Fig. S2†). A high aspect-ratio of 200 has been successfully achieved. Fig. 4(a) shows a photograph of one partially treated sample, where the le (right) part is the untreated (treated) area. The untreated area shows grey color due to the diffuse reection

FESEM image of the nanograsses left on the scotch tape after the peeling off process.

Fig. 3


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Fig. 4 (a) Photograph of partially treated anodic TiO2 NT arrays, (b) a magnified SEM image showing the boundary of treated and untreated areas, and (c) a typical SEM image of large area top-open TiO2 NT arrays, obtained by lifting off the nanograss top layer.

from the disordered nanograss top-layer, while the treated area turns into dark yellow. The boundary between the treated and untreated areas can be easily discerned by the naked eye. The boundary was imaged by SEM and shown in Fig. 4(b), which clearly shows the nanograss-top-layer/nanotube-under-layer interface. Fig. 4(c) shows the top view of the uncapped TiO2 NTs on a large scale. No nanograss residual and other remnants were found on the top of the NTs. The weak bonding between the nanograss top-layer and TiO2 NT bottom layer is the critical factor that allows the successful exfoliation of the disordered top-layer using a scotch tape.21 This approach is highly reliable and efficient, which can be used to easily prepare top-open TiO2 NT arrays with an area of 1 1.5 cm2 (see Fig. S4 in the ESI†). Meanwhile, this scotch tape technique is found to be quite easy to repeat; in order to show the facility of this approach, we patterned the TiO2 nanotube array lms with stripes and rectangular arrays, as shown in Fig. 5. Thanks to this simple method, anodic TiO2 NTs with open top and a high aspect ratio can be much easily obtained in contrast to its predecessor.

Fig. 5 Patterned TiO2 NT arrays films by using the scotch tape technique.



To examine the effect of the treatment on the performances of devices, the anodic TiO2 NT arrays were assembled into dye sensitized solar cells (DSSCs). Since the metal Ti substrate is opaque, a back-illuminated DSSC structure was used in this study, as shown in Fig. 6(a). Prior to the cell assembly, the TiO2 NT arrays were sintered at 450 C for 2 h to induce crystallinity. The X-ray diffraction indicates that the TiO2 NT arrays were pure anatase (see Fig. S5 in the ESI†). It should be noted there that amorphousness is also preferred in certain applications.28 The performance of DSSCs fabricated with capped and uncapped anodic TiO2 NT arrays is compared. Fig. 6(a) shows the I–V curves of the assembled DSSCs under AM 1.5 sunlight illumination (100 mW cm2), and the detailed photovoltaic parameters are summarized in the Table 1. For capped TiO2 NT arrays, short-circuit current density (JSC), open-circuit voltage (VOC) and energy conversion efficiency (h) are 12.6 mA cm2, 643 mV, and 3.31, respectively. For uncapped TiO2 NT arrays, the DSSCs show an enhanced performance. JSC and h increase to 13.7 mA cm2 and 4.28, respectively. Noticed that VOC and the ll factor have small discrepancies, the enhancement of h (29.3%) could be mainly attributed to the variation of the increased JSC. VOC can be expressed by the following equation: VOC ¼

EF;TiO2 EF;redox q

(1)

where EF,TiO2 is the electro-Fermi level under illumination, and EF,redox is the redox level of the electrolyte. JSC can be given by JSC ¼ ekc,rednscox

(2)

where ns is the electron density at the surface and cox is the concentration of the oxidized species of the redox system.29 According to eqn (1), the VOC is determined by using the intrinsic properties of the materials, well explaining the small variation of VOC. JSC is proportional to the ns and cox, which is directly related to the incident photons and the concentration of the oxidized species in the redox system. Since the micromechanical exfoliation treatment has a negligible effect on the length of the NTs (see Fig. 1(a) and (b)), the removal of the cap layer of TiO2 NT arrays should be responsible for the enhanced photovoltaic performance. It has been demonstrated that the nanograss layers have a crucial role in the charge carrier transportation and recombination performances.15,19 In addition, the nanograss layer could also affect the performance of the DSSCs by two additional reasons. First, the nanograss layer would cause light scattering and decrease the absorbed photons. For a back-illuminated DSSC structure (see Fig. S6, ESI†), the incident light rstly interact with the nanograss toplayer before penetrating the underneath TiO2 NT arrays. The nanograss layer reduces the numbers of photons reaching the TiO2 NTs. As a result, less hot electrons were produced, leading to decreased ns. Second, the mouth of the TiO2 NTs is blocked by the randomly bundled nanograsses (see Fig. 1(c), (e) and 4(b)), which would adversely affect the diffusion of redox species, leading to a decrease of cox. In contrast, uncapped TiO2 NT arrays benet the diffusion of redox species and lead to an

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Fig. 6 (a) J–V curves of the DSSCs based on the capped and uncapped anodic TiO2 NT arrays, where the inset shows the geometry of a back illuminated DSSC and (b) photocatalytic degradation kinetics of MO by using the anodic TiO2 NT arrays before and after lifting off the nanograss layer as catalysis panels.

increase of ns and cox. According to eqn (2), both increased ns and cox would result in an enhanced photocurrent and energy conversion efficiency (h). Incident photo-to-current conversion efficiency (IPCE) results, as shown in Fig. 7(a), further reveal that a higher efficiency in the conversion of UV light (200) have been successfully obtained by simply liing off the scotch tape. This approach was found to be very reliable and efficient, allowing the preparation of >1 1.5 cm2 uncapped TiO2 NT arrays. Both DSSCs and photocatalysis show improvement by using uncapped TiO2 NT arrays due to the removal of disorder nanograsses. Our work facilitates the growth and applications of high aspect-ratio anodic TiO2 NT arrays in many devices and areas.

Experimental Micromechanical exfoliation of “nanograss” from anodic TiO2 NT arrays TiO2 NT arrays were synthesized by electrochemical oxidation of titanium sheets as we described previously.32,33 Briey, the titanium sheets were oxidized at 50 V at 7 C with a common electrolyte recipe (ethylene glycol containing 0.25 wt% NH4F and 1.0 wt% deionized (DI) water). Aer 3 h anodization, TiO2 NT array lms were obtained. In order to remove the disordered nanograss from the top of the anodic TiO2 NT arrays, a scotch tape (basically any type) was gently attached and then peeled off from the surface of the as-prepared lm. The nanograss will be attached to the scotch tape during peeling, leaving uncapped TiO2 NT arrays on the metal Ti substrate. The exfoliation treatment process is schematically illustrated in Fig. 2. The treated lm immediately turned dark yellow (see Fig. S3 in the ESI†). All the reagents were of analytical grade (Sinopharm Group Chemical Reagent Co. Ltd., China).


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Cell assembly and characterization DSSCs based on the micromechanically exfoliated anodic TiO2 NT arrays were assembled. Before the cell assembly, the anodic TiO2 NT arrays were sintered at 450 C for 2 h at a heating rate of 2 C min. The sintered TiO2 NT arrays were dye sensitized by soaking in a 0.5 mM absolute ethanol solution of the ruthenium complex Ru[LL0 -(NCS)2](N-719) for 24 h at room temperature. Pt sputtered FTO substrates were used as the counter electrode. The process to assemble DSSCs is similar to those reported previously.34 The electrolytes consisted of 0.5 M LiI, 0.05 M I2, and 0.5 M tertbutylpyridine in acetonitrile. Average photovoltaic data were obtained by studying ve effective devices for each type of studied DSSCs. During the assembly process, the active area of the resulting cell exposed to light was approximately 0.25 cm2 (0.5 0.5 cm). The incident photonto-electron conversion efficiency (IPCE) of the electrodes was measured at 0.2 V vs. Ag/AgCl in 1 M KOH (pH ¼ 13.6) by using a QE/IPCE measurement system (Newport, USA).

Acknowledgements This work was nancially supported by the National Basic Research Program of China under Grant no. 2012CB933104, National Nature Science Foundation of China under Grant no. 61021061 and no. 61271037, International Cooperation Projects under Grant no. 2013HH0003 and no. 2012DFR10730, and 111Project under Grant no. B13042.

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