Introduction

The fabrication of TiO₂ nanotube arrays directly on substrates are highly sought after in applications such as sensing,^1-3 photovoltaics,^4-7 and photocatalysis.^8-10 For gas-sensing applications, TiO₂ nanotube arrays, with wall thicknesses in order of their Debye length, show extreme changes in electrical resistance on exposure to hydrogen.¹ Their morphology and wall thickness are considered the main factors for this extreme conductance response. For photovoltaics, highly ordered transparent TiO₂ nanotube arrays are found to have a superior electron lifetime in dye-sensitized solar cells due to improved charge separation and transport by highly ordered architectures.^6,7 Last but not least, TiO₂ nanotube arrays, as photocatalysts, are able to degrade a wide range of substances with enhanced performances.^8-10 All these applications demand the ability to prepare highly ordered TiO₂ nanotube arrays with controllable dimensions grown on substrates.

Several methods such as hydrothermal synthesis,^11-13 anodization of Ti sheets,^1,14-17 template-assisted growth,^18-20 and seeded growth²¹ have been used to fabricate TiO₂ nanotubes and nanotube arrays. Among these, the porous anodic alumina (PAA)-assisted approach distinguishes itself from others as a simple and well-controlled method.^18-20 This nanoporous template can be easily prepared by anodization of Al sheets at room temperature, and the removal of the template after fabrication of the nanotubes can be achieved by a gentle chemical etching.^22,23 The hexagonally packed nanopores provide not only templates to confined growth of nanotubes but also an ability to simultaneously arrange these nanotubes into a well-aligned, hexagonally packed array. The diameter and length of the nanotubes, as well as the distance between two neighboring nanotubes, can be well controlled by varying the corresponding dimensions of the template, which is realized by varying the anodization conditions. Recently, Sander et al.¹⁹ have prepared dense and well-aligned TiO₂ nanotube arrays on Si substrate by using atomic layer deposition (ALD) on a PAA template that was directly integrated on the substrate (PAA-substrate). This method shows a number of advantages over those previously reported. For example, the PAA-substrate method allows for direct growth of nanostructures on various substrates, which is of great practical importance in applications. Next, ALD enables conformal coating of dense and pinhole-free TiO₂ films over the template and allows precise control over the nanotubes' wall thickness at the angstrom scale. As such, well-aligned arrays of TiO₂ nanotubes with controllable dimensions are grown directly on substrates, which exhibit size-dependent optical adsorption shifts due to this precise wall-thickness control. For further device applications, we have recently prepared highly ordered and uniform TiO₂ nanotube arrays on glass using ALD on a "soft imprinted" template.²⁴ The transparent glass substrate allows for direct measurement of its size-dependent optical behaviors by UV-vis spectroscopy.

Compared with the method of using a free-standing PAA template as a mask for nanostructure fabrication, the PAA-substrate method has an advantage of having intimate contact between the template and the substrate, which allows for both vapor- and solution-based fabrication.^24-26 Nevertheless, in this method, anodization of Al films deposited on the substrate will contaminate the surfaces of the substrate and also alter its surface morphology. Because, at the end, anodization will continue to attack the substrate, resulting in electrolyte-relevant contaminations, modification of surface compositions, formation of anodized pores and pits on the underlying substrate, and even detachment of the template from the substrate, in particular on conductive substrates.^25,27,28 This anodization of the underlying substrate is highly undesirable in many applications that require unmodified surfaces.^29,30 In addition to the anodization on the substrate, to prepare highly ordered template on the substrate, special procedures are usually needed.^19,24 Conversely, fabrication using a free-standing PAA template attached on substrates will not cause any chemical contaminations and surface morphology modifications. Next, fabrication of a highly ordered free-standing PAA template is simple and well-established by two-step anodization of Al foil.^22,23 The free-standing PAA template has been widely used as a mask for nanostructure fabrication on various substrates via evaporation or etching.^30-32 In this study, we are interested in whether the free-standing PAA can be used as a template for fabricating TiO₂ nanotube arrays on various substrates via ALD. Unlike the PAA-substrate method in which the PAA is supported by the substrate, the method using a free-standing PAA template involves a transfer of a brittle and rigid template, which is very difficult to be handled during fabrication and leads to formation of voids between the template and the substrate. ALD is a non-line-of-sight film-coating technique that relies on a series of self-limiting surface reactions by alternating the exposures of two precursors on the surface.¹⁹ Therefore, the voids between the template and the substrate might not be filled by ALD, which will result in poor contact between the deposited nanotubes and the substrate and lead to collapse of the prepared TiO₂ nanotubes after template removal.

In this paper, we demonstrate the fabrication of dense, uniform, highly ordered, and well-aligned arrays of TiO₂ nanotubes on various substrates, over large areas, via ALD on free-standing PAA templates. In comparison with a PAA-substrate method, this method can easily fabricate highly ordered TiO₂ nanotube arrays on various substrates (Si, glass, and flexible polyimide substrates are demonstrated) without changing the surface morphology and producing chemical contaminations on the underlying substrate. The large-area free-standing PAA (>4.5 cm²) template can be easily transferred and attached onto the substrates due to a PMMA protective layer. The adhesion of the template and the substrate are improved by pretreating the substrate with mild O₂ plasma and applying acetone to the template after the attachment. The ALD TiO₂ nanotube arrays prepared by this free-standing PAA template also show intimate contact with the substrate and demonstrate size-dependent optical adsorption behaviors due to the precise wall-thickness control offered by the ALD technique.

Experimental Methods

Attaching Free-Standing PAA onto Substrates. Free-standing PAA templates were prepared by two-step anodization of high purity (99.999%) Al foils (Goodfellow Cambridge) as published previously.²² Briefly, the electrochemically polished Al foil was anodized in 0.3 M oxalic acid at room temperature for 6-10 h at 40 V. After removal of the as-prepared PAA film with a mixture of 3.5 vol % H₃PO₄ and 45 g/L CrO₃ at ~60 C, a second anodization at 2 C was performed. The desired template thickness depends on the anodization time, with a rate of ~40 nm/min. The pores of the as-prepared template were widened in 5 wt % H₃PO₄ for ~50 min before applying a protective poly(methyl methacrylate) (PMMA) layer. This layer was subject to drying at 120 C for ~30 min prior to separating the template from the Al foil with 0.1 M CuSO₄ and 10 wt % HCl. The barrier layer at the bottom of the template was etched in 5 wt % H₃PO₄ solution for another 30-40 min. The template was then carefully attached to three different substrates: p-type Si (100) with 50 nm SiO₂ layer, glass, and flexible polyimide (Dupont-Kapton) film. The substrates were pretreated with mild O₂ plasma (80 sccm O₂, 80 mTorr, 150 W) for ease of attaching PAA to them. The PMMA protective layer was removed by UV-ozone in a dry stripper (Samco UV-1) at 200 C for ~30 min. Finally, drops of acetone were added to the attached template to further reduce any voids.

ALD of TiO₂ Nanotube Arrays. Free-standing PAA templates attached on the substrates were deposited with TiO₂ films in a home-built ALD setup at room temperature. The substrates were alternately exposed to vapors of TiCl₄ (Merck, 99%) and deionized H₂O at a base pressure of 1 × 10^-3 Torr. Both precursors have a 2-s exposure time and a 30-s interval between the two exposures. Reactive ion etching (RIE, Oxford Plasmalab 80, 55 sccm CHF₃, and 5 sccm O₂) was employed to etch the TiO₂ overlay on top of the template. The nanotube arrays were then released from the template by immersion in 1 M KOH for ~15 min followed by a rinse in deionized water.

Characterization Methods. Field emission scanning electron microscopy (FESEM, JEOL-6700F) was used to characterize morphology of the free-standing PAA template and TiO₂ nanotube array. Individual TiO₂ nanotubes and cross-sectional arrays of as-deposited TiO₂ nanotubes in the free-standing PAA template were studied by high-resolution transmission electron microscopy (HRTEM, Philips CM300). The arrays of TiO₂ nanotubes were scrapped off from the Si substrate and dispersed in ethanol and onto a TEM grid. The cross-sectional TEM sample was prepared by bonding the top surfaces of 2 pieces of 3 mm × 1 mm samples with epoxy glue. The sample was cured and subsequently polished using graphite lapping film until about 10-50 m thick. The sample was dimpled and finally ion milled until transparent. X-ray diffractograms of the TiO₂ film on Si were measured with a Bruker D8 GADDS XRD. UV-vis spectrum for TiO₂ nanotube arrays on glass substrate were performed using a Shimadzu UV-3101PC instrument.

Results and Discussions

A schematic of the fabrication process is shown in Figure 1. A large-area free-standing PAA template supported by a PMMA layer is attached onto the substrate. Subsequently, the protective layer is removed to form a through-hole PAA template on the substrate for TiO₂ ALD. The contact between the template and the substrate is improved by pretreating the substrate with O₂ plasma and addition of acetone to the attached template. TiO₂ films are deposited via ALD on the template at room temperature. The TiO₂ overlay on top of the PAA template is dry etched by reactive ion etching, and the template is removed by wet chemical etch to finally release highly ordered TiO₂ nanotube arrays on the substrate.

Figure 1 Schematic of the process to fabricate highly ordered TiO2 nanotube arrays on substrates using ALD on the free-standing PAA template. Free-standing PAA supported with a PMMA layer was first attached onto the substrate. The PMMA was then removed for ALD on the PAA template. The TiO2 overlayer was etched by RIE, and finally highly ordered ALD TiO2 nanotube arrays were released from the PAA template.

Figure 2 demonstrates the versatility of our method by attaching the free-standing PAA template on various substrates. In the PAA-substrate method, various substrates show different anodization behaviors at the end of anodization, and great cares need to be taken to avoid formation of pores and pits on the substrate, or worse, template detachment from the substrate.^25,26 However, the free-standing PAA method is substrate-friendly and shows no damages to the substrate. As shown in Figure 2a, the free-standing PAA template with an average area >4.5 cm² are firmly attached on the substrates such as glass (left), Si (middle), and flexible polyimide films (right). To make a large-area free-standing PAA template, a thin film of PMMA was used as a support layer for the ultrathin (~300 nm) and brittle template.^30,32 UV-ozone treatment was applied to remove this protective layer, and the free-standing template is still intact after the removal. Voids between the template and substrate are not desirable in this process. To avoid these voids, substrates such as glass and polyimide films were pretreated with mild O₂ plasma to modify surfaces hydrophilicity before attachment. In addition, drops of acetone were added onto the attached template as capillary forces produced during the evaporation of acetone can further reduce the voids. The attachment of free-standing PAA template can also be further extended to polymer substrates such as flexible polyimide film. Direct anodization of Al on the flexible film is however difficult to be achieved.²⁴ Unlike the PAA-substrate method, in principle, our method can be applied to any substrates, though only three types of substrates were illustrated here. Furthermore, the highly ordered pores in the free-standing PAA template are easily achieved by a well-established two-step anodization, and no special cares are required as compared with the fabrication of highly ordered PAA-substrate.^22,23 As shown in Figure 2b, the free-standing PAA template via two-step anodization shows hexagonally packed pores with a diameter of ~65 nm and interpore distance of ~110 nm. The shape, size, and interpore distance can be well-controlled by varying the anodization conditions as reported elsewhere.²² Although the PAA-substrate method has numerous advantages, the fabrication of free-standing PAA template is much easier, simpler, and more flexible.

Figure 2 (A) Free-standing PAA template attached onto glass, Si, and flexible polyimide film (from left to right, respectively) with an average area >4.5 cm2. (B) SEM image of the highly ordered free-standing PAA template.

The free-standing PAA template attached on the Si substrate after removal of the PMMA protective layer is shown in Figure 3a (oblique-view SEM image). The sample is cleaved to reveal the cross-sectional structure on the Si substrate. The template is in good contact with the substrate after cleavage, and no voids are observed along the interface between the template and the substrate, indicating firm contact between them. The free-standing PAA template is estimated to be ~300 nm thick. It is apparent in the SEM image that the surface morphology of the Si substrate remains unmodified, unlike the PAA-substrate method that will produce anodized pores and pits on the substrate at the end of Al anodization.²⁷ The image also shows that the PMMA protective layer has been completely removed by UV-ozone treatment, whereas the template remains intact. Using a PMMA protective layer to facilitate the transfer of ultrathin PAA films has been previously reported,^30,32 but in these reports, removal of the protective layer by acetone rinse tends to cause cracks and detachment of the template. Our dry-etching-based method overcomes this drawback and makes it possible to fabricate intact free-standing PAA templates on substrates over large areas. After the removal of the protective layer from the template, it was then subject to TiO₂ ALD. As shown in Figure 3b, the pores are significantly reduced after 150-cycle ALD at a growth rate of 1.0 Å/cycle, and the template is still firmly in contact with the substrate. This is attributed to the conformal coating of ALD not only on in-plane surfaces but also along walls of PAA pores. The TiO₂ overlayer can be removed by RIE etching, and the free-standing PAA template is easily removed by chemical etching. After these processes, the TiO₂ nanotube arrays on Si substrate are fully released from the template, as shown in Figure 3c. The dimensions of the TiO₂ nanotube can be finely tuned by correspondingly varying the height and interpore distance of the template in Al anodization,²² while the nanotube wall thickness is determined by the number of ALD cycles. The TiO₂ nanotube arrays in Figure 3c are highly ordered, dense, and well-aligned, inheriting the patterns of the free-standing PAA template. The cross section of the well-aligned TiO₂ nanotube arrays was illustrated in Figure 3d.

Figure 3 Dense, uniform, highly ordered, and well-aligned ALD TiO2 nanotube arrays on Si and flexible polyimide film. SEM images of highly ordered free-standing PAA template (~300 nm) attached on Si before (A) and after (B) 150-cycle TiO2 ALD. (C) Highly ordered TiO2 nanotube arrays released from the template on the Si substrate. (D) Cross-section of well-aligned TiO2 nanotube arrays on a bending polyimide film.

The morphologies of the TiO₂ nanotubes were examined under TEM by scrapping the nanotubes from the Si substrate (Figure 4a) and cleaving the sample for cross sectional imaging (Figure 4b). As shown in Figure 4a, the nanotube wall is uniform in thickness along the length of the nanotube and its bottom is closed due to ALD also occurring on the underlying substrate. The bottom layers of the nanotubes are direct evidence of good contacts between the nanotube arrays and the substrate. The intimate contact between them is vital for device applications. Free-standing PAA templates are widely used as masks for line-of-sight deposition such as sputtering and evaporation. In these processes, materials can be deposited only on the top and bottom surfaces of the template. As such, small voids between the template and the substrate may not have any impact for the formation of nanoparticle arrays. However, these voids can seriously affect the contact between nanotube arrays and the substrate because ALD is a surface-limiting and conformal film-coating technique. Micrometer voids between the interfaces cannot be completely filled by ALD. To further check the contact, the interface between as-deposited TiO₂ nanotube arrays and its substrate is examined with cross-sectional TEM imaging. The TEM image in Figure 4b reveals that the as-deposited nanotube arrays in the free-standing PAA template are in good contact with the Si substrate (a thin layer of SiO₂ (~50 nm) is deposited on the Si substrate). This might be attributed to the conformal film coating of ALD and the good contact between the free-standing template and the substrate. With the pretreatment of the substrate with mild O₂ plasma and the use of acetone to attach the template to the substrate, we are able to achieve close contact between the template and the substrate. Therefore, ALD might be a convenient and promising method as nanoglues for bonding free-standing PAA templates on substrates.³³

Figure 4 TiO2 nanotube is uniform in wall thickness and in good contact with the substrate. (A) TEM image of a single TiO2 nanotube with uniform wall thickness and closed bottom end. (B) Cross-sectional TEM image to demonstrate good contact between the as-deposited nanotube arrays and the Si substrate. There is a SiO2 layer (~50 nm) between the template and the substrate.

Figure 5 shows the XRD spectrum of ~30 nm TiO₂ film grown by ALD on thermally oxidized Si(100). There are no sharp diffraction peaks for the as-deposited film, films annealed at 100 and 200 C for 1 h in air. This is in good agreement with previous studies since amorphous films are deposited at room temperature and postdeposition annealing is needed to improve crystallinity.^34,35 However, crystalline TiO₂ films are possible an with increase of deposition temperature.³⁵ After annealing at 300 C and above for 1 h in air, diffraction peaks corresponding to anatase TiO₂ were apparent. The crystallinity of the ALD TiO₂ film is crucial in many applications.

Figure 5 XRD spectra of ~30-nm ALD TiO2 film on Si (100). The as-deposited ALD film is amorphous, and anatase peaks were seen after annealing at 300 C and above for 1 h in air.

The optical absorption behaviors of the as-deposited ALD TiO₂ nanotube arrays in the free-standing PAA template on transparent glass substrate were investigated by UV-vis spectroscopy. By variation of the ALD cycles, the normalized absorption spectra of the nanotubes demonstrate a red shift with increasing nanotube wall thickness, as shown in Figure 6. A tunable red-shift in the adsorption edge was observed with increasing ALD cycles from 0 to 100 cycles as similar to previous reports.²⁶ Although reasons for this phenomenon are still controversial,^26,36 it is clear that the precise wall-thickness control offered by ALD gave rise to tunable optical adsorption properties of the nanotube arrays. The ability to control nanotubes' electronic structures provided by ALD is very useful in applications such as photocatalysis and photovoltaics, where wall thickness affects photoconversion efficiency.

Figure 6 UV-vis spectra of highly ordered ALD TiO2 nanotube arrays on the glass substrate. A tunable red-shift to lower energies is shown with increasing the thickness of nanotube walls.

Conclusions

In summary, we have demonstrated the fabrication of highly ordered TiO₂ nanotube arrays on various substrates via ALD on free-standing PAA templates attached on the substrate. As compared to the PAA-substrate method, this is a simple method to create highly ordered nanotube arrays on substrates over large areas without affecting the surface morphology and causing chemical contamination on the substrate. As no anodization of the substrate is involved, this method can be used for fabrication of nanotubes on a wide range of substrates, such as Si, glass, and flexible polyimide films, as demonstrated. The conformal film coating and precise wall-thickness control by ALD give rise to dense, uniform, well-aligned, and dimension-tunable TiO₂ nanotube arrays that show size-dependent optical adsorption behaviors and have an intimate contact with substrates. This is also a general method that can fabricate nanotube arrays of various materials on a wide range of substrates.