Transparent TiO2 nanotube array photoelectrodes prepared via two-step anodization
© Kim et al.; licensee Springer 2014
Received: 28 January 2014
Accepted: 5 February 2014
Published: 4 April 2014
Two-step anodization of transparent TiO2 nanotube arrays has been demonstrated with aid of a Nb-doped TiO2 buffer layer deposited between the Ti layer and TCO substrate. Enhanced physical adhesion and electrochemical stability provided by the buffer layer has been found to be important for successful implementation of the two-step anodization process. With the proposed approach, the morphology and thickness of NT arrays could be controlled very precisely, which in turn, influenced their optical and photoelectrochemical properties.
Aligned TiO2 nanotube (NT) arrays have attracted considerable interest owing to their versatility in a number of optoelectronic applications, such as solar cells [1–3], photocatalysis [4, 5], and electrochromics . For instance, when used as electrodes in dye-sensitized solar cells (DSSCs), they were found to show higher light-harvesting and charge-collection efficiencies compared to their nanoparticle-based counterparts . However, most anodized TiO2 NT arrays have been prepared using non-transparent Ti-foil substrates, limiting the light illumination geometry to just one side of the electrode. The limited illumination geometry lowers the performance of the optoelectronic devices because the incident light is attenuated by the device components (e.g. electrolyte) other than the TiO2 NT electrode . Therefore, the benefits of TiO2 NT-based electrodes can only be fully utilized by using transparent TiO2 NT arrays. The challenges associated with preparing transparent TiO2 NT arrays have thus far limited the numbers of routes for fabricating these electrodes [9, 10]. Recently, we found that the transparent TiO2 NT array films could be prepared via a simple and reproducible approach involving the deposition of a conducting buffer layer between the Ti layer and transparent conducting oxide (TCO) substrate . The buffer layer protects the TCO from degradation through a self-terminating mechanism that arrests the TiO2 NT growth on the Ti layer and improves the adhesion and electrical contact between the NTs and TCO.
A second challenge in TiO2 NT array synthesis has been the ability to precisely control the microstructure dimensions, extent of alignment, pore ordering, and degree of overlayer formation. These properties have a substantial impact on the device properties. For instance, we recently showed that removing the structural disorder of TiO2 NT arrays promoted more efficient charge transport in DSSCs . Also, ordering of the NTs can affect their optical properties, in particular their ability to scatter light that affects the light-harvesting properties of the electrode. Strategies to control the NT dimensions and the extent of alignment have involved using different anodization potentials [13, 14], electrolytes [15, 16], drying conditions , and bath temperatures . However, only a few studies have focused on the control of pore ordering  and overlayer formation [9, 18]. On the other hand, it is well known that highly ordered pore structures in alumina can be achieved via a two-step process [19, 20]. However, only a paucity of studies have investigated the use of the two-step anodization of TiO2 NT arrays [14, 17, 21], mostly owing to the absence of proper chemical etchants that dissolve only oxide NTs without dissolving the underlying substrate. Furthermore, all of these examples used non-transparent Ti foil as the substrate for growing NTs. For the reason given above, it would be highly useful to extend this two-step anodization technique to transparent TiO2 NT arrays.
The biggest challenge for the successful implementation of the two-step procedure for preparing transparent TiO2 NT arrays is overcoming the poor adhesion between the Ti layer and TCO substrate. During a typical two-step anodization, TiO2 NT arrays are grown to a certain thickness and then mechanically removed either by gas evolution  or by adhesive tapes . We find that these approaches are not applicable to TiO2 NT arrays grown on TCO substrates. Removal of the initially grown NT layer with these approaches results in delamination of the remaining Ti film from the TCO substrate. This situation is largely attributed to the poor adhesion between the Ti film and the TCO substrate, unlike the case of NTs grown on Ti foil.
In this communication, we demonstrate the first two-step anodization procedure for growing transparent TiO2 NT array films on TCO substrates. Key to the synthesis is the aid of a buffer layer (i.e. Nb-doped TiO2; NTO), which is found to improve both the physical adhesion between the Ti layer and TCO and the electrochemical stability of the NT films during the anodization process. We also report on simple and versatile approaches to control the morphology of the NT arrays by using a post-growth pore widening procedure, which involves changing the anodization time.
In summary, we have demonstrated that the highly ordered transparent TiO2 NT arrays can be prepared by a two-step anodization with the aid of a Nb-doped TiO2 (NTO) buffer layer. Strong physical adhesion and electrochemical stability provided by the NTO layer are found to be crucial for the success of this approach. The morphology and thickness of the transparent TiO2 NT arrays can be controlled precisely by the post-growth pore widening and the anodization time between first and second step anodization. The controlled nanostructure, in turn, affects the optical properties of the transparent TiO2 NT arrays, which are shown to impact the photoelectrochemical properties of NT-based sensitized solar cells.
Transparent TiO2 NT arrays were prepared via an electrochemical anodization process . Before depositing Ti metal films, 10 at% Nb-doped TiO2 (NTO) thin layers were deposited on the F-doped SnO2 (TEC15, Pilkington) substrates by RF-magnetron sputtering at 400°C using a ceramic target with the same composition under Ar flow (20 sccm) with a working pressure of 5 mTorr. Ti metal films with a thickness of 2.6 μ m were deposited on the NTO/TCO substrates using a Ti metal target (99.9%) under the same sputtering conditions. The Ti thin films were anodized in a two-electrode cell at 50 V (room temperature, Pt counter electrode) using 0.25 wt% NH4F (Aldrich, 99.9%) electrolyte in ethylene glycol (Aldrich, 99%) with 1 wt% of H2O. The anodization process was controlled by a source meter (Keithley, Model: 2425) connected to a computer. The applied voltage was increased with a ramp rate of 1 V/sec and 5 V/sec for first and second anodization processes, respectively . After washing with water, the first NT layer was removed by applying a cathodic potential (−3 ~ −5 V) in 1 M H2SO4 aqueous solution  using a two-electrode cell with a platinum counter electrode, and then anodized again after washing with water and ethanol. The as-prepared samples were rinsed with water and ethanol, and dried with a gentle stream of N2. Then, for the pore widening, the as-anodized NT films were immersed in the formamide solution consisting of 0.15 M NH4F and 3.5 wt% H2O at 80°C for 5 min . After rinsing and drying using the same conditions, the NT films were annealed at 400°C for 1 h under an ambient atmosphere.
For fabricating DSSCs, the films were stained with a 0.3 mM ethanolic solution of Z907 dye (cis-bis(isothiocyanato)(2,2′-bipyridyl-4,4′-dicarboxylato)(4,4′-di-nonyl-2′-bipyridyl)ruthenium(II)), and sandwich type devices were fabricated with Pt-loaded TCO substrates using the similar procedure described previously . The electrolyte was composed of 1.0 M 1-methyl-3-propylimidazolium iodide, 30 mM I2, 0.5 M 1-butyl-1H-benzimidazole, and 0.1 M guanidinium thiocyanate in 3-methoxypropionitrile .
The microstructure and thickness of the NT arrays were characterized by field-emission scanning electron microscopy (FE-SEM). The optical absorption/transmission spectra and external quantum efficiencies were obtained using an IPCE measurement system (PV Measurements, Inc.).
This work was supported by the KIST internal fund, the New & Renewable Energy Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry Of Trade, Industry & Energy (No. 20113020010040), the Nano∙Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2012M3A7B4049989), and the “National Agenda Project” program of Korea Research Council of Fundamental Science & Technology (KRCF). The work done at Green School was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MSIP) (2013, University-Institute cooperation program). KZ, NRN, and AJF were supported by the Solar Photochemistry Program, Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy, under contract No. DEAC36-08GO28308.
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