Fabrication of TiO2 nanotubes by anodization of Ti thin films for VOC sensing
Introduction
Anodic porous TiO2 and TiO2 nanotubes were first synthesized using hydrofluoric acid (HF) electrolyte by Zwilling et al. [1] and Gong et al. [2] respectively. Thereafter, many studies succeeded in controlling and extending the nanotube morphology, the length, pore size, and the wall thickness. The choice of electrolyte plays a critical role in determining the resultant nanotube array. There have been four generations according to the electrolyte used to fabricate TiO2 nanotube array up to now [3]. The first generation electrolyte was fluoride containing solution, the second was pH dependent solution, the third was organic polar electrolyte and the fourth was fluoride free solution. Several electrolytes, such as HF/H2O [2], [4], HF/H2SO4/H2O [5], NH4F/H2SO4[6], KF/NaF/H2O [7], NH4F/(NH4)2SO4/H2O [8], and fluoride-free HCl aqueous electrolyte [9], were used to fabricate TiO2 nanotube arrays. Long nanotube arrays up to 1000 μm were produced in presence of fluoride ion containing baths in combination with a variety of non-aqueous organic polar electrolytes, including dimethyl sulfoxide, formamide, ethylene glycol, and N-methylformamide [10], [11], [12], [13]. The geometrical features of TiO2 nanotube arrays are controlled by a variety of anodization parameters (potential, time, and temperature) and a variety of electrolyte parameters (composition, pH, viscosity and primarily conductivity). In order to fabricate TiO2 nanotubes, most of the works have focused on the anodization of Ti-foils, but there were few studies about anodization of Ti thin film [14], [15], [16], [17], [18]. For electro-optical and electrochromic applications of devices, direct fabrication of nanotube arrays on the desired substrate using thin film deposition is needed. In this paper, TiO2 nanotube arrays were successfully fabricated by the anodization of Ti thin film deposited on glass substrate.
TiO2 nanotube arrays have a wide range of applications in areas such as photoelectrochemical materials, dye-sensitized solar cells, hydrogen (H2) sensors, oxygen (O2) sensors, bio-sensing and biomedical applications, and catalyst support because of their various functional properties [3]. Many investigations have been performed for hydrogen gas sensing properties of TiO2 nanotube arrays at room temperature and the results showed excellent response to hydrogen [19], [20], [21], [22], [23], [24]. For instance, undoped micrometer length TiO2 nanotube arrays prepared by anodization of Ti foil demonstrate an unprecedented change in electrical resistance, of 8.7 orders of magnitude [23]. The H2 sensing properties of TiO2 nanotube arrays were studied in terms of the nanotube length, the catalysis coating electrode, and the carrier gas. The highest sensitivity was observed for micrometer-length TiO2 nanotubes with a catalytic (Pd, or Pt) nanofilm coating and using dry air as a carrier gas. On the other hand, there are few studies about oxidizing and reducing gas sensing properties of TiO2 nanotubes. Lu et al. fabricated TiO2 nanotubes by anodization using fluoride containing electrolyte for low-temperature oxygen sensing [25]. Seo et al. prepared various TiO2 nanotube morphologies by hydrothermal treatment of TiO2 nanoparticles at different temperatures and investigated volatile organic compound sensing properties at high temperature (450–550 °C) [26], [27]. They observed high sensitivity to toluene at this temperature.
In this study, we synthesized highly ordered TiO2 nanotubes by anodic oxidation of Ti thin film on glass substrate depending on anodization conditions and investigated their volatile organic compound sensing properties.
Section snippets
Fabrication of TiO2 nanotubes
Ti thin films of 1 μm thickness were evaporated on pre-cleaned microscope glass substrates in a Leybold Univex 450 coater system with an Inficon Deposition Monitor (XTM/2). The substrate temperature was room temperature. Then two different electrolytes were used for anodization. Anodization was performed in an organic polar electrolyte medium of 0.5 wt.% NH4F/ethylene glycol solution and an aqueous electrolyte of 0.5 wt.% HF in a thermostated bath using a dc power supply, a platinum foil as a
Aqueous HF Electrolyte
Current density versus time for anodized Ti thin film with a constant anodization voltage of 10 V at anodization temperature of 0 °C during anodization process is shown in Fig. 2. The current density decreases sharply from 60 mA/cm2 to 7 mA/cm2 in 27 s and then increases slowly to 8 mA/cm2 in 120 s as seen in Fig. 2. After this slow increase, a rapid decrease in current density starts. When this second decrease in current density starts, the anodization is finalized. The formation of TiO2 nanotubes
Conclusions
TiO2 nanotube arrays are fabricated by anodization of Ti thin films with different anodization solutions depending on anodization temperatures and anodization voltages. For aqueous HF electrolyte, TiO2 nanotube formation is observed at 0 °C with anodization voltage of 10 V (N-I). For NH4F/Ethylene glycol electrolyte, TiO2 nanotube formation is observed at room temperature with anodization voltage above 40 V (N-II). The maximum sensor response to VOCs is observed for N-I TiO2 nanotube sample due to
Acknowledgements
This study was supported by Scientific Research Project of Gebze Institute of Technology Project number: (2010-A-11) and by DPT (State Planning Organization of Turkey) through the Project number: 2009K120730. The authors would like to thank the researcher Ahmet Nazım at Gebze Institute of Technology, Department of Materials Science and Engineering for SEM facilities.
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