Optical and electrical characterization of TiO2

Applied Surface Science 252 (2005) 1101–1106 www.elsevier.com/locate/apsusc

Optical and electrical characterization of TiO2 nanotube arrays on titanium substrate Y.K. Lai a, L. Sun a, C. Chen b, C.G. Nie a, J. Zuo a, C.J. Lin a,* a

College of Chemistry and Chemical Engineering, State Key Laboratory of Physical Chemistry of Solid Surface, Xiamen University, Xiamen 361005, China b Department of Physics, Xiamen University, Xiamen 361005, China Received 19 July 2004; received in revised form 2 February 2005; accepted 2 February 2005 Available online 9 March 2005

Abstract Novel oriented aligned TiO2 nanotube (TN) arrays were fabricated by anodizing titanium foil in 0.5% HF electrolyte solution. It is indicated that the sizes of the TNs greatly depended on the applied voltages to some extent. The electrical properties of the TN arrays were characterized by current–voltage (I–V) measurements. It exhibits a nonlinear, asymmetric I–V characterization, which can be explained that there exists an n-type semiconductor/metal Schottky barrier diode between TN arrays and titanium substrate interface. The absorption edges shift towards shorter wavelengths with the decrease of the anodizing voltages, which is attributed to the quantum size effects. At room temperature, a novel wide PL band consisting of four overlapped peaks was observed in the photoluminescence (PL) measurements of the TN arrays. Such peaks were proposed to be resulted from the direct transition X1 ! X2/X1, indirect transition G1 ! X2/X1, self-trapped excitons and oxygen vacancies, respectively. # 2005 Elsevier B.V. All rights reserved. Keywords: TiO2 nanotube arrays; Schottky barrier diode; Photoluminescence; Optical and electrical characterization

1. Introduction In the last decade, TiO2 was one of the most widely studied semiconductor materials due to its favorable physical, optical and electrical properties and its many important applications, such as photocatalyst [1,2], solar cells [3,4], gas sensors [5] and even biomaterials * Corresponding author. Tel.: +86 592 2184655; fax: +86 592 2189354. E-mail address: [email protected] (C.J. Lin).

[6,7]. The morphology of TiO2 is a crucial factor in these applications because many important chemical and physical events take place on the surface. Onedimension (1-D) TiO2 nanotubes (TNs) have attracted extraordinary attention for its novel physical properties comparing to the bulk TiO2, and the potential applications in constructing nanoscale electronic and optoelectronic devices. Thin films of the oriented nanotubes are often more desirable for practical applications involving solar cells, gas sensors and photocatalyst and high surface area electrodes.

0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.02.035

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Y.K. Lai et al. / Applied Surface Science 252 (2005) 1101–1106

Recently, the TiO2 nanotube array film has been widely synthesized by various methods [8–11]. However, only little attention has been paid to its electronic and optical properties, particularly for the well-organized TiO2 nanotube (TN) arrays structure formed directly on titanium (Ti) substrate by anodic oxidation. To our knowledge, it is usually difficult to observe any PL phenomenon at room temperature for bulk TiO2 due to its indirect transition nature [12,13]. By contrast, some special nanocrystal materials have been reported to exhibit PL at room temperature [14–16]. In this article, a simple electrochemical approach was used to prepare TN arrays, and the investigation of the electrical properties and the photoluminescence (PL) phenomenon of the highly ordered and uniform TN arrays at room temperature was carried out. The mechanism of the intense rectifying characterization and the novel PL spectra were also discussed.

2. Experimental The TN arrays on Ti substrate were prepared by using an electrochemical process [10,11]. In a typical preparation procedure, the Ti foils (>99% purity, thickness of 0.1 mm) were washed in turn with distilled water and anhydrous ethanol in an ultrasonic bath and dried in air. Following, the Ti foils were anodized in 0.5% hydrofluoric acid electrolyte

solution with a platinum counter electrode. The anodizing experiments were conducted with a magnetic agitation at room temperature. The surface morphologies of TN arrays were observed using a scanning electron microscope (SEM, LEO-1530). The I–V characterization was measured by Bio-Rad ECV equipment (model PN4300) employed 1 M ethylenediaminetetraacetic acid (EDTA) disodium salt solution as electrolyte. The UV–vis absorption spectra were recorded on the Varian Cary-5000 UV–VIS–NIR spectrophotometer. The PL spectra were measured in an Edinburgh FLS 920 spectrophotometer with a 325 nm He–Cd laser as the excitation light source at room temperature.

3. Results and discussion Fig. 1a and b show typical SEM images of the highly dense TN arrays prepared at 10 and 20 V, respectively. It can be seen that the TNs exhibits uniform tube morphology, the tubes open on the top and close at the bottom, nearly parallel and closed side by side. The sizes of TNs are about 38 and 82 nm in inner diameter, 10 and 14 nm in wall thickness at 10 and 20 V, respectively. It indicates that the sizes of the TNs are greatly depend on the applied voltages to some extent. Fig. 2 presents a cross-section image of the TN arrays prepared at 20 V for 30 min. It is clear that the well-aligned TN (about 400 nm length, and

Fig. 1. SEM images of the top view of TN arrays under the anodization voltage of 10 V (a) and 20 V (b).


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Fig. 2. SEM cross-sectional image of TN arrays under a 20 V anodization voltage.

45 nm thickness of barrier layer) arrays have been coated vertically on the Ti substrate. It is well-known that the Schottky barrier diode has been fabricated by n-type semiconductor (Si, GaN and SiC) on the different kinds of metals (Pt, Au, Al and Ti) by various methods [17–19]. The nonstoichiometric TiO2 resulted from the oxygen deficiency can be used to explain the n-type semiconductor behavior of as-prepared TN arrays film. The I–V curve of the TN/Ti interface (Fig. 3a) exhibits a typical asymmetric conductivity characterization for the TN arrays film on Ti substrate. When a negative voltage is applied, the current appears cut-off in a wide range, and with the decrease of the voltage, the current decays abruptly when the voltage is smaller than 4 V. Contrarily, in positive direction, the current increases exponentially when the voltage is beyond the low turn-on point (about 0.6 V). The contact resistance of the TN/Ti interface varies greatly with the direction of applied electric field, implying that a strong nano size effect played an important role in the I–V characterization. Such intense rectifying behavior of semiconductor– metal junction at interface is similar to the I–V relationship of an n-type semiconductor–metal Schottky barrier diode. In our case, this might result from the presence of the special nonstoichiometric n-type TN array thin film on metal Ti substrate. Comparatively, the I–V measurement of an air formed TiO2 film on the Ti substrate shows a nonlinear, symmetric I–V characteristic (Fig. 3b).

Fig. 3. I–V characteristics of TiO2/Ti interface (a) and TiO2/Ti interface (b).

The UV–vis spectra of TN arrays formed under the different anodizing voltages are showed in Fig. 4. It indicates that the absorption edges shift toward shorter wavelengths with the decrease of the nanotube inner diameter, which is attributed to the quantum size effects. The UV–vis absorption increases with the increasing of the applied voltages for the TN formation, because the thickness of TN array increases with the applied voltages (10–23 V) in range of 200– 500 nm. It is usually difficult to accurately determine the UV–vis absorption for the pure TN arrays because of the effect of Ti substrate. The absorption peaks appearing in the wavelength between 400 and 800 nm are not due to the interference fringes of the TN arrays because the theoretical calculation of the 20 V sample (263.6 nm) is not agreed with the experimental

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Fig. 4. UV–vis spectra of the TN arrays with different anodization voltages.

observation by SEM (about 400 nm) [20,21]. Bahnemann et al. [22] reported the superposition of the spectra associated with trapped charge carriers in TiO2 nanoparticles exhibited extremely strong and broad optical absorption peaks. The trapped hole exhibits the most absorption at wavelengths around 430 nm or even shorter, while the trapped electron shows a strong absorption around 650 nm. The TN array exhibits the similar absorption peaks in the tested samples, so these phenomena may be identified to be the sub-band gap states of the TN array due to the special nanotube structure. The PL technique as an important tool has been widely used to investigate the energy level of materials and to provide fundamental information on the properties of the energy levels lying within the band gap. The PL spectra of TN arrays were obtained by using an excitation wavelength of 325 nm in the range from 350 to 600 nm at room temperature. The original PL curves of the samples prepared at 10 and 20 V, and their four PL emission peaks by Gaussian fitting are illustrated in Fig. 5a and b, respectively. It can be found that the Gaussian curves fitted the original PL curves perfectly. The data of the Gaussian fitting multipeaks are shown in Table 1, in which tentative assignments are indicated as well. The PL emission peaks are located at 3.66, 3.00 eV (Fig. 5a) and 3.69, 3.13 eV (Fig. 5b), which are proposed to be attributed to the highest energy direct transition X1 ! X2/X1 and the lowest energy indirect transition G1 ! X2/X1, respectively [23].

Fig. 5. PL spectra and their Gaussian fit bands of sample 10 and 20 V (excitation wavelength: 325 nm).

In general, the PL spectra of nanocrystal materials might mainly close to three kinds of physical origins within the bandgap: self-trapped excitons [24], oxygen vacancies (OVs) [12,25] and surface states [26,27]. The emission peaks originated from transitions within the band gap at the long wavelength were also observed in our experiments. Both samples have the similar constant PL band at about 2.74–2.79 eV, indicating that this band probably originated from the intrinsic states rather than the surface states. Therefore, we may assign this PL band to self-trapped excitons localized on TiO6 octahedra [24]. Some authors reported that not less than eight shallow trap levels at energies from 0 to 1 eV below the conduction band [12,28–30]. The shallow trap levels concerned OVs at various energy levels were


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Table 1 PL bands calculated and experimental energies (eV) for TiO2

Acknowledgments

Calculated energy (eV) [23]

Experimental energy Sample 10 V

Sample 20 V

3.59, 3.45 3.05, 2.91 Shallow levels

3.66 3.00 2.74, 2.21

3.69 3.13 2.79, 2.39

The financial supports from National Science Foundation of China for this project (20127302, 20021002) and Xiamen R&D Program (3502Z200 21087) are gratefully acknowledged.

Origin [23]

X1 ! X2/X1 G1 ! X2/X1 See text

established at 2.21 eV [31] and 2.40 eV [28,29]. The PL peaks in our experiment at 2.21 and 2.39 eV are very closed to these two shallow traps. Therefore, it is reasonable to assign these two peaks to the OVs on the surface for the TN arrays. The process of the PL emission identified with OVs might occur with photogenerated conduction band electrons trapped by ionized oxygen vacancy levels in TN arrays, and subsequently recombine with the holes in valence band radiatively. Comparing with the ultrafine TiO2 particles [13], TiO2 nanosheet crystallites [16] and the oriented organized nanocrystalline TiO2 [32], the well-aligned ordered TN arrays do give an interesting PL behavior at room temperature. Further studies including experiments and theoretical calculations are required to ascertain the exact origin of the PL bands for the application of such novel TN arrays.

4. Conclusions In summary, novel well-aligned TN arrays were successfully prepared by using electrochemical method in 0.5% HF electrolyte solution. The SEM images shows that the TN sizes are controllable by adjusting the anodization voltage. Intense rectifying characterization of the TN/Ti interface is resulted from the n-type semiconductor/metal Schottky barrier diode. With the decreasing of the anodization voltages, the absorption edges shift towards shorter wavelengths, which is attributed to the quantum size effects for the TN on Ti substrate. The wide superposition of the PL spectra of the TN arrays are composed of direct transition X1 ! X2/X1, indirect transition G1 ! X2/X1, self-trapped excitons and OVs at room temperature. It is expected that such special wide PL bands of the TN arrays may provide potential applications in the field of photo/electronic devices.

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