Electronic properties of oxygen vacancies in titania nanotubes

ARTICLE IN PRESS Physica B 404 (2009) 127–130

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Physica B journal homepage: www.elsevier.com/locate/physb

Electronic properties of oxygen vacancies in titania nanotubes J.M. Cho a, J.M. Seo a, J.-K. Lee a,b,, H. Zhang c, R. Lamb c a b c

Department of Physics, Chonbuk National University, Jeonju 561-756, Republic of Korea Research Institute of Physics and Chemistry, Chonbuk National University, Jeonju 561-756, Republic of Korea School of Chemistry, University of Melbourne, Parkville, Vic 3010, Australia

a r t i c l e in fo

abstract

Article history: Received 19 June 2008 Received in revised form 9 October 2008 Accepted 12 October 2008

The electronic properties of oxygen vacancies in titania nanotubes were examined using electron spin resonance spectroscopy. The single-electron-trapped oxygen vacancy (SETOV) showed a Curie behavior at low temperatures (140 KoTo300 K), which is indicative of localized electrons at the SETOV sites. On the other hand, the Ti3+ sites showed a thermally activated behavior with an activation energy of 120 meV. Microwave saturation measurements revealed the SETOV signal to show apparent saturation with increasing microwave power. In contrast, the Ti3+ signal showed almost unsaturated behavior, suggesting Ti3+ had faster spin-relaxation times than SETOV. The difference between Ti3+ and SETOV appears to be related to the difference between the bulk and surface spins. & 2008 Elsevier B.V. All rights reserved.

PACS: 61.46.Fg 61.72.jd 76.30.v Keywords: Titania nanotube Electron spin resonance Oxygen vacancy

1. Introduction Titania nanotubes have attracted considerable attention for their potential applications in a variety of areas, such as solar cells [1], lithium storage [2], gas sensors [3], photocatalysis [4], and spintronics [5]. Differing structures of nanotubes, such as H2Ti3O7 [6,7], H2Ti3O7  nH2O (no3) [8], orthorhombic [9] and anatase [10,11], have been reported under various synthesis conditions. The crystal structure of annealed titania nanotubes is known to be oxygen deficient. The existence of oxygen vacancies was noted by sub-band gap photoluminescence [12]. Co-doped TiO2 nanotubes showed ferromagnetism due to the existence of oxygen vacancies [5]. The single-electron-trapped oxygen vacancy (SETOV) and Ti3+ sites were previously reported using electron spin resonance (ESR) measurements of annealed titania nanotubes. A symmetric (g ¼ 2.003) ESR signal for the SETOV [5,10], and the asymmetric Ti3+ ESR signals (g ¼ 1.97–1.98) were reported from the titania nanotubes [10,13]. The bulk oxygen vacancy in the TiO2 lattice serves as a doubly charged positive center that is capable of trapping two electrons, and can accept an electron to form an SETOV. Ti3+ usually  Corresponding author at: Department of Physics, Chonbuk National University, Dukjin-dong, Dukjin-Gu, Jeonju 561-756, Republic of Korea. Tel.: +82 63 270 3445; fax: +82 63 270 3320. E-mail address: [email protected] (J.-K. Lee).

0921-4526/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2008.10.027

originated from surface defects. Two Ti3+ sites are present for each oxygen vacancy on the TiO2(11 0) surface. One missing oxygen atom at the bridging-O site leaves two Ti3+ sites on the TiO2(11 0) surface [14,15]. This study examined the different electronic localization properties and spin-relaxation behavior of the SETOV and Ti3+ electrons in titania nanotubes using ESR.

2. Experiment Titania nanotubes were prepared using a hydrothermal reaction. TiO2 nanoparticles (Degussa P-25 powder) were mixed with 10 M NaOH. The mixture was placed in a sealed Teflon container at 130 1C and maintained at that temperature for 40 h. The product was thoroughly washed with deionized water and 0.1 M HCl to neutralize it. X-ray diffraction (XRD) showed the phase of the titania nanotubes to be anatase. The as-prepared titania nanotubes showed poor crystalline quality. The crystalline structure was improved by annealing at 300–500 1C for 1 h. Above 500 1C, the nanotube structure was demolished and changed to anatase nanocrystals, which was previously reported elsewhere [10]. The morphology of the samples was examined by highresolution transmission electron microscopy (HRTEM, JEOL JEM3010). The overall phase of the samples was analyzed by XRD (RIGAKU D/Max-2500).

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Electron spin resonance (ESR) measurements were made using a Bruker EMX-8 apparatus with an X-band. For the lowtemperature ESR measurements, a liquid-N2 Dewar was set in the ESR cavity with flowing liquid N2 gas. The g-values of the ESR signals were obtained by taking g ¼ 2.0036 of diphenyl picryl hydrazyl (DPPH) as a reference. For ESR measurements, the nanotube samples were annealed in a vacuum (1 105 Torr) at 400–500 1C for 2 h. After cooling the samples in the vacuum chamber, they were sealed in ambient N2 for the Ti3+ measurements or in air for the SETOV measurements.

3. Results and discussion

0. 7

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Fig. 1 shows a TEM image of the as-prepared titania nanotubes. The typical length of the nanotubes was approximately 500–700 nm. The outer and inner diameter was 10 nm and 5 nm, respectively. The interlayer spacing was 0.77 nm, as obtained from HRTEM (Fig. 1, inset). The as-prepared titania nanotubes showed poor crystalline quality. The crystalline structure was improved by annealing at 300–500 1C for 1 h. XRD showed the phase of the titania nanotubes to be anatase (Fig. 2).

Fig. 3(a) shows the ESR spectra of the annealed titania nanotubes measured at low temperature. The lineshape of SETOV (g ¼ 2.003) was well fitted with a Lorentzian derivative lineshape. The SETOV signal increased with decreasing temperature. The linewidth (DHpp ¼ 5 G) did not change with temperature. The DHpp was defined as the linewidth between the derivative peaks. Fig. 3(b) shows the ESR intensity as a function of temperature. The ESR intensity of the signals was calculated using a double integration of the first derivative spectra. The integrated ESR intensity (which is proportional to spin density) varied linearly with reciprocal temperature (Curie’s law). This Curie behavior suggests that the electronic wavefunction of SETOV is well localized in the temperature range (140–300 K). The number of trapped electrons does not vary over this temperature range. Fig. 4(a) shows the low-temperature ESR spectra of titania nanotubes with two different oxygen vacancies, SETOV (g ¼ 2.003) and Ti3+ (g ¼ 1.98). The broad left signal (gE2.01) at 300 K was previously assigned to the paramagnetic surface states affected by the ambient gases [10]. However, it is possible that the signal was affected by impurities, such as oxides of nitrogen in the evacuated sample. The ESR intensity of the signals was calculated by integrating the spectra separated into three resonances. The inset in Fig. 4(a) shows a fit of the integrated ESR spectrum. In Fig. 4(b), the SETOV exhibits Curie behavior. However, the intensity of the Ti3+ signal shows a deviation from Curie behavior. The Ti3+ signal shows a decrease in the spin density at lower

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Fig. 1. TEM image of the titania nanotubes. Inset shows a nanotube with an interlayer spacing of 0.77 nm.

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Fig. 2. XRD patterns of the as-prepared and calcined titania nanotubes at different temperatures. A indicates anatase.

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1000/T [K-1] Fig. 3. Temperature dependence of the (a) ESR spectrum of titania nanotubes and (b) integrated ESR intensity for SETOVs.

ARTICLE IN PRESS J.M. Cho et al. / Physica B 404 (2009) 127–130

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Fig. 4. (a) The ESR spectra of the titania nanotubes with SETOV and Ti3+. The titania nanotubes were annealed at 400 1C in a vacuum for 2 h and then sealed in ambient N2 gas. The inset shows a fit for the resonances. (b) Temperature dependence of the integrated ESR intensities for SETOVs (K), Ti3+ sites (m), and the surface-related left signal (+). The smooth curve is the fit for the thermally activated spin density.

Fig. 5. (a) The ESR spectra of titania nanotubes at different microwave powers. The nanotubes were annealed in a vacuum at 500 1C (2 h) and sealed in ambient N2. (b) Dependence of the ESR intensities on the microwave power for SETOV (K), Ti3+(m), and the left signal (+). The intensities were normalized to be the same at low power.

temperatures (To200 K), which is a thermally activated behavior. The thermally activated behavior of the ESR spin density for Ti3+, N(T), could be well fitted to the following equation:

that the pool (B sites) takes the paramagnetic electrons from the surface-related left resonance as the temperature is decreased. In order to compare the spin-relaxation properties, a series of ESR spectra was recorded with microwave powers varying from the condition of negligible saturation to one of pronounced saturation. Fig. 5(a) shows the ESR spectra obtained at room temperature. For SETOV (g ¼ 2.003), the linewidth (DHpp) increased from 5.0 G at low power (10 mW–0.3 mW) to 6.0 G at high power (100 mW). Fig. 5(b) shows the power saturation curves for the SETOV and the Ti3+. The ESR intensities for the different sites were normalized to the same values at low microwave power. The results show that SETOV and Ti3+ have different spinrelaxation properties. The spin–spin-relaxation time (T2) of SETOV was estimated to be 1.3  108 s using the equation, pffiffiffi  T 2 ¼ 2= 3gDHpp , where g is the gyromagnetic ratio and DHpp

NðTÞ / 1=½1 þ ðA=BÞ expðEa =kTÞ

(1)

where A is the number of states for Ti3+, B is the number of surrounding acceptor states, and k is Boltzmann constant. The best fit (smooth curve) was given by Ea (activation energy) E120 meV and A/BE105. The large number of B states may be the conduction band or a pool of surface states. However, the origin of the pool is not clear. It was reported that defect states (shallow traps) such as Ti3+ are located close to the conduction band in the TiO2 system [16,17]. This might have occurred in the annealed titania nanotube system. In other words, as the temperature is increased (at around 200 K), one electron of a doubly occupied surface oxygen vacancy (Ti2+) is thermally excited to an enormous number of B states at a higher energy level of 120 meV, leaving Ti3+ sites. Further studies will be needed to determine the origin exclusively. The spin density of the left signal decreased with decreasing temperature. It is also possible

here is the linewidth below saturation [18]. The spin–lattice relaxation time (T1) of SETOV was estimated to be no faster than 7.9  106 s using the power saturation curve [18]. The intensity of Ti3+ is approximately proportional to the square root of the microwave power. This suggests that the spin-relaxation times are faster for Ti3+ than with SETOV. The specific value for Ti3+ is not

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reported because the saturation method for determining the relaxation times ensures the validity of the Bloch equations and a Lorentzian lineshape [18]. The faster relaxation times of Ti3+ compared with SETOV might be related to the difference between the bulk and surface spins in their spin-relaxation properties. The fact that both the left (surface-related states) and the Ti3+ signals showed similar unsaturated linear-dependence on the microwave power suggests that both Ti3+ and the left resonance originated from a similar surface environment. The surface and bulk spins have different relaxation mechanisms [19,20], and the relaxation time might be different.

4. Conclusion The electronic properties of oxygen vacancies in titania nanotubes were examined using electron spin resonance (ESR). The single-electron-trapped oxygen vacancy (SETOV) showed a Curie behavior for the temperature range (140–300 K), which is indicative of localized electrons at SETOV sites. Ti3+ showed a thermally activated behavior with an estimated activation energy of 120 meV. The microwave saturation measurements showed that the spin-relaxation times were faster for Ti3+ than with SETOV. For the SETOV, the spin–spin-relaxation time (T2) was estimated to be 1.3  108 s and the spin–lattice relaxation time (T1) was no faster than 7.9  106 s.

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