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Optical properties of sol–gel processed TiO2 thin films up to the vacuum ultraviolet energy region
Journal of Non-Crystalline Solids 356 (2010) 1300–1304
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Optical properties of sol–gel processed TiO2 thin films up to the vacuum ultraviolet energy region Reiko Sato a, Taketoshi Kawai a,⁎, Kouichi Kifune a,b a b
Department of Physical Science, Graduate School of Science, Osaka Prefecture University, Gakuen-cho 1-1, Naka-ku, Sakai City, Osaka 599-8531, Japan Faculty of Liberal Arts and Sciences, Osaka Prefecture University, Gakuen-cho 1-1, Naka-ku, Sakai City, Osaka 599-8531, Japan
a r t i c l e
i n f o
Article history: Received 13 May 2009 Received in revised form 11 February 2010 Available online 27 April 2010 Keywords: TiO2 thin film; Structural morphology; Absorption bands; Recombination luminescence; Multiplication process
a b s t r a c t We have investigated structural and optical properties of TiO2 thin films prepared by a conventional sol–gel method. X-ray diffraction and transmission electron microscopy demonstrate that the TiO2 films are polycrystallized and consist of anatase phase. The absorption spectra above the band-edge of the TiO2 thin films exhibit the shoulder structures around 3.9 and 4.3 eV and the broad bands around 4.8 and 6.2 eV. Several absorption bands revealed by decomposing the absorption spectra can be assigned to the transitions from oxygen 2p to titanium 3d states in TiO2. Excitation spectra for a visible luminescence peaking at 2.23 eV exhibit a gradual increase with increasing energy from 8.0 eV, whose energy corresponds to the beginning of the transitions from oxygen 2p to titanium 4s and 4p states. The result gives the characteristics of the relaxation processes from titanium 4s and 4p to the relaxed excited states. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide (TiO2) is one of the technologically important materials and has been widely used as pigments, photocatalyst, optical coatings, and in other applications, owing to chemical durability, high refractive index, and mechanical stability [1–3]. During the past decade, there has been considerable interest in the structural, optical, chemical, and electrical properties of TiO2 thin films from the viewpoint of applications, such as optical filter [4–6], gas sensor [7–10], ceramic membrane [11,12], photocatalyst [13–15], and dye sensitized solar cells [16–20]. A variety of physical and chemical techniques, for example, electron beam evaporation [21,22], DC or RF magnetron sputtering [10,15,18,23–26], chemical vapor deposition [14,19,27,28], pulsed laser deposition [8,9,29–31], atomic layer deposition [20,32–34], spray pyrolysis [17,35–38], and sol–gel method [4,7,11,39,40], have been applied in preparation for TiO2 thin films. Among these techniques, the sol–gel method offers several advantages such as controllability, reliability, reproducibility, low consumption of energy, and easy and speedy deposition on large area with good homogeneity without the requirement of expensive equipment. In the present study, we have prepared TiO2 thin films on quartz glass substrates by using a conventional dip-coating sol–gel method and have investigated optical properties of the TiO2 thin films up to the vacuum ultraviolet (VUV) energy region. Though a lot of studies on structural and photocatalytic properties have been performed for TiO2 thin films [14,15,21–24,28,34,36,40], the studies on optical properties of
⁎ Corresponding author. Tel.: +81 72 254 9196; fax: +81 72 254 9931. E-mail address: [email protected] (T. Kawai). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.03.008
TiO2 thin films in the energy region above the band edge have been considerably limited, especially, up to the VUV energy region. The crystalline structure of the TiO2 thin films obtained is examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The absorption spectra of the thin films are measured in order to clarify the transitions from the valence to conduction bands in TiO2. Furthermore, the luminescence and excitation spectra up to the VUV energy region would provide significant information on relaxation processes of the photo-excited carriers from the higher excited states to the stable relaxed excited states. 2. Experimental procedures TiO2 films were prepared according to the sol-lyophilization and dipcoating processes shown in the flow chart of Fig. 1. The precursors of TiO2 sol were synthesized by the partial hydrolysis and polycondensation of titanium isopropoxide (Ti(iso-OC3H7)4) with deionized water (H2O), where ethanol (EtOH), ammonium acetate (CH3COONH4), and polyethylene glycol (HO(CH2CH2O)nH: PEG) were used as solvent, catalyst, and as adjustment of viscosity, respectively. Firstly, ammonium acetate and ethanol were mixed together, and the solution was stirred for 2 h. Next, titanium isopropoxide was added into the solution, and the mixed solution was stirred for 24 h. Deionized water was added into the mixed solution, and the mixed solution containing deionized water was stirred for 24 h. Lastly, polyethylene glycol was added into the mixed solution containing deionized water, and it was stirred for 24 h. The precursors of TiO2 sol obtained were coated on a quartz glass and were dried at 130 °C for 15 min. Then, the films were annealed at 600 °C for 30 min by using an electric furnace. The heat treatments were preformed under atmospheric conditions.
R. Sato et al. / Journal of Non-Crystalline Solids 356 (2010) 1300–1304
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Fig. 2. (a) Shows XRD patterns of the quartz glass, the TiO2 film dried at 130 °C, and the TiO2 film annealed at 600 °C. (b) and (c) are calculation results based on the structural parameters of the anatase and rutile TiO2 phases, respectively.
Fig. 1. Flow chart of the fabrication of the TiO2 thin film by a conventional dip-coating method.
The obtained TiO2 thin films color owing to the interference of light with visible wavelengths. The structural studies of the TiO2 films were performed by using TEM (Jeol JEM-100C) and X-ray diffractometer (Rigaku RINT2000) with a monochromatized Cu-Kα1 radiation. The optical measurements up to the VUV energy region were carried out using synchrotron radiation at BL-1B line of the Ultraviolet Synchrotron Orbital Radiation (UVSOR) facility, Institute for Molecular Science (Okazaki, Japan). The light beam from the 750 MeV electron storage ring of the synchrotron radiation was monochromatized through a 1 m Seya-Namioka type monochromator. The transmitted light was detected by the combination of a photomultiplier (Hamamatsu R105) and a sodium salicylate phosphor. The luminescence from the sample was detected by using a grating monochromator (Action SpectraPro-300i) equipped with a charge coupled device camera (Roper Scientific LN/CCD-100 EB-GI). The excitation spectra were measured by using an optical detection system composed of the grating monochromator and a photomultiplier (Hamamatsu R4220). Corrections for the intensity of excitation light were made using the excitation spectrum for sodium salicylate. The TiO2 films on the quartz glass substrates were mounted on the cold stage of a He-flow-type cryostat and cooled down to 10 K.
phase. Therefore, the TiO2 film annealed at 600 °C consists of the anatase phase. Electron diffraction pattern of the TiO2 thin film annealed at 600 °C is shown in Fig. 3. Several rings are obviously distinguishable. From the ring radii, the respective rings can be specified by the Miller indices of the plane of the anatase TiO2 crystal. The Miller indices of the crystal plane are represented by the black bold numbers in figure. Fig. 4 shows TEM micrograph of the TiO2 thin film. The nanoparticles of anatase TiO2 can be confirmed as the gray area in the TEM image. The average diameter of nanoparticles in the TEM image is estimated to be about 10 nm. The thickness of the films is estimated to several hundred nm, from the interference spectra in the visible light region and the observation of the cross section of the films. 3.2. Optical properties of TiO2 thin films Fig. 5 shows the absorption spectrum of the TiO2 thin film obtained by the conventional sol–gel method. The absorption spectrum exhibits the rise at 3.32 eV, whose value is almost equal to the band gap of anatase TiO2 crystals [41,42] and the rise of the TiO2 films reported previously [19,22,26,30,36,38,39]. Reflecting the thinness of the sample, the several broad peaks and shoulders can be seen in the
3. Experimental results 3.1. Crystalline structure and microstructure of TiO2 films Fig. 2(a) shows the XRD patterns of the quartz glass substrate, the TiO2 film dried at 130 °C, and the TiO2 film annealed at 600 °C in the diffraction angle range of 10–75°. The XRD patterns of the films are noisy owing to the thinness on the quartz glass substrate. Fig. 2(b) and (c) are calculated intensities based on the structural parameters of the anatase and rutile phases of TiO2 crystals, respectively. The broad bands observed around 21° in Fig. 2(a) come from the quartz glass substrate, which is amorphous. Since the TiO2 film dried at 130 °C exhibits no peak except the broad band around 21°, the film dried at 130 °C is also amorphous. On the other hand, the TiO2 film annealed at 600 °C has the typical crystalline peaks at 2θ = 25° and 38° in the XRD pattern. The calculation results shown in Fig. 2(b) and (c) indicate that the crystalline peaks at 2θ = 25° and 38° arise from the anatase TiO2
Fig. 3. Electron diffraction pattern of TiO2 thin film. The numbers on the rings are the Miller indices of the plane of the anatase TiO2 crystal.
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R. Sato et al. / Journal of Non-Crystalline Solids 356 (2010) 1300–1304
Fig. 4. TEM image of TiO2 thin film.
energy region above the absorption band edge of anatase TiO2. These structures are consistent with the absorption spectra calculated by means of Kramers–Kronig (K–K) transformations from the reflection spectra of TiO2 crystals reported previously [43]. Fig. 6 shows the luminescence spectra of the TiO2 thin film at 10 K under the various excitation energies. Under the excitation above the absorption edge of the thin film, the luminescence bands peaking at 2.23 eV and having the halfwidth of about 0.6 eV are observed. The peak energy and halfwidth of the luminescence bands are independent of the excitation photon energy in the energy range between 3.2 and 10 eV. The fact implies that the photo-excited carriers in the higher energy states rapidly relax to the stable relaxed excited states and undergo the radiative recombination. The luminescence bands of the TiO2 thin film have the similar peak position and halfwidth to those of the anatase TiO2 single crystals [42,44,45], powders [46,47], and films [27,32]. The origin of the luminescence bands is considered to the recombination transition of the self-trapped excitons, the excitons or electrons trapped around oxygen defects [42,44–47]. Since the TiO2 thin films are poly-crystallized and consist of the anatase nanoparticles with about 10 nm in a diameter, the luminescence bands in our thin film may be not intrinsic origin but extrinsic one. Fig. 7 shows the excitation spectrum for the luminescence band at 2.23 eV in the TiO2 thin film in the energy region from 2.5 to 12.0 eV. The excitation spectrum exhibits the response rising sharply from
Fig. 5. The solid thick curve is absorption spectrum of TiO2 thin film prepared by the conventional sol–gel method. Thin curves represent six Gaussian components decomposing the absorption spectrum. Broken line corresponds to the background consisting of the absorption tail of the higher energy bands and the quartz glass substrate.
Fig. 6. Luminescence spectra of TiO2 thin film at 10 K under the various excitation energies. The excitation photon energies are indicated in the figure.
3.32 eV, which corresponds to the rise on the absorption spectrum of the TiO2 thin film. In the energy region from 3.5 to 7.0 eV, the excitation spectrum has no remarkable structure. In the energy region below 4.5 eV, the excitation spectrum in Fig. 7 is similar to those of the anatase TiO2 single crystals [42,45] and films [27]. 4. Discussion 4.1. Assignment of the absorption bands above the band edge According to the experimental and theoretical studies for TiO2 crystals [43,48–51], the highest valence band is composed of oxygen 2p states and the conduction band below about 8.0 eV consists mainly of titanium 3d states. The relative energy levels in the vicinity of the band gap of TiO2 crystals are calculated by Daude et al. [48], and the several bands observed in the energy region above the band edge have been assigned to the transitions between these energy levels [48,52–55]. Now, we decompose the absorption spectrum of the TiO2 thin film into the several Gaussian shape bands in order to clear the position of the absorption bands. The Gaussian shape bands obtained by decomposing the absorption spectrum are represented by thin curves in Fig. 5. The peak energy, peak intensity, and halfwidth of each Gaussian shape
Fig. 7. Excitation spectrum for the luminescence band at 2.23 eV in TiO2 thin film over the wide range from ultraviolet to vacuum ultraviolet energy at 10 K.
R. Sato et al. / Journal of Non-Crystalline Solids 356 (2010) 1300–1304 Table 1 Parameters of Gaussian components decomposing the absorption spectrum of TiO2 thin film. The right column is the assignment to the transition between the relative energy levels calculated by Daude et al. [48]. Com.
Peak [eV]
Int. [O.D.]
Width [eV]
Transition (energy [eV])
A B C D E F
3.53 3.84 4.17 4.77 5.50 6.00
0.11 0.41 0.46 1.83 0.17 1.33
0.26 0.27 0.51 1.15 0.46 1.21
X2b → X1b (3.59) Γ5’a → Γ1b (4.05) Γ2’ → Γ1b (4.3) Γ3 → Γ5′b (4.81) X2a → X1b (5.84) Γ5 → Γ5′b (6.20)
bands are summarized in Table 1. As shown in the right column of Table 1, each absorption bands in our thin film can also be assigned to the transitions between the relative energy levels calculated by Daude et al. The intense (B, C, D, F) and weak (A, E) absorption bands are correlated with the transitions at the Γ and X points, respectively.
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the rise at 3.32 eV in the similar way to the absorption spectra of the TiO2 thin films. In the energy region from 3.5 to 7.0 eV, where the transitions from oxygen 2p to titanium 3d states occur, the excitation spectra exhibit no remarkable peak and an almost constant response. With increasing photon energy from 8.0 eV, the increase of the excitation spectra is observed owing to the secondary exciton and/or secondary e–h pairs created by the relaxation energy from the titanium 4s and 4p states to the bottom of the conduction band. Acknowledgments One of the authors (T.K.) is extremely thankful to Dr. T. Hirai of Ritsumeikan University and Prof. N. Ohno of Osaka Electro-Communication University for supporting the optical measurements at UVSOR. Optical measurements of this work were supported by the Joint Studies Program of the Institute for Molecular Science, Japan. References
4.2. Relaxation processes from the higher energy levels In the excitation spectrum in Fig. 7, it should be noted that the small peak around 7.8 eV and the increase of the intensity with increasing photon energy from 8.0 eV are observed. We cannot obtain the transmittance spectrum of the TiO2 thin film above 7.0 eV, because of the absorption of the quartz glass substrate. According to the absorption spectra calculated from the reflection spectra by means of K–K transformations [43], the absorption peak due to the transitions from oxygen 2p to titanium 3d states is located around 9.0 eV and the absorption coefficient decreases with increasing photon energy from 9.0 to 12.0 eV. This fact implies that the increase of the excitation spectrum from 8.0 eV in Fig. 7 is not attributed to that of the light intensity absorbed in the sample. The increase of the excitation spectrum would come from the change of the relaxation processes. As mentioned above, the photo-excitation in the energy range up to 8.0 eV brings about the transitions from the valence band composing of oxygen 2p states to the conduction band consisting of the titanium 3d states. On the other hand, the cluster and/or band calculation for TiO2 demonstrate that the conduction band composing mainly of the titanium 4s and 4p states is located around the energy region above 8.0 eV from the top of the valence band [43,49,50]. The fact implies that the photo-excitation above 8.0 eV creates the carries in the conduction band composing of the titanium 4s and 4p states. Since the photo-excitation energy of 8.0 eV exceeds twice the band gap of anatase TiO2, the relaxation energy from the titanium 4s and 4p states to the bottom of the conduction band can be nonradiatively transferred for the formation of secondary exciton and/or secondary electron–hole (e–h) pairs. The recombination luminescence of the secondary exciton and/or secondary e–h pairs would lead to the increase of the luminescence intensity. Therefore, the increase of the excitation spectrum from 8.0 eV might be attributed to the formation of secondary exciton and/or secondary e–h pairs. In order to make the relaxation processes clear, the further measurements of the luminescence and excitation spectra over the VUV energy region would be needed for anatase TiO2. 5. Conclusions TiO2 thin films were prepared by the conventional sol–gel method and were characterized as polycrystalline anatase phase from the analysis of the XRD and TEM. In the TiO2 thin films, the rise of the absorption spectra is located at 3.32 eV of the band gap of anatase TiO2 crystals and several absorption bands due to the transitions from oxygen 2p to titanium 3d states are confirmed in the energy region above the absorption edge. Under the excitation above the absorption edge of the TiO2 thin films, the luminescence band peaking at 2.23 eV appears at 10 K. The excitation spectra for the 2.23 eV luminescence band exhibit
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Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Optical properties of sol–gel processed TiO2 thin films up to the vacuum ultraviolet energy region Reiko Sato a, Taketoshi Kawai a,⁎, Kouichi Kifune a,b a b
Department of Physical Science, Graduate School of Science, Osaka Prefecture University, Gakuen-cho 1-1, Naka-ku, Sakai City, Osaka 599-8531, Japan Faculty of Liberal Arts and Sciences, Osaka Prefecture University, Gakuen-cho 1-1, Naka-ku, Sakai City, Osaka 599-8531, Japan
a r t i c l e
i n f o
Article history: Received 13 May 2009 Received in revised form 11 February 2010 Available online 27 April 2010 Keywords: TiO2 thin film; Structural morphology; Absorption bands; Recombination luminescence; Multiplication process
a b s t r a c t We have investigated structural and optical properties of TiO2 thin films prepared by a conventional sol–gel method. X-ray diffraction and transmission electron microscopy demonstrate that the TiO2 films are polycrystallized and consist of anatase phase. The absorption spectra above the band-edge of the TiO2 thin films exhibit the shoulder structures around 3.9 and 4.3 eV and the broad bands around 4.8 and 6.2 eV. Several absorption bands revealed by decomposing the absorption spectra can be assigned to the transitions from oxygen 2p to titanium 3d states in TiO2. Excitation spectra for a visible luminescence peaking at 2.23 eV exhibit a gradual increase with increasing energy from 8.0 eV, whose energy corresponds to the beginning of the transitions from oxygen 2p to titanium 4s and 4p states. The result gives the characteristics of the relaxation processes from titanium 4s and 4p to the relaxed excited states. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide (TiO2) is one of the technologically important materials and has been widely used as pigments, photocatalyst, optical coatings, and in other applications, owing to chemical durability, high refractive index, and mechanical stability [1–3]. During the past decade, there has been considerable interest in the structural, optical, chemical, and electrical properties of TiO2 thin films from the viewpoint of applications, such as optical filter [4–6], gas sensor [7–10], ceramic membrane [11,12], photocatalyst [13–15], and dye sensitized solar cells [16–20]. A variety of physical and chemical techniques, for example, electron beam evaporation [21,22], DC or RF magnetron sputtering [10,15,18,23–26], chemical vapor deposition [14,19,27,28], pulsed laser deposition [8,9,29–31], atomic layer deposition [20,32–34], spray pyrolysis [17,35–38], and sol–gel method [4,7,11,39,40], have been applied in preparation for TiO2 thin films. Among these techniques, the sol–gel method offers several advantages such as controllability, reliability, reproducibility, low consumption of energy, and easy and speedy deposition on large area with good homogeneity without the requirement of expensive equipment. In the present study, we have prepared TiO2 thin films on quartz glass substrates by using a conventional dip-coating sol–gel method and have investigated optical properties of the TiO2 thin films up to the vacuum ultraviolet (VUV) energy region. Though a lot of studies on structural and photocatalytic properties have been performed for TiO2 thin films [14,15,21–24,28,34,36,40], the studies on optical properties of
⁎ Corresponding author. Tel.: +81 72 254 9196; fax: +81 72 254 9931. E-mail address: [email protected] (T. Kawai). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.03.008
TiO2 thin films in the energy region above the band edge have been considerably limited, especially, up to the VUV energy region. The crystalline structure of the TiO2 thin films obtained is examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The absorption spectra of the thin films are measured in order to clarify the transitions from the valence to conduction bands in TiO2. Furthermore, the luminescence and excitation spectra up to the VUV energy region would provide significant information on relaxation processes of the photo-excited carriers from the higher excited states to the stable relaxed excited states. 2. Experimental procedures TiO2 films were prepared according to the sol-lyophilization and dipcoating processes shown in the flow chart of Fig. 1. The precursors of TiO2 sol were synthesized by the partial hydrolysis and polycondensation of titanium isopropoxide (Ti(iso-OC3H7)4) with deionized water (H2O), where ethanol (EtOH), ammonium acetate (CH3COONH4), and polyethylene glycol (HO(CH2CH2O)nH: PEG) were used as solvent, catalyst, and as adjustment of viscosity, respectively. Firstly, ammonium acetate and ethanol were mixed together, and the solution was stirred for 2 h. Next, titanium isopropoxide was added into the solution, and the mixed solution was stirred for 24 h. Deionized water was added into the mixed solution, and the mixed solution containing deionized water was stirred for 24 h. Lastly, polyethylene glycol was added into the mixed solution containing deionized water, and it was stirred for 24 h. The precursors of TiO2 sol obtained were coated on a quartz glass and were dried at 130 °C for 15 min. Then, the films were annealed at 600 °C for 30 min by using an electric furnace. The heat treatments were preformed under atmospheric conditions.
R. Sato et al. / Journal of Non-Crystalline Solids 356 (2010) 1300–1304
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Fig. 2. (a) Shows XRD patterns of the quartz glass, the TiO2 film dried at 130 °C, and the TiO2 film annealed at 600 °C. (b) and (c) are calculation results based on the structural parameters of the anatase and rutile TiO2 phases, respectively.
Fig. 1. Flow chart of the fabrication of the TiO2 thin film by a conventional dip-coating method.
The obtained TiO2 thin films color owing to the interference of light with visible wavelengths. The structural studies of the TiO2 films were performed by using TEM (Jeol JEM-100C) and X-ray diffractometer (Rigaku RINT2000) with a monochromatized Cu-Kα1 radiation. The optical measurements up to the VUV energy region were carried out using synchrotron radiation at BL-1B line of the Ultraviolet Synchrotron Orbital Radiation (UVSOR) facility, Institute for Molecular Science (Okazaki, Japan). The light beam from the 750 MeV electron storage ring of the synchrotron radiation was monochromatized through a 1 m Seya-Namioka type monochromator. The transmitted light was detected by the combination of a photomultiplier (Hamamatsu R105) and a sodium salicylate phosphor. The luminescence from the sample was detected by using a grating monochromator (Action SpectraPro-300i) equipped with a charge coupled device camera (Roper Scientific LN/CCD-100 EB-GI). The excitation spectra were measured by using an optical detection system composed of the grating monochromator and a photomultiplier (Hamamatsu R4220). Corrections for the intensity of excitation light were made using the excitation spectrum for sodium salicylate. The TiO2 films on the quartz glass substrates were mounted on the cold stage of a He-flow-type cryostat and cooled down to 10 K.
phase. Therefore, the TiO2 film annealed at 600 °C consists of the anatase phase. Electron diffraction pattern of the TiO2 thin film annealed at 600 °C is shown in Fig. 3. Several rings are obviously distinguishable. From the ring radii, the respective rings can be specified by the Miller indices of the plane of the anatase TiO2 crystal. The Miller indices of the crystal plane are represented by the black bold numbers in figure. Fig. 4 shows TEM micrograph of the TiO2 thin film. The nanoparticles of anatase TiO2 can be confirmed as the gray area in the TEM image. The average diameter of nanoparticles in the TEM image is estimated to be about 10 nm. The thickness of the films is estimated to several hundred nm, from the interference spectra in the visible light region and the observation of the cross section of the films. 3.2. Optical properties of TiO2 thin films Fig. 5 shows the absorption spectrum of the TiO2 thin film obtained by the conventional sol–gel method. The absorption spectrum exhibits the rise at 3.32 eV, whose value is almost equal to the band gap of anatase TiO2 crystals [41,42] and the rise of the TiO2 films reported previously [19,22,26,30,36,38,39]. Reflecting the thinness of the sample, the several broad peaks and shoulders can be seen in the
3. Experimental results 3.1. Crystalline structure and microstructure of TiO2 films Fig. 2(a) shows the XRD patterns of the quartz glass substrate, the TiO2 film dried at 130 °C, and the TiO2 film annealed at 600 °C in the diffraction angle range of 10–75°. The XRD patterns of the films are noisy owing to the thinness on the quartz glass substrate. Fig. 2(b) and (c) are calculated intensities based on the structural parameters of the anatase and rutile phases of TiO2 crystals, respectively. The broad bands observed around 21° in Fig. 2(a) come from the quartz glass substrate, which is amorphous. Since the TiO2 film dried at 130 °C exhibits no peak except the broad band around 21°, the film dried at 130 °C is also amorphous. On the other hand, the TiO2 film annealed at 600 °C has the typical crystalline peaks at 2θ = 25° and 38° in the XRD pattern. The calculation results shown in Fig. 2(b) and (c) indicate that the crystalline peaks at 2θ = 25° and 38° arise from the anatase TiO2
Fig. 3. Electron diffraction pattern of TiO2 thin film. The numbers on the rings are the Miller indices of the plane of the anatase TiO2 crystal.
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Fig. 4. TEM image of TiO2 thin film.
energy region above the absorption band edge of anatase TiO2. These structures are consistent with the absorption spectra calculated by means of Kramers–Kronig (K–K) transformations from the reflection spectra of TiO2 crystals reported previously [43]. Fig. 6 shows the luminescence spectra of the TiO2 thin film at 10 K under the various excitation energies. Under the excitation above the absorption edge of the thin film, the luminescence bands peaking at 2.23 eV and having the halfwidth of about 0.6 eV are observed. The peak energy and halfwidth of the luminescence bands are independent of the excitation photon energy in the energy range between 3.2 and 10 eV. The fact implies that the photo-excited carriers in the higher energy states rapidly relax to the stable relaxed excited states and undergo the radiative recombination. The luminescence bands of the TiO2 thin film have the similar peak position and halfwidth to those of the anatase TiO2 single crystals [42,44,45], powders [46,47], and films [27,32]. The origin of the luminescence bands is considered to the recombination transition of the self-trapped excitons, the excitons or electrons trapped around oxygen defects [42,44–47]. Since the TiO2 thin films are poly-crystallized and consist of the anatase nanoparticles with about 10 nm in a diameter, the luminescence bands in our thin film may be not intrinsic origin but extrinsic one. Fig. 7 shows the excitation spectrum for the luminescence band at 2.23 eV in the TiO2 thin film in the energy region from 2.5 to 12.0 eV. The excitation spectrum exhibits the response rising sharply from
Fig. 5. The solid thick curve is absorption spectrum of TiO2 thin film prepared by the conventional sol–gel method. Thin curves represent six Gaussian components decomposing the absorption spectrum. Broken line corresponds to the background consisting of the absorption tail of the higher energy bands and the quartz glass substrate.
Fig. 6. Luminescence spectra of TiO2 thin film at 10 K under the various excitation energies. The excitation photon energies are indicated in the figure.
3.32 eV, which corresponds to the rise on the absorption spectrum of the TiO2 thin film. In the energy region from 3.5 to 7.0 eV, the excitation spectrum has no remarkable structure. In the energy region below 4.5 eV, the excitation spectrum in Fig. 7 is similar to those of the anatase TiO2 single crystals [42,45] and films [27]. 4. Discussion 4.1. Assignment of the absorption bands above the band edge According to the experimental and theoretical studies for TiO2 crystals [43,48–51], the highest valence band is composed of oxygen 2p states and the conduction band below about 8.0 eV consists mainly of titanium 3d states. The relative energy levels in the vicinity of the band gap of TiO2 crystals are calculated by Daude et al. [48], and the several bands observed in the energy region above the band edge have been assigned to the transitions between these energy levels [48,52–55]. Now, we decompose the absorption spectrum of the TiO2 thin film into the several Gaussian shape bands in order to clear the position of the absorption bands. The Gaussian shape bands obtained by decomposing the absorption spectrum are represented by thin curves in Fig. 5. The peak energy, peak intensity, and halfwidth of each Gaussian shape
Fig. 7. Excitation spectrum for the luminescence band at 2.23 eV in TiO2 thin film over the wide range from ultraviolet to vacuum ultraviolet energy at 10 K.
R. Sato et al. / Journal of Non-Crystalline Solids 356 (2010) 1300–1304 Table 1 Parameters of Gaussian components decomposing the absorption spectrum of TiO2 thin film. The right column is the assignment to the transition between the relative energy levels calculated by Daude et al. [48]. Com.
Peak [eV]
Int. [O.D.]
Width [eV]
Transition (energy [eV])
A B C D E F
3.53 3.84 4.17 4.77 5.50 6.00
0.11 0.41 0.46 1.83 0.17 1.33
0.26 0.27 0.51 1.15 0.46 1.21
X2b → X1b (3.59) Γ5’a → Γ1b (4.05) Γ2’ → Γ1b (4.3) Γ3 → Γ5′b (4.81) X2a → X1b (5.84) Γ5 → Γ5′b (6.20)
bands are summarized in Table 1. As shown in the right column of Table 1, each absorption bands in our thin film can also be assigned to the transitions between the relative energy levels calculated by Daude et al. The intense (B, C, D, F) and weak (A, E) absorption bands are correlated with the transitions at the Γ and X points, respectively.
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the rise at 3.32 eV in the similar way to the absorption spectra of the TiO2 thin films. In the energy region from 3.5 to 7.0 eV, where the transitions from oxygen 2p to titanium 3d states occur, the excitation spectra exhibit no remarkable peak and an almost constant response. With increasing photon energy from 8.0 eV, the increase of the excitation spectra is observed owing to the secondary exciton and/or secondary e–h pairs created by the relaxation energy from the titanium 4s and 4p states to the bottom of the conduction band. Acknowledgments One of the authors (T.K.) is extremely thankful to Dr. T. Hirai of Ritsumeikan University and Prof. N. Ohno of Osaka Electro-Communication University for supporting the optical measurements at UVSOR. Optical measurements of this work were supported by the Joint Studies Program of the Institute for Molecular Science, Japan. References
4.2. Relaxation processes from the higher energy levels In the excitation spectrum in Fig. 7, it should be noted that the small peak around 7.8 eV and the increase of the intensity with increasing photon energy from 8.0 eV are observed. We cannot obtain the transmittance spectrum of the TiO2 thin film above 7.0 eV, because of the absorption of the quartz glass substrate. According to the absorption spectra calculated from the reflection spectra by means of K–K transformations [43], the absorption peak due to the transitions from oxygen 2p to titanium 3d states is located around 9.0 eV and the absorption coefficient decreases with increasing photon energy from 9.0 to 12.0 eV. This fact implies that the increase of the excitation spectrum from 8.0 eV in Fig. 7 is not attributed to that of the light intensity absorbed in the sample. The increase of the excitation spectrum would come from the change of the relaxation processes. As mentioned above, the photo-excitation in the energy range up to 8.0 eV brings about the transitions from the valence band composing of oxygen 2p states to the conduction band consisting of the titanium 3d states. On the other hand, the cluster and/or band calculation for TiO2 demonstrate that the conduction band composing mainly of the titanium 4s and 4p states is located around the energy region above 8.0 eV from the top of the valence band [43,49,50]. The fact implies that the photo-excitation above 8.0 eV creates the carries in the conduction band composing of the titanium 4s and 4p states. Since the photo-excitation energy of 8.0 eV exceeds twice the band gap of anatase TiO2, the relaxation energy from the titanium 4s and 4p states to the bottom of the conduction band can be nonradiatively transferred for the formation of secondary exciton and/or secondary electron–hole (e–h) pairs. The recombination luminescence of the secondary exciton and/or secondary e–h pairs would lead to the increase of the luminescence intensity. Therefore, the increase of the excitation spectrum from 8.0 eV might be attributed to the formation of secondary exciton and/or secondary e–h pairs. In order to make the relaxation processes clear, the further measurements of the luminescence and excitation spectra over the VUV energy region would be needed for anatase TiO2. 5. Conclusions TiO2 thin films were prepared by the conventional sol–gel method and were characterized as polycrystalline anatase phase from the analysis of the XRD and TEM. In the TiO2 thin films, the rise of the absorption spectra is located at 3.32 eV of the band gap of anatase TiO2 crystals and several absorption bands due to the transitions from oxygen 2p to titanium 3d states are confirmed in the energy region above the absorption edge. Under the excitation above the absorption edge of the TiO2 thin films, the luminescence band peaking at 2.23 eV appears at 10 K. The excitation spectra for the 2.23 eV luminescence band exhibit
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