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J. Phys. Chem. B 2006, 110, 25266-25272
Preparation of Nitrogen-Substituted TiO2 Thin Film Photocatalysts by the Radio Frequency Magnetron Sputtering Deposition Method and Their Photocatalytic Reactivity under Visible Light Irradiation† Masaaki Kitano,‡ Keisho Funatsu,§ Masaya Matsuoka,§ Michio Ueshima,‡ and Masakazu Anpo*,§ Industry-UniVersity Cooperation Organization, Osaka Prefecture UniVersity, 1-2 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8570, Japan, and Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture UniVersity, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan. ReceiVed: July 31, 2006; In Final Form: September 21, 2006
Nitrogen-substituted TiO2 (N-TiO2) thin film photocatalysts have been prepared by a radio frequency magnetron sputtering (RF-MS) deposition method using a N2/Ar mixture sputtering gas. The effect of the concentration of substituted nitrogen on the characteristics of the N-TiO2 thin films was investigated by UV-vis absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and scanning electron microscopy (SEM) analyses. The absorption band of the N-TiO2 thin film was found to shift smoothly to visible light regions up to 550 nm, its extent depending on the concentration of nitrogen substituted within the TiO2 lattice in a range of 2.0-16.5%. The N-TiO2 thin film photocatalyst with a nitrogen concentration of 6.0% exhibited the highest reactivity for the photocatalytic oxidation of 2-propanol diluted in water even under visible (λ g 450 nm) or solar light irradiation. Moreover, N-TiO2 thin film photocatalysts prepared on conducting glass electrodes showed anodic photocurrents attributed to the photooxidation of water under visible light, its extent depending on wavelengths up to 550 nm. The absorbed photon to current conversion efficiencies reached 25.2% and 22.4% under UV (λ ) 360 nm) and visible light (λ ) 420 nm), respectively. UV-vis and photoelectrochemical investigations also confirmed that these thin films remain thermodynamically and mechanically stable even under heat treatment at 673 K. In addition, XPS and XRD studies revealed that a significantly high substitution of the lattice O atoms of the TiO2 with the N atoms plays a crucial role in the band gap narrowing of the TiO2 thin films, enabling them to absorb and operate under visible light irradiation as a highly reactive, effective photocatalyst.
Introduction TiO2 thin film photocatalysts have been intensively investigated for such significant reactions as the purification of toxic compounds in polluted water and air,1 applications in photoelectrochemical solar cells,2 self-cleaning surfaces,3 and materials that can utilize their photoinduced super-hydrophilic properties.4,5 However, conventional TiO2 photocatalysts operate only under UV light of wavelengths shorter than 400 nm and utilize only 3-4% of the solar beams that reach the earth. It is, thus, of great significance to design visible-light-responsive TiO2 photocatalysts by narrowing the band gap of the TiO2.6 During the past decade, many new approaches have been applied to extend the response of TiO2 into the visible region.7 One approach is the doping of transition metal cations within the TiO2, and the development of such visible-light-responsive TiO2 has been successfully carried out by the physical doping of transition metal cations such as V or Cr into the lattice position of Ti4+ in TiO2 by advanced metal-ion-implantation techniques.8-12 Another representative approach is anion doping within TiO2. In 1986, Sato et al. reported that nitrogen-doped †
Part of the special issue “Arthur J. Nozik Festschrift”. * Author to whom correspondence should be addressed. Phone: +8172-254-9282. Fax: +81-72-254-9910. E-mail:
[email protected]. ‡ Industry-University Cooperation Organization, Osaka Prefecture University. § Department of Applied Chemistry, Osaka Prefecture University.
TiO2 prepared by a wet method exhibited photocatalytic activity under visible light irradiation.13 Numerous studies on anion doping14-17 have been carried out since the pioneering work of Asahi et al. in which the doping of nitrogen as a substitute for oxygen in the TiO2 lattice was suggested.18 The visible light sensitivity of the nitrogen-doped TiO2 was due to the band gap narrowing caused by the mixing of the N 2p and O 2p states. However, most of the nitrogen-doped TiO2 exhibited visible light absorption as a shoulder in the wavelength range of 400600 nm,14,18-23 indicating that the isolated N 2p orbitals are formed above the O 2p orbitals since the concentration of nitrogen that could be doped into the TiO2 lattice was limited in a very low range of 4), despite the large shift of the steep absorption edge into the visible region. Ti3+ species were formed in the N-TiO2(X) films, especially in regions of X values larger than 4, as shown in Figure 1. Such Ti3+ species are considered to act as the recombination centers for the excited electrons and holes that decrease the photocatalytic activity.41 These results show that nitrogen substitution of the TiO2 thin film is a crucial factor in the development of the visible-light-responsive TiO2 films, although the substitution of excess amounts of nitrogen led to the undesirable formation of Ti3+ species as the recombination centers, which decreased the photocatalytic activity. Field experiments for the photocatalytic activity of the N-TiO2 thin film were also carried out under natural outdoor sunlight on a clear sunny day in June. The reaction time profiles of the photocatalytic decomposition of 2-propanol on Pt-loaded N-TiO2(4) and Pt-loaded TiO2(O2/ Ar) are shown in Figure 6. Under these conditions, Pt-loaded N-TiO2(4) exhibited much higher photocatalytic activity than Pt-loaded TiO2(O2/Ar). The decline in the reaction rates in the late afternoon can be attributed to a decline in the intensity of the sunlight. It was, thus, demonstrated that a significantly high substitution of nitrogen with the lattice oxygen of TiO2 effectively narrowed the band gap energy of TiO2, enabling Ptloaded N-TiO2(4) to absorb light in visible regions and act as an efficient photocatalyst even under natural sunlight. The effective narrowing of the band gap of TiO2 can be ascribed to the negative shift of the valence band edge through the efficient hybridization of the N 2p orbitals of the substituted nitrogen atoms and the O 2p orbitals of the bulk TiO2.18 Photoelectrochemical Properties of N-TiO2 Thin Films. The photoelectrochemical properties of the N-TiO2(4)/ITO
Figure 7. Relative photocurrent as a function of the cut-off wavelength of the incident light for the N-TiO2(4)/ITO electrode measured in 0.25 M K2SO4 aqueous solution at +1.0 V vs SCE. The broken line shows the UV-vis diffuse reflectance spectrum of the N-TiO2(4)/ITO. The inset shows the cyclic voltammogram of the N-TiO2(4)/ITO electrode under chopped visible light irradiation (λ g 420 nm).
electrode were examined using an aqueous solution of 0.25 M K2SO4 (pH ) 6.7). The inset of Figure 7 shows the cyclic voltammogram of the N-TiO2(4)/ITO electrode under chopped visible light irradiation (λ g 420 nm). Dark currents were negligible under scanning potentials from -1.0 to +1.5 V vs SCE, while the anodic photocurrent increased with an increase in the anodic bias. The generation of anodic photocurrents indicates the n-type semiconducting nature of this material. Figure 7 shows the photocurrent observed for the N-TiO2(4)/ ITO electrode as a function of the incident light wavelength, which was controlled by cut-off filters. These measurements were carried out with a bias of +1.0 V vs SCE. The photoelectrochemical onset of N-TiO2(4)/ITO electrode was located at approximately 550 nm and showed a good parallel relationship between the photoresponse and the absorption spectrum. These results clearly show that the observed photocurrent originated from a band gap transition. The quantum yields (APCEs) at 1.0 V were determined to be 25.2% at 360 nm and 22.4% at 420 nm. In addition, the value of the photocurrent remained constant even during approximately 300 min of consecutive measurement under visible light longer than 450 nm. It was also found that Pt-loaded N-TiO2(4) exhibited photocatalytic activity for the O2 evolution reaction from AgNO3 aqueous solution even under visible light, as shown in the
Preparation of N-TiO2 Thin Film Photocatalysts
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Figure 8. Reaction time profiles of the photoelectrochemical decomposition of water on N-TiO2(4)/Ti and the Pt electrode under visible light irradiation (λ g 450 nm). The applied bias was 1.5 V vs the Pt counter electrode.
Figure 10. Effect of calcination temperatures on photocurrent observed for N-TiO2(4)/ITO electrode under (A) UV light (λ g 300 nm) and (B) visible light (λ g 450 nm). Figure 9. XPS spectra of the N 1s peaks of the N-TiO2(4) thin film (a) before and (b) after photoelectrolysis for about 300 min and (c) after Ar+ ion etching following photoelectrolysis.
Supporting Information (Figure S1). Moreover, photoelectrochemical decomposition of water was carried out under bias conditions. As shown in Figure 8, H2 and O2 evolved in a stoichiometric ratio under visible light irradiation (λ g 450 nm), while no gas evolution was observed in the dark under the same experimental conditions. The total amount of evolved H2 was estimated to be 13 µmol from the photocurrent (35 µA) observed for 20 h. The calculated value was in rough agreement with the experimental value. These results indicate that the observed photocurrent is attributed to the photooxidation of water into O2 and not to the photooxidation of the N-TiO2(4) thin film itself. To evaluate the stability of the N-TiO2(4) film under light irradiation, the surface of the N-TiO2(4)/ITO electrode before and after photoelectrolysis was investigated by XPS. Figure 9 shows the XPS spectra of the N-TiO2(4)/ITO electrode before and after photoelectrolysis under visible light irradiation (λ g 450 nm) for 300 min. The intensity of the N 1s peak decreased after photoelectrolysis, indicating that the surface of the electrode is oxidized during photoelectrolysis. However, the intensity of the N 1s peak could be recovered after Ar+ ion etching of the film for 1 min. Furthermore, no noticeable differences were obtained in the UV-vis spectra of N-TiO2(4) before and after photoelectrolysis, indicating that the bulk of N-TiO2(4) remained stable throughout photoelectrolysis. These results showed that N-TiO2 thin films can act as efficient and stable photoanodes for the decomposition of water even under visible light up to 550 nm. Effect of Calcination Temperature on the Photoelectrochemical Properties of N-TiO2 Thin Films. To investigate
Figure 11. UV-vis absorption spectra of TiO2(O2/Ar) and N-TiO2(4) thin films calcined in air at various temperatures. Calcination temperature (K): (a) 473, (b) 673, (c) 773.
the thermal stability of the N-TiO2 films under calcination treatment, the N-TiO2(4)/ITO electrode was calcined in air at various temperatures. Figure 10 shows the photocurrents observed for the electrode before and after calcination at various temperatures. XRD investigations revealed that all the samples consisted only of an anatase phase. Under full arc irradiation from a 500 W Xe lamp, the anodic photocurrent increased with an increase in the calcination temperatures (TC), as shown in Figure 10A. In fact, the N-TiO2(4)/ITO electrodes calcined at 673 and 773 K exhibited higher photocurrent than the TiO2(O2/Ar)/ITO electrode. However, as shown in Figure 10B, under visible light irradiation of longer than 450 nm, the photocurrent increased with an increase in the TC until the maximum values were attained at 673 K and then decreased at a TC of 773 K. The UV-vis absorption spectra of N-TiO2(4) calcined at various temperatures are shown in Figure 11. The absorption band at wavelengths longer than 500 nm, perhaps due to the Ti3+ species, was found to decrease with an increase in the TC.
25272 J. Phys. Chem. B, Vol. 110, No. 50, 2006 And in fact, the amount of the Ti3+ species, which act as recombination centers for the photoformed electrons and holes, decreased with an increase in TC, leading to an increase in the photocurrent under UV light irradiation. The absorption band in visible light regions up to 500 nm also decreased with an increase in the calcination temperature. XPS investigations revealed that the surface nitrogen concentration of N-TiO2(4) after calcination decreased with an increase in the calcination temperature, as shown in the Supporting Information (Table S1). In the case of heat treatment up to 673 K, it was noted that the nitrogen concentration recovered to approximately 6% after Ar+ ion etching of the film for 5 min, indicating that the bulk of the N-TiO2 thin films remained stable even after calcination. However, the nitrogen concentration of N-TiO2(4) calcined at 773 K could not be recovered after Ar+ ion etching of the film for 5 min, suggesting that the bulk of the film was partially oxidized after calcination. The dramatic decrease in the photocurrent of N-TiO2(4) calcined at 773 K under visible light irradiation can be attributed to the decrease in the absorption band in visible light regions. These results show that N-TiO2 thin film photocatalysts remain stable even under heat treatment until 673 K. Conclusions N-TiO2 thin films were prepared by an RF-MS method using a N2/Ar mixture as the sputtering gas, and their photocatalytic activity and photoelectrochemical properties were investigated. The absorption band of the N-TiO2 thin film was found to shift smoothly toward visible light regions, the extent of the red shift depending on the concentration of nitrogen in the sputtering gas mixture. XPS N 1s investigations revealed that the concentrations of nitrogen substituted within TiO2 were determined to be 2.0-16.5%, the value increasing with an increase in the N2/(N2 + Ar) ratio of the sputtering gas. Moreover, XRD investigations suggested that the lattice constant of the TiO2 thin films expands by nitrogen substitution. These results showed the oxygen sites of the TiO2 lattice to be substituted by nitrogen atoms, leading to the appearance of steep absorption edges in the visible light region. These N-TiO2 thin films were found to exhibit effective and efficient photocatalytic activity for the liquid-phase degradation of 2-propanol diluted in water under visible or sunlight irradiation, even in outdoor field experiments. Furthermore, photoelectrochemical studies revealed that N-TiO2 thin films act as efficient and stable photoanodes for the decomposition of water even under visible light of up to 550 nm, and the APCE at 1.0 V was 25.2% at 360 nm and 22.4% at 420 nm. The photocatalytic activity of N-TiO2 thin films was also observed to be enhanced by calcination in air at 673 K through the oxidation of the Ti3+ species within the N-TiO2 bulk that acts as the recombination center of the photoformed electrons and holes. It was, thus, clearly demonstrated that the magnetron sputtering method can be applied to prepare highly nitrogen-substituted TiO2 thin films that can act as photocatalysts for such significant reactions as the purification of toxic compounds in polluted water and air as well as the production of H2 in the photoelectrochemical splitting of water under sunlight irradiation. Supporting Information Available: The reaction time profile of the photocatalytic evolution of O2 from a 0.05 M AgNO3 aqueous solution on Pt-loaded N-TiO2(4) under visible light irradiation and surface and bulk nitrogen concentration of N-TiO2(4) calcined at various temperatures. This material is available free of charge via the Internet at http://pubs.acs.org.
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