NANO LETTERS
Enhanced Nitrogen Doping in TiO2 Nanoparticles
2003 Vol. 3, No. 8 1049-1051
Clemens Burda,*,† Yongbing Lou,† Xiaobo Chen,† Anna C. S. Samia,† John Stout,‡ and James L. Gole‡ Center for Chemical Dynamics and Nanomaterials Research, Department of Chemistry, Case Western ReserVe UniVersity, CleVeland, Ohio 44106, and School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332 Received May 22, 2003; Revised Manuscript Received June 10, 2003
ABSTRACT TiO2 - xNx nanoparticles were prepared by employing the direct amination of 6−10-nm-sized titania particles. Doping on the nanometer scale led to an enhanced nitrogen concentration of up to 8%, compared to e2% in thin films and micrometer-scale TiO2 powders. The synthesized TiO2 - xNx nanocrystals are catalytically active and absorb well into the visible region up to 600 nm, thus exemplifying the use of a nanostructurebased synthesis as a means of producing novel photocatalytic materials.
The efficient utilization of solar energy is one of the major goals of modern science and engineering that will have a great impact on technological applications.1-5 Of the materials being developed for photocatalytic applications, titanium dioxide (TiO2) remains the most promising because of its high efficiency, low cost, chemical inertness, and photostability.6-10 However, the widespread technological use of TiO2 is impaired by its wide band gap (3.2 eV), which requires ultraviolet irradiation for photocatalytic activation. Because UV light accounts for only a small fraction (8%) of the sun’s energy compared to visible light (45%), any shift in the optical response of TiO2 from the UV to the visible spectral range will have a profound positive effect on the photocatalytic efficiency of the material.11 Here we report a simple nitrogen-doping method for nanometer-sized visible-light TiO2 photocatalysts. The prepared photocatalysts show an enhancement in the photodegradation efficiency of methylene blue under visible light (wavelength g 390 nm) irradiation compared to commercially available TiO2 catalyst. An initial approach to shifting the optical response of TiO2 from the UV to the visible spectral range has been the doping of TiO2 with transition-metal elements.12-18 However, metal doping has several drawbacks. The doped materials have been shown to suffer from thermal instability, and the metal centers act as electron traps, which reduces the photocatalytic efficiency. Furthermore, the preparation of transition-metaldoped TiO2 requires more expensive ion-implantation facilities.19,20 Recently, it was shown that the desired band gap narrowing of TiO2 can be better achieved by using anionic * Corresponding author. E-mail:
[email protected]. † Case Western Reserve University. ‡ Georgia Institute of Technology. 10.1021/nl034332o CCC: $25.00 Published on Web 07/09/2003
© 2003 American Chemical Society
dopant species rather than metals ions.11,21,22 Substitutional doping of nitrogen was found to be most effective because its p states contribute to the band gap narrowing by mixing with O 2p states. Recently, Asahi et al. showed that TiO2 films can be doped with nitrogen by sputtering methods and exhibit thereafter enhanced photoactivity in the visible spectral range.11 Considerable effort has been undertaken to dope TiO2 thin films and powders with nitrogen by annealing TiO2 at elevated temperature under NH3 flow for several hours. Nevertheless, the doping process on these micrometersized TiO2 systems resulted in only small amounts (e2%) of nitrogen incorporation.11 We have developed an alternative nanoscale synthesis route that leads to increased nitrogen dopant concentration (up to 8%) in titania. TiO2 - xNx has been synthesized at room temperature by employing the direct amination of TiO2 nanoparticles. The synthesized TiO2 - xNx nanoparticles are photocatalytically active, with absorbance that extends into the visible region up to 600 nm. Small TiO2 nanocrystals were prepared by the controlled hydrolysis of titanium (IV) isopropoxide in water under controlled pH.23,24 By adjusting the pH of the solution, TiO2 nanocrystals in the size range of 3 to 10 nm can be synthesized as transparent colloidal solutions, which are stable for extended periods.24 To introduce the nitrogen dopant into the titania nanoparticles, triethylamine is added to the colloidal nanoparticle solution. The addition of amine to the nanoparticle solution results in the formation of yellow nanocrystals (mean diameter of ∼10 nm).23 X-ray powder diffractometry (XRD) and high-resolution transmission electron microscopy (HRTEM) demonstrate that the treated
Figure 1. XPS spectrum of a TiO2 - xNx nanoparticle sample with an average diameter of 10 nm measured on a carbon support.
Figure 2. Reflectance measurements showing the red shift in optical response due to the nitrogen doping of TiO2 nanoparticles.
nanostructures are of the anatase crystalline phase. In addition, X-ray photoelectron spectroscopy (XPS) confirms enhanced nitrogen incorporation (Figure 1). The change in color of the nanocrystals upon nitrogen incorporation demonstrates a profound effect on their optical response in the visible wavelength range. In contrast to the nanoparticle reactivity, which we have described above, no significant reaction was observed when TiO2 micropowders were treated with triethylamine. Furthermore, the treatment of Degussa P25 “nanopowder” (particle size g 30 nm) resulted in a much slower doping reaction. Shown in Figure 2 are the UV-visible reflectance spectra of pure TiO2 and nitrogen-doped TiO2 nanoparticles. The reflectance measurements on the doped TiO2 nanoparticles show that the band gap absorption onset of the nanocrystals shifted from 380 to 600 nm. This Figure compares the optical reflectance spectra of commercial Degussa P25 TiO2, with an onset at 380 nm (spectrum a), and the reflectance spectrum
Figure 3. Comparison of the photocatalytic decomposition of methylene blue in the presence of doped and undoped titania nanoparticles, as monitored by the changes in absorbance at 650 nm after (a) 390-nm laser excitation and (b) 540-nm excitation. The inset in a shows the photodegradation of methylene blue in water at neutral pH. 1050
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for TiO2 - xNx nanocrystals with size = 10 nm, rising at 600 nm (spectrum b). The photocatalytic activity of the nanoparticles was evaluated by measuring the decomposition of methylene blue at 650 nm upon photoexcitation with 390- and 540-nm visible light using a Clark MXR 2001 femtosecond laser system. The laser beam (800 fs, 1 kHz, 120-fs laser pulse train) was sent either through a BBO crystal to generate second harmonic 390-nm (10 mW) light or to an optical parametric amplifier to generate stable 540-nm (4 mW) light. The pulse train was guided into a quartz cuvette filled with 2 mL of an aqueous solution of methylene blue (optical density of 0.8) and 10 mg of the new catalyst to excite a pump volume of about 5 nL (0.5 mm is the diameter of the excitation beam at the reaction cell). The decomposition of the solute was followed by measuring the decolorization of the methylene blue in solution with a Varian Cary Bio50 UV-vis spectrometer. The analysis of the monochromatic photon flux and taking into account the excitation volume show that about 1 photon/particle was used for excitation, which is in the low-intensity regime and compares with bright sunlight excitation. Shown in Figure 3 is the observed photodegradation of methylene blue in water at neutral pH. The nitrogen-doped nanoparticles showed enhanced photocatalytic activity. However, the undoped TiO2 nanoparticles (Degussa P25) did not show much activity under visible-light radiation compared to the reference experiment without nanoparticles. The observed gradual decrease in the absorption of methylene blue over time is attributed to the direct decomposition of the dye upon laser irradiation and is not due to the excitation of the Degussa P25 TiO2 nanocrystals.25 The difference in the photocatalytic activity of the nanoparticles observed after 390- and 540-nm excitation can be correlated to their reflectance spectra presented in Figure 2. At 390 nm, a larger difference in optical response is observed, which explains the significant difference in the photocatalytic activity at 390-nm irradiation (Figure 3a). However, at wavelengths >500 nm, the differences in the optical responses are smaller, hence the photocatalytic activity under 540-nm irradiation is less pronounced. Nevertheless, the nitrogen-doped nanocrystals still exhibited higher photocatalytic activity. In conclusion, this study demonstrates the effectiveness of using nanometer-scale materials in developing efficient visible-light-activated photocatalysts. Furthermore, it is shown that the modification of the TiO2 photocatalyst can be realized by using a simple, room-temperature synthesis process, which results in enhanced nitrogen incorporation that is most efficient on the nanometer scale. This study could provide a pathway for the production of environmentally benign photocatalysts that exceed the efficiency of current catalysts, particularly for visible-light activation. Acknowledgment. C.B. gratefully acknowledges financial support from an NSF CAREER Grant (CHE-0239688), ACS PRF (39881-G5M), and the Ohio Board of Regents. Nano Lett., Vol. 3, No. 8, 2003
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