Proceedings of the Eighth German-Vietnamese Seminar on Physics and Engineering, Erlangen, 03 -08, April, 2005 Visible Light Photocatalysis in Nitrogen – doped Titanium Oxides for Environmental Applications V.T.Bicha, T.T. Ducb, N.T.Tinhb, T.B. Ngoca, N.L.Lamb, V.T.M.Hanhb and T.X. Hoaib a Center for Quantum Electronics, Institute of Physics and Electronics, VAST 10, Daotan Road, Badinh District, Hanoi, Vietnam firstname.lastname@example.org b Institute of Applied Physics and Scientific Instruments, VAST 18, Hoang Quoc Viet Road, Caugiay District, Hanoi, Vietnam email@example.com Abstract. The status of the study on titanium oxide photocalysis for environmental protection in Vietnam has been showed. The transparent and colorless TiO2 nanoparticle coating films on tiles, glass or painted wall show sterilizing and self-cleaning properties thank to its photocatalysis and hydrophylicity. A new photocatalysis visible- light have been desired to make use the main part of the solar irradiation. This TiO2-xNx nanopowders were prepared by annealing the mixture of TiO2 powder (Degussa P25) and Urea in the air at 500°C for 3 hours. The characteristics of the prepared visible-light TiO2 photocatalysts were evaluated by the Xray diffraction (XRD), Raman spectroscopy and UV-visible reflectance spectroscopy. The photocatalytic activity of the prepared samples has been tested by decoloration of methylene blue under visible light. All these results confirm the presence of the nitrogen doping in the titania structure caused significant absorption shift to the visible region compared to pure TiO2 powder. The addition of nitrogen to titanium oxide extends the optical absorption of these oxides into visible range, enabling its use on the self-cleaning and self- sterilizing surfaces. 1. Introduction. The efficient utilization of solar energy is one of major goal of modern science and engineering. Of the materials being developed for photocatalytic applications, titanium dioxide remains the most promising because of the relatively high reactivity and chemical stability of the oxide under UV light excitation (λ< 387 nm). The sun can provide an abundant source of photons; however UV light accounts for only a small fraction (∼ 5%) of the sun’s energy compared to the visible region. An efficient process that shifts the optical response of active TiO2 from UV to the visible spectral range can provide a framework to more easily incorporate the photocatalytic and solar efficiency of this materials. Further the cost and accessibility of UV photons make it desirable to develop photocatalysts which are very high active under visible light excitation utilizing the solar spectrum or even interior room lighting. The photocatalytic activity of TiO2 is greatly influenced by its crystal structure, particle size and the doping. Recently, it is found that nitrogen doped titanium oxide photocatalyst TiO2-x Nx is also activated by visible light irradiation as well as ultraviolet irradiation [1-2]. Therefore, many techniques had been used to produce the visible- light- active TiO2-x Nx films, such as spray pyrolysis, sol- gel method, ion-implatation, plasma-treatment ect. However the quality of visible- light- active TiO2-x Nx films was not high enough and the cost for technology is so high. In Vietnam, the application domain with low cost is the aim of our research. Therefore, the first step we have paid much attention in preparing TiO2 photocatalyst thin film and applied it for self-cleaning, self- sterilizing and anti-fogging and the second step we are focusing on nitrogen doping study to convert the TiO2 absorption onset from the UV to the visible region to use the main part of the solar irradiation. In this paper, we will show the status of the study on titanium oxides photocalysis for environmental protection in Vietnam. 2. Experimental results and discussions 2.1 Synthesis and structural characterization of nanostructured TiO2 thin films To enable extension the applications of the photocatalyst in many fields of environmental clean up, nanostructured titanium dioxide thin films were prepared using the solgel method. The precursor solutions were prepared through the controlled hydrolysis of titanium tetraisopropoxide in water . The stable TiO2 photocatalyst sols are resulted from this procedure. The sol can be coated as a film with strong adhesion Fig. 1: TEM image of TiO2 strength, hardness and transparency. Using transmission particles in the TiO2 colloidal sol electron microscopy (TEM 125K Russia - Japan) it has been demostrated that the size of the colloidal particles was ca. 5 nm (Fig.1). X-ray diffraction analysis also confirmed that they are dominantly of the anatase nanocrystallites. After the heat treatment of TiO2 coating at 500oC the higher crystallinity was given and the particle size increased to about 25 nm. 2.2 Self-sterilizing photocatalytic coatings TiO2 has strong oxidation affects to single-celled organism that includes all bacteria and fungus. Fig.2a show that on the TiO2 coated glass, 90% of the bacteria were killed in Fig. 2: Sterilization effect of TiO2 coated 45 min under the light of 1000 lux (which is equal to the brightness of the surface of a study desk) and killed completely in about 3 h. We have seen also the in the Fig. 2b E. coli grows fast without TiO2 thin film under irradiation of fluorescent lamp(right side), but no bacteria were observed with the TiO2 films (left side). The results confirmed that the prepared nanostructured TiO2 coatings have the anti-bacteria properties even under the normal lightning condition. 2.3 Super-hydrophilic property–self-cleaning and anti-fogging When the surface of photocatalytic film is exposed to UV light, the contact angle of the TiO2 film with water is reduced gradually. After enough exposure to light, the surface reaches super-hydrophilic. Hydrophilic action makes the water to penetrate under the stains on the surface. The water lies flat on the surface in sheets instead of forming droplets (Fig.3). Dust and other contaminants are wiped away with water. Therefore, there are not dirty marks on the surface even after it is dry. When the original buildings materials are coated with a photocatalyst, the dirt on the walls will wash with rainfall, keeping the building exterior clean at all time. Photo catalyst coating Non coating (normal con.) Fig. 3 Water droplets are on right half of mirror, but water sheet formed on TiO2 coated left half, so it remains transparent 2.4 Synthesis and characterization of new photocatalyst - visible light sensitivity. To convert the TiO2 absorption onset from the UV to the visible region, visible-light TiO2 photocatalyst powder were prepared by annealing the mixture of commercial TiO2 powder (Degussa P25) and Urea (Guangzhou-China) in the air at 500°C for 3 hours, followed by washing in sulphuric acid to remove the residual urea. The obtained powder was a yellowish one. White TiO2 (Degussa P25) powder was also annealed in air at 500°C for 3 h as a reference sample (Fig.4). Fig. 4 TiO2 powder P25 (a); N-doped (b) Fig. 5 XDR patterns for untreated (top) and nitrided TiO2 nanopowder (bottom) The X-ray diffraction (XRD) measurements were performed on a Seimens/ D5000 X-ray diffractometer using CuKα radiation. XDR patterns of visible-light TiO2 and the reference were presented in the Fig.5. It is have seen from the figure that both structures correspond dominantly of the anatase phase and a small amount of the rutile, but not TiN were observed. FT-Raman spectra of white TiO2 powder and prepared yellow nitrogen- doped TiO2 that were carried out on a Nicolet in the 100- 1200 cm-1 frequency range with 1064 nm exciting line have seen in the Fig.6. Five peaks can be observed: a strong sharp peak at 144 cm-1, tree wide midintensity peaks at 397, 515 and 637 cm-1 and weak peak at 196 cm-1. The three peaks at 637, 196 and 144 cm-1 are assigned to the Eg modes and the band at 397 cm-1 to the B1g mode of the anatase TiO2 phase. The band at 515 cm-1 is a Fig. 6 Raman spectrum of commercial TiO2 double of A1g and B1g modes of TiO2 anatase. The (Degussa P25) and TiO2 nitrided data in the Fig.6 demonstrate the fit to the Raman nanoparticles and the fit for TiO2 nitrided lines of the nitrided TiO showing two features at 2 nanoparticles 445 cm-1 and 612 cm-1 from rutile structure of both powders and a feature at 550 cm-1 (TO mode) that has been attributed to the first-order scattering of non-stoichiometric titanium nitride. This band originates from N atoms surrounding Ti vacancies . Fig.6 also presents that all positions and the widths of the principal Raman bands were not changed. Thus, in agreement with the XRD data, all Raman peaks can be associated with the dominantly anatase powder containing a small amount of rutile in the nanometer-size, the crystalline structure is not changed by the doping procedure and the observed nitrogen-doping sample was confirmed. Since the XRD did not indicate the formation of TiN bonds, it was determinated that O-Ti-N bonds were formed. Therefore, we propose these powders can be described as TiO2-xNx. Fig.7 compares the optical reflectance spectrum for Degussa P25 TiO2 (reported at an average size of 25 1 nm) that were recorded on a Varian Cary 5 UV-VisNIR spectrometer on setting sharply at ~380 nm and the reflectance spectrum for yellow N-doped TiO2 2 which is showing the red-shift effect of nitrogen doping on the absorption of the nanocrystallites. This significant shift on the sample must be associated with the presence of nitrogen ion in the titania structure in the case not change of size of Fig. 7 Reflection spectra for Degussa P25 nanoparticles, as supported by XRD and FT-Raman TiO2 (1) ; -doped yellow TiO2 (2) measurements . The site that give rise to this shift in the reflectance may correspond to titanium ion (Ti4+)1 2 3 O-hole centers which are perturbed by nitrogen . Photocatalytic activity of synthesized nanopowders has been evaluated by measuring the decoloration of methylene blue in using Halogen lamp as visible light source. The blue color of aqueous solution changed markedly by naked eye during irradiation under visible Fig. 8. The photodegradation of light with the irradiation time in the case of the present methylene blue: before (1) and under of the nitrogen component of titanium dioxide, as show visible light after 90 min with:2)TiO2; in Fig.8. 3) N-doped TiO2. 3. Conclusion The status of the study on titanium oxides photocalysis for environmental protection in Vietnam have been showed. The TiO2 nanoparticle coating agents were produced and coated on various materials. Photocalysis technology is easy of setup, low consumption of energy, operation at ambient temperatures and consequently low cost. The anti-bacteria properties even under the normal lightning condition and the sterilizing, self-cleaning properties thank to its photocatalysis and hydrophylicity. A new nitrogen doping TiO2 photocatalyst can be realized by annealing a TiO2 nano powders and Urea at 500°C. The XRD and Raman results were confirmed the observed Ndoping sample is dominantly anatase powder containing a small amount of rutile phase in the nanometer-size. The photocatalytic activity of the prepared samples has been tested by decoloration of methylene blue under visible light. All these results confirm the presence of the nitrogen doping in the titania structure caused significant absorption shift to the visible region compared to pure TiO2 powder and this N-doping powder extends enabling its use on the self-cleaning and self- sterilizing surfaces. 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