Chaoyang District, Beijing , P.R of China. Tohoku University, Sendai , Japan

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1 Solid State Phenomena Vols (2009) pp Online available since 2009/Jan/06 at (2009) Trans Tech Publications, Switzerland doi: / Visible-light Induced Photocatalytic Activity of Vanadium and Nitrogen Co-doped SrTiO3 Jinshu Wang 1, a Hui Li 1, Hongyi Li 1, Shu Yin 2, b, Tsugio Sato 2 1 School of Materials Science and Engineering, Beijing University of Technology, Chaoyang District, Beijing , P.R of China 2 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai , Japan a wangjsh@bjut.edu.cn; b shuyin@tagen.tohoku.ac.jp Keywords: photocatalyst, photocatalytic efficiency, Dopant, visible light Abstract. Vanadium doped, nitrogen doped, and vanadium and nitrogen co-doped SrTiO 3 powders with size of nm in diameter were prepared by high energy milling method respectively. It was found that compared with those of pure SrTiO 3, the photocatalytic activities of these three doped SrTiO 3 samples for decomposing NO under visible light (λ>400 nm) and near ultraviolet light (λ>290 nm) irradiation were improved, and the co-doped sample exhibited the highest photocatalytic activity. 43.2% NO could be eliminated under the irradiation of light with wavelength larger than 400nm, about 3 times higher than that by pure SrTiO 3. The photocatalytic activity of this sample near ultraviolet range is about 1.7 times higher than that of pure SrTiO 3. The high visible light photocatalytic activity of this substance may be due to the high visible light absorption and large specific surface area. Introduction Photoelectrochemical processes at semiconductor colloid-electrolyte interfaces have been received special attention because of their possible application to conversion of solar energy into chemical energy and pollution control. Now, photocatalytic reaction systems have been commercially supplied to conduct self-cleaning, deodorant, anti-bacteria, etc., and are expected to be applied to the pollution control to decompose toxic materials in air and waste water using sun light and indoor light. Until now so many photocatalysts have been developed. SrTiO 3, as one of important photocatalysts, has been used for water splitting and mineralization of organic pollutants under UV radiation [1-3]. However, it can not utilize visible light and make use of ultraviolet (UV) light which is less than 5% of the solar beams on the earth because of relatively large band gap energy of 3.2 ev. In the past, many researchers have attempted to modify the electronic properties of another photocatalyst TiO 2 in order to extend its optical absorption edge into the visible light region and to improve the photocatalytic activity. Some groups have carried out the studies on the doping of transition metal ions and nonmetal elements into TiO 2 lattice to red shift the absorption edge [4-7]. It was reported that Cr or V ion-implanted TiO 2 absorbed visible light efficiently and could decompose NO by visible light irradiation. We found that nitrogen doping is also effective to improve visible light response photocatalytic activity of SrTiO 3 [8, 9]. It is suspected, however, that replacing O 2- with N 3- would result in the formation of anion defects for the charge compensation and the anion defects would act as electron-hole recombination centers. It is expected that the charge compensation is satisfied if O 2- is replaced with N 3- together with replacing Sr 2+ or Ti 4+ with a higher valence metal ion. Since it was reported that V was favorable for the improvement of photocatalytic activity of TiO 2, we took V ion as the substitution ion to maintain the charge balance caused by the substitution of N 3- to O 2-. Therefore, we prepared a novel stable photocatalyst, nitrogen and vanadium co-doped SrTiO 3 by a simple and efficient way for the synthesis of material without heating, mechanochemical reaction. The photocatalytic activity of this sample will be introduced in this paper. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: , Pennsylvania State University, University Park, United States of America-25/06/14,01:08:58)

2 852 Mechatronic Systems and Materials III Experimental methods SrTiO 3 was prepared by solid state reaction of SrCO 3 and TiO 2 at 1100 o C for 2h in air. Vanadium nitride, vanadium oxide and urea were used as nitrogen and vanadium co-doped source, vanadium source and nitrogen source, respectively. SrTiO 3 powder was mixed with 0.5mol%, 1mol% of VN, 0.5mol%V 2 O 5, 15wt% urea and these samples were assigned as Samples 1, 2, 3 and 4. A planetary ball mill (Pulverisette-7, Fritsch, Germany) was used to grind the mixture at 700 rpm rotation speed for 2 h at room temperature in air. Three grams of the mixed powder was charged in a zirconia pot of 45 cm 3 inner volume with seven zirconia balls of 15 mm in diameter. The binding energies of N and V were measured at room temperature using an electron spectrometer (Perkin Elmer PHI 5600).The UV-Visible spectra were recorded by UV-VISIBLE spectrometer(uv-2550,shimadzu, Japan).The particle size and shape were evaluated by transmission electron microscope (JEM-2000EX). The photocatalytic activity for the oxidative decomposition of nitrogen monoxide was determined by measuring the concentration of NO gas by using a NO x analyzer (ATI 200E) under the irradiation of light. Results and discussion Fig. 1 shows the binding state of vanadium of different samples. It could be seen that V2s XPS spectra of pure VN displayed that two kinds of vanadium existed in the sample, see Fig. 1a. The peak around 514.3eV was assigned to vanadium in VN and the latter peak around 517.4eV suggested the presence of vanadium in V 2 O 5, indicating that part of vanadium in VN had been oxidized into V 2 O 5. One V2s peak of the milling product of SrTiO 3 and VN located at 515.7eV, which could be assigned as the doping state of vanadium in SrTiO 3 lattice. Fig. 2 displayed N1s spectra of the samples. Using VN as co-doping nitrogen and vanadium source, one N1s peak positioned at 398.4eV existed in the Sample 2 spectra, which could be assigned as doping state of nitrogen in SrTiO 3 lattice by comparison with that obtained for starting material VN since the binding energy of N1s of VN located at 1.0eV lower energy side, ev (Fig. 2a). Our former experimental result showed that N1s in nitrogen doped SrTiO 3 had the binding energy of 398.5eV [9], which also confirmed the existence of nitrogen in the lattice of SrTiO 3 prepared. Figs. 1-2 indicate that nitrogen and vanadium have been doped into SrTiO 3 lattice. b b a a Binding energy/ev Fig. 1. XPS spectrum of V2s of (a) Pure VN and (b) Sample Bingding enenrgy/ev Fig. 2. XPS spectrum of N1s of (a) Pure VN and (b) Sample 2 Fig. 3 illustrates the diffuse reflection spectra of starting material SrTiO 3 and the doped SrTiO 3. SrTiO 3 showed absorption edge at 390 nm corresponding to the band gap energy of 3.18 ev (see Fig. 3a), which was consistent with the band gap of 3.2eV of SrTiO 3 reported by Cardona[10].Such large band gap resulted in that SrTiO 3 particles must absorb light energy high enough to form electron-holes pairs. It could be seen in Fig. 3, all the doped samples have higher visible light absorption than pure SrTiO 3. The visible light absorption capabilities of the samples were in the

3 Solid State Phenomena Vols order: vanadium and nitrogen co-doped SrTiO 3 > nitrogen doped SrTiO 3 > vanadium doped SrTiO 3 > pure SrTiO 3. Fig. 4 shows TEM observation of vanadium and nitrogen co-doped SrTiO 3 and pure SrTiO 3. It showed that pure SrTiO 3 consisted of relatively large particles of about m in diameter (see Fig. 4a) whereas the nitrogen and vanadium co-doped SrTiO 3 consisted of two kinds of particles in size (Fig. 4b). One was the spherical nano-sized particles about nm in diameter. The other one was spherical aggregated particles in the size about 0.1 m with the surface was covered by tiny crystallites. The tiny particles seemed to be brought by high energy grinding. During high energy milling, SrTiO 3 powders were subjected to severe mechanical deformation by the collisions with milling balls and pot. The deformation is localized in shear bands consisting of an array of dislocations with high density. At a certain strain level, these dislocations annihilate and recombine as small-angle grain boundaries separating the individual grains. The subgrains produced by this route are in the nanometer range and the average grain size could be reduced to about nm. Fig. 3. The diffuse reflection spectra of SrTiO 3 (a), vanadium doped SrTiO 3 (b), nitrogen doped SrTiO 3 (c) and vanadium and nitrogen co-doped SrTiO 3 (d) Fig. 4. Transmission electron micrographs of (a) pure SrTiO 3 and (b) Sample 2 Fig. 5 illustrates the relationship between the wavelength of light source and photoreactivity of vanadium and nitrogen co-doped samples prepared with different amount of vanadium nitride (0.5mol%VN and 1mol%VN) and SrTiO 3, nitrogen doped SrTiO 3, vanadium doped SrTiO 3 as well as pure SrTiO 3 under irradiation of visible light ( > 400 nm) and UV light ( > 290 nm). It took about 10 min to reach the steady state after light irradiation. It was also found that when the light was turned off, NO concentration returned to its initial level of 1 ppm within 10 min. These results suggested that light energy is essential for the oxidation of NO, i.e., NO was photocatalytically eliminated. As expected from its large band gap energy, pure SrTiO 3 exhibited little visible light photocatalytic activity. It shows that all the doped SrTiO 3 samples especially the co-doped samples showed higher NO elimination ratio than pure SrTiO 3. Fig. 5a and b shows that the increase of the amount of VN is favorable for the improvement for the photocatalytic activity. When SrTiO 3 was grounded with 1mol% VN for 2h followed by heat treatment at 150 o C, Fig. 5c, the powder presented excellent photocatalytic ability on NO destruction especially in the visible light range. 43.2% NO could be eliminated in the light wavelength larger than 400nm, ca. 3 times higher than that by pure SrTiO 3. The photocatalytic activity of this sample near ultraviolet range is about 1.7 times higher than that of pure SrTiO 3. Fig. 5b and c show that heat treatment at 150 o C is favorable for the improvement of the photocatalytic activity. It is well known that the high energy grinding may result in the formation of defects in the sample which may become the recombination centers of the photo-generated electrons and holes. Heat treatment could reduce the number of these defects. In addition, such treatment could

4 854 Mechatronic Systems and Materials III decrease the number of hydroxyl groups on the surface of the powder since hydroxyl groups are easy to react with holes to form peroxides which could also become the recombination centers of photo-generated electrons and holes. Fig. 5. The photocatalytic activity of V and N co-doped SrTiO 3 prepared with (a) 0.25mol%VN and SrTiO 3, (b) 1mol%VN and SrTiO 3, (c) Heated (b) at 150 o C, (d) nitrogen doped SrTiO 3 prepared with 15wt%urea and SrTiO 3 followed by calcinations at 400 o C, (e) vanadium doped SrTiO 3 prepared with 0.5mol% V 2 O 5 and SrTiO 3 and (f) pure SrTiO 3 under irradiation light with different wavelength During the preparation of nitrogen doped SrTiO 3, the milled product should be calcined at 400 o C since taking urea as the nitrogen source, the following two reactions might take place during the grinding: 2(NH 2 ) 2 CO NH 3 + NH 2 CONHCONH 2 (biuret) (1) 3(NH 2 ) 2 CO 3NH 3 + C 3 H 3 N 3 O 3 (cyanuric acid) (2) NH 3 produced by these two reactions seems to react with SrTiO 3 to dope nitrogen. The solid-state interdiffusion reaction during reactive ball milling was triggered by fragmentation of SrTiO 3 powder thus creating new surfaces. These freshly created surfaces reacted with NH 3 to form a nitrogen doped SrTiO 3 surface layer over the unreacted core particles. However, it should be noted that the byproducts of NH 2 CONHCONH 2 and C 3 H 3 N 3 O 3 remained in the sample, therefore, the milled sample should be calcined at high temperature to remove these unfavorable products. According to the previous results in TiO 2 system [11], it is clear that in the presence of oxygen the photo-generated electrons in the conduction band of SrTiO 3 are trapped immediately by the molecular oxygen to form O 2 -, which can then generate high active OOH radicals. The NO reacts with these reactive oxygen radicals, hole, molecular oxygen and water to produce HNO 2 and HNO 3. The photocatalytic activities under visible light and ultraviolet irradiation ( >400 and >290 nm) were in the order: the V and N co-doped SrTiO 3 prepared with 1mol%VN > the co-doped SrTiO 3 prepared with 0.25mol%VN > nitrogen doped SrTiO 3 > vanadium doped SrTiO 3 >pure SrTiO 3. The vanadium and nitrogen co-doped SrTiO 3 possessed the highest visible light absorption capability among all the doped SrTiO 3 samples and the nitrogen doped sample were in the second place. Therefore, the co-doped sample exhibited the best performance in the removal of nitrogen monoxide and the order of the photocatalytic activity of these samples followed the sequence shown above. Specific surface area of the sample is another important factor which affects the photocatalytic activity. It is well known that the larger specific surface area results in higher adsorption ability of NO. In addition, it needs less time for photo-generated carriers to diffuse from the inside of a sample to the surface. As a result, the tendency for the recombination of photo-generated electrons and holes could be decreased with decreasing crystallite size. Vanadium and nitrogen co-doped SrTiO 3 had larger specific surface area

5 Solid State Phenomena Vols (22.0m 2 g -1 ) than pure SrTiO 3 (4.1m 2 g -1 ).On other hand, the co-doped sample possessed a larger pore volume than pure SrTiO 3, i.e., the total pore volume of the co-doped sample and pure SrTiO 3 were 26.8 and 7.4 cm 3 g -1, respectively. The mesopores are responsible for the effective adsorption of NO. The photocatalytic activity results indicate that co-doping of vanadium and nitrogen in SrTiO 3 could generate a stable visible light-responsive photocatalyst with good nitrogen monoxide destruction potential. Conclusions Three kinds of doped SrTiO 3, namely, nitrogen doped SrTiO 3, vanadium doped SrTiO 3 and vanadium and nitrogen co-doped SrTiO 3 have been prepared by a sample way of high energy milling of SrTiO 3 and the related element sources such as urea (nitrogen source), vanadium oxide (vanadium source) and vanadium nitride (vanadium and nitrogen co-doping source). The co-doped sample shows the highest absorption capability in visible light range and functions as a visible light responsible photocatalyst for the photo-oxidation of nitrogen monoxide. The photocatalytic activities under visible light and ultraviolet irradiation were in the order: the V and N co-doped SrTiO 3 > nitrogen doped SrTiO 3 > vanadium doped SrTiO 3 >pure SrTiO 3. The photocatalytic activity of SrTiO 3 in NO elimination was improved by 3 and 1.7 times under visible light ( >400 nm) and UV light ( >290 nm) irradiation, respectively after the co-doping. Acknowledgement This work is sponsored by New Century Excellent Talents in University, Program for Scientific development proposed by Beijing education committee (KM ). References [1] K. Domen, A.Kudo, T. Onishi, J. Catal.Vol.102(1986),p.92 [2] Q.S.Li,, K.Domen, S.Naito, T.Onishi and K.Tamaru, Chem. Lett.Vol.3(1983),p.321 [3] S. Ahuja, T.R.N. Kutty, J. Photochem. Photobio. A: Chem. Vol.97(1996),p.99 [4] J. Zhu, Z. Deng, F. Chen, et al, Appl. Catal. B: Environ. Vol.62(2006),p.329 [5] M. Anpo, M. Takeuchi, J. Catal. Vol.216(2003),p.505 [6] R. Asahi, T. MOrikawa, T Ohwaki et al, Science Vol.293(2001),p.269 [7] J. Jang, H. Kim, S. Ji, et al, J. Solid State Chem. Vol.179(2006),p.1064 [8] J. Wang, Y. Shu, T.Sato, Appl. Catal. B: Environ. Vol.52 (2004), p.11 [9] J. Wang, Y. Shu, T.Sato, J. Photochem.& Photobio. A: Chem. Vol.187(2007),p.72 [10] M. Cardona, Phys.Rev. Vol.140(1965),p.651 [11] S.Yin, and T.Sato, Ind.Eng.Chem.Res. Vol.39 (2000),p.4526

6 Mechatronic Systems and Materials III / Visible-Light Induced Photocatalytic Activity of Vanadium and Nitrogen Co-Doped SrTiO /