Materials Chemistry and Physics

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1 Materials Chemistry and Physics 117 (2009) Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: Preparation and characterization of N I co-doped nanocrystal anatase TiO 2 with enhanced photocatalytic activity under visible-light irradiation Liang Zhou a, Jian Deng b,, Yubao Zhao b, Wanbing Liu c, Lin An b, Fei Chen b a School of Public Health, University of South China, Hengyang Hunan , China b School of Chemistry and Chemical Engineering, University of South China, Hengyang Hunan , China c Central of quality technology supervision inspection test of Henan Pingdingshan, Pingdingshan Henan , China article info abstract Article history: Received 18 January 2009 Received in revised form 22 May 2009 Accepted 27 June 2009 Keywords: TiO 2 Co-doping Nitrogen Iodine Hydrolysis method Visible-light Photocatalytic activity N I co-doped TiO 2 nanoparticles were prepared by hydrolysis method, using ammonia and iodic acid as the doping sources and Ti(OBu) 4 as the titanium source. The prepared catalysts were characterized by X- ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and ultraviolet visible diffuse reflection spectroscopy (UV vis DRS). XRD spectra show that N I TiO 2 samples calcined at 673 K for 3 h are of anatase structure. XPS analysis of N I TiO 2 samples indicates that some N atoms replace O atoms in TiO 2 lattice, and I exist in I 7+,I and I 5+ chemical states in the samples. UV vis DRS results reveal that N I TiO 2 had significant optical absorption in the region of nm. The photocatalytic activity of catalysts was evaluated by monitoring the photocatalytic degradation of methyl orange (MO). Compared with P25 and monodoped TiO 2, N I TiO 2 powder shows higher photocatalytic activity under both visible-light ( > 420 nm) and UV vis light irradiation. Furthermore, N I TiO 2 also displays higher COD removal rate under UV vis light irradiation Elsevier B.V. All rights reserved. 1. Introduction Titanium dioxide (TiO 2 ) has been widely studied and used as a photocatalyst in virtue of its low cost, chemical stability, non-toxicity and favorable optoelectronic properties [1 5]. But a serious disadvantage of TiO 2 is that as a wide band gap semiconductor (3.2 ev for anatase TiO 2 ), it can only absorb UV light ( 387 nm), which accounts for only a small part (3 5%) of solar energy [1]. To improve the photocatalytic activity of TiO 2 under visible-light irradiation, many attempts have been made, such as initial method of doping TiO 2 with transition metals (Fe, Ce, La, etc.)[6 9]. After Asahi et al. [10] reported in 2001 that doping TiO 2 with nitrogen can narrow its band gap and induce visible photocatalysis, much attention has been paid on investigating nitrogen doping TiO 2 [11 15]. It was confirmed that N-doping can extend the optical absorption edge of TiO 2 into visible-light region, but the practical application of N TiO 2 is still hindered due to its low reactivity and quantum efficiency. To solve these problems, TiO 2 photocatalysts co-doped with nitrogen and metal or other non-metal ions have been actively studied recently. It was reported that TiO 2 co-doped with nitrogen and other elements, such as La[16], V[17], F[18], S[19],W[20] etc., Corresponding author. Tel.: ; fax: address: dengjfp@163.com (J. Deng). usually show superior photocatalytic activity under visible-light irradiation. Other studies demonstrated that TiO 2 doped with I showed high photocatalytic activity under UV and visible-light irradiation. Hong et al. [21] claimed that certain amount of Ti 3+ surface states/cation vacancies have been generated to maintain the electroneutrality by substituting Ti 4+ with I 5+ in the lattice, which can slow down the recombination of the electron hole pairs. Su et al. [22] argued that iodine exists in both I 7+ /I chemical states on the surface of the I-doped TiO 2 and a sub-band-gap transition corresponding to the excitation from the valence band of TiO 2 to the doped iodine species contributes to the visible-light response of I-doped TiO 2. Recently, Song et al. [23] synthesized I Ce co-doped TiO 2 powders, and claimed that the doping iodine exists as I 5+ /I in the TiO 2. The chemical state of iodine in TiO 2 can affect its photocatalytic activity to some degree. Taking into account of the fact that N-doping enhances TiO 2 s optical response in the visible region and I-doping enhances photocatalytic activity in both UV and visible region (although their mechanisms are debatable), it could be expected that N I codoping of TiO 2 might generate a significant effect to improve further the photocatalytic activity of TiO 2 and induce a significant visiblelight activity. In this study, a UV-enhanced and visible-light-driven TiO 2 photocatalyst co-doped with nitrogen and iodine was synthesized for the first time and characterized by XRD, XPS, FTIR and UV vis DRS, and its photocatalytic activities were examined /$ see front matter 2009 Elsevier B.V. All rights reserved. doi: /j.matchemphys

2 L. Zhou et al. / Materials Chemistry and Physics 117 (2009) through degradation of MO under visible-light irradiation and UV vis light irradiation respectively. 2. Experimental 2.1. Sample preparation The N I co-doped TiO 2 nanoparticals were synthesized by hydrolysis method. Tetrabutyl titanate was used as a starting material, ammonia as nitrogen source and iodic acid as iodine source, respectively. All chemicals used in the experiments were of analytical reagent grade. 10 ml of tetrabutyl titanate was mixed with 20 ml of ethanol in a 150 ml funnel by sonic oscillation for 10 min to form solution A; 2 g of iodic acid (atomic ratio of I/Ti = 0.4) was dissolved in 100 ml of ultrapure deionized water to form solution B. Solution A and 4 ml of ammonia were added dropwise into solution B under vigorously stirring condition. After continuous stirring for 2 h, the reaction mixture was placed in a 353 K water bath for 12 h. A wet precipitation was obtained and dried at 363 K for 24 h in an oven. After calcination at 673 K for 3 h, N I co-doped powders were prepared. For comparison, undoped TiO 2, N doped TiO 2, and I doped TiO 2 were synthesized respectively by a similar procedure Characterization The size and crystalline structure of all samples were determined using a Bruker D8 advanced X-ray diffractometer equipped with graphite monochromatized Cu K radiation ( = ). The Brunauer Emmett Teller (BET) surface areas (S BET)ofthe samples were measured with a surface area and pore analyzer (F-Sorb 2400, Beijing). The particle size was also determined by a transmission electron microscope (JEM-1230, Japan) operating at 100 kv. Chemical composition and chemical state were determined by X-ray photoelectron spectra (XPS) with a Kratos Axis Ultra DLD spectrometer, employing a monochromatic Al Ka X-ray source ( ev), hybrid (magnetic/electrostatic) optics, a multi-channel plate, and a delay line detector (DLD). FTIR spectra on pellets of the samples mixed with KBr were recorded on a Hitachi U-3010 (Tokyo, Japan) spectrometer. The ultraviolet visible diffusion reflectance spectra (UV vis DRS) of all samples were measured with the Hitachi U-3010 spectrophotometer at room temperature in the wavelength range of nm Photocatalytic activity measurements Photocatalytic activity of all samples was evaluated through the photodegradation of MO aqueous solution under UV vis and visible-light irradiation respectively. An artificial photo-reactor was used to conduct photocatalytic experiments. A 125 W Fig. 1. XRD powder diffraction patterns for TiO 2, N TiO 2, I TiO 2 and N I TiO 2. Table 1 Crystalline properties of the prepared TiO 2 photocatalysts. Nanophotocatalyst Crystalline structure Crystal size(nm) S BET(m 2 g 1 ) TiO 2 Anatase, brookite N TiO 2 Anatase I TiO 2 Anatase N I TiO 2 Anatase high pressure Hg lamp (GYZ125) with main emission wavelength in the range of nm was used as UV vis light source, while the lamp connected to a UV cutoff filter ( > 420 nm) was used for visible-light source. This artificial photo-reactor was surrounded by circulating water to control the temperature during the reaction. The reaction suspension was prepared by adding 50 mg of samples into 50 ml Fig. 2. TEM images of (a) TiO 2; (b) N TiO 2, (c) I TiO 2 and (d) N I TiO 2.

3 524 L. Zhou et al. / Materials Chemistry and Physics 117 (2009) of MO solution with concentration of 50 mol L 1 and adjusting ph to 3 with nitric acid. The distance between light source and the surface of the solution was 30 cm. Prior to photoreaction, the suspension was magnetically stirred in dark for 30 min to reach adsorption/desorption equilibrium. At the given time intervals, the analytical samples were taken from the suspension and immediately centrifuged at 4000 rpm for 10 min. Then its absorbance was measured by UV vis 8500 spectrometer at the wavelength of 504 nm to detect MO concentrations. Meanwhile, the mineralization of MO was determined by COD Mn concentration of the reaction solution [24]. 3. Results and discussion 3.1. Crystal structure and morphology analysis Fig. 3. XPS spectrum of N I doped TiO 2. The XRD patterns for all samples are shown in Fig. 1. It can be seen that pure TiO 2 is a mixed structure of anatase (90.6%) and brookite (9.4%) crystalline phases, and all other three doped samples are of anatase structure. The average grain size is calculated using the Scherrer s equation based on the fullwidth at half-maximum (FWHM) of the (1 0 1) peak of anatase. The crystalline structure, crystal size and BET surface areas of all samples are listed in Table 1. It is observed that doping of I reduced the grain size and enlarged the surface areas of nanoparticles. TEM results (as shown in Fig. 2) are in good agreement with the XRD data, N I TiO 2 exists not only in Fig. 4. High-resolution XPS of N I TiO 2: (a) N1s; (b) I3d; (c) Ti 2p and (d) O1s.

4 L. Zhou et al. / Materials Chemistry and Physics 117 (2009) quite small size, but also less agglomerates than other samples XPS analysis XPS analysis is performed to determine the chemical states and the possible main elements existing in the samples. Fig. 3 shows the XPS survey spectra of the N I co-doped TiO 2. Elements of Ti, O, N, I and C are confirmed to exist in the doped nanoparticles. N and I are resulted from the doping solution, and C probably is originated from the calcination residue of organic materials [25]. The molar content of these five elements Ti, O, N, C and I are 20.2, 63.7, 0.8, 7.9, 7.4%, respectively. Fig. 4a shows N1s XPS spectrum. Three peaks are observed at the binding energy around ev, ev and ev. Several reports demonstrated that the peak at 396 ev corresponds to N atoms in Ti N bonds, indicating that some N atoms substitute for O atoms in TiO 2 lattice, which correlates with the visible-light activity of doped TiO 2 [10,11,14]. The peak around 400 ev is due to the chemisorbed N 2 molecules, which are adsorbed onto the surface of TiO 2 [13,15]. As shown in Fig. 4b, three peaks of I3d are distinctly detected, suggesting that I exists in multi-valency in the N I doped TiO 2. The peaks at ev and ev can be ascribed to I 7+ and I, respectively, in agreement with the results reported by Su et al. [22]. Self-disproportionation reaction can occur to form heptavalent iodine (I 7+ ) and negatively charged iodine (I ) species during the heat-treatment process. The reaction can be expressed in Eq. (1). 4IO 3 = 3IO 4 + I (1) r G m = r H m T r S m (2) r G m = RT ln K (3) We can calculate the r H m (89.8 J mol 1 ) and r S m (32.4 J mol 1 K) of reaction 1 using Lange s Handbook of Chemistry[26,22], and the equilibrium constant K ( ) at 353 K according to Eqs. (2) and (3). It can be seen form the constant K that I 5+ coexist with I 7+ and I at the same time in the N I TiO 2, but the concentration of I 5+ is much lower than I 7+ and I when the reaction reaches equilibrium during the heat-treatment process. On the other hand, due to equivalent ionic radius of I 5+ (0.062 nm) and Ti 4+ (0.064 nm), it is easy for some I 5+ to substitute for Ti 4+ and enter into the TiO 2 lattice. Thus, the amount of I 5+ on the surface of TiO 2 decreases [21,23,27]. As a result, if I dopant concentration in TiO 2 is low (such as 1.1% in the report of Su), I 5+ might not be detected by XPS, as reported by Su [22]. While the concentration of iodine atom in this study reached 7.9%, I 5+ signal is intensive enough to be detected, so the weak peak at ev in Fig. 4b should be ascribed to I 5+. Fig. 4c shows the Ti2p XPS spectrum of N I TiO 2. The Ti2p3/2 peak can be split into two peaks: one peak at ev corresponding to Ti 4+ (75.5%), another peak at ev ascribed to Ti 3+ (24.5%), because the reduction of Ti 4+ Ti 3+ is resulted from a charge imbalance caused by I 5+ substituting Ti 4+ [28]. It can be seen from Fig. 4d that the prominent peak of O1s XPS at ev is due to the lattice oxygen. The other peak at ev can be attributed to the chemisorbed oxygen on the surface of N I TiO 2 [25,29]. Fig. 5. FT-IR spectra of the prepared samples: (a) pure TiO 2, (b)n TiO 2, (c) I TiO 2, (d) N I TiO cm 1 can be attributed to O H bending of adsorbed water molecules; the peaks at cm 1 can be assigned to stretching vibration of hydroxyl groups on the catalysts [31 33]. Dataofboth N TiO 2 and N I TiO 2 show a weak absorption peak at cm 1, which is the character of N H stretching vibration due to nitrogen doping [20,34]. Compared with other samples, there are two new peaks appearing around cm 1,1137.1cm 1 for N I TiO 2.T. bezrodna et al. [32] confirmed that these two bands were attributed to the absorption of different Ti OH surface groups, which were more stronger H-bonded with adsorbed water molecules, and not influenced by increasing temperature. Obviously, codoping with N and I increases the amount of surface-adsorbed water and hydroxyl groups, which would be beneficial for the photocatalytic process UV vis DRS The UV vis diffuse reflectance spectra of all samples are shown in Fig. 6. Pure TiO 2 is not able to respond to visiblelight, whereas N TiO 2, I TiO 2 and N I TiO 2 extend the absorption 3.3. FTIR analysis FTIR spectra of the prepared samples in the cm 1 region are shown in Fig. 5. For all samples, the intensive and broad bands between 800 cm 1 and 400 cm 1 are caused by the strong stretching vibrations of Ti O and Ti O Ti bonds [30]; the peaks at Fig. 6. UV vis DRS of prepared TiO 2 photocatalysts.

5 526 L. Zhou et al. / Materials Chemistry and Physics 117 (2009) Fig. 7. (F(R)E) 1/2 versus photon energy for prepared TiO 2 photocatalysts. edges to visible-light region. Moreover, N I TiO 2 exhibits the highest UV vis light absorption of the four samples. There exist two inflection points in the DRS curve of the N I TiO 2. The first edge ( nm) is well known to be related to the natural width of energy level. The second one is around nm, which indicates that the new energy levels are successfully formed in the forbidden band of TiO 2. As show in Fig. 7, the Kubelka Munk function is used to estimate the band gap energy of the prepared TiO 2 by plotting [F(R)E] 1/2 versus energy of light. The extrapolation of the linear portion of the modified spectra to zero absorption determines the band gap energies of the as-prepared photocatalysts. The optical band gap energies for TiO 2, N TiO 2, I TiO 2, N I TiO 2 are 3.20 ev, 3.08 ev, 2.80 ev and 2.65 ev, respectively. This means that doped TiO 2, especially N I TiO 2, have a narrower band gap than pure TiO 2. Fig. 8. Comparison of the photocatalytic degradation of MO under visible-light irradiation ( > 420 nm) by: p25, pure TiO 2, N TiO 2, I TiO 2 and N I TiO Photocatalytic activity The photocatalytic activities of prepared TiO 2 photocatalysts are measured by monitoring the photo-discoloration of MO. The concentration of MO is obtained by assuming the linear relation between the absorbance and the concentration of MO solution. The commercial TiO 2 photocatalyst (Degussa P25), which is thought to be a high active photocatalyst, is chosen as standard photocatalyst to contrast the photoactivity of the samples. As show in Fig. 8, under visible-light irradiation ( > 420 nm), all the doped TiO 2 photocatalysts show effective vis-photocatalytic activities, comparing with P25 and pure TiO 2. In particular, N I TiO 2 shows the best photocatalytic activity among all samples, and the decoloration rate of which is as high as 96.3% under visible-light irradiation for 2h. As shown in Fig. 9, under UV vis light irradiation, P25 shows better photocatalytic activity than N TiO 2, and is not significantly different from I TiO 2. These agree with the report of Hong [21], who pointed out that UV light makes a great contribution to the photocatalytic activity of P25 and I TiO 2. To our surprise, we found that the photocatalytic activity of N I TiO 2 is higher than those of mono-doped TiO 2 and P25. It was 3.0, 2.2, 1.4 and 1.3 times higher than that of undoped TiO 2, N TiO 2, P25 and I TiO 2, respectively. This indicates that not only UV lights but also visible-light play a dominant role in photocatalyic activity of N I TiO 2. Fig. 10 shows the COD Mn removal rate of all samples, which is used to evaluate the mineralization of organic compounds in reaction solution. Compared with P25 and I TiO 2, N I TiO 2 shows Fig. 9. Comparison of the photocatalytic degradation of MO under UV vis light irradiation (: nm) by: p25, pure TiO 2, N TiO 2, I TiO 2 and N I TiO 2. Fig. 10. Comparision of COD removal rate under UV vis light irradiation (: nm) by: p25, pure TiO 2, N TiO 2, I TiO 2 and N I TiO 2.

6 L. Zhou et al. / Materials Chemistry and Physics 117 (2009) better COD removal abilities, and the COD removal rate of which is as high as 41.7% under UV visible-light irradiation for 4 h. Compared with the decoloration rate (in Fig. 9), the speed of COD removing is much slower. This indicates that degradation of the MO takes place in a two-step manner: the first step might be the breakdown of the chromophore ( N N ) and the second step is mineralization, which will take more time than the decolorization step Discussion The high rate of decoloration and COD removal rate of the N I doped TiO 2 demonstrate that the synergistic effect of N and I in TiO 2 improves the photocatalytic activity of N I TiO 2. The XRD results indicate that the crystalline structure of the N I TiO 2 is anatase with smaller mean diameter. It is well known that the anatase phase and small mean diameter are very beneficial for photocatalytic degradation of organic contamination [35]. The XPS confirms that N atoms substitute at some of O sites in TiO 2, forming a narrow N2p band above the valence band, which was determined to be responsible for visible-light sensitivity [10,11,14]. Upon iodine doping, some iodine energy bands appear in the band gap of TiO 2. The sub-band-gap transition (I 7+ /I 5+ /I ) can further improve the visiblelight response of N I TiO 2 [22].I 7+ can accept e, and I can accept h +, thus can prevent electron hole recombination. Moreover, the existence of Ti 3+ on the surface of N I TiO 2 can slow down the recombination of the electron hole pairs [21,23,25]. The FTIR spectra show that there are large amount of hydroxyl groups and water molecules on the surface of N I TiO 2, which are considered beneficial for the photocatalytic process. On one hand, when the OH or H 2 O species accept h + under light irradiation, they may be oxidized to form OH; on the other hand, the surface hydroxyl groups can also act as absorption centers for O 2 molecules. The e from the conduction band can be captured by O 2 molecules to produce O 2. It is OH and O 2, which possess high oxidizability, that oxidize and mineralize MO. 4. Conclusions A UV-enhanced and visible-light-driven TiO 2 photocatalyst codoped with nitrogen and iodine was synthesized and characterized in this study. The XRD analysis shows that the crystal type of prepared N I TiO 2 is anatase with average grain size of 10.9 nm. The XPS analysis indicates that the N I TiO 2 photocatalyst mainly consists of Ti (Ti 4+,Ti 3+ ), O, N and I (I 7+,I and I 5+ ). The N I codoping can narrow the band gap of TiO 2, enhance the optical absorption in the range of nm, and hence lead to a higher vis-photocatalytic activity than that of mono-doped TiO 2. 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