Characterization of Visible Light Response N-F Codoped TiO2 Photocatalyst Prepared by Acid Catalyzed Hydrolysis

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1 ACTA PHYSICO-CHIMICA SINICA Volume 24, Issue 11, November 2008 Online English edition of the Chinese language journal Cite this article as: Acta Phys. -Chim. Sin., 2008, 24(11): ARTICLE Characterization of Visible Light Response N-F Codoped TiO2 Photocatalyst Prepared by Acid Catalyzed Hydrolysis Xiaohui Li, Shouxin Liu* College of Material Science and Engineering, Northeast Forestry University, Harbin , P. R. China Abstract: N-F codoped TiO 2 (TONF) photocatalysts were prepared using acid catalyzed hydrolysis method from mixed aqueous solution of TiCl 4 and NH 4F. The photocatalytic activity of the TONF was evaluated through the degradation of phenol under both visible and UV light irradiation. X-ray photoelectron spectroscopy (XPS), UV-Vis diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), scanning electron microscope (SEM), and N 2 adsorption isotherm were used to characterize the obtained powders. The results showed that N-F codoped TiO 2 exhibited significant improvement of visible light catalytic activity. N-F codoping could improve dispersion of TiO 2, inhibit particle size agglomeration, and retard phase transformation. Doped N could extend the light response of TiO 2 to visible light region. In addition, narrower band gap formed by F-doping was beneficial to the high visible light photocatalytic activity. Key Words: TiO 2; N-F codoping; Visible light; Photocatalyst Visible light driven photocatalysis has attracted much attention in the field of water split for green energy and pollutants degradation for environmental conservation. For the former, the main research target is the development of a novel photocatalytic material system. For the latter, it mainly focuses on the modification of TiO 2. Some UV-active oxides were used as visible light photocatalysts by substitutional doping of metal ions [1,2], ion implantation [3], organic dye sensitization [4], hydrogen plasma reduction of TiO 2 [5], and hydroxide or surface coordination [6]. However, these modified photocatalysts, in general, show a small absorption in the visible light region and deactivate easily [7]. Band-gap narrowing by the introduction of nonmetal anions (N, C, S, and F) into TiO 2 was recently found to be more efficient than the traditional methods to yield catalyst with high catalytic activity under visible light irradiation [7]. N-doping is one of the most efficient ways to enhance the visible light response. Asahi et al. [8] prepared TiO 2 x N x with adsorption edge red shift to 550 nm by sputtering the TiO 2 target in N 2 /Ar gas atmosphere followed by heat treatment at 600 C for 3 h. Kobayakawa et al. [9] heated the mixed aqueous solution of titanium hydroxide, obtained from tetraisopropyl titanate (TTIP) hydrolysis, and urea, the prepared photocatalysts exhibited significant visible light response (<550 nm). However, due to the decrease of UV-activity, most of these catalysts do not have sufficiently high levels of activities for a wide light region. Nosaka et al. [10] prepared N doped TiO 2 (TON) visible light responsive photocatalysts by using guanidine carbonate, guanidinium chloride, and urea as N atom sources on commercial TiO 2. Liu et al. [11] obtained N doped TiO 2 by calcinating the TiO 2 precursor in the atmosphere of NH 3 /N 2. The atomic radius of F is close to that of N. Therefore, F doping can influence the energy band structure and activity of TiO 2. Yu et al. [12] hydrolyzed TTIP in NH 4 F solution and found that F doping can enhance surface area and crystalline of TiO 2. Hattori et al. [13] confirmed that F doping can improve the activity of TiO 2 significantly due to the fact that doped F can increase the content of anatase. Nukumizu et al. [14] prepared N-F codoped TiO 2 (TONF) with an absorption edge at 570 nm using (NH 4 ) 2 TiF 6 and SiO 2 as sources. However, this experiment should be carried out at high temperature. Huang et al. [15] prepared N-F codoped TiO 2 by sol-gel method. The Received: May 14, 2008; Revised: July 3, *Corresponding author. liushouxin@126.com; Tel/Fax: The project was supported by the Program for New Century Excellent Talents in University, the Specialized Research Fund for the Doctoral Program of Higher Education ( ), and the National Natural Science Foundation of China ( ). Copyright 2008, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University. Published by Elsevier BV. All rights reserved. Chinese edition available online at

2 authors proposed that the synergy between N and F could result in higher degradation for parachlorophenol. Yu et al. [16] believed that due to charge compensation effect, doped F can convert part of Ti 4+ to Ti 3+ and the latter can inhibit the recombination of photogenerated electron and hole, then enhance the activity. Li et al. [17] proposed that the higher activity of F doped TiO 2 synthesized by spray pyrolysis method should attribute to the stronger surface acidity and the generation of oxygen vacancies and more active sites. Xie et al. [18] obtained N-F codoped TiO 2, which was more active than commercial P-25, they proposed that N-F codoping can inhibit the recombination of electrons and holes. Li et al. [19] believed that both N and F can replace the oxygen in TiO 2 lattice for the N-F codoped catalyst prepared by spray pyrolysis method. Ihara et al. [20] indicated that the grain boundary (13 nm) is important for the visible light activity of the TiO 2 photocatalyst. The acid-catalyzed hydrolysis method is effective for the preparation of TiO 2 particles with a size of nm [21]. In this work, NH 4 F was used as raw material; the acid catalyzed hydrolysis method was used for the preparation of visible light response N-F codoped TiO 2 photocatalysts. The catalysts were characterized by X-ray photoelectron spectroscopy (XPS), UV-Vis diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), scanning electron microscope (SEM), and N 2 adsorption isotherm. The mechanism of N-F codoping for improving the visible light activity of TiO 2 is discussed. 1 Experimental 1.1 Preparation of TONF photocatalysts In an ice bath, 25 ml of TiCl 4 (AR, Tianjin Dongda Chemical Factory, China) was added drop wise into 250 ml H 2 O under vigorous stirring (300 r min 1 ) at 5 C for 1 h. A mixture solution of (NH 4 ) 2 SO 4 (AR, Tianjin Kemiou Chemical Reagent Limited Company, China) and HCl (AR, Harbin Xinda Chemical Factory, China) was then added and further stirred for 0.5 h, and the molar ratio of TiCl 4 :(NH 4 ) 2 SO 4 :HCl was maintained at 1:2:10 during the reaction. The mixture was heated to 98 C at a rate of 5 C min 1 and was maintained at this temperature for 1 h. Thereafter, NH 3 H 2 O (AR, Tianjin Kemiou Chemical Reagent Limited Company, China) was used to adjust the ph to 8 and the mixture was kept at 98 C for 1 h. The TiO 2 precursor thus obtained was aged at room temperature for 10 h and then washed with double-deionized water until no Cl and SO 4 2 were detected, followed by washing with ethanol. Finally, the prepared TiO 2 precursor was added into the mixture of NH 4 F (AR, Shanghai Fengcheng Reagent Factory, China) and ethanol (AR, Beijing Chemical Factory, China) (with starting mass ratio of TiO 2 to NH 4 F of 1:0, 1:0.05, 1:0.1, 1:0.2, 1:0.5), stirred for 1 h, and aged for 12 h, then vacuum dried at 85 C. The obtained powder was heated under air atmosphere to a certain temperature and maintained for 2 h. The prepared catalysts were denoted as TONF(w, T), where w is the starting mass ratio of TiO 2 to NH 4 F, and T is the calcination temperature. The unmodified TiO 2 was prepared under the identical conditions except that NH 4 F. P-25 (Degussa Company, German) was used for comparison. 1.2 Photocatalytic activity test The artificial light photocatalytic activity test was carried out in a cylindrical quartz photoreactor (250 ml) using phenol as the model compound [11]. An 8-W UV lamp (λ max =365 nm) or a 380-W Xe lamp (λ max =510 nm, λ<450 nm was cut off by a UV filter) was positioned inside the reactor as the irradiation source. Atmosphere was aerated into the photoreactor vessel with 80 ml min 1. The photoreactor vessel was maintained at (25±1) C by circulating cool water. Before the photocatalytic degradation, the suspension was magnetically stirred in dark for 30 min to establish a phenol adsorption/desorption equilibrium. Samples of 5 ml were collected from the suspension and were immediately centrifuged at 4000 r min 1 for 10 min. The concentration of phenol after illumination was determined spectrophotometrically using a TU-1901 UV spectrometer at 270 nm. 1.3 Characterization of TONF photocatalysts The surface composition and valence state of the obtained photocatalysts were detected by a PHI5700 X-ray photoelectron spectroscopy (XPS) using Al K α X-ray (hv= ev). The binding energy was referenced to the C 1s line at ev for calibration. The energy threshold and photo properties of TONF catalysts were tested using TU-1901 UV-Vis diffuse reflectance spectroscopy with integrating sphere, BaSO 4 as the reference. X-ray diffraction analysis was performed on a D/max-rB X-ray diffractometer with Cu K α radiation (45 kv, 40 ma). The TiO 2 crystallite size was calculated from the Scherrer equation. The surface structure of the photocatalyst was observed using a QUANTA 200 scanning electron microscope. The specific surface area of the sample was measured using an ST-2000 automated apparatus based on the Brunauer-Emmett-Teller (BET) method at 77 K with N 2 as the adsorbent and H 2 as the carrier gas, and α-al 2 O 3 was used as reference. 2 Results and discussion 2.1 Photocatalytic activity Figs.1a and 2a are the results of the activity test under visible light irradiation. It is clear that N-F codoping can increase the visible light activity. The degradation of phenol over TONF(1:0.1, 700 C) was the most effective, and the complete degradation could be achieved within 60 min. However, only 15.5% and 13.0% phenol were degradated on naked TiO 2 and

3 Table 1 Pseudo-first order reaction constant of P-25, TiO 2, and TONF for phenol photocatalytic degradation TONF k/min 1 Vis-light irradiation UV-light irradiation 1:0.05 * (700 C) # :0.1 (600 C) :0.1 (700 C) :0.1 (800 C) :0.1 (900 C) :0.2 (700 C) :0.5 (700 C) TiO P * starting mass ratio of TiO 2 to NH 4F; # calcination temperature Fig.1 Photocatalytic activity of TiO 2 and N-F codoped TiO 2 (TONF) calcined at different temperatures (a) visible light, (b) UV light; starting mass ratio of TiO 2 to NH 4F is 1:0.1. commercial P-25 under the same condition within 100 min. From Figs.1b and 2b, it could be observed that the activity of TONF was also higher than the naked TiO 2 and commercial P-25 when UV light irradiation was adopted. With further increasing the NH 4 F content, the activity decreased. The analysis of kinetics showed that the process agreed with the pseudo-first order reaction kinetics. The reaction rate constant (k) is illustrated in Table 1. It was obvious that TONF exhibited considerably higher activity. 2.2 Results of XRD The XRD patterns of TONF calcined at different temperatures (Fig.3) showed that there was no new diffraction peaks due to N-F codoping. Also, no significant peak shift of anatase or rutile was observed. For N-F codoped TiO 2, the amorphous precursor was almost completely transformed into anatase at 500 C. The phase transformation of anatase to rutile started at 900 C. The rutile mass fraction was 2% for 900 C and 3% for 1000 C calcination sample. It can be seen that the content of rutile phase was almost not increased with the increase of calcination temperature. However, for the naked TiO 2 and the N doped TiO 2, the phase transformation of anatase to rutile both started at 700 C. The rutile mass fractions of 99% [22] and 61% [11] were observed for naked and N doped TiO 2 calcined at Fig.2 Photocatalytic activity of TiO 2 and TONF (700 C) with different starting mass ratio of TiO 2 to NH 4F (a) visible light, (b) UV light Fig.3 XRD patterns of TONF photocatalyst calcined at different temperatures

4 codoping can improve the dispersion and reduce the agglomerate size of TiO 2. The agglomerate size was nm and nm for the catalyst calcined at 500 C of TONF and naked TiO 2, respectively. With the increase in the calcination temperature, the TiO 2 crystals agglomerated more significantly. These results demonstrated that N-F codoping can inhibit the agglomeration of TiO 2 powder effectively, which was beneficial to the higher activity. 2.4 Results of DRS Fig.4 Effect of calcination temperature on the crystallite size 900 C, respectively. From these results, it can be observed that N-F codoping can inhibit the phase transformation of TiO 2 from anatase to rutile. It is known that TiO 2 phase transformation from anatase to rutile is controlled by the formation of Ti 4+ complexing [23]. Compared with TON, the inhibition of phase transformation from anatase to rutile due to F doping may attribute to the similar atom radius of F and O. Doped F can form complex with Ti 4+. The formed complex can prevent the transformation of spiral chains of anatase to linear chains structure of rutile TiO 6 octahedra [23]. Fig.4 shows the influence of calcination temperature on the BET surface area and crystallite size of the TONF. With the increase of calcination temperature, for both TONF and TiO 2, the surface area decreased and the crystallite size increased, which was more significant for naked TiO 2. When the calcination temperature varied from 500 to 600 C, for the naked TiO 2, the surface area declined to m 2 g 1 and the crystallite size increased to 20 nm. However, for TONF, the values were 10 m 2 g 1 and 3 nm, respectively. In addition, compared with N doped TiO 2 [11], N-F codoping can enhance the catalyst dispersion. According to the result of XRD, for the TONF calcined at 500 and 600 C, complete anatase structure has not formed. Hence, it is not difficult to explain the higher activity of TONF sample calcined at 700 C. 2.3 Results of SEM and BET surface area of TONF and TiO 2 SEM images of TONF and TiO 2 calcined at different temperatures are presented in Fig.5. It can be seen that N-F The DRS spectra of TONF with different mass ratios of TiO 2 /NH 4 F are shown in Fig.6, the insert is the enlarged diagram of the region from 380 to 430 nm. Similar to TON, with increasing the NH 4 F/TiO 2 mass ratio, the edge shifted to longer wavelength. Li [23] and Yamaki [24] et al. reported that F doping could not influence the energy band structure of TiO 2. Therefore, we attribute the visible light absorption of TONF to the doped N. For the TON catalysts, it was indicated that with increasing the N contents, the visible light absorption of catalysts increased [11]. Combined with the activity result, it was obvious that the activity was not proportional to the content of codoped N-F, however, an optimum content existed. 2.5 Results of XPS The XPS spectra of TONF and naked TiO 2 are presented in Fig.7. It can be seen that TONF contained predominantly Ti, O, and trace amount of N, F, and C. C 1s peak at ev was organic polluted carbon and was used for calibration. Combined with the high-resolution XPS spectra of Ti 2p (Fig.8a), it indicated that the Ti atoms existed mainly as Ti 4+ due to the appearance of Ti 2p characteristics peak with binding energies of and ev. The spectrum of O 1s is shown in Fig.8b, the peak at ev corresponded to lattice oxygen of TiO 2, and a shoulder located at higher binding energy of ev was assigned to mixed contributions from surface hydroxides [24]. N 1s and F 1s spectra are shown in Fig.8c and Fig.8d, respectively. The N 1s can be deconvoluted by two peaks, and ev. Similar to TON, part of N atoms entered into the lattice of TiO 2 and formed a new narrow energy level, which could induce visible light response. The F 1s was also deconvoluted by two peaks, the peak at Fig.5 SEM images of TiO 2 (a, b) and TONF (c, d) catalysts prepared at different calcination temperatures (a) 500 C, (b) 700 C, (c) 500 C, (d) 700 C

5 Fig.6 Diffuse reflectance spectra of TiO 2 and TONF catalysts Fig.7 XPS survey spectra of TiO 2 and TONF Fig.8 XPS spectra of TONF (a) Ti 2p, (b) O 1s, (c) N 1s, (d) F 1s Table 2 XPS results of TiO 2 and TONF(1:0.1) prepared at different calcination temperatures T/ C Ti O N F TONF % 71.12% 0.85% 0.59% % 71.58% 0.67% 0.18% % 73.38% 0.30% 0.00% TiO % 71.42% 0.00% 0.00% ev was similar with the peak of F atoms in TiOF 2 [18], the other peak at ev should attribute to the doped F atoms in TiO 2. Because the atomic radius of F is similar to that of O, F atoms replaced O atoms in the lattice of TiO 2 and then formed Ti F bonds. The charge imbalance caused by the absence of O was represented by decreasing the valence state of Ti 4+ ; hence, a shallow energy band formed, which could induce visible light response. The results of elemental contents of TiO 2 and TONF (1:0.1) calcined at different temperatures are summarized and listed in Table 2. 3 Conclusions N-F codoped TiO 2 photocatalysts were prepared using acid catalyzed hydrolysis method from mixed aqueous solution of TiCl 4 and NH 4 F. TONF(with starting mass ratio of 1:0.1 of TiO 2 to NH 4 F and calcination temperature of 700 C) exhibited the highest activity. Both the N and F atoms entered into the lattice of TiO 2. N doping can enhance the absorption in visible light region. F doping can narrow the energy gap of TiO 2. Due to the synergy effect between doped N and F, TONF catalysts presented much better photocatalytic per-

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