- Email: [email protected]
Synthesis and characterization of F-N-W-codoped TiO2 photocatalyst with enhanced visible light response
Materials Research Bulletin 47 (2012) 4347–4352
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Synthesis and characterization of F-N-W-codoped TiO2 photocatalyst with enhanced visible light response Xiaoliang Shi a,b,*, Haibo Qin b, Xingyong Yang b, Qiaoxin Zhang a,b a b
School of Mechanical and Electronic Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China School of Materials Science and Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 April 2012 Received in revised form 1 August 2012 Accepted 5 September 2012 Available online 12 September 2012
Anatase F-N-W-codoped TiO2 photocatalyst was successfully prepared by a method combining sol–gel with hydrothermal treatment. Effects of F, N and W ion dosage concentration on the crystallinity, morphology, grain size and chemical status of the photocatalyst were investigated. The results showed that the F-7NW-TiO2 photocatalyst composed of uniform ellipsoidal particles around 20 nm in length and 10 nm in width, and the photocatalyst displayed enhanced visible-light absorption and photocatalytic activities. Using the photocatalyst and under visible irradiation for 1.5 h, the decoloration percent of RB and carbon removal rate were about 98% and 94% respectively, which were much higher than that of commercial P25, TiO2, N-W-TiO2 and F-TiO2. The high visible-light photocatalytic activity of F-7NW-TiO2 might result from narrowing the band gap and lowing charge pairs recombination rate for the WxTi1xO2, O–Ti–N and valence variation of W ions existing. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Semiconductors B. Chemical synthesis C. Electron microscopy D. Catalytic properties
1. Introduction It is well known that nano TiO2 is one of the most suitable semiconductors as photocatalysts because of its high photoreactivity, biological and chemical inertness, cost effectiveness, nontoxicity, long-term stability against photocorrosion and chemical corrosion [1]. And nano TiO2 has been widely applied in various photocatalytic fields such as environmental purification, decomposition of organic contaminants, and water photosplitting into H2 and O2 [2]. However, TiO2 as photocatalyst has two main defects. Firstly, pure TiO2, whose band gap energy is 3.2 eV, can only absorb ultraviolet (UV) light, and then its photoefficiency is not sufficient. The fraction of UV light in the solar energy spectrum is only about 4%. Hence, it is necessary to shift the absorption threshold of TiO2 towards the visible region. Secondly, the high recombination rate between excited electron and hole pairs results in the low quantum yield rate and undesirable photooxidation rate [3]. Recently, various modifications have been performed to extend the optical absorption edge of TiO2 into the visible light region. The initial approach was the doping of TiO2 with metal elements. W ion doping was helpful to enhance the visible photocatalytic performance of nano TiO2 [4]. Y. Yang et al. [5] prepared W6+-doped TiO2
* Corresponding author at: School of Materials Science and Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China. Tel.: +86 27 87651793; fax: +86 27 87651793. E-mail address: [email protected] (X. Shi). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.09.006
thin film photocatalyst, and they indicated that the higher photocatalytic performances were attributed to efficiently inhibiting the recombination of photoinduced electrons and holes. Once optical excitation occurs, the photogenerated electrons can be transferred to the lower-lying conduction band of WO3. Meanwhile, the holes will accumulate in the valence band of TiO2 [6]. H. Tian et al. [3] considered that W-dopant might increase the charge separation efficiency and the presence of surface acidity. However, D.G. Huang et al. [7] pointed out that metal doping had several drawbacks because the doped materials were not thermally stable, and the metal centers acted as electron trapping centers to reduce the photocatalytic efficiency. Other approaches were adopted to narrow the band gap of TiO2 by replacing lattice oxygen with anionic dopant species, such as N [8], C [9], S [10] and so on. F-doping was also found to have a significant enhancement on the photocatalytic activity of nano TiO2 by preventing the grain growth and inhibiting the transformation of anatase to rutile phase. The doped F atoms that were incorporated into TiO2 lattice might take a positive role in photocatalysis [11]. S.J. Pan et al. [12] reported that the doped F atoms increased the surface acid sites and the formation of Ti3+ ions. The surface acidic sites could not only improve the adsorption of organics, but also act as an electron acceptor to reduce the electron and hole recombination rate. The doped F atoms might also trap the photogenerated electrons, as presented by J. Xu et al. [13]. The N doping of TiO2 substitutive sites has been proven to be helpful for band-gap narrowing and improving photocatalytic activity [8]. Moreover, N-F-codoped TiO2 showed higher absorption and water splitting activities than N-doped TiO2 under visible
4348
X. Shi et al. / Materials Research Bulletin 47 (2012) 4347–4352
light irradiation [7,12]. Y.F. Shen et al. [14] synthesized N-Wcodoped TiO2 photocatalyst by a two-step method combining sol– gel with mechanical alloying. And the photocatalyst had excellent properties of strong absorbance in visible range. T. Mishra et al. [15] synthesized a series of WN co-doped titania nanomaterials with varying tungsten content and dealt with the increased and selective nitrate reduction under visible light over a WN co-doped titania photocatalyst. The results showed that visible light absorption increased with the increasing tungsten amount. Overall high nitrate reduction could be attributed to the synergistic effect of tungsten and nitrogen co-doping, optimum surface hydroxyl group, mesoporosity and appreciable visible light absorption of the materials. To the best of our knowledge, meager information is available as regards the development of the F-N-W-codoped TiO2 photocatalyst by use of a method combining sol–gel with hydrothermal treatment (sol–gel-hydrothermal method) and the investigation of the photocatalytic performances. In this work, the F-N-W-codoped TiO2 photocatalyst was successfully synthesized by the sol–gel-hydrothermal method. Effects of the N, F and W ions on the crystallinity, morphology and grain size of the photocatalyst, as well as the photocatalytic performances under visible light irradiation were systematically investigated. 2. Experimental 2.1. photocatalyst preparation The F-N-W-codoped TiO2 photocatalyst was prepared by the sol–gel-hydrothermal method. Tetrabutyl titanate was used as the starting material. Hydrofluoric acid (40 wt.%) was used as the fluorine source. And ammonium tungstate (92 wt.%) was used as the tungsten and nitrogen source. All chemicals used in the experiment were analytical reagent grade. Firstly, 0.1 mL hydrofluoric acid (RF = 4.0%, the atomic percentage of F to Ti was designated as RF) and a certain amount of ammonium Tungstate (the atomic percentage of W to Ti was designated as RW, and the atomic percentage of N to Ti was designated as RN) were dissolved into 5 mL deionized water with stirring for 10 min to form solution A. 20 mL tetrabutyl titanate was dissolved into the mixture of 180 mL anhydrous ethanol and 8 mL acetic acid, and then stirred for 30 min to form solution B. Secondly, the solution A was slowly dripped into solution B under vigorous stirring. Thirdly, the resultant mixture was slowly stirred at room temperature for at least 24 h until a white transparent immobile sol was formed. Finally, the deionized water was chosen to be solvent. The concentration of Ti4+ in sol mixture was kept in the range of 0.04– 0.06 mol/L. The sol mixture was transferred into a 300 mL Tefloninner-liner stainless steel autoclave. The autoclave was heated in an oven and kept at 150 8C for 2 h, which had been confirmed to be the applicable reaction conditions by H. Tian et al. [3] and D.G. Huang et al. [7]. After the hydrothermal treatment, the gained precipitates were washed by deionized water several times, dried at 80 8C in vacuum. The obtained photocatalysts were denoted as F-N-W-codoped TiO2, including F-3NW-TiO2 (RF = 4.0%, RW = 0.3%, RN/RW = 6/7), F-7NW-TiO2 (RF = 4.0%, RW = 0.7%, RN/RW = 6/7) and F-15NW-TiO2 (RF = 4.0%, RW = 1.5%, RN/RW = 6/7). For comparison, the TiO2 was prepared by the similar method in the absence of solution A. TiO2 with RF = 4.0% prepared in the absence of ammonium tungstate, was denoted as F-TiO2. And TiO2 with Rw = 0.7% and RN/RW = 6/7 prepared in the absence of hydrofluoric acid, was denoted as 7NW-TiO2. Commercial TiO2 Degussa P25 (75% anatase and 25% rutile, specific area of 50 m2/g, average particle size 30 nm, non-porous) was used for comparison.
2.2. Analysis The purity and crystallinity of the as-prepared samples were examined by X-ray diffractometer (XRD, XD-3A, Shimadazu Corporation, Japan) using graphite monochromatic copper radiation (Cu Ka) at 40 kV, 30 mA over the 2u range of 10–808 [11]. The average grain size was determined according to the Scherrer equation: D¼
0:89l B cos u
(1)
where l is the wavelength of the characteristic X-ray applied; B is the half-value width of anatase (1 0 1) peak obtained by XRD; u is the diffraction angle [7,16]. An ultraviolet–visible (UV–Vis) spectrophotometer (Shimadzu UV-2100) was used to record the diffuse reflectance spectra of the samples. In order to investigate the recombination and lifespan of photogenerated electrons and holes in the photocatalysts, the photoluminescence (PL) emission spectra of the samples were measured at room temperature by LS55 (Perkin-Elmer) illuminated with a 325 nm He–Cd laser. The morphologies and grain size of the photocatalyst were observed by a JEM-2100F field emission transmission electron microscope (FETEM). X-ray photoelectron spectra (XPS) of the samples were measured using a PHI5300 photoelectron spectrometer system with an Mg source (1486.6 eV). The shift of binding energy due to relative surface charging was corrected using the C1s level at 287.1 eV as an internal standard of the surface adventitious carbon. N2 adsorption analysis was conducted on an ASAP 2010 Surface Area to get the Brunauer–Emmett–Teller (BET) surface area. 2.3. Photocatalytic activity measurements The photocatalytic activity of the photocatalyst was estimated by measuring the decomposition percentage of RB in an aqueous solution. The photocatalytic degradation experiments were performed with the following procedure: A 160 W tungsten halogen lamp was connected to a light filter (wavelength: 400– 550 nm), and the concentration of RB was 12 mg/L. The reaction was carried out in a 250 mL beaker with an electromagnetic stirrer. The amount of the photocatalyst was 0.5 g/L. Prior to photoreaction, the system was kept in the dark for 30 min to reach adsorption–desorption equilibrium condition. Then the reactants were irradiated for 1.5 h. The samples were taken every 15 min, centrifuged and filtrated, orderly. The dye was analyzed on a UV– vis spectrophotometer (723, Shanghai spectrum Instruments Co., Ltd.), with the maximum absorption wavelength of RB at 554 nm [17]. The degradation rate of RB was measured by determining the chemical oxygen demand (COD) of reaction liquid using K2CrO7, it is could be calculated by: ðCOD0 CODÞ D¼ (2) COD0 where COD0 and COD are the chemical oxygen demand of reaction liquid before and after the reaction respectively [7,14]. 3. Results and discussion 3.1. Characterization of F-N-W-codoped TiO2 photocatalyst The XRD patterns of the photocatalysts were shown in Fig. 1. The XRD analysis showed that all peaks (major peaks: 25.128, 37.788 and 47.888) were ascribed to well-crystallized anatase TiO2, and there were not rutile and brookite existing. Moreover, TiOF2, TiF4, TiN, WOx, O–Ti–N and WxTi1xO2 also were not found because the content was little, and the similar results have been observed by other authors [2,5,7,11,13,14]. And the doping F atoms did not
X. Shi et al. / Materials Research Bulletin 47 (2012) 4347–4352
4349
(see Fig. 2(a)) shows that the F-TiO2 possessed spherical particles. While FETEM image (see Fig. 2(b)) displayed that the appearance of F-7NW-TiO2 was ellipsoidal particles around 20 nm in length and 10 nm in width, which was close to the average particle size calculated from the XRD analysis as shown in Table 1. Besides, FETEM image of the F-TiO2 (see Fig. 2(c)) illustrated that the fringe spacings of 3.580 A˚, 1.872 A˚ and 2.302 A˚ could be indexed to the (1 0 1), (2 0 0) and (0 0 4) planes of anatase TiO2, respectively. While FETEM image of F-7NW-TiO2 (see Fig. 2(d)) revealed the fringe spacings of (1 0 1), (2 0 0) and (0 0 4) planes were reduced to 3.458 A˚, 1.784 A˚ and 2.206 A˚, respectively. It further demonstrated that the W6+ might replace Ti4+ to form W–O–Ti bonds in TiO2 crystal lattice, thus the d(h k l) value of F-7NW-TiO2 was decreased. 3.3. BET specific surface area analysis Fig. 1. XRD patterns of the samples: (a) TiO2; (b) F-TiO2; (c) 7NW-TiO2; (d) F-3NWTiO2; (e) F-7NW-TiO2; (f) F-15NW-TiO2.
cause a shift of the peak position of anatase TiO2, which was consistent with the previous results [1,11]. They thought that the ion radius of fluorine atom (0.133 nm) was virtually same as that of the oxygen atom (0.132 nm). Bragg’s Law refers to the simple equation: dðh k lÞ ¼
l
(3)
ð2 sin u Þ
where d(h k l) is the distance between crystal planes of (h k l); l is the X-ray wave length; u is the diffraction angle of the crystal plane (h k l). According to Bragg’s law, the d(h k l) value was not changed after oxygen atoms were replaced by F atoms. Thus, 2u also was not shifted. As shown in Fig. 1, the average grain sizes of the samples (a), (b), (c), (d), (e) and (f) were calculated by Scherrer Equation. As shown in Table 1, the average grain size of TiO2 doped with F atom was smaller than that of pure TiO2. It could be concluded that Fdoping could inhibit the TiO2 grain growth during the hydrothermal process. In addition, the diffraction peaks gradually became broader with the increasing of the doped amount of W6+, which indicated the smaller grain size. An obvious difference of the (1 0 1) diffraction peak between pure TiO2 and W doped TiO2 showed that the peak position of the photocatalysts with various contents of W6+ slightly shifted toward a higher 2u value. According to Bragg’s law, the d(h k l) value decreases with the increasing of 2u value. It is well known that the ionic radius of W6+ (41 pm) is smaller than that of Ti4+ (53 pm) [5]. When Ti4+ in TiO2 crystal lattice is displaced by W6+, the lattice parameter d(h k l) value would become smaller [3]. Thus, the W6+ should enter into TiO2. Nevertheless, Y. Yang et al. [5] observed that the peaks of anatase TiO2 doped with W6+ shifted toward smaller 2u value, and they thought W6+ might be located at interstitial sites. 3.2. Microstructure analysis Fig. 2 shows the FETEM images of the F-TiO2 and F-7NW-TiO2 prepared by the sol–gel-hydrothermal method. The FETEM image Table 1 The BET surface areas and average particle size of the samples. Photocatalysts
Crystal composition
Average grain size (nm)
SBET (m2/g)
TiO2 F-TiO2 7NW-TiO2 F-3NW-TiO2 F-7NW-TiO2 F-15NW-TiO2
Anatase Anatase Anatase Anatase Anatase Anatase
17 11 18 12 12 13
148 192 143 186 183 162
BET specific surface areas (SBET) (m2/g) of the samples were shown in Table 1. It demonstrated that SBET met the following order: F-TiO2 > F-3NW-TiO2 > F-7NW-TiO2 > F-15NW-TiO2 > TiO2 > 7NW-TiO2. The SBET of F-TiO2 was as high as 192 m2/g. It could be that F played an important role in suppressing the grain growth. The SBET (m2/g) of F-N-W-TiO2 decreased with the increasing of W6+ dopant, and the highest SBET of F-7NW-TiO2 was about 183 m2/g. Moreover, the SBET of 7NW-TiO2 was 143 m2/ g, which was lower than that of pure TiO2. It was in line with the results of XRD analysis. Thus, N and W dopants might result in the larger particle size and particle aggregations. 3.4. Chemical status of the F, N and W atoms in the F-N-W-codoped TiO2 photocatalyst Fig. 3 shows the XPS spectra of the F-7NW-TiO2. As shown in Fig. 3(a), the XPS survey spectra indicated that the peaks were related to Ti, O, F, N, W and a trace amount of carbon. The presence of carbon was ascribed to the residual carbon from the precursor solution and the adventitious hydrocarbon from the XPS instrument itself. XPS data also showed that the peaks of Ti 2p shifted toward higher wavelength and appeared at 462.0 eV and 467.8 eV. The XPS signal of Ti 2p could be fitted with two components. 462.0 eV could attribute to Ti (II) species. However, 467.8 eV was much higher than that of Ti (IV) species for the incorporating of F, N and W species [18]. Fig. 3(b) shows the F 1s XPS spectrum of F-7NW-TiO2 samples. The peaks located at 683.02 eV and 686.75 eV were the characteristics of F 1s peak, which especially meant that there were two chemical forms of F existing. Using Gaussian distributions, one peak indicated that F ions were adsorbed on the surfaces of TiO2 particles, and the other peak stated that F entered into TiO2 crystal lattices [19]. The peak observed at 683.02 eV was originated from the F-containing compounds that adsorbed on the surface of TiO2, and the peak located at 686.75 eV was attributed to the F atoms that replaced the oxygen in the TiO2 crystal lattice. Since any evidences of TiF4 and TiOF2 existing were not found in the XRD patterns, the two peaks indicated that F atoms could be incorporated into TiO2 crystal lattice, and O–Ti–F bonds perhaps had been formed by the sol–gel-hydrothermal method. The W4f peak was used to determine the oxidation state of W, and the binding energy of W 4f7/2 corresponded to the characteristic of W6+ [20]. As shown in Fig. 3(c), the W4f peak located at about 36.03 eV was observed, which was similar to the results of Y.F. Shen et al. [14]. Thus, the W6+ might replace Ti4+ to form W–O–Ti bonds. As shown in Fig. 3(d), the large broadness of the N 1s XPS spectrum was obvious, and main peak emerged at 400.75 eV. M. Pelaez et al. [21] and M. Sathish et al.
4350
X. Shi et al. / Materials Research Bulletin 47 (2012) 4347–4352
Fig. 2. FETEM images of the samples: (a and b) F-TiO2; (c and d) F-7NW-TiO2.
[22] prepared N-doped TiO2 by a sol–gel method, and they suggested that the N 1s peak was at about 400 eV for the formation of the O–Ti–N bonds. Thus, it was concluded that N atoms were incorporated into TiO2 crystal lattice, and O–Ti–N bonds had been formed by the sol–gel-hydrothermal method.
3.5. UV–Vis diffuse reflectance spectra (DRS) and PL spectra Fig. 4 shows the absorption spectra of the photocatalysts. As to 7NW-TiO2, the absorption onset appeared at about 425 nm (2.92 eV), it obviously had red shift when compared with the onset of anatase TiO2 (l = 387 nm). The result was in accordance
Fig. 3. X-ray photoelectron spectra of the F-7NW-TiO2 photocatalyst: (a) global range XPS; (b) F 1s peaks; (c) W 4f peak; (d) N 1s peak.
X. Shi et al. / Materials Research Bulletin 47 (2012) 4347–4352
Fig. 4. UV–Vis spectra of the samples: (a) 7NW-TiO2; (b) F-TiO2; (c) TiO2; (d) F7NW-TiO2.
4351
The photoexcited electrons in the conduction band of TiO2 can be accepted by W6+ [24]. R. Asahi et al. [8] concluded that N was doped into the substitutive sites of TiO2, and the N2p accepter states contributed to the band gap narrowing by mixing with O2p states. Thus, as shown in Fig. 5, the doping N and W atoms could enhance separation efficiency of TiO2 charge pairs. Meanwhile, Fdopant could convert some Ti4+ to Ti3+ by charge compensation. The Ti3+ surface states, which formed a donor level between the band gaps of TiO2, would trap the photogenerated electrons [1] and then transfer them to the state of oxygen vacancies on the surface of TiO2. The F-7NW-TiO2 photocatalyst had the strongest absorption ability of visible light and highest separation efficiency of charge pairs, which mostly were attributed to a synergetic effect of the doped N, W and F atoms. Doped F ions generated greater specific surface area. Doped N and W atoms contributed to more absorption of visible light, and valence variation of W6+ enhanced separation efficiency of charge pairs. 3.6. Photocatalytic activity
with that obtained by some other authors [2,14]. In addition, the absorption edge of F-TiO2 slightly shifted toward longer wavelengths, and the absorption onset appeared at about 410 nm (3.02 eV). Especially for F-7NW-TiO2, the absorption onset appeared at about 500 nm. The band gap energy was about 2.48 eV, which was much lower than that of pure TiO2 (3.2 eV). Consequently, the F-N-W-codopant was beneficial to the visible light absorption of TiO2 as shown in Fig. 4. PL emission spectra had been used to investigate the efficiency of charge carrier transfer and the recombination rate of electron and hole pairs in semiconductor particles [1,11]. The lower excitonic PL intensity corresponded to the higher separation rate of electron and hole pairs [23]. Fig. 5 shows that the PL spectra of TiO2, F-TiO2, 7NW-TiO2 and F-7NW-TiO2, and their excitonic PL intensity obeyed the following order: TiO2 > F-TiO2 > 7NWTiO2 > F-7NW-TiO2. It revealed that a certain amount of F-N-Wcodopants could sharply decrease the recombination rate of electron and hole pairs. According to the recent study, the measured band-gap energy of WO3 2.75 eV was more easily excited by visible light than that of TiO2, and the W6+ doping of TiO2 could generate electron and hole pairs at the tail states of conduction band and valence band under visible light [5], following the scheme [3]: W6þ þ e ðTiO2 Þcb ! W5þ
The degradation experiment of 12 mg/L RB solution (COD0 = 115 mg/L, the measurement error of all data is less than 1%) was chosen to investigate the photocatalytic activity of the photocatalysts with different dopants. The kinetic curves of photodecoloration rate and carbon removal rate & histograms of COD of RB in the visible light (400–550 nm) were shown in Figs. 6 and 7 respectively. As shown in Fig. 6(c), the moderate photodegradation activity of F-TiO2 under visible illumination was observed. The decoloration percent was 70%. As shown in Fig. 7(B) and (c), under visible light irritation for 90 min, the COD decreased from 115 to 35 mg/L using the F-TiO2 photocatalyst, and carbon removal rate was up to 70%. It might be attributed to the enhancement of surface acidity and the creation of oxygen vacancies. Then hydroxyl radicals (OH) were formed, which were generally regarded as the important active species for initiating the photocatalytic reaction [24]. The decoloration rate of RB was up to 83% using 7NW-TiO2, and the COD decreased to 22 mg/L, which was much better than that of P25, pure TiO2 and F-TiO2. Compared with 7NW-TiO2 and F-TiO2, the photoactivity of F7NW-TiO2 evidently was improved. The decoloration percent of RB was up to about 98%. As shown in Fig. 6(f) and Fig. 7(A, B)(f), the COD was only about 7 mg/L, and carbon removal rate reached to 94%. However, the decoloration rate and carbon removal rate of
W5þ þ hþ ðTiO2 Þcb ! W6þ
Fig. 5. PL spectra of the samples: (a) TiO2; (b) F-TiO2; (c) 7NW-TiO2; (d) F-7NWTiO2.
Fig. 6. Kinetic curves of photodecoloration rate of RB under visible light (450– 550 nm) (C0 = The initial concentration of RB; C = The concentration of RB at various times): (a) P25; (b) TiO2; (c) F-TiO2; (d) 7NW-TiO2; (e) F-3NW-TiO2; (f) F-7NWTiO2; (g) F-15NW-TiO2.
4352
X. Shi et al. / Materials Research Bulletin 47 (2012) 4347–4352
4. Conclusions A simple sol–gel-hydrothermal method was developed to prepare highly photoactive anatase F-N-W-codoped TiO2 photocatalyst. It appeared uniform ellipsoidal particles around 20 nm in length and 10 nm in width. The finer particles resulted in larger surface area (about 180 m2/g) as obtained from BET. Compared with the 7NW-TiO2 and F-TiO2, the higher RB degradation rate of the F-7NW-TiO2 demonstrated that the photoexcited wavelength range had stronger red shift, which was ascribed to a synergetic effect of the doped F, N and W atoms: (I) Doped F ions could create larger specific surface area and higher surface acidity; (II) W-doping formed the donor level, and W6+ could accept photoelectrons to suppress the recombination of charge pairs; (III) N-doping induced the acceptor level. All made contributions to the visible light response of the F-7NW-codoped TiO2 photocatalyst. The decoloration percent of RB was up to about 98%, and carbon removal rate reached 94% under visible illumination. Thus, the preparation and photocatalytic activity of the F-N-W-codoped TiO2 photocatalyst were worthy to be further investigated. Acknowledgements This work was supported by the Academic Leader Program of Wuhan City (201150530146); the Nature Science Foundation of Hubei Province (20101j0018); the Project for Science and Technology Plan of Wuhan City (200910321092); and the Fundamental Research Funds for the Central Universities (2010II-020). References
Fig. 7. Kinetic curves of carbon removal rate (A) and histograms of COD (mg/L) of RB after 30 min, 60 min and 90 min photoreaction (COD0 = 115 mg/L; COD = The chemical oxygen demand of RB at various times) (B): (a) P25; (b) TiO2; (c) F-TiO2; (d) 7NW-TiO2; (e) F-3NW-TiO2; (f) F-7NW-TiO2; (g) F-15NW-TiO2.
F-15NW-TiO2 only were about 92% and 88% respectively. It was noteworthy that excessive W amounts seemed to be detrimental to the photodegradation of RB because too high-loaded W would act as recombination centers for electron and hole pairs [3,5], which was in line with the results of UV–Vis spectra as shown in Fig. 5. Thus, it was concluded that high photodegradation activity of F-NW-codoped TiO2 was ascribed to a synergetic effect of the doped N, W and F atoms. The larger specific surface area was created by the doped-F ions. Then more RB molecules could be absorbed on the particle surface because RB was a cationic dye. Meanwhile, more photoexcited electrons were created from WxTi1xO2 or O–Ti–N in the photocatalyst under visible light, and then W6+ accepted photoelectrons to suppress the recombination of charge pairs, thus more photoelectrons were transferred to the particle surface. More hydroxyl radicals (OH) were generated, which could disrupt RB molecules. And the photocatalytic activity of F-N-W-codoped TiO2 was improved drastically under visible illumination.
[1] J.C. Yu, J.G. Yu, W.G. Ho, Z.T. Jiang, L.Z. Zhang, Chem. Mater. 14 (2004) 3808–3816. [2] H.M. Yang, R.R. Shi, K. Zhang, Y.H. Hu, A.D. Tang, X.W. Li, J. Alloys Compd. 398 (2005) 200–202. [3] H. Tian, J.F. Ma, K. Li, J.J. Li, Mater. Chem. Phys. 112 (2008) 47–51. [4] W.Y. Choi, A. Termin, M.R. Hoffmann, J. Phys. Chem. 98 (1994) 13669–13679. [5] Y. Yang, H.Y. Wang, X. Li, C. Wang, Mater. Lett. 63 (2009) 331–333. [6] D.N. Ke, H.J. Liu, T.Y. Peng, X. Liu, K. Dai, Mater. Lett. 62 (2008) 447–450. [7] D.G. Huang, S.J. Liao, J.M. Liu, Z. Dang, L. Petrik, J. Photochem. Photobiol. A 184 (2006) 282–288. [8] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269–271. [9] S. Sakthivel, H. Kisch, Chem. Int. Ed. 42 (2003) 4908–4911. [10] T. Ohno, M. Akiyoshi, T. Umebayashi, K. Asai, T. Mitsui, M. Matsumura, Appl. Catal. A 265 (2004) 115–121. [11] J.J. Xu, Y.H. Ao, D.G. Fu, C.W. Yuan, Appl. Surf. Sci. 254 (2008) 3033–3038. [12] S.J. Pan, H.C. Liang, C.Q. Wang, F.W. Yang, X.D. Li, S. Yang, Sci. China Ser. B: Chem. 52 (2009) 2043–2046. [13] J. Xu, B.F. Yang, M. Wu, Z.P. Fu, Y. Lv, Y.X. Zhao, J. Phys. Chem. C 114 (2010) 15251–15259. [14] Y.F. Shen, T.Y. Xiong, T.F. Li, K. Yang, Appl. Catal. B 83 (2008) 177–185. [15] T. Mishra, M. Mahato, N. Aman, J.N. Patel, R.K. Sahu, Catal. Sci. Technol. 1 (2011) 609–615. [16] K. Demeestere, J. Dewulf, H.V. Langenhove, B. Sercu, Chem. Eng. Sci. 58 (2003) 2255–2267. [17] C. Wang, B.Q. Xu, X.M. Wang, J.C. Zhao, J. Solid State Chem. 178 (2005) 3500–3506. [18] J.G. Yu, X.J. Zhao, Q.N. Zhao, J.C. Du, Chin. J. Mater. Res. 14 (2000) 203–209. [19] H.W. Park, W.Y. Choi, J. Phys. Chem. B 108 (2004) 4086–4093. [20] S. Penner, X.J. Liu, B. Klotzer, F. Klauser, B. Jenewein, E. Bertel, Thin Solid Films 516 (2008) 2829–2836. [21] M. Pelaez, A.A. de la Cruz, E. Stathatos, P. Falaras, D.D. Dionysiou, Catal. Today 144 (2009) 19–25. [22] M. Sathish, B. Viswanathan, R.P. Viswanath, Appl. Catal. B 74 (2007) 307–312. [23] Y.S. Luo, J.P. Liu, X.H. Xia, X.Q. Li, T. Fang, S.Q. Li, Q.F. Ren, J.L. Li, Z.J. Jia, Mater. Lett. 61 (2007) 2467–2472. [24] P. Cheng, C.S. Deng, D.N. Liu, X.M. Dai, Appl. Surf. Sci. 254 (2008) 3391–3396.
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Synthesis and characterization of F-N-W-codoped TiO2 photocatalyst with enhanced visible light response Xiaoliang Shi a,b,*, Haibo Qin b, Xingyong Yang b, Qiaoxin Zhang a,b a b
School of Mechanical and Electronic Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China School of Materials Science and Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 April 2012 Received in revised form 1 August 2012 Accepted 5 September 2012 Available online 12 September 2012
Anatase F-N-W-codoped TiO2 photocatalyst was successfully prepared by a method combining sol–gel with hydrothermal treatment. Effects of F, N and W ion dosage concentration on the crystallinity, morphology, grain size and chemical status of the photocatalyst were investigated. The results showed that the F-7NW-TiO2 photocatalyst composed of uniform ellipsoidal particles around 20 nm in length and 10 nm in width, and the photocatalyst displayed enhanced visible-light absorption and photocatalytic activities. Using the photocatalyst and under visible irradiation for 1.5 h, the decoloration percent of RB and carbon removal rate were about 98% and 94% respectively, which were much higher than that of commercial P25, TiO2, N-W-TiO2 and F-TiO2. The high visible-light photocatalytic activity of F-7NW-TiO2 might result from narrowing the band gap and lowing charge pairs recombination rate for the WxTi1xO2, O–Ti–N and valence variation of W ions existing. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Semiconductors B. Chemical synthesis C. Electron microscopy D. Catalytic properties
1. Introduction It is well known that nano TiO2 is one of the most suitable semiconductors as photocatalysts because of its high photoreactivity, biological and chemical inertness, cost effectiveness, nontoxicity, long-term stability against photocorrosion and chemical corrosion [1]. And nano TiO2 has been widely applied in various photocatalytic fields such as environmental purification, decomposition of organic contaminants, and water photosplitting into H2 and O2 [2]. However, TiO2 as photocatalyst has two main defects. Firstly, pure TiO2, whose band gap energy is 3.2 eV, can only absorb ultraviolet (UV) light, and then its photoefficiency is not sufficient. The fraction of UV light in the solar energy spectrum is only about 4%. Hence, it is necessary to shift the absorption threshold of TiO2 towards the visible region. Secondly, the high recombination rate between excited electron and hole pairs results in the low quantum yield rate and undesirable photooxidation rate [3]. Recently, various modifications have been performed to extend the optical absorption edge of TiO2 into the visible light region. The initial approach was the doping of TiO2 with metal elements. W ion doping was helpful to enhance the visible photocatalytic performance of nano TiO2 [4]. Y. Yang et al. [5] prepared W6+-doped TiO2
* Corresponding author at: School of Materials Science and Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China. Tel.: +86 27 87651793; fax: +86 27 87651793. E-mail address: [email protected] (X. Shi). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.09.006
thin film photocatalyst, and they indicated that the higher photocatalytic performances were attributed to efficiently inhibiting the recombination of photoinduced electrons and holes. Once optical excitation occurs, the photogenerated electrons can be transferred to the lower-lying conduction band of WO3. Meanwhile, the holes will accumulate in the valence band of TiO2 [6]. H. Tian et al. [3] considered that W-dopant might increase the charge separation efficiency and the presence of surface acidity. However, D.G. Huang et al. [7] pointed out that metal doping had several drawbacks because the doped materials were not thermally stable, and the metal centers acted as electron trapping centers to reduce the photocatalytic efficiency. Other approaches were adopted to narrow the band gap of TiO2 by replacing lattice oxygen with anionic dopant species, such as N [8], C [9], S [10] and so on. F-doping was also found to have a significant enhancement on the photocatalytic activity of nano TiO2 by preventing the grain growth and inhibiting the transformation of anatase to rutile phase. The doped F atoms that were incorporated into TiO2 lattice might take a positive role in photocatalysis [11]. S.J. Pan et al. [12] reported that the doped F atoms increased the surface acid sites and the formation of Ti3+ ions. The surface acidic sites could not only improve the adsorption of organics, but also act as an electron acceptor to reduce the electron and hole recombination rate. The doped F atoms might also trap the photogenerated electrons, as presented by J. Xu et al. [13]. The N doping of TiO2 substitutive sites has been proven to be helpful for band-gap narrowing and improving photocatalytic activity [8]. Moreover, N-F-codoped TiO2 showed higher absorption and water splitting activities than N-doped TiO2 under visible
4348
X. Shi et al. / Materials Research Bulletin 47 (2012) 4347–4352
light irradiation [7,12]. Y.F. Shen et al. [14] synthesized N-Wcodoped TiO2 photocatalyst by a two-step method combining sol– gel with mechanical alloying. And the photocatalyst had excellent properties of strong absorbance in visible range. T. Mishra et al. [15] synthesized a series of WN co-doped titania nanomaterials with varying tungsten content and dealt with the increased and selective nitrate reduction under visible light over a WN co-doped titania photocatalyst. The results showed that visible light absorption increased with the increasing tungsten amount. Overall high nitrate reduction could be attributed to the synergistic effect of tungsten and nitrogen co-doping, optimum surface hydroxyl group, mesoporosity and appreciable visible light absorption of the materials. To the best of our knowledge, meager information is available as regards the development of the F-N-W-codoped TiO2 photocatalyst by use of a method combining sol–gel with hydrothermal treatment (sol–gel-hydrothermal method) and the investigation of the photocatalytic performances. In this work, the F-N-W-codoped TiO2 photocatalyst was successfully synthesized by the sol–gel-hydrothermal method. Effects of the N, F and W ions on the crystallinity, morphology and grain size of the photocatalyst, as well as the photocatalytic performances under visible light irradiation were systematically investigated. 2. Experimental 2.1. photocatalyst preparation The F-N-W-codoped TiO2 photocatalyst was prepared by the sol–gel-hydrothermal method. Tetrabutyl titanate was used as the starting material. Hydrofluoric acid (40 wt.%) was used as the fluorine source. And ammonium tungstate (92 wt.%) was used as the tungsten and nitrogen source. All chemicals used in the experiment were analytical reagent grade. Firstly, 0.1 mL hydrofluoric acid (RF = 4.0%, the atomic percentage of F to Ti was designated as RF) and a certain amount of ammonium Tungstate (the atomic percentage of W to Ti was designated as RW, and the atomic percentage of N to Ti was designated as RN) were dissolved into 5 mL deionized water with stirring for 10 min to form solution A. 20 mL tetrabutyl titanate was dissolved into the mixture of 180 mL anhydrous ethanol and 8 mL acetic acid, and then stirred for 30 min to form solution B. Secondly, the solution A was slowly dripped into solution B under vigorous stirring. Thirdly, the resultant mixture was slowly stirred at room temperature for at least 24 h until a white transparent immobile sol was formed. Finally, the deionized water was chosen to be solvent. The concentration of Ti4+ in sol mixture was kept in the range of 0.04– 0.06 mol/L. The sol mixture was transferred into a 300 mL Tefloninner-liner stainless steel autoclave. The autoclave was heated in an oven and kept at 150 8C for 2 h, which had been confirmed to be the applicable reaction conditions by H. Tian et al. [3] and D.G. Huang et al. [7]. After the hydrothermal treatment, the gained precipitates were washed by deionized water several times, dried at 80 8C in vacuum. The obtained photocatalysts were denoted as F-N-W-codoped TiO2, including F-3NW-TiO2 (RF = 4.0%, RW = 0.3%, RN/RW = 6/7), F-7NW-TiO2 (RF = 4.0%, RW = 0.7%, RN/RW = 6/7) and F-15NW-TiO2 (RF = 4.0%, RW = 1.5%, RN/RW = 6/7). For comparison, the TiO2 was prepared by the similar method in the absence of solution A. TiO2 with RF = 4.0% prepared in the absence of ammonium tungstate, was denoted as F-TiO2. And TiO2 with Rw = 0.7% and RN/RW = 6/7 prepared in the absence of hydrofluoric acid, was denoted as 7NW-TiO2. Commercial TiO2 Degussa P25 (75% anatase and 25% rutile, specific area of 50 m2/g, average particle size 30 nm, non-porous) was used for comparison.
2.2. Analysis The purity and crystallinity of the as-prepared samples were examined by X-ray diffractometer (XRD, XD-3A, Shimadazu Corporation, Japan) using graphite monochromatic copper radiation (Cu Ka) at 40 kV, 30 mA over the 2u range of 10–808 [11]. The average grain size was determined according to the Scherrer equation: D¼
0:89l B cos u
(1)
where l is the wavelength of the characteristic X-ray applied; B is the half-value width of anatase (1 0 1) peak obtained by XRD; u is the diffraction angle [7,16]. An ultraviolet–visible (UV–Vis) spectrophotometer (Shimadzu UV-2100) was used to record the diffuse reflectance spectra of the samples. In order to investigate the recombination and lifespan of photogenerated electrons and holes in the photocatalysts, the photoluminescence (PL) emission spectra of the samples were measured at room temperature by LS55 (Perkin-Elmer) illuminated with a 325 nm He–Cd laser. The morphologies and grain size of the photocatalyst were observed by a JEM-2100F field emission transmission electron microscope (FETEM). X-ray photoelectron spectra (XPS) of the samples were measured using a PHI5300 photoelectron spectrometer system with an Mg source (1486.6 eV). The shift of binding energy due to relative surface charging was corrected using the C1s level at 287.1 eV as an internal standard of the surface adventitious carbon. N2 adsorption analysis was conducted on an ASAP 2010 Surface Area to get the Brunauer–Emmett–Teller (BET) surface area. 2.3. Photocatalytic activity measurements The photocatalytic activity of the photocatalyst was estimated by measuring the decomposition percentage of RB in an aqueous solution. The photocatalytic degradation experiments were performed with the following procedure: A 160 W tungsten halogen lamp was connected to a light filter (wavelength: 400– 550 nm), and the concentration of RB was 12 mg/L. The reaction was carried out in a 250 mL beaker with an electromagnetic stirrer. The amount of the photocatalyst was 0.5 g/L. Prior to photoreaction, the system was kept in the dark for 30 min to reach adsorption–desorption equilibrium condition. Then the reactants were irradiated for 1.5 h. The samples were taken every 15 min, centrifuged and filtrated, orderly. The dye was analyzed on a UV– vis spectrophotometer (723, Shanghai spectrum Instruments Co., Ltd.), with the maximum absorption wavelength of RB at 554 nm [17]. The degradation rate of RB was measured by determining the chemical oxygen demand (COD) of reaction liquid using K2CrO7, it is could be calculated by: ðCOD0 CODÞ D¼ (2) COD0 where COD0 and COD are the chemical oxygen demand of reaction liquid before and after the reaction respectively [7,14]. 3. Results and discussion 3.1. Characterization of F-N-W-codoped TiO2 photocatalyst The XRD patterns of the photocatalysts were shown in Fig. 1. The XRD analysis showed that all peaks (major peaks: 25.128, 37.788 and 47.888) were ascribed to well-crystallized anatase TiO2, and there were not rutile and brookite existing. Moreover, TiOF2, TiF4, TiN, WOx, O–Ti–N and WxTi1xO2 also were not found because the content was little, and the similar results have been observed by other authors [2,5,7,11,13,14]. And the doping F atoms did not
X. Shi et al. / Materials Research Bulletin 47 (2012) 4347–4352
4349
(see Fig. 2(a)) shows that the F-TiO2 possessed spherical particles. While FETEM image (see Fig. 2(b)) displayed that the appearance of F-7NW-TiO2 was ellipsoidal particles around 20 nm in length and 10 nm in width, which was close to the average particle size calculated from the XRD analysis as shown in Table 1. Besides, FETEM image of the F-TiO2 (see Fig. 2(c)) illustrated that the fringe spacings of 3.580 A˚, 1.872 A˚ and 2.302 A˚ could be indexed to the (1 0 1), (2 0 0) and (0 0 4) planes of anatase TiO2, respectively. While FETEM image of F-7NW-TiO2 (see Fig. 2(d)) revealed the fringe spacings of (1 0 1), (2 0 0) and (0 0 4) planes were reduced to 3.458 A˚, 1.784 A˚ and 2.206 A˚, respectively. It further demonstrated that the W6+ might replace Ti4+ to form W–O–Ti bonds in TiO2 crystal lattice, thus the d(h k l) value of F-7NW-TiO2 was decreased. 3.3. BET specific surface area analysis Fig. 1. XRD patterns of the samples: (a) TiO2; (b) F-TiO2; (c) 7NW-TiO2; (d) F-3NWTiO2; (e) F-7NW-TiO2; (f) F-15NW-TiO2.
cause a shift of the peak position of anatase TiO2, which was consistent with the previous results [1,11]. They thought that the ion radius of fluorine atom (0.133 nm) was virtually same as that of the oxygen atom (0.132 nm). Bragg’s Law refers to the simple equation: dðh k lÞ ¼
l
(3)
ð2 sin u Þ
where d(h k l) is the distance between crystal planes of (h k l); l is the X-ray wave length; u is the diffraction angle of the crystal plane (h k l). According to Bragg’s law, the d(h k l) value was not changed after oxygen atoms were replaced by F atoms. Thus, 2u also was not shifted. As shown in Fig. 1, the average grain sizes of the samples (a), (b), (c), (d), (e) and (f) were calculated by Scherrer Equation. As shown in Table 1, the average grain size of TiO2 doped with F atom was smaller than that of pure TiO2. It could be concluded that Fdoping could inhibit the TiO2 grain growth during the hydrothermal process. In addition, the diffraction peaks gradually became broader with the increasing of the doped amount of W6+, which indicated the smaller grain size. An obvious difference of the (1 0 1) diffraction peak between pure TiO2 and W doped TiO2 showed that the peak position of the photocatalysts with various contents of W6+ slightly shifted toward a higher 2u value. According to Bragg’s law, the d(h k l) value decreases with the increasing of 2u value. It is well known that the ionic radius of W6+ (41 pm) is smaller than that of Ti4+ (53 pm) [5]. When Ti4+ in TiO2 crystal lattice is displaced by W6+, the lattice parameter d(h k l) value would become smaller [3]. Thus, the W6+ should enter into TiO2. Nevertheless, Y. Yang et al. [5] observed that the peaks of anatase TiO2 doped with W6+ shifted toward smaller 2u value, and they thought W6+ might be located at interstitial sites. 3.2. Microstructure analysis Fig. 2 shows the FETEM images of the F-TiO2 and F-7NW-TiO2 prepared by the sol–gel-hydrothermal method. The FETEM image Table 1 The BET surface areas and average particle size of the samples. Photocatalysts
Crystal composition
Average grain size (nm)
SBET (m2/g)
TiO2 F-TiO2 7NW-TiO2 F-3NW-TiO2 F-7NW-TiO2 F-15NW-TiO2
Anatase Anatase Anatase Anatase Anatase Anatase
17 11 18 12 12 13
148 192 143 186 183 162
BET specific surface areas (SBET) (m2/g) of the samples were shown in Table 1. It demonstrated that SBET met the following order: F-TiO2 > F-3NW-TiO2 > F-7NW-TiO2 > F-15NW-TiO2 > TiO2 > 7NW-TiO2. The SBET of F-TiO2 was as high as 192 m2/g. It could be that F played an important role in suppressing the grain growth. The SBET (m2/g) of F-N-W-TiO2 decreased with the increasing of W6+ dopant, and the highest SBET of F-7NW-TiO2 was about 183 m2/g. Moreover, the SBET of 7NW-TiO2 was 143 m2/ g, which was lower than that of pure TiO2. It was in line with the results of XRD analysis. Thus, N and W dopants might result in the larger particle size and particle aggregations. 3.4. Chemical status of the F, N and W atoms in the F-N-W-codoped TiO2 photocatalyst Fig. 3 shows the XPS spectra of the F-7NW-TiO2. As shown in Fig. 3(a), the XPS survey spectra indicated that the peaks were related to Ti, O, F, N, W and a trace amount of carbon. The presence of carbon was ascribed to the residual carbon from the precursor solution and the adventitious hydrocarbon from the XPS instrument itself. XPS data also showed that the peaks of Ti 2p shifted toward higher wavelength and appeared at 462.0 eV and 467.8 eV. The XPS signal of Ti 2p could be fitted with two components. 462.0 eV could attribute to Ti (II) species. However, 467.8 eV was much higher than that of Ti (IV) species for the incorporating of F, N and W species [18]. Fig. 3(b) shows the F 1s XPS spectrum of F-7NW-TiO2 samples. The peaks located at 683.02 eV and 686.75 eV were the characteristics of F 1s peak, which especially meant that there were two chemical forms of F existing. Using Gaussian distributions, one peak indicated that F ions were adsorbed on the surfaces of TiO2 particles, and the other peak stated that F entered into TiO2 crystal lattices [19]. The peak observed at 683.02 eV was originated from the F-containing compounds that adsorbed on the surface of TiO2, and the peak located at 686.75 eV was attributed to the F atoms that replaced the oxygen in the TiO2 crystal lattice. Since any evidences of TiF4 and TiOF2 existing were not found in the XRD patterns, the two peaks indicated that F atoms could be incorporated into TiO2 crystal lattice, and O–Ti–F bonds perhaps had been formed by the sol–gel-hydrothermal method. The W4f peak was used to determine the oxidation state of W, and the binding energy of W 4f7/2 corresponded to the characteristic of W6+ [20]. As shown in Fig. 3(c), the W4f peak located at about 36.03 eV was observed, which was similar to the results of Y.F. Shen et al. [14]. Thus, the W6+ might replace Ti4+ to form W–O–Ti bonds. As shown in Fig. 3(d), the large broadness of the N 1s XPS spectrum was obvious, and main peak emerged at 400.75 eV. M. Pelaez et al. [21] and M. Sathish et al.
4350
X. Shi et al. / Materials Research Bulletin 47 (2012) 4347–4352
Fig. 2. FETEM images of the samples: (a and b) F-TiO2; (c and d) F-7NW-TiO2.
[22] prepared N-doped TiO2 by a sol–gel method, and they suggested that the N 1s peak was at about 400 eV for the formation of the O–Ti–N bonds. Thus, it was concluded that N atoms were incorporated into TiO2 crystal lattice, and O–Ti–N bonds had been formed by the sol–gel-hydrothermal method.
3.5. UV–Vis diffuse reflectance spectra (DRS) and PL spectra Fig. 4 shows the absorption spectra of the photocatalysts. As to 7NW-TiO2, the absorption onset appeared at about 425 nm (2.92 eV), it obviously had red shift when compared with the onset of anatase TiO2 (l = 387 nm). The result was in accordance
Fig. 3. X-ray photoelectron spectra of the F-7NW-TiO2 photocatalyst: (a) global range XPS; (b) F 1s peaks; (c) W 4f peak; (d) N 1s peak.
X. Shi et al. / Materials Research Bulletin 47 (2012) 4347–4352
Fig. 4. UV–Vis spectra of the samples: (a) 7NW-TiO2; (b) F-TiO2; (c) TiO2; (d) F7NW-TiO2.
4351
The photoexcited electrons in the conduction band of TiO2 can be accepted by W6+ [24]. R. Asahi et al. [8] concluded that N was doped into the substitutive sites of TiO2, and the N2p accepter states contributed to the band gap narrowing by mixing with O2p states. Thus, as shown in Fig. 5, the doping N and W atoms could enhance separation efficiency of TiO2 charge pairs. Meanwhile, Fdopant could convert some Ti4+ to Ti3+ by charge compensation. The Ti3+ surface states, which formed a donor level between the band gaps of TiO2, would trap the photogenerated electrons [1] and then transfer them to the state of oxygen vacancies on the surface of TiO2. The F-7NW-TiO2 photocatalyst had the strongest absorption ability of visible light and highest separation efficiency of charge pairs, which mostly were attributed to a synergetic effect of the doped N, W and F atoms. Doped F ions generated greater specific surface area. Doped N and W atoms contributed to more absorption of visible light, and valence variation of W6+ enhanced separation efficiency of charge pairs. 3.6. Photocatalytic activity
with that obtained by some other authors [2,14]. In addition, the absorption edge of F-TiO2 slightly shifted toward longer wavelengths, and the absorption onset appeared at about 410 nm (3.02 eV). Especially for F-7NW-TiO2, the absorption onset appeared at about 500 nm. The band gap energy was about 2.48 eV, which was much lower than that of pure TiO2 (3.2 eV). Consequently, the F-N-W-codopant was beneficial to the visible light absorption of TiO2 as shown in Fig. 4. PL emission spectra had been used to investigate the efficiency of charge carrier transfer and the recombination rate of electron and hole pairs in semiconductor particles [1,11]. The lower excitonic PL intensity corresponded to the higher separation rate of electron and hole pairs [23]. Fig. 5 shows that the PL spectra of TiO2, F-TiO2, 7NW-TiO2 and F-7NW-TiO2, and their excitonic PL intensity obeyed the following order: TiO2 > F-TiO2 > 7NWTiO2 > F-7NW-TiO2. It revealed that a certain amount of F-N-Wcodopants could sharply decrease the recombination rate of electron and hole pairs. According to the recent study, the measured band-gap energy of WO3 2.75 eV was more easily excited by visible light than that of TiO2, and the W6+ doping of TiO2 could generate electron and hole pairs at the tail states of conduction band and valence band under visible light [5], following the scheme [3]: W6þ þ e ðTiO2 Þcb ! W5þ
The degradation experiment of 12 mg/L RB solution (COD0 = 115 mg/L, the measurement error of all data is less than 1%) was chosen to investigate the photocatalytic activity of the photocatalysts with different dopants. The kinetic curves of photodecoloration rate and carbon removal rate & histograms of COD of RB in the visible light (400–550 nm) were shown in Figs. 6 and 7 respectively. As shown in Fig. 6(c), the moderate photodegradation activity of F-TiO2 under visible illumination was observed. The decoloration percent was 70%. As shown in Fig. 7(B) and (c), under visible light irritation for 90 min, the COD decreased from 115 to 35 mg/L using the F-TiO2 photocatalyst, and carbon removal rate was up to 70%. It might be attributed to the enhancement of surface acidity and the creation of oxygen vacancies. Then hydroxyl radicals (OH) were formed, which were generally regarded as the important active species for initiating the photocatalytic reaction [24]. The decoloration rate of RB was up to 83% using 7NW-TiO2, and the COD decreased to 22 mg/L, which was much better than that of P25, pure TiO2 and F-TiO2. Compared with 7NW-TiO2 and F-TiO2, the photoactivity of F7NW-TiO2 evidently was improved. The decoloration percent of RB was up to about 98%. As shown in Fig. 6(f) and Fig. 7(A, B)(f), the COD was only about 7 mg/L, and carbon removal rate reached to 94%. However, the decoloration rate and carbon removal rate of
W5þ þ hþ ðTiO2 Þcb ! W6þ
Fig. 5. PL spectra of the samples: (a) TiO2; (b) F-TiO2; (c) 7NW-TiO2; (d) F-7NWTiO2.
Fig. 6. Kinetic curves of photodecoloration rate of RB under visible light (450– 550 nm) (C0 = The initial concentration of RB; C = The concentration of RB at various times): (a) P25; (b) TiO2; (c) F-TiO2; (d) 7NW-TiO2; (e) F-3NW-TiO2; (f) F-7NWTiO2; (g) F-15NW-TiO2.
4352
X. Shi et al. / Materials Research Bulletin 47 (2012) 4347–4352
4. Conclusions A simple sol–gel-hydrothermal method was developed to prepare highly photoactive anatase F-N-W-codoped TiO2 photocatalyst. It appeared uniform ellipsoidal particles around 20 nm in length and 10 nm in width. The finer particles resulted in larger surface area (about 180 m2/g) as obtained from BET. Compared with the 7NW-TiO2 and F-TiO2, the higher RB degradation rate of the F-7NW-TiO2 demonstrated that the photoexcited wavelength range had stronger red shift, which was ascribed to a synergetic effect of the doped F, N and W atoms: (I) Doped F ions could create larger specific surface area and higher surface acidity; (II) W-doping formed the donor level, and W6+ could accept photoelectrons to suppress the recombination of charge pairs; (III) N-doping induced the acceptor level. All made contributions to the visible light response of the F-7NW-codoped TiO2 photocatalyst. The decoloration percent of RB was up to about 98%, and carbon removal rate reached 94% under visible illumination. Thus, the preparation and photocatalytic activity of the F-N-W-codoped TiO2 photocatalyst were worthy to be further investigated. Acknowledgements This work was supported by the Academic Leader Program of Wuhan City (201150530146); the Nature Science Foundation of Hubei Province (20101j0018); the Project for Science and Technology Plan of Wuhan City (200910321092); and the Fundamental Research Funds for the Central Universities (2010II-020). References
Fig. 7. Kinetic curves of carbon removal rate (A) and histograms of COD (mg/L) of RB after 30 min, 60 min and 90 min photoreaction (COD0 = 115 mg/L; COD = The chemical oxygen demand of RB at various times) (B): (a) P25; (b) TiO2; (c) F-TiO2; (d) 7NW-TiO2; (e) F-3NW-TiO2; (f) F-7NW-TiO2; (g) F-15NW-TiO2.
F-15NW-TiO2 only were about 92% and 88% respectively. It was noteworthy that excessive W amounts seemed to be detrimental to the photodegradation of RB because too high-loaded W would act as recombination centers for electron and hole pairs [3,5], which was in line with the results of UV–Vis spectra as shown in Fig. 5. Thus, it was concluded that high photodegradation activity of F-NW-codoped TiO2 was ascribed to a synergetic effect of the doped N, W and F atoms. The larger specific surface area was created by the doped-F ions. Then more RB molecules could be absorbed on the particle surface because RB was a cationic dye. Meanwhile, more photoexcited electrons were created from WxTi1xO2 or O–Ti–N in the photocatalyst under visible light, and then W6+ accepted photoelectrons to suppress the recombination of charge pairs, thus more photoelectrons were transferred to the particle surface. More hydroxyl radicals (OH) were generated, which could disrupt RB molecules. And the photocatalytic activity of F-N-W-codoped TiO2 was improved drastically under visible illumination.
[1] J.C. Yu, J.G. Yu, W.G. Ho, Z.T. Jiang, L.Z. Zhang, Chem. Mater. 14 (2004) 3808–3816. [2] H.M. Yang, R.R. Shi, K. Zhang, Y.H. Hu, A.D. Tang, X.W. Li, J. Alloys Compd. 398 (2005) 200–202. [3] H. Tian, J.F. Ma, K. Li, J.J. Li, Mater. Chem. Phys. 112 (2008) 47–51. [4] W.Y. Choi, A. Termin, M.R. Hoffmann, J. Phys. Chem. 98 (1994) 13669–13679. [5] Y. Yang, H.Y. Wang, X. Li, C. Wang, Mater. Lett. 63 (2009) 331–333. [6] D.N. Ke, H.J. Liu, T.Y. Peng, X. Liu, K. Dai, Mater. Lett. 62 (2008) 447–450. [7] D.G. Huang, S.J. Liao, J.M. Liu, Z. Dang, L. Petrik, J. Photochem. Photobiol. A 184 (2006) 282–288. [8] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269–271. [9] S. Sakthivel, H. Kisch, Chem. Int. Ed. 42 (2003) 4908–4911. [10] T. Ohno, M. Akiyoshi, T. Umebayashi, K. Asai, T. Mitsui, M. Matsumura, Appl. Catal. A 265 (2004) 115–121. [11] J.J. Xu, Y.H. Ao, D.G. Fu, C.W. Yuan, Appl. Surf. Sci. 254 (2008) 3033–3038. [12] S.J. Pan, H.C. Liang, C.Q. Wang, F.W. Yang, X.D. Li, S. Yang, Sci. China Ser. B: Chem. 52 (2009) 2043–2046. [13] J. Xu, B.F. Yang, M. Wu, Z.P. Fu, Y. Lv, Y.X. Zhao, J. Phys. Chem. C 114 (2010) 15251–15259. [14] Y.F. Shen, T.Y. Xiong, T.F. Li, K. Yang, Appl. Catal. B 83 (2008) 177–185. [15] T. Mishra, M. Mahato, N. Aman, J.N. Patel, R.K. Sahu, Catal. Sci. Technol. 1 (2011) 609–615. [16] K. Demeestere, J. Dewulf, H.V. Langenhove, B. Sercu, Chem. Eng. Sci. 58 (2003) 2255–2267. [17] C. Wang, B.Q. Xu, X.M. Wang, J.C. Zhao, J. Solid State Chem. 178 (2005) 3500–3506. [18] J.G. Yu, X.J. Zhao, Q.N. Zhao, J.C. Du, Chin. J. Mater. Res. 14 (2000) 203–209. [19] H.W. Park, W.Y. Choi, J. Phys. Chem. B 108 (2004) 4086–4093. [20] S. Penner, X.J. Liu, B. Klotzer, F. Klauser, B. Jenewein, E. Bertel, Thin Solid Films 516 (2008) 2829–2836. [21] M. Pelaez, A.A. de la Cruz, E. Stathatos, P. Falaras, D.D. Dionysiou, Catal. Today 144 (2009) 19–25. [22] M. Sathish, B. Viswanathan, R.P. Viswanath, Appl. Catal. B 74 (2007) 307–312. [23] Y.S. Luo, J.P. Liu, X.H. Xia, X.Q. Li, T. Fang, S.Q. Li, Q.F. Ren, J.L. Li, Z.J. Jia, Mater. Lett. 61 (2007) 2467–2472. [24] P. Cheng, C.S. Deng, D.N. Liu, X.M. Dai, Appl. Surf. Sci. 254 (2008) 3391–3396.