- Email: [email protected]
Dual-responsive F–N–W-codoped TiO2 nanoparticles with strong visible light response and low-temperature thermo-oxidation activity
Materials Chemistry and Physics 135 (2012) 818e825
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Dual-responsive FeNeW-codoped TiO2 nanoparticles with strong visible light response and low-temperature thermo-oxidation activity Xiaoliang Shi a, b, *, Haibo Qin b, Qiaoxin Zhang a, b, Song Zhang a 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
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< FeNeW-codoped TiO2 NPs were synthesized by a simple solegelmixing method. < The NPs showed high visible photocatalytic performance. < The NPs showed excellent lowtemperature thermal-oxidation ability in the dark. < The NPs could be applied as environmental catalyst without light illumination.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 September 2011 Received in revised form 20 April 2012 Accepted 19 May 2012
FeNeW-codoped TiO2 nanoparticles were successfully prepared by a solegel-mixing method without hydrothermal or calcining treatment. Effects of the F, N and W ion dosage concentration on crystallinity, morphology, grain size and chemical status of TiO2 were investigated. The results showed that the appearance of FeNeW-codoped TiO2 powders was uniformly spheroidal nanoparticles, whose average particle size was about 7 nm. The FeNeW-codoped TiO2 nanoparticles owned high catalytic activity under visible light irradiation and low-temperature thermo-oxidation activity in the dark. The highest degradation rate of Rhodamine B dye was obtained by TiO2 powders modified with 0.5 at.% W (N/W ¼ 6/ 7) and 4.0 at.% F. Even in the dark, the degradation rate increased with the increasing of temperature. It was as high as 92% when the mixing liquid was kept at 60 C for 2 h. The high catalytic activity of the FeN eW-codoped TiO2 might result from larger specific surface area and high surface acidity because of surface F ions, the band gap narrowing caused by WxTi1xO2 or OeTieN, as well as lower recombination rate of charge carriers because of valence variation of W ions (W6þ / W5þ) in the forbidden band. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Semiconductors Chemical synthesis Electron microscopy Oxidation
1. Introduction TiO2 is a typical n-type semiconductor, which is considered as the most suitable photocatalyst because of biological and chemical inertness, non-toxicity, and long-term stability [1]. And it had been
* Corresponding author. School of Mechanical and Electronic Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China. Tel./fax: þ86 27 87651793. E-mail address: [email protected] (X. Shi). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.05.064
widely applied in various fields, such as decomposition of organic contaminants, development of efficient dye-sensitized solar cells and water photosplitting into H2 and O2 [2]. However, pure TiO2, whose band gap energy is 3.2 eV, only absorbs ultraviolet (UV) light. The fraction of UV light in the solar energy spectrum is only about 4%. Thus, its catalytic efficiency is very limited. Moreover, the high recombination rate of photoexcited charge carries results in the low quantum yield rate and undesirable photooxidation rate [3]. Hence, it is necessary to shift the absorption threshold toward the visible region and decrease recombination rate of excited
X. Shi et al. / Materials Chemistry and Physics 135 (2012) 818e825
electron/hole pairs. Researchers have made various modifications to improve the photocatalytic activity. The initial approach was the doping of TiO2 with metal elements [4]. W ion doped TiO2 is used to enhance photocatalytic properties in the visible range by some authors, and they indicated that its higher photocatalytic performance owed to the band gap narrowing created from WxTi1xO2 and efficiently inhibiting the recombination of photoinduced electrons and holes [5e8]. Once optical excitation occurs, the photogenerated electrons can be transferred to the lower-lying conduction band of WO3, while the holes will accumulate in the valence band of TiO2 [9]. H. Tian et al. considered that W-dopant might increase the charge separation efficiency and the presence of surface acidity [3]. V. Iliev et al. have prepared Au/WO3/TiO2 with high photocatalytic activity under visible light irradiation via calcining treatment [7]. However, D.G. Huang et al. pointed out that the doped metal materials were not thermally stable and acted as electron trapping centers to reduce the photocatalytic efficiency [10]. Other approaches were adopted to narrow the desired band gap of TiO2 by replacing lattice oxygen with anionic dopant species, such as N [11,12], C [13] and S [14]. R. Asahi et al. indicated that nitrogen doped into substitutional sites of TiO2 had been proven to be indispensable for band-gap narrowing and photocatalytic activity [8]. Y.F. Shen et al. [12] have synthesized NeW-codoped TiO2 nano-powders by a two-step method combining with solegel and mechanical alloying method, and the photocatalyst had excellent properties of strong absorbance in visible range, as long as 650 nm. F-doping could prevent the grain growth, enhance surface acidity and inhibit the transformation of anatase to rutile phase [1]. S.J. Pan et al. [15] reported the doped-F atoms also caused the increasing of surface acid sites and forming of Ti3þ ions. More surface acidic sites could not only adsorb more organics, but also act as an electron acceptor to reduce the eeh recombination rate. NeF-codoped TiO2 showed higher absorption and water splitting activity than N-doped TiO2 under visible light irradiation [10,15]. TiO2eWO3 combined system protected stainless steel from corrosion for 5 h after 1 h UV irradiation, due to the stored charge [16]. T. Tatsuma et al. reported that TiO2eWO3 combined system generated H2O2 by reducing oxygen to retain a bactericidal effect even after the light was turned off [17]. When TiO2eWO3 was irradiated by UV light, WO3 could be reduced even in pure water or humid air (relative humidity 25%) [18]. Although the mechanism has not yet been known, it means that one of the limitations of TiO2 is overcome. As we know, the TiO2 nanoparticles (NPs) with various doped species were usually synthesized by solegel-hydrothermal or solegel-calcining process. On one hand, these methods can guarantee intact crystal and high purity. On the other hand, they also may bring about many defects, such as grain growth, particles agglomeration and thermal invalidation of metal ions, which go actively against quantum size effect and valence variation of metal ions. To the best of our knowledge, this work may be the first report about synthesizing FeNeW-codoped TiO2 NPs by a simple solegel-mixing method without hydrothermal or calcining process in this work. And the visible light photocatalytic performance and thermal-oxidation ability in the dark of FeNeWcodoped TiO2 NPs were investigated by measuring the decomposition percentage of Rhodamine B dye (RB). Effects of F, N and W atoms dosage concentration on the crystallinity, morphology, chemical status and grain size of FeNeW-codoped TiO2 NPs was systematically characterized by various analysis and testing techniques. A tentative elucidation of the mechanism for the high visible light photocatalytic activity and especial low-temperature thermo-oxidation performance of the FeNeW-codoped TiO2 NPs was presented.
819
2. Experimental section 2.1. Catalyst preparation The fluorine, nitrogen and tungsten codoped TiO2 samples were prepared by a solegel-mixing 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 sources. All chemicals used in the experiment were analytical reagent grade. Firstly, a certain amount of ammonium tungstate (RW ¼ 0.25%, 0.5%, 1.0%, and 2.0%, the atomic percentage of W to Ti was designated as RW, RN/RW ¼ 6/ 7) was completely dissolved into 8 mL deionized water with 0.1 mL hydrofluoric acid (RF ¼ 4.0%, the atomic percentage of F to Ti was designated as RF), and then the mixing solution was slowly dripped into 80 mL anhydrous ethanol to form ivory sol-A during stirring. Secondly, 20 mL tetrabutyl titanate was dissolved into the mixture of 100 mL anhydrous ethanol & 5.0 mL acetic acid, and stirred for 30 min to form uniform transparent solution-B. Finally, the sol-A was slowly dripped into the solution-B under vigorous stirring. The resultant mixture gradually became a white transparent sol under ceaselessly stirring, and then the sol started to become colloidal particles slowly. During these processes as above, the temperature of solution was maintained below 5 C. After stewing for more than one day, the gained colloidal solution were diluted several times with deionized water, and then heated in water bath at 80 C for 2 h to become precipitate. The precipitates were washed by deionized water, then dried at 60 C in vacuum. The obtained catalyst was denoted as FeNeWeTiO2, including Fe5NWeTiO2, Fe10NWeTiO2 and Fe2.5NWeTiO2, Fe20NWeTiO2. For comparison, TiO2 was prepared by the similar method in the absence of solution A. TiO2 with only RF ¼ 4.0% was prepared in the absence of ammonium tungstate, denoted as FeTiO2. And TiO2 with Rw ¼ 0.5% (RN/RW ¼ 6/7) was prepared in the absence of hydrofluoric acid, denoted as 5NWeTiO2. TiO2 Degussa P25 (75% anatase and 25% rutile, specific area of 50 m2 g1, average particle size 30 nm, non-porous) was used for the purpose of comparison. 2.2. Analysis Purity and crystallinity of the as-prepared samples were examined by X-ray diffractometer (XRD, XD-3A, Shimadzu Corporation, Japan) using graphite monochromatic copper radiation (Cu Ka) at 40 kV, 30 mA over the 2q range of 10e80 [19]. The average crystallite size was determined according to the Scherrer equation:
D ¼ 0:89l=B cos q
(1)
where l is the wavelength of the characteristic X-ray applied; B is the half-value width of anatase (101) peak obtained by XRD; q is the diffraction angle [10,20]. An ultravioletevisible (UVeVis) 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/holes in the photocatalyst, the photoluminescence (PL) emission spectra of the samples were measured at room temperature by LS-55 (PerkineElmer) illuminated with a 325 nm HeeCd laser. Morphology and grain size of the NPs were observed by a JEM2100F field emission transmission electron microscope (FETEM) and field emission scanning electron microscope (FESEM). X-ray photoelectron spectra (XPS) of the samples were measured using a PHI5300 photoelectron spectrometer system with a Mg source (1486.6 eV). The shift of binding energy due to relative surface charging was corrected using the C1s level at 286.7 eV as an internal
820
X. Shi et al. / Materials Chemistry and Physics 135 (2012) 818e825
standard of the surface adventitious carbon. N2 adsorption analysis was conducted on an ASAP 2010 Surface Area to get the BrunauereEmmetteTeller (BET) surface area. 2.3. Photocatalytic and thermal-oxidation activity measurements Decomposition percentage of RB in an aqueous solution was studied to estimate the visible light photocatalytic activity and lowtemperature thermal-oxidation performance (60 C) of the FeNeW-codoped TiO2 NPs. The visible light source was 160 W tungsten halogen lamp connected to a light filter (wavelength: 400e550 nm), and the concentration of RB was 30 mg L1. The reaction was carried out in a 250 mL beaker with an electromagnetic stirrer. The amount of catalyst was 0.5 g L1. Prior to photoreaction, the system was kept at dark room for 60 min to reach adsorptionedesorption equilibrium condition, and then the illumination reaction lasted for 1.5 h. In the thermal-oxidation experiment, the catalyst was exposed to visible light (400e550 nm) irradiation for 15 min before mixing with RB solution, and then the mixing solution was placed at different temperature (5, 60 C) for 2 h in the dark after ultrasonic dispersing. The aim that the catalyst was exposed to visible light (400e550 nm) irradiation for 15 min before mixing with RB solution was to produce photoexcited electrons in the catalyst. The distinct characteristic of the catalyst was that the photoexcited electrons in the catalyst could be rapidly released with the increasing of environment temperature in the dark, which was defined as thermooxidation activity in this article. The samples were taken every 15 min, centrifuged and filtrated, orderly. The filtrates were analyzed by using a spectrophotometer (723, Shanghai spectrum Instruments Co., Ltd.) to detect the discoloration rate at its maximum absorption wavelength of 554 nm for RB solution [21]. If the chromophoric groups of RB are destroyed, and decolorization would happen. However, intermediate products, even more toxic, could be formed. Of course, RB also could have been degradated and mineralized during the process of decolorization. Thus, the relevant photocatalytic performance of degradation is to be evaluated with respect to the mineralization of RB, i.e. its conversion to CO2 and water by the chemical oxygen demand (COD) determinations of reaction liquid using K2CrO7, it could be calculated by:
D ¼ ½ðCOD0 CODÞ=COD0
(hkl). According to Bragg’s law, when Ti4þ in TiO2 crystal lattice was displaced by W6þ, the lattice parameter d(hkl) value would become smaller, because the ionic radius of W6þ (41 pm) is smaller than that of Ti4þ (53 pm) [3]. Nevertheless, Y. Yang et al. [5] observed that the peaks of anatase TiO2 with W6þ shifted toward smaller 2q value, and they thought W6þ might be located at interstitial sites. Thus, the results in our study indicated that some W6þ ions were introduced into TiO2 crystal lattice by the solegel-mixing method. Moreover, as shown in Fig. 1(b) and Fig. 1(f), the disparity between the peaks indicated that superfluous doped W atoms suppressed crystallization processes to cause bad crystallization, which was related to the interfacial charge neutralization. Then the d(hkl) value became larger. The WO3 nanoclusters with negative charges would be absorbed on the surface of TiO2 nanoclusters with positive charges when they were mixed [23]. 3.2. Morphology analysis
(2)
where COD0 and COD are the chemical oxygen demand of reaction liquid before and after the reaction, respectively [10]. 3. Result and discussion 3.1. Characterization of FeNeW-codoped TiO2 powders The XRD patterns of FeTiO2, 5NWeTiO2 and FeNeW-codoped TiO2 were shown in Fig. 1. The XRD analysis showed that all peaks (major peaks: 25.12 , 37.78 and 47.88 ) were ascribed to imperfectly crystallized anatase TiO2 without rutile and brookite in all samples. However, TiOF2, TiF4, TiN, WOx and WxTi1xO2 were not found, because the content was little. The similar results had been also observed by other authors [2,5,10,12,19,22]. It was obvious that F atoms efficiently suppressed the TiO2 grain growth, and the doping W atoms caused shifting of peak positions of anatase TiO2 to higher 2q value, which were consistent with the previous results [3]. Bragg’s Law refers to the simple equation:
dðhklÞ ¼ l=ð2 sin qÞ
Fig. 1. XRD patterns of the samples: (a) FeTiO2; (b) 5NWeTiO2 (RN/RW ¼ 6/7); (c) Fe2.5NWeTiO2; (d) Fe5NWeTiO2; (e) Fe10NWeTiO2; and (f) Fe20NWeTiO2.
(3)
where d(hkl) is the distance between crystal planes of (hkl); l is the X-ray wavelength; q is the diffraction angle of the crystal plane
Fig. 2 showed FESEM and FETEM images of Fe5NW-codoped TiO2 powders prepared by the solegel-mixing method. The FESEM image (see Fig. 2(a)) showed that the agglomeration phenomenon of FeNeW-codoped TiO2 NPs was serious. The FETEM image (see Fig. 2(b)) illustrated that the fringe spacing of 1.89 A could be indexed to the (200) plane of anatase TiO2. It indicated that the crystalline state of FeWeTiO2 at (200) plane was prone to grow. Besides, FETEM image (see Fig. 2(c)) displayed that a mass of Fe5NWeTiO2 NPs got together to form aggregated spheres with different sizes. And the grain sizes of these particles ranged from 5 nm to 20 nm. It was close to the value of average particle size calculated from the XRD analysis as shown in Table 1. 3.3. BET specific surface area analysis BET specific surface areas (SBET) (m2 g1) of the as-prepared samples were also shown in Table 1. It showed that the SBET met the following order: FeTiO2, Fe2.5NWeTiO2, Fe5NWeTiO2, Fe10NWeTiO2, Fe10NWeTiO2, and 5NWeTiO2. The SBET of FeTiO2 NPs was highest, up to 228.6 m2 g1. The SBET (m2 g1) of FeNeWeTiO2 NPs decreased with the increasing of W6þ dosage concentration, and the SBET of Fe5NWeTiO2 NPs was up to 210.5 m2 g1. The SBET of 5NWeTiO2 NPs was 187.6 m2 g1, which
X. Shi et al. / Materials Chemistry and Physics 135 (2012) 818e825
821
Table 1 BET surface areas and average particle sizes of the as-prepared samples.
Photocatalysts
Crystal composition
Average crystal size (nm)
SBET (m2 g1)
FeTiO2 5NWeTiO2 Fe2.5NWeTiO2 Fe5NWeTiO2 Fe10NWeTiO2 Fe20NWeTiO2
Anatase Anatase Anatase Anatase Anatase Anatase
6 15 7 9 12 14
228.6 183.9 217.8 210.5 200.1 187.6
and and and and and and
amorphous amorphous amorphous amorphous amorphous amorphous
was the lowest and in accordance with the result of XRD spectra. Thus, it was concluded that F played an important role in suppressing the grain growth, and N & W ion dopants might result in aggregation of NPs. 3.4. Chemical status of the F, N and W atoms in the FeNeWcodoped TiO2 catalyst
Fig. 2. (a) FESEM image of Fe7NWeTiO2; (b) and (c) FETEM images of Fe7NWeTiO2.
Fig. 3 showed the XPS spectra of the Fe5NW-codoped 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. The peaks of Ti2p were observed at 461.55 eV and 467.22 eV, which shifted toward to higher wavelength. The XPS signal of Ti2p3/2 could be fitted with two components. One attributed to Ti (IV) species, and the other assigned to Ti (III) species [24]. Thus, the peaks of Ti (IV) and Ti (III) might exist in the region of 462.0 eV. The peaks of F1s and N1s also showed that F and W atoms were incorporated into the TiO2 crystal lattice or adsorbed on the surface of the nanoclusters. Fig. 3(b) showed the F1s XPS spectra of the Fe5NW-codoped TiO2 NPs. The peaks located at 682.7 eV and 686.9 eV were designed to the characteristics of F1s peak, and it especially meant that there were two chemical forms of F existing in the Fe5NWcodoped TiO2 NPs. Using Gaussian distributions, the F1s peak was divided into two separated peaks [10]. One peak indicated that F was adsorbed on the surfaces of TiO2 particles, and the other peak stated that F entered into TiO2 crystal lattices [25]. Thus, as shown in Fig. 3(b), the peak observed at 682.7 eV was originated from the F-containing compounds adsorbing on the surface, and the peak located around 686.9 eV was attributed to the F atoms replacing the oxygen in the TiO2 crystal lattice [10]. Although any evidence of TiF4 and TiOF2 was not found in the XRD pattern, these two XPS peaks at 682.7 eV and 686.9 eV indicated that F atoms were incorporated into the FeNeW-codoped TiO2 crystal lattice to form OeTieF bonds by nucleophilic substitution reaction of F ions during the hydrolysis process. Two energy levels of tungsten were measured in the W4d and W4f region as shown in Fig. 3(a). The W4f peak was used to determine the oxidation state of tungsten, and the binding energy of W4f7/2 corresponded to the characteristic of W6þ [26]. The W4f peak in Fig. 3(c) located at about 35.9 eV was observed, which was similar to the results of V. Iliev et al. [7]. So we could propose that the W6þ existed, and was absorbed on the surface of TiO2 NPs or entered into TiO2 crystal lattices. As shown in Fig. 3(d), the N1s XPS spectrum displayed a large broadness, and main peaks emerged at 400.5 eV. M. Pelaez et al. [27] and M. Sathish et al. [28] prepared Ncodoped TiO2 powders by a solegel method, and they suggested that the N1s peak at about 400 eV was due to the formation of the OeTieN bonds. So it was concluded that N atoms were incorporated into TiO2 crystal lattice, and OeTieN bonds had been formed by the solegel-mixing method.
822
X. Shi et al. / Materials Chemistry and Physics 135 (2012) 818e825
Fig. 3. X-ray photoelectron spectra of Fe5NWeTiO2 NPs: (a) Global range XPS; (b) F1s peaks; (c) W4f peak; (d) N1s peak.
3.5. UVeVis diffuse reflectance spectra (DRS) and PL spectra Fig. 4 showed the absorption spectra of the various NPs. As to Fdoped TiO2 (curve (e)), the onset of the absorption appeared at about 405 nm (3.06 eV). Compared with the onset of anatase TiO2 (l ¼ 387 nm), it had obvious red-shift, which was in accordance
Fig. 4. UVeVis spectra of the samples: (a) Fe5NWeTiO2; (b) Fe10NWeTiO2; (c) 5NWeTiO2; (d) Fe2.5NWeTiO2; (e) FeTiO2.
with results obtained by other authors [2,7,12]. In addition, the absorption edge of 5NWeTiO2 slightly shifted toward higher wavelengths, and the onset of the absorption appeared at about 450 nm (2.76 eV), which was close to the band-gap energies of WO3 (2.75 eV). Especially for FeNeW-doped TiO2, the onset of the absorption changed with the increasing of W and N atoms. The absorption edge of Fe5NWeTiO2 could locate at about 550 nm, and the band gap energy was 2.26 eV, which was much lower than that of anatase TiO2 and others. It may contribute to the quantum size effect in nanoclusters. The OeTieN and WxTi1xO2 made NPs be efficiently excited by visible light. And interfacial charges were easily transferred. However, the absorption edge of both Fe10NWeTiO2 and Fe20NWeTiO2 decreased, because superfluous W and N atoms would act as the recombination center of charge carries. PL emission spectra had been used to investigate the efficiency of charge transfer and recombination rate of electronehole pairs in semiconductor particles [1,19]. The lower excitonic PL intensity corresponded to the higher separation rate of electrons and holes [29]. Fig. 5 showed the PL spectra of the various samples. The excitonic PL intensity obeyed the following order: FeTiO2 > 5NW eTiO2 > Fe2.5NWeTiO2 > Fe10NWeTiO2 > Fe5NWeTiO2. Compared with FeTiO2, TiO2 with a certain amount of F, N and W atoms could sharply decrease the excitonic PL intensity. According to the recent study, TiO2 has slightly larger band gap than WO3. Both the top of the valence band and the bottom of the conduction band of WO3 are lower than that of TiO2 (as indicated in Fig. 6). As to FeTiO2, the photogenerated electrons and holes usually combined quickly. When FeNeW-codoped TiO2 NPs was excited by light, the photogenerated electrons could be transferred
X. Shi et al. / Materials Chemistry and Physics 135 (2012) 818e825
823
3.6. Photocatalytic activity The decoloration and degradation experiments of RB solution (30 mg L1, COD0 ¼ 287.5 mg L1) were chosen to investigate the photocatalytic activity of the composite catalysts, and their kinetic curves of photodecoloration of RB in the visible light (400e550 nm) were shown in Fig. 7. The decoloration reaction was closely related to the reaction time. It was interesting that the FeNeW-doped TiO2 NPs became red in uniformly mixed solution, and then gradually became yellowgreen after illumination for about 60 min. The yellowgreen color was very obvious after about 120 min. But when the illumination time extended to about 150 min, the yellowgreen NPs changed back white. While the red FeTiO2 NPs did not become yellowgreen, and directly became white during decoloration experiment. According to the phenomenon, we deduced W6þ on the surface of the NPs accepted some photoexcited electrons to be converted to W5þ, and then yellowgreen MxWO3 was created after a period of illumination time, following the scheme: Fig. 5. PL spectra of the samples: (a) FeTiO2; (b) 5NWeTiO2; (c) Fe2.5NWeTiO2; (d) Fe10NWeTiO2; (e) Fe5NWeTiO2.
from the TiO2 conduction band to the WO3 conduction band, and holes can be transferred from the WO3 valence band to the TiO2 valence band, following the scheme [3]:
TiO2 =WO3 þ hv/TiO*2 =WO*3 þ e þ hþ
(4)
W6þ þ e ðTiO2 Þcb /W5þ
(5)
The photoexcited electrons in the conduction band of TiO2 can be accepted by W6þ [30]. Thus, as shown in Fig. 5, a certain range of doping W6þ could enhance separation efficiency of charge carries for TiO2. However, too high-loaded W would act as recombination centers for electronehole pairs and result in the increasing of the excitonic PL intensity of Fe10NWeTiO2. The absorption edge of Fe5NW-doped TiO2 could expand to as large as 550 nm, which mostly owed to the synergetic effect of the doped N, W and F atoms. The larger specific surface area is, the more there are oxygen vacancies existing. OeTieN and WxTi1xO2 facilitated to generate and transfer more excited electrons, and the valence transition between W6þ & W5þ in the NPs could effectively separate photogenerated carriers.
Fig. 6. Band gap structure diagram of TiO2eWO3 and the separation process of electronehole pairs.
WO3 þ xe þ MHþ /Mx WO3 ðyellowgreenÞ
(6)
It was observed that the photodecoloration activity of FeTiO2 photocatalyst was up to 60% after 120 min under visible light irradiation as shown in Fig. 7(b). It might be attributed to the enhancement of surface acidity, higher specific surface areas and the creation of oxygen vacancies, which made hydroxyl radicals (OH) seize more RB molecules [1,30]. Compared with FeTiO2, the photoactivity of FeNeW-codoped TiO2 evidently augmented, which was in line with the result of UVeVis spectra and PL spectra as shown in Figs. 4 and 5. It was noteworthy that superfluous W6þ seemed to be detrimental to the photodecoloration of RB, and the optimal doping amount of W6þ in the NPs was found to be about 0.5 at.%. The decoloration percent of RB was as high as 99% using Fe5NWeTiO2 (see Fig. 7(f)), which was remarkably higher than that of the others. It demonstrated that the moderate dopant of F, N and W would absorb more visible light and reduce the recombination rate of electronehole pairs. The COD was only about 14.38 mg L1, and carbon removal rate reached to about 95%. Thus, the high photodecoloration and photodegradation activities of Fe5NW-codoped TiO2 were ascribed to the synergetic effect of the doping N, W
Fig. 7. Kinetic curves of photodecoloration of RB in the presence of visible light (400e550 nm) (C0 ¼ The initial concentration of RB; C ¼ The concentration of RB at various times): (a) P25; (b) FeTiO2; (c) Fe20NWeTiO2; (d) Fe2.5NWeTiO2; (e) Fe10NWeTiO2; (f) Fe5NWeTiO2.
824
X. Shi et al. / Materials Chemistry and Physics 135 (2012) 818e825
and F atoms. The larger specific surface area was, the more oxygen vacancies were created from doped-F ions. RB molecules could be absorbed on the particle surface, because RB was a cationic dye. Meanwhile, OeTieN and WxTi1xO2 facilitated to generate more excited electrons, and W6þ in the NPs efficiently suppressed the recombination of electronehole pairs. Thus, more photoexcited electrons were transferred to the particle surface, and then more hydroxyl radicals were generated. Afterward, a mass of OH disrupted RB molecules, and then decomposition was completed under visible light irradiation. 3.7. Low-temperature thermo-oxidation activity The decoloration and degradation experiments of RB solution (30 mg L1, COD0 ¼ 287.5 mg L1) were chosen to investigate the low-temperature thermo-oxidation activity of the NPs, and their kinetic curves of thermo-oxidation decoloration of RB at about 5 C and 60 C in the dark were shown in Fig. 8(A) and (B), respectively. As shown in Fig. 8(A), lots of RB molecules mostly absorbed on the surface of the NPs during initial 60 min. The process of thermooxidation of RB happened mostly in first 24 h at 5 C, and the decoloration rate of RB with Fe5NW-codoped TiO2 was as high as 94% in the dark. The COD was about 28.75 mg L1, and carbon removal rate reached to about 90%. While the oxidizing
Fig. 8. Kinetic curves of thermo-oxidation decoloration of RB at 5 C (A) and 60 C (B) in the dark (all catalysts were exposed to visible light (400e550 nm) for 15 min): (a) P25; (b) FeTiO2; (c) Fe20NWeTiO2; (d) Fe2.5NWeTiO; (e) Fe10NWeTiO2; (f) Fe5NWeTiO2.
temperature was risen to 60 C, the decoloration rate of RB rapidly increased, which was up to 96% in the dark after 120 min. The COD was only about 23.0 mg L1, and carbon removal rate reached to about 92%. So we affirmed that the decoloration and degradation mechanisms of FeNeWeTiO2 were closely linked to thermaloxidation, and the decoloration and degradation speeds increased with the increasing of oxidizing temperature. Compared with pure P25 and FeTiO2, both Fig. 8(A) and (B) showed that the decoloration and degradation rates of FeNeWeTiO2 rapidly increased in the dark. Superfluous W6þ seemed to be detrimental to the photodecoloration of RB, which was similar to all other analytic results. As shown in Fig. 9(a), the mechanism of reductive energy storage was used to account for low-temperature thermo-oxidation activity of FeNeW-codoped TiO2. WO3 and W6þ could gain photogenerated electrons from TiO2 conduction band, and were translated to HxWO3 and W5þ under visible light irradiation,
Fig. 9. (a) Mechanism of reductive energy storage of FeNeW-codoped TiO2; (b) Proposed model of electron and ion transfer in the charging and self-discharging processes of the FeNeW-codoped TiO2 NPs in solution.
X. Shi et al. / Materials Chemistry and Physics 135 (2012) 818e825
respectively. The number of HxWO3 and W5þ rapidly increased with the increasing of illumination time, and this was the process of energy storage of the catalyst. When the catalyst was placed in the dark, HxWO3 and W5þ were oxidized, and electrons were released (see Fig. 9(b)):
825
Acknowledgment
Mx WO3 /WO3 þ xe þ xMþ
(7)
This work was supported by the Academic Leader Program of Wuhan City (201150530146); the Fundamental Research Funds for the Central Universities (2010-II-020); the Nature Science Foundation of Hubei Province (20101j0018); and the Project for Science and Technology Plan of Wuhan City.
W5þ /W6þ þ e
(8)
References
2H2 O þ O2 þ 2e /H2 O2 þ 2OH
(9)
H2 O2 þ e /,OH þ OH
(10)
Some hydroxyl radicals with very strong oxidizability were generated. Subsequently, lots of RB molecules would combine with OH, and be oxidized to be CO2, H2O, as well as other small molecules. The creation speed of OH from chemical reaction rapidly increased with the increasing of the oxidation temperature, and then RB molecules were quickly decomposed. 4. Conclusions A simple solegel-mixing method was developed for the preparation of FeNeW-codoped TiO2 NPs, and the high photocatalytic performance and thermal-oxidation ability might result from the synergetic effect of tungsten, nitrogen and fluorine atoms in the catalyst. After performance test and analysis, the following results could be concluded: (1) F-dopant in the catalyst suppressed grain growth, and caused larger specific surface area & higher surface acidity. (2) The dopant of W and N atoms would generate WxTi1xO2 and OeTieN, which narrowed the band gap. Thus, the absorption edge of FeNeW-codoped TiO2 expanded to 550 nm, and valence variation of W ions (W6þ / W5þ) in the forbidden band decreased the recombination rate of electronehole pairs. Besides, FeNeWeTiO2 nanoclusters led to interfacial charge transfer. (3) W5þ could store some electrons while the FeNeWeTiO2 NPs were irradiated by visible light, and then provided a certain number of electrons to create some hydroxyl radicals (OH) in the dark. And those hydroxyl radicals owned very strong oxidizability to complete the decomposition of RB solution. (4) The FeNeWeTiO2 NPs could be widely applied in dyesensitized cells, water splitting into H2 and O2, and degradation of organic dye, especially under the environment without light illumination.
[1] C.J. Yu, J.G. Yu, W.G. Ho, Z.T. Jiang, L.Z. Zhang, Chem. Mater. 14 (2004) 3808e3816. [2] H.M. Yang, R.R. Shi, K. Zhang, Y.H. Hu, A.D. Tang, X.W. Li, J. Alloys Compd. 398 (2005) 200e202. [3] H. Tian, J.F. Ma, K. Li, J.J. Li, Mater. Chem. Phys. 112 (2008) 47e51. [4] W.Y. Choi, A. Termin, M.R. Hoffmann, J. Phys. Chem. 98 (1994) 13669e13679. [5] Y. Yang, H.Y. Wang, X. Li, C. Wang, Mater. Lett. 63 (2009) 331e333. [6] H.Y. Song, H.F. Jiang, X.Q. Liu, G.Y. Meng, J. Photochem. Photobiol. A 181 (2006) 421e428. [7] V. Iliev, D. Tomova, S. Rakovsky, A. Eliyas, G.L. Puma, J. Mol. Catal. A Chem. 327 (2010) 51e57. [8] J. Kim, O. Bondarchuk, B.D. Kay, Catal. Today 120 (2007) 186e195. [9] D.N. Ke, H.J. Liu, T.Y. Peng, X. Liu, K. Dai, Mater. Lett. 62 (2008) 447e450. [10] D.G. Huang, S.J. Liao, J.M. Liu, Z. Dang, L. Petrik, J. Photochem. Photobiol. A 184 (2006) 282e288. [11] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269e271. [12] Y.F. Shen, T.Y. Xiong, T.F. Li, K. Yang, Appl. Catal. B 83 (2008) 177e185. [13] S. Sakthivel, H. Kisch, Angew. Chem. Int. Ed. 42 (2003) 4908e4911. [14] T. Ohno, M. Akiyoshi, T. Umebayashi, K. Asai, T. Mitsui, M. Matsumura, Appl. Catal. A 265 (2004) 115e121. [15] S.J. Pan, H.C. Liang, C.Q. Wang, F.W. Yang, X.D. Li, S. Yang, Sci. China Ser. B Chem. 52 (2009) 2043e2046. [16] T. Tatsuma, S. Saitoh, Y. Ohko, A. Fujishima, Chem. Mater. 13 (2001) 2838e2842. [17] T. Tatsuma, S. Takeda, S. Saitoh, Y. Ohko, A. Fujishima, Electrochem. Commun. 5 (2003) 793e796. [18] T. Tatsuma, S. Saitoh, P. Ngaotrakanwiwat, Y. Ohko, A. Fujishima, Langmuir 18 (2002) 7777e7779. [19] J.J. Xu, Y.H. Ao, D.G. Fu, C.W. Yuan, Appl. Surf. Sci. 254 (2008) 3033e3038. [20] K. Demeestere, J. Dewulf, H.V. Langenhove, B. Sercu, Chem. Eng. Sci. 58 (2003) 2255e2267. [21] C. Wang, B.Q. Xu, X.M. Wang, J.C. Zhao, J. Solid State Chem. 178 (2005) 3500e3506. [22] J. Xu, B.F. Yang, M. Wu, Z.P. Fu, Y. Lv, Y.X. Zhao, J. Phys. Chem. C 114 (2010) 15251e15259. [23] Y.P He, Z.Y. Wu, L.M. Fu, C.R. Li, Y.M. Miao, L. Cao, H.M. Fan, B.S. Zou, Chem. Mater. 15 (2003) 4039e4045. [24] X.L. Yan, W.L. Dai, C.W. Guo, H. Chen, Y. Cao, H.X. Li, H.Y. He, K.N. Fan, J. Catal. 234 (2005) 438e450. [25] H.W. Park, W.Y. Choi, J. Phys. Chem. B 108 (2004) 4086e4093. [26] S. Penner, X.J. Liu, B. Klotzer, F. Klauser, B. Jenewein, E. Bertel, Thin Solid Films 516 (2008) 2829e2836. [27] M. Pelaez, A.A. de la Cruz, E. Stathatos, P. Falaras, D.D. Dionysiou, Catal. Today 144 (2009) 19e25. [28] M. Sathish, B. Viswanathan, R.P. Viswanath, Appl. Catal. B 74 (2007) 307e312. [29] 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) 2467e2472. [30] P. Cheng, C.S. Deng, D.N. Liu, X.M. Dai, Appl. Surf. Sci. 254 (2008) 3391e3396.
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Dual-responsive FeNeW-codoped TiO2 nanoparticles with strong visible light response and low-temperature thermo-oxidation activity Xiaoliang Shi a, b, *, Haibo Qin b, Qiaoxin Zhang a, b, Song Zhang a 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
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< FeNeW-codoped TiO2 NPs were synthesized by a simple solegelmixing method. < The NPs showed high visible photocatalytic performance. < The NPs showed excellent lowtemperature thermal-oxidation ability in the dark. < The NPs could be applied as environmental catalyst without light illumination.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 September 2011 Received in revised form 20 April 2012 Accepted 19 May 2012
FeNeW-codoped TiO2 nanoparticles were successfully prepared by a solegel-mixing method without hydrothermal or calcining treatment. Effects of the F, N and W ion dosage concentration on crystallinity, morphology, grain size and chemical status of TiO2 were investigated. The results showed that the appearance of FeNeW-codoped TiO2 powders was uniformly spheroidal nanoparticles, whose average particle size was about 7 nm. The FeNeW-codoped TiO2 nanoparticles owned high catalytic activity under visible light irradiation and low-temperature thermo-oxidation activity in the dark. The highest degradation rate of Rhodamine B dye was obtained by TiO2 powders modified with 0.5 at.% W (N/W ¼ 6/ 7) and 4.0 at.% F. Even in the dark, the degradation rate increased with the increasing of temperature. It was as high as 92% when the mixing liquid was kept at 60 C for 2 h. The high catalytic activity of the FeN eW-codoped TiO2 might result from larger specific surface area and high surface acidity because of surface F ions, the band gap narrowing caused by WxTi1xO2 or OeTieN, as well as lower recombination rate of charge carriers because of valence variation of W ions (W6þ / W5þ) in the forbidden band. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Semiconductors Chemical synthesis Electron microscopy Oxidation
1. Introduction TiO2 is a typical n-type semiconductor, which is considered as the most suitable photocatalyst because of biological and chemical inertness, non-toxicity, and long-term stability [1]. And it had been
* Corresponding author. School of Mechanical and Electronic Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China. Tel./fax: þ86 27 87651793. E-mail address: [email protected] (X. Shi). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.05.064
widely applied in various fields, such as decomposition of organic contaminants, development of efficient dye-sensitized solar cells and water photosplitting into H2 and O2 [2]. However, pure TiO2, whose band gap energy is 3.2 eV, only absorbs ultraviolet (UV) light. The fraction of UV light in the solar energy spectrum is only about 4%. Thus, its catalytic efficiency is very limited. Moreover, the high recombination rate of photoexcited charge carries results in the low quantum yield rate and undesirable photooxidation rate [3]. Hence, it is necessary to shift the absorption threshold toward the visible region and decrease recombination rate of excited
X. Shi et al. / Materials Chemistry and Physics 135 (2012) 818e825
electron/hole pairs. Researchers have made various modifications to improve the photocatalytic activity. The initial approach was the doping of TiO2 with metal elements [4]. W ion doped TiO2 is used to enhance photocatalytic properties in the visible range by some authors, and they indicated that its higher photocatalytic performance owed to the band gap narrowing created from WxTi1xO2 and efficiently inhibiting the recombination of photoinduced electrons and holes [5e8]. Once optical excitation occurs, the photogenerated electrons can be transferred to the lower-lying conduction band of WO3, while the holes will accumulate in the valence band of TiO2 [9]. H. Tian et al. considered that W-dopant might increase the charge separation efficiency and the presence of surface acidity [3]. V. Iliev et al. have prepared Au/WO3/TiO2 with high photocatalytic activity under visible light irradiation via calcining treatment [7]. However, D.G. Huang et al. pointed out that the doped metal materials were not thermally stable and acted as electron trapping centers to reduce the photocatalytic efficiency [10]. Other approaches were adopted to narrow the desired band gap of TiO2 by replacing lattice oxygen with anionic dopant species, such as N [11,12], C [13] and S [14]. R. Asahi et al. indicated that nitrogen doped into substitutional sites of TiO2 had been proven to be indispensable for band-gap narrowing and photocatalytic activity [8]. Y.F. Shen et al. [12] have synthesized NeW-codoped TiO2 nano-powders by a two-step method combining with solegel and mechanical alloying method, and the photocatalyst had excellent properties of strong absorbance in visible range, as long as 650 nm. F-doping could prevent the grain growth, enhance surface acidity and inhibit the transformation of anatase to rutile phase [1]. S.J. Pan et al. [15] reported the doped-F atoms also caused the increasing of surface acid sites and forming of Ti3þ ions. More surface acidic sites could not only adsorb more organics, but also act as an electron acceptor to reduce the eeh recombination rate. NeF-codoped TiO2 showed higher absorption and water splitting activity than N-doped TiO2 under visible light irradiation [10,15]. TiO2eWO3 combined system protected stainless steel from corrosion for 5 h after 1 h UV irradiation, due to the stored charge [16]. T. Tatsuma et al. reported that TiO2eWO3 combined system generated H2O2 by reducing oxygen to retain a bactericidal effect even after the light was turned off [17]. When TiO2eWO3 was irradiated by UV light, WO3 could be reduced even in pure water or humid air (relative humidity 25%) [18]. Although the mechanism has not yet been known, it means that one of the limitations of TiO2 is overcome. As we know, the TiO2 nanoparticles (NPs) with various doped species were usually synthesized by solegel-hydrothermal or solegel-calcining process. On one hand, these methods can guarantee intact crystal and high purity. On the other hand, they also may bring about many defects, such as grain growth, particles agglomeration and thermal invalidation of metal ions, which go actively against quantum size effect and valence variation of metal ions. To the best of our knowledge, this work may be the first report about synthesizing FeNeW-codoped TiO2 NPs by a simple solegel-mixing method without hydrothermal or calcining process in this work. And the visible light photocatalytic performance and thermal-oxidation ability in the dark of FeNeWcodoped TiO2 NPs were investigated by measuring the decomposition percentage of Rhodamine B dye (RB). Effects of F, N and W atoms dosage concentration on the crystallinity, morphology, chemical status and grain size of FeNeW-codoped TiO2 NPs was systematically characterized by various analysis and testing techniques. A tentative elucidation of the mechanism for the high visible light photocatalytic activity and especial low-temperature thermo-oxidation performance of the FeNeW-codoped TiO2 NPs was presented.
819
2. Experimental section 2.1. Catalyst preparation The fluorine, nitrogen and tungsten codoped TiO2 samples were prepared by a solegel-mixing 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 sources. All chemicals used in the experiment were analytical reagent grade. Firstly, a certain amount of ammonium tungstate (RW ¼ 0.25%, 0.5%, 1.0%, and 2.0%, the atomic percentage of W to Ti was designated as RW, RN/RW ¼ 6/ 7) was completely dissolved into 8 mL deionized water with 0.1 mL hydrofluoric acid (RF ¼ 4.0%, the atomic percentage of F to Ti was designated as RF), and then the mixing solution was slowly dripped into 80 mL anhydrous ethanol to form ivory sol-A during stirring. Secondly, 20 mL tetrabutyl titanate was dissolved into the mixture of 100 mL anhydrous ethanol & 5.0 mL acetic acid, and stirred for 30 min to form uniform transparent solution-B. Finally, the sol-A was slowly dripped into the solution-B under vigorous stirring. The resultant mixture gradually became a white transparent sol under ceaselessly stirring, and then the sol started to become colloidal particles slowly. During these processes as above, the temperature of solution was maintained below 5 C. After stewing for more than one day, the gained colloidal solution were diluted several times with deionized water, and then heated in water bath at 80 C for 2 h to become precipitate. The precipitates were washed by deionized water, then dried at 60 C in vacuum. The obtained catalyst was denoted as FeNeWeTiO2, including Fe5NWeTiO2, Fe10NWeTiO2 and Fe2.5NWeTiO2, Fe20NWeTiO2. For comparison, TiO2 was prepared by the similar method in the absence of solution A. TiO2 with only RF ¼ 4.0% was prepared in the absence of ammonium tungstate, denoted as FeTiO2. And TiO2 with Rw ¼ 0.5% (RN/RW ¼ 6/7) was prepared in the absence of hydrofluoric acid, denoted as 5NWeTiO2. TiO2 Degussa P25 (75% anatase and 25% rutile, specific area of 50 m2 g1, average particle size 30 nm, non-porous) was used for the purpose of comparison. 2.2. Analysis Purity and crystallinity of the as-prepared samples were examined by X-ray diffractometer (XRD, XD-3A, Shimadzu Corporation, Japan) using graphite monochromatic copper radiation (Cu Ka) at 40 kV, 30 mA over the 2q range of 10e80 [19]. The average crystallite size was determined according to the Scherrer equation:
D ¼ 0:89l=B cos q
(1)
where l is the wavelength of the characteristic X-ray applied; B is the half-value width of anatase (101) peak obtained by XRD; q is the diffraction angle [10,20]. An ultravioletevisible (UVeVis) 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/holes in the photocatalyst, the photoluminescence (PL) emission spectra of the samples were measured at room temperature by LS-55 (PerkineElmer) illuminated with a 325 nm HeeCd laser. Morphology and grain size of the NPs were observed by a JEM2100F field emission transmission electron microscope (FETEM) and field emission scanning electron microscope (FESEM). X-ray photoelectron spectra (XPS) of the samples were measured using a PHI5300 photoelectron spectrometer system with a Mg source (1486.6 eV). The shift of binding energy due to relative surface charging was corrected using the C1s level at 286.7 eV as an internal
820
X. Shi et al. / Materials Chemistry and Physics 135 (2012) 818e825
standard of the surface adventitious carbon. N2 adsorption analysis was conducted on an ASAP 2010 Surface Area to get the BrunauereEmmetteTeller (BET) surface area. 2.3. Photocatalytic and thermal-oxidation activity measurements Decomposition percentage of RB in an aqueous solution was studied to estimate the visible light photocatalytic activity and lowtemperature thermal-oxidation performance (60 C) of the FeNeW-codoped TiO2 NPs. The visible light source was 160 W tungsten halogen lamp connected to a light filter (wavelength: 400e550 nm), and the concentration of RB was 30 mg L1. The reaction was carried out in a 250 mL beaker with an electromagnetic stirrer. The amount of catalyst was 0.5 g L1. Prior to photoreaction, the system was kept at dark room for 60 min to reach adsorptionedesorption equilibrium condition, and then the illumination reaction lasted for 1.5 h. In the thermal-oxidation experiment, the catalyst was exposed to visible light (400e550 nm) irradiation for 15 min before mixing with RB solution, and then the mixing solution was placed at different temperature (5, 60 C) for 2 h in the dark after ultrasonic dispersing. The aim that the catalyst was exposed to visible light (400e550 nm) irradiation for 15 min before mixing with RB solution was to produce photoexcited electrons in the catalyst. The distinct characteristic of the catalyst was that the photoexcited electrons in the catalyst could be rapidly released with the increasing of environment temperature in the dark, which was defined as thermooxidation activity in this article. The samples were taken every 15 min, centrifuged and filtrated, orderly. The filtrates were analyzed by using a spectrophotometer (723, Shanghai spectrum Instruments Co., Ltd.) to detect the discoloration rate at its maximum absorption wavelength of 554 nm for RB solution [21]. If the chromophoric groups of RB are destroyed, and decolorization would happen. However, intermediate products, even more toxic, could be formed. Of course, RB also could have been degradated and mineralized during the process of decolorization. Thus, the relevant photocatalytic performance of degradation is to be evaluated with respect to the mineralization of RB, i.e. its conversion to CO2 and water by the chemical oxygen demand (COD) determinations of reaction liquid using K2CrO7, it could be calculated by:
D ¼ ½ðCOD0 CODÞ=COD0
(hkl). According to Bragg’s law, when Ti4þ in TiO2 crystal lattice was displaced by W6þ, the lattice parameter d(hkl) value would become smaller, because the ionic radius of W6þ (41 pm) is smaller than that of Ti4þ (53 pm) [3]. Nevertheless, Y. Yang et al. [5] observed that the peaks of anatase TiO2 with W6þ shifted toward smaller 2q value, and they thought W6þ might be located at interstitial sites. Thus, the results in our study indicated that some W6þ ions were introduced into TiO2 crystal lattice by the solegel-mixing method. Moreover, as shown in Fig. 1(b) and Fig. 1(f), the disparity between the peaks indicated that superfluous doped W atoms suppressed crystallization processes to cause bad crystallization, which was related to the interfacial charge neutralization. Then the d(hkl) value became larger. The WO3 nanoclusters with negative charges would be absorbed on the surface of TiO2 nanoclusters with positive charges when they were mixed [23]. 3.2. Morphology analysis
(2)
where COD0 and COD are the chemical oxygen demand of reaction liquid before and after the reaction, respectively [10]. 3. Result and discussion 3.1. Characterization of FeNeW-codoped TiO2 powders The XRD patterns of FeTiO2, 5NWeTiO2 and FeNeW-codoped TiO2 were shown in Fig. 1. The XRD analysis showed that all peaks (major peaks: 25.12 , 37.78 and 47.88 ) were ascribed to imperfectly crystallized anatase TiO2 without rutile and brookite in all samples. However, TiOF2, TiF4, TiN, WOx and WxTi1xO2 were not found, because the content was little. The similar results had been also observed by other authors [2,5,10,12,19,22]. It was obvious that F atoms efficiently suppressed the TiO2 grain growth, and the doping W atoms caused shifting of peak positions of anatase TiO2 to higher 2q value, which were consistent with the previous results [3]. Bragg’s Law refers to the simple equation:
dðhklÞ ¼ l=ð2 sin qÞ
Fig. 1. XRD patterns of the samples: (a) FeTiO2; (b) 5NWeTiO2 (RN/RW ¼ 6/7); (c) Fe2.5NWeTiO2; (d) Fe5NWeTiO2; (e) Fe10NWeTiO2; and (f) Fe20NWeTiO2.
(3)
where d(hkl) is the distance between crystal planes of (hkl); l is the X-ray wavelength; q is the diffraction angle of the crystal plane
Fig. 2 showed FESEM and FETEM images of Fe5NW-codoped TiO2 powders prepared by the solegel-mixing method. The FESEM image (see Fig. 2(a)) showed that the agglomeration phenomenon of FeNeW-codoped TiO2 NPs was serious. The FETEM image (see Fig. 2(b)) illustrated that the fringe spacing of 1.89 A could be indexed to the (200) plane of anatase TiO2. It indicated that the crystalline state of FeWeTiO2 at (200) plane was prone to grow. Besides, FETEM image (see Fig. 2(c)) displayed that a mass of Fe5NWeTiO2 NPs got together to form aggregated spheres with different sizes. And the grain sizes of these particles ranged from 5 nm to 20 nm. It was close to the value of average particle size calculated from the XRD analysis as shown in Table 1. 3.3. BET specific surface area analysis BET specific surface areas (SBET) (m2 g1) of the as-prepared samples were also shown in Table 1. It showed that the SBET met the following order: FeTiO2, Fe2.5NWeTiO2, Fe5NWeTiO2, Fe10NWeTiO2, Fe10NWeTiO2, and 5NWeTiO2. The SBET of FeTiO2 NPs was highest, up to 228.6 m2 g1. The SBET (m2 g1) of FeNeWeTiO2 NPs decreased with the increasing of W6þ dosage concentration, and the SBET of Fe5NWeTiO2 NPs was up to 210.5 m2 g1. The SBET of 5NWeTiO2 NPs was 187.6 m2 g1, which
X. Shi et al. / Materials Chemistry and Physics 135 (2012) 818e825
821
Table 1 BET surface areas and average particle sizes of the as-prepared samples.
Photocatalysts
Crystal composition
Average crystal size (nm)
SBET (m2 g1)
FeTiO2 5NWeTiO2 Fe2.5NWeTiO2 Fe5NWeTiO2 Fe10NWeTiO2 Fe20NWeTiO2
Anatase Anatase Anatase Anatase Anatase Anatase
6 15 7 9 12 14
228.6 183.9 217.8 210.5 200.1 187.6
and and and and and and
amorphous amorphous amorphous amorphous amorphous amorphous
was the lowest and in accordance with the result of XRD spectra. Thus, it was concluded that F played an important role in suppressing the grain growth, and N & W ion dopants might result in aggregation of NPs. 3.4. Chemical status of the F, N and W atoms in the FeNeWcodoped TiO2 catalyst
Fig. 2. (a) FESEM image of Fe7NWeTiO2; (b) and (c) FETEM images of Fe7NWeTiO2.
Fig. 3 showed the XPS spectra of the Fe5NW-codoped 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. The peaks of Ti2p were observed at 461.55 eV and 467.22 eV, which shifted toward to higher wavelength. The XPS signal of Ti2p3/2 could be fitted with two components. One attributed to Ti (IV) species, and the other assigned to Ti (III) species [24]. Thus, the peaks of Ti (IV) and Ti (III) might exist in the region of 462.0 eV. The peaks of F1s and N1s also showed that F and W atoms were incorporated into the TiO2 crystal lattice or adsorbed on the surface of the nanoclusters. Fig. 3(b) showed the F1s XPS spectra of the Fe5NW-codoped TiO2 NPs. The peaks located at 682.7 eV and 686.9 eV were designed to the characteristics of F1s peak, and it especially meant that there were two chemical forms of F existing in the Fe5NWcodoped TiO2 NPs. Using Gaussian distributions, the F1s peak was divided into two separated peaks [10]. One peak indicated that F was adsorbed on the surfaces of TiO2 particles, and the other peak stated that F entered into TiO2 crystal lattices [25]. Thus, as shown in Fig. 3(b), the peak observed at 682.7 eV was originated from the F-containing compounds adsorbing on the surface, and the peak located around 686.9 eV was attributed to the F atoms replacing the oxygen in the TiO2 crystal lattice [10]. Although any evidence of TiF4 and TiOF2 was not found in the XRD pattern, these two XPS peaks at 682.7 eV and 686.9 eV indicated that F atoms were incorporated into the FeNeW-codoped TiO2 crystal lattice to form OeTieF bonds by nucleophilic substitution reaction of F ions during the hydrolysis process. Two energy levels of tungsten were measured in the W4d and W4f region as shown in Fig. 3(a). The W4f peak was used to determine the oxidation state of tungsten, and the binding energy of W4f7/2 corresponded to the characteristic of W6þ [26]. The W4f peak in Fig. 3(c) located at about 35.9 eV was observed, which was similar to the results of V. Iliev et al. [7]. So we could propose that the W6þ existed, and was absorbed on the surface of TiO2 NPs or entered into TiO2 crystal lattices. As shown in Fig. 3(d), the N1s XPS spectrum displayed a large broadness, and main peaks emerged at 400.5 eV. M. Pelaez et al. [27] and M. Sathish et al. [28] prepared Ncodoped TiO2 powders by a solegel method, and they suggested that the N1s peak at about 400 eV was due to the formation of the OeTieN bonds. So it was concluded that N atoms were incorporated into TiO2 crystal lattice, and OeTieN bonds had been formed by the solegel-mixing method.
822
X. Shi et al. / Materials Chemistry and Physics 135 (2012) 818e825
Fig. 3. X-ray photoelectron spectra of Fe5NWeTiO2 NPs: (a) Global range XPS; (b) F1s peaks; (c) W4f peak; (d) N1s peak.
3.5. UVeVis diffuse reflectance spectra (DRS) and PL spectra Fig. 4 showed the absorption spectra of the various NPs. As to Fdoped TiO2 (curve (e)), the onset of the absorption appeared at about 405 nm (3.06 eV). Compared with the onset of anatase TiO2 (l ¼ 387 nm), it had obvious red-shift, which was in accordance
Fig. 4. UVeVis spectra of the samples: (a) Fe5NWeTiO2; (b) Fe10NWeTiO2; (c) 5NWeTiO2; (d) Fe2.5NWeTiO2; (e) FeTiO2.
with results obtained by other authors [2,7,12]. In addition, the absorption edge of 5NWeTiO2 slightly shifted toward higher wavelengths, and the onset of the absorption appeared at about 450 nm (2.76 eV), which was close to the band-gap energies of WO3 (2.75 eV). Especially for FeNeW-doped TiO2, the onset of the absorption changed with the increasing of W and N atoms. The absorption edge of Fe5NWeTiO2 could locate at about 550 nm, and the band gap energy was 2.26 eV, which was much lower than that of anatase TiO2 and others. It may contribute to the quantum size effect in nanoclusters. The OeTieN and WxTi1xO2 made NPs be efficiently excited by visible light. And interfacial charges were easily transferred. However, the absorption edge of both Fe10NWeTiO2 and Fe20NWeTiO2 decreased, because superfluous W and N atoms would act as the recombination center of charge carries. PL emission spectra had been used to investigate the efficiency of charge transfer and recombination rate of electronehole pairs in semiconductor particles [1,19]. The lower excitonic PL intensity corresponded to the higher separation rate of electrons and holes [29]. Fig. 5 showed the PL spectra of the various samples. The excitonic PL intensity obeyed the following order: FeTiO2 > 5NW eTiO2 > Fe2.5NWeTiO2 > Fe10NWeTiO2 > Fe5NWeTiO2. Compared with FeTiO2, TiO2 with a certain amount of F, N and W atoms could sharply decrease the excitonic PL intensity. According to the recent study, TiO2 has slightly larger band gap than WO3. Both the top of the valence band and the bottom of the conduction band of WO3 are lower than that of TiO2 (as indicated in Fig. 6). As to FeTiO2, the photogenerated electrons and holes usually combined quickly. When FeNeW-codoped TiO2 NPs was excited by light, the photogenerated electrons could be transferred
X. Shi et al. / Materials Chemistry and Physics 135 (2012) 818e825
823
3.6. Photocatalytic activity The decoloration and degradation experiments of RB solution (30 mg L1, COD0 ¼ 287.5 mg L1) were chosen to investigate the photocatalytic activity of the composite catalysts, and their kinetic curves of photodecoloration of RB in the visible light (400e550 nm) were shown in Fig. 7. The decoloration reaction was closely related to the reaction time. It was interesting that the FeNeW-doped TiO2 NPs became red in uniformly mixed solution, and then gradually became yellowgreen after illumination for about 60 min. The yellowgreen color was very obvious after about 120 min. But when the illumination time extended to about 150 min, the yellowgreen NPs changed back white. While the red FeTiO2 NPs did not become yellowgreen, and directly became white during decoloration experiment. According to the phenomenon, we deduced W6þ on the surface of the NPs accepted some photoexcited electrons to be converted to W5þ, and then yellowgreen MxWO3 was created after a period of illumination time, following the scheme: Fig. 5. PL spectra of the samples: (a) FeTiO2; (b) 5NWeTiO2; (c) Fe2.5NWeTiO2; (d) Fe10NWeTiO2; (e) Fe5NWeTiO2.
from the TiO2 conduction band to the WO3 conduction band, and holes can be transferred from the WO3 valence band to the TiO2 valence band, following the scheme [3]:
TiO2 =WO3 þ hv/TiO*2 =WO*3 þ e þ hþ
(4)
W6þ þ e ðTiO2 Þcb /W5þ
(5)
The photoexcited electrons in the conduction band of TiO2 can be accepted by W6þ [30]. Thus, as shown in Fig. 5, a certain range of doping W6þ could enhance separation efficiency of charge carries for TiO2. However, too high-loaded W would act as recombination centers for electronehole pairs and result in the increasing of the excitonic PL intensity of Fe10NWeTiO2. The absorption edge of Fe5NW-doped TiO2 could expand to as large as 550 nm, which mostly owed to the synergetic effect of the doped N, W and F atoms. The larger specific surface area is, the more there are oxygen vacancies existing. OeTieN and WxTi1xO2 facilitated to generate and transfer more excited electrons, and the valence transition between W6þ & W5þ in the NPs could effectively separate photogenerated carriers.
Fig. 6. Band gap structure diagram of TiO2eWO3 and the separation process of electronehole pairs.
WO3 þ xe þ MHþ /Mx WO3 ðyellowgreenÞ
(6)
It was observed that the photodecoloration activity of FeTiO2 photocatalyst was up to 60% after 120 min under visible light irradiation as shown in Fig. 7(b). It might be attributed to the enhancement of surface acidity, higher specific surface areas and the creation of oxygen vacancies, which made hydroxyl radicals (OH) seize more RB molecules [1,30]. Compared with FeTiO2, the photoactivity of FeNeW-codoped TiO2 evidently augmented, which was in line with the result of UVeVis spectra and PL spectra as shown in Figs. 4 and 5. It was noteworthy that superfluous W6þ seemed to be detrimental to the photodecoloration of RB, and the optimal doping amount of W6þ in the NPs was found to be about 0.5 at.%. The decoloration percent of RB was as high as 99% using Fe5NWeTiO2 (see Fig. 7(f)), which was remarkably higher than that of the others. It demonstrated that the moderate dopant of F, N and W would absorb more visible light and reduce the recombination rate of electronehole pairs. The COD was only about 14.38 mg L1, and carbon removal rate reached to about 95%. Thus, the high photodecoloration and photodegradation activities of Fe5NW-codoped TiO2 were ascribed to the synergetic effect of the doping N, W
Fig. 7. Kinetic curves of photodecoloration of RB in the presence of visible light (400e550 nm) (C0 ¼ The initial concentration of RB; C ¼ The concentration of RB at various times): (a) P25; (b) FeTiO2; (c) Fe20NWeTiO2; (d) Fe2.5NWeTiO2; (e) Fe10NWeTiO2; (f) Fe5NWeTiO2.
824
X. Shi et al. / Materials Chemistry and Physics 135 (2012) 818e825
and F atoms. The larger specific surface area was, the more oxygen vacancies were created from doped-F ions. RB molecules could be absorbed on the particle surface, because RB was a cationic dye. Meanwhile, OeTieN and WxTi1xO2 facilitated to generate more excited electrons, and W6þ in the NPs efficiently suppressed the recombination of electronehole pairs. Thus, more photoexcited electrons were transferred to the particle surface, and then more hydroxyl radicals were generated. Afterward, a mass of OH disrupted RB molecules, and then decomposition was completed under visible light irradiation. 3.7. Low-temperature thermo-oxidation activity The decoloration and degradation experiments of RB solution (30 mg L1, COD0 ¼ 287.5 mg L1) were chosen to investigate the low-temperature thermo-oxidation activity of the NPs, and their kinetic curves of thermo-oxidation decoloration of RB at about 5 C and 60 C in the dark were shown in Fig. 8(A) and (B), respectively. As shown in Fig. 8(A), lots of RB molecules mostly absorbed on the surface of the NPs during initial 60 min. The process of thermooxidation of RB happened mostly in first 24 h at 5 C, and the decoloration rate of RB with Fe5NW-codoped TiO2 was as high as 94% in the dark. The COD was about 28.75 mg L1, and carbon removal rate reached to about 90%. While the oxidizing
Fig. 8. Kinetic curves of thermo-oxidation decoloration of RB at 5 C (A) and 60 C (B) in the dark (all catalysts were exposed to visible light (400e550 nm) for 15 min): (a) P25; (b) FeTiO2; (c) Fe20NWeTiO2; (d) Fe2.5NWeTiO; (e) Fe10NWeTiO2; (f) Fe5NWeTiO2.
temperature was risen to 60 C, the decoloration rate of RB rapidly increased, which was up to 96% in the dark after 120 min. The COD was only about 23.0 mg L1, and carbon removal rate reached to about 92%. So we affirmed that the decoloration and degradation mechanisms of FeNeWeTiO2 were closely linked to thermaloxidation, and the decoloration and degradation speeds increased with the increasing of oxidizing temperature. Compared with pure P25 and FeTiO2, both Fig. 8(A) and (B) showed that the decoloration and degradation rates of FeNeWeTiO2 rapidly increased in the dark. Superfluous W6þ seemed to be detrimental to the photodecoloration of RB, which was similar to all other analytic results. As shown in Fig. 9(a), the mechanism of reductive energy storage was used to account for low-temperature thermo-oxidation activity of FeNeW-codoped TiO2. WO3 and W6þ could gain photogenerated electrons from TiO2 conduction band, and were translated to HxWO3 and W5þ under visible light irradiation,
Fig. 9. (a) Mechanism of reductive energy storage of FeNeW-codoped TiO2; (b) Proposed model of electron and ion transfer in the charging and self-discharging processes of the FeNeW-codoped TiO2 NPs in solution.
X. Shi et al. / Materials Chemistry and Physics 135 (2012) 818e825
respectively. The number of HxWO3 and W5þ rapidly increased with the increasing of illumination time, and this was the process of energy storage of the catalyst. When the catalyst was placed in the dark, HxWO3 and W5þ were oxidized, and electrons were released (see Fig. 9(b)):
825
Acknowledgment
Mx WO3 /WO3 þ xe þ xMþ
(7)
This work was supported by the Academic Leader Program of Wuhan City (201150530146); the Fundamental Research Funds for the Central Universities (2010-II-020); the Nature Science Foundation of Hubei Province (20101j0018); and the Project for Science and Technology Plan of Wuhan City.
W5þ /W6þ þ e
(8)
References
2H2 O þ O2 þ 2e /H2 O2 þ 2OH
(9)
H2 O2 þ e /,OH þ OH
(10)
Some hydroxyl radicals with very strong oxidizability were generated. Subsequently, lots of RB molecules would combine with OH, and be oxidized to be CO2, H2O, as well as other small molecules. The creation speed of OH from chemical reaction rapidly increased with the increasing of the oxidation temperature, and then RB molecules were quickly decomposed. 4. Conclusions A simple solegel-mixing method was developed for the preparation of FeNeW-codoped TiO2 NPs, and the high photocatalytic performance and thermal-oxidation ability might result from the synergetic effect of tungsten, nitrogen and fluorine atoms in the catalyst. After performance test and analysis, the following results could be concluded: (1) F-dopant in the catalyst suppressed grain growth, and caused larger specific surface area & higher surface acidity. (2) The dopant of W and N atoms would generate WxTi1xO2 and OeTieN, which narrowed the band gap. Thus, the absorption edge of FeNeW-codoped TiO2 expanded to 550 nm, and valence variation of W ions (W6þ / W5þ) in the forbidden band decreased the recombination rate of electronehole pairs. Besides, FeNeWeTiO2 nanoclusters led to interfacial charge transfer. (3) W5þ could store some electrons while the FeNeWeTiO2 NPs were irradiated by visible light, and then provided a certain number of electrons to create some hydroxyl radicals (OH) in the dark. And those hydroxyl radicals owned very strong oxidizability to complete the decomposition of RB solution. (4) The FeNeWeTiO2 NPs could be widely applied in dyesensitized cells, water splitting into H2 and O2, and degradation of organic dye, especially under the environment without light illumination.
[1] C.J. Yu, J.G. Yu, W.G. Ho, Z.T. Jiang, L.Z. Zhang, Chem. Mater. 14 (2004) 3808e3816. [2] H.M. Yang, R.R. Shi, K. Zhang, Y.H. Hu, A.D. Tang, X.W. Li, J. Alloys Compd. 398 (2005) 200e202. [3] H. Tian, J.F. Ma, K. Li, J.J. Li, Mater. Chem. Phys. 112 (2008) 47e51. [4] W.Y. Choi, A. Termin, M.R. Hoffmann, J. Phys. Chem. 98 (1994) 13669e13679. [5] Y. Yang, H.Y. Wang, X. Li, C. Wang, Mater. Lett. 63 (2009) 331e333. [6] H.Y. Song, H.F. Jiang, X.Q. Liu, G.Y. Meng, J. Photochem. Photobiol. A 181 (2006) 421e428. [7] V. Iliev, D. Tomova, S. Rakovsky, A. Eliyas, G.L. Puma, J. Mol. Catal. A Chem. 327 (2010) 51e57. [8] J. Kim, O. Bondarchuk, B.D. Kay, Catal. Today 120 (2007) 186e195. [9] D.N. Ke, H.J. Liu, T.Y. Peng, X. Liu, K. Dai, Mater. Lett. 62 (2008) 447e450. [10] D.G. Huang, S.J. Liao, J.M. Liu, Z. Dang, L. Petrik, J. Photochem. Photobiol. A 184 (2006) 282e288. [11] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269e271. [12] Y.F. Shen, T.Y. Xiong, T.F. Li, K. Yang, Appl. Catal. B 83 (2008) 177e185. [13] S. Sakthivel, H. Kisch, Angew. Chem. Int. Ed. 42 (2003) 4908e4911. [14] T. Ohno, M. Akiyoshi, T. Umebayashi, K. Asai, T. Mitsui, M. Matsumura, Appl. Catal. A 265 (2004) 115e121. [15] S.J. Pan, H.C. Liang, C.Q. Wang, F.W. Yang, X.D. Li, S. Yang, Sci. China Ser. B Chem. 52 (2009) 2043e2046. [16] T. Tatsuma, S. Saitoh, Y. Ohko, A. Fujishima, Chem. Mater. 13 (2001) 2838e2842. [17] T. Tatsuma, S. Takeda, S. Saitoh, Y. Ohko, A. Fujishima, Electrochem. Commun. 5 (2003) 793e796. [18] T. Tatsuma, S. Saitoh, P. Ngaotrakanwiwat, Y. Ohko, A. Fujishima, Langmuir 18 (2002) 7777e7779. [19] J.J. Xu, Y.H. Ao, D.G. Fu, C.W. Yuan, Appl. Surf. Sci. 254 (2008) 3033e3038. [20] K. Demeestere, J. Dewulf, H.V. Langenhove, B. Sercu, Chem. Eng. Sci. 58 (2003) 2255e2267. [21] C. Wang, B.Q. Xu, X.M. Wang, J.C. Zhao, J. Solid State Chem. 178 (2005) 3500e3506. [22] J. Xu, B.F. Yang, M. Wu, Z.P. Fu, Y. Lv, Y.X. Zhao, J. Phys. Chem. C 114 (2010) 15251e15259. [23] Y.P He, Z.Y. Wu, L.M. Fu, C.R. Li, Y.M. Miao, L. Cao, H.M. Fan, B.S. Zou, Chem. Mater. 15 (2003) 4039e4045. [24] X.L. Yan, W.L. Dai, C.W. Guo, H. Chen, Y. Cao, H.X. Li, H.Y. He, K.N. Fan, J. Catal. 234 (2005) 438e450. [25] H.W. Park, W.Y. Choi, J. Phys. Chem. B 108 (2004) 4086e4093. [26] S. Penner, X.J. Liu, B. Klotzer, F. Klauser, B. Jenewein, E. Bertel, Thin Solid Films 516 (2008) 2829e2836. [27] M. Pelaez, A.A. de la Cruz, E. Stathatos, P. Falaras, D.D. Dionysiou, Catal. Today 144 (2009) 19e25. [28] M. Sathish, B. Viswanathan, R.P. Viswanath, Appl. Catal. B 74 (2007) 307e312. [29] 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) 2467e2472. [30] P. Cheng, C.S. Deng, D.N. Liu, X.M. Dai, Appl. Surf. Sci. 254 (2008) 3391e3396.